Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and 2019, with several lava flows appearing in late 2019. This report covers continuing activity from January-June 2020 when ongoing explosive events produced ash plumes, pyroclastic flows, and the growth of new dome inside the crater. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Explosions with ash plumes rising up to three kilometers above the summit area were intermittent from late January through early June 2020. Some of the larger explosions produced pyroclastic flows that traveled down multiple flanks. Thermal anomalies within the Nicanor crater were recorded in satellite data several times each month from February through June. A reduction in overall activity led SERNAGEOMIN to lower the Alert Level from Orange to Yellow (on a 4-level, Green-Yellow-Orange-Red scale) during the first week of March, although tens of explosions with ash plumes were still recorded during March and April. Explosive activity diminished in early June and SERNAGEOMIN reported the growth of a new dome inside the Nicanor crater. By the end of June, a new flow had extended about 100 m down the N flank. Thermal activity recorded by the MIROVA project showed a drop in thermal energy in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in thermal and explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June (figure 52).

Figure 52. MIROVA thermal anomaly data for Nevados de Chillan from 8 September 2019 through June 2020 showed a drop in thermal activity in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June. Courtesy of MIROVA.

Weak gas emissions were reported daily during January 2020 until a series of explosions began on the 21st. The first explosion rose 100 m above the active crater; the following day, the highest explosion rose 1.6 km above the crater. The Buenos Aires VAAC reported pulse emissions visible in satellite imagery on 21 and 24 January that rose to 3.9-4.3 km altitude and drifted SE and NE, respectively. Intermittent explosions continued through 26 January. Incandescent ejecta was observed during the night of 28-29 January. The VAAC reported an isolated emission on 29 January that rose to 5.2 km altitude and drifted E. A larger explosion on 30 January produced an ash plume that SERNAGEOMIN reported at 3.4 km above the crater (figure 53). It produced pyroclastic flows that traveled down ravines on the NNE and SE flanks. The Washington VAAC reported on behalf of the Buenos Aires VAAC that an emission was observed in satellite imagery on 30 January that rose to 4.9 km altitude and was moving rapidly E, reaching 15 km from the summit at midday. The altitude of the ash plume was revised two hours later to 7.3 km, drifting NNE and rapidly dissipating. Satellite images identified two areas of thermal anomalies within the Nicanor crater that day. One was the same emission center (CE4) identified in November 2019, and the second was a new emission center (CE5) located 60 m NW.

Figure 53. A significant explosion and ash plume from the Nicanor crater at Nevados de Chillan on 30 January 2020 produced an ash plume reported at 7.3 km altitude. The left image was taken within one minute of the initial explosion. Images posted by Twitter accounts #EmergenciasÑuble (left) and T13 (right); original photographers unknown.

When the weather permitted, low-altitude mostly white degassing was seen during February 2020, often with traces of fine-grained particulate material. Incandescence at the crater was observed overnight during 4-5 February. The Buenos Aires VAAC reported an emission on 14 February visible in the webcam. The next day, an emission was visible in satellite imagery at 3.9 km altitude that drifted E. Episodes of pulsating white and gray plumes were first observed by SERNAGEOMIN beginning on 18 February and continued through 25 February (figure 54). The Buenos Aires VAAC reported pulses of ash emissions moving SE on 18 February at 4.3 km altitude. Ash drifted E the next day at 3.9 km altitude and a faint plume was briefly observed on 20 February drifting N at 3.7 km altitude before dissipating. Sporadic pulses of ash moved SE from the volcano on 22 February at 4.3 km altitude, briefly observed in satellite imagery before dissipating. Thermal anomalies were visible from the Nicanor crater in Sentinel-2 satellite imagery on 23 and 28 February.

Figure 54. An ash emission at Nevados de Chillan on 18 February 2020 was captured in Sentinel-2 satellite imagery drifting SE (left). Thermal anomalies within the Nicanor crater were measured on 23 (right) and 28 February. Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Only low-altitude degassing of mostly steam was reported for the first half of March 2020. When SERNAGEOMIN lowered the Alert Level from Orange to Yellow on 5 March, they reduced the affected area from 5 km NE and 3 km SW of the crater to a radius of 2 km around the active crater. Thermal anomalies were recorded at the Nicanor crater in Sentinel-2 imagery on 4, 9, 11, 16, and 19 March (figure 55). A new series of explosions began on 19 March; 44 events were recorded during the second half of the month (figure 56). Webcams captured multiple explosions with dense ash plumes; on 25 and 30 March the plumes rose more than 2 km above the crater. Fine-grained ashfall occurred in Las Trancas (10 km SW) on 25 March. Pyroclastic flows on 25 and 30 March traveled 300 m NE, SE, and SW from the crater. Incandescence was observed at night multiple times after 20 March. The Buenos Aires VAAC reported several discrete pulses of ash that rose to 4.3 km altitude and drifted SE on 20 and 21 March, SW on 25 March, and SE on 29 and 30 March. Another ash emission rose to 5.5 km altitude later on 30 March and drifted SE.

Figure 55. Sentinel-2 Satellite imagery of Nevados de Chillan during March 2020 showed thermal anomalies on five different dates at the Nicanor crater, including on 9, 11, and 16 March. A second thermal anomaly of unknown origin was also visible on 11 March about 2 km SW of the crater (center). Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Figure 56. Forty-four explosive events were recorded at Nevados de Chillan during the second half of March 2020 including on 19 March. Courtesy of SERNAGEOMIN webcams and chillanonlinenoticia.

In their semi-monthly reports for April 2020, SERNAGEOMIN reported 94 explosive events during the first half of the month and 49 during the second half; many produced dense ash plumes. The Buenos Aires VAAC reported frequent intermittent ash emissions during 1-13 April reaching altitudes of 3.7-4.3 km (figure 57). They reported the plume on 8 April visible in satellite imagery at 7.3 km altitude drifting SE. An emission on 13 April was also visible in satellite imagery at 6.1 km altitude drifting NE.

Figure 57. Sentinel-2 satellite imagery captured a strong thermal anomaly and an ash plume drifting SE from Nevados de Chillan on 10 April 2020. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

During the second half of April 2020, SERNAGEOMIN reported that only one plume exceeded 2 km in height; on 21 April, it rose to 2.4 km above the crater (figure 58). The Buenos Aires VAAC reported isolated pulses of ash on 18, 26, 28, and 30 April. During the second half of April SERNAGEOMIN also reported that a pyroclastic flow traveled about 1,200 m from the crater rim down the SE flank. The ash from the pyroclastic flow drifted SE and S as far as 3.5 km. Satellite images showed continued activity from multiple emission centers around the crater. Pronounced scarps were noted on the internal walls of the crater, attributed to the deepening of the crater from explosive activity.

Figure 58. Tens of explosions were reported at Nevados de Chillan during the second half of April 2020 that produced dense ash plumes. The plume on 21 April rose 2.4 km above the Nicanor crater. Photo by Josefa Carrasco Acuña from San Fabián de Alico; posted by Noticias Valpo Express.

Intermittent explosive activity continued during May 2020. The plumes contained abundant particulate material and were accompanied by periodic pyroclastic flows and incandescent ejecta around the active crater, especially visible at night. The Buenos Aires VAAC reported several sporadic weak ash emissions during the first week of May that rose to 3.7-5.2 km altitude and drifted NE. SERNAGEOMIN reported that only one explosion produced an ash emission that rose more than two km above the crater during the first two weeks of the month; on 6 May it rose to 2.5 km above the crater and drifted NE. They also observed pyroclastic flows on the E and SE flanks that day. Additional pyroclastic flows traveled 450 m down the S flank during the first half of the month, and similar deposits were observed to the N and NE. Satellite observations showed various emission points along the NW-trending lineament at the summit and multiple erosion scarps. Major erosion was noted at the NE rim of the crater along with an increase in degassing around the rim.

During the second half of May 2020 most of the ash plumes rose less than 2 km above the crater; a plume from one explosion on 22 May rose 2.2 km above the crater; the Buenos Aires VAAC reported the plume at 5.5 km altitude drifting NW (figure 59). Continuing pyroclastic emissions deposited material as far as 1.5 km from the crater rim on the NNW flank. There were also multiple pyroclastic deposits up to 500 m from the crater directed N and NE during the period. SERNAGEOMIN reported an increase in steam degassing between Nuevo-Nicanor and Nicanor-Arrau craters.

Figure 59. Explosions produced dense ash plumes and pyroclastic flows at Nevados de Chillan multiple times during May 2020 including on 22 May. Courtesy of SERNAGEOMIN.

Webcam images during the first two weeks of June 2020 indicated multiple incandescent explosions. On 3 and 4 June plumes from explosions reached heights of over 1.25 km above the crater; the Buenos Aires VAAC reported them drifting NW at 3.9 km altitude. Incandescent ejecta on 6 June rose 760 m above the vent and drifted NE. In addition, pyroclastic flows were distributed on the N, NW, E and SE flanks. Significant daytime and nighttime incandescence was reported on 6, 9, and 10 June (figure 60). The VAAC reported emission pulses on 6 and 9 June drifting E and SE at 4.3 km altitude.

Figure 60. Multiple ash plumes with incandescence were reported at Nevados de Chillan during the first ten days of June 2020 including on 6 June, after which explosive activity decreased significantly. Courtesy of SERNAGEOMIIN and Sismo Alerta Mexicana.

SERNAGEOMIN reported that beginning on the afternoon of 9 June 2020 a tremor-type seismic signal was first recorded, associated with continuous emission of gas and dark gray ash that drifted SE (figure 61). A little over an hour later another tremor signal began that lasted for about four hours, followed by smaller discrete explosions. A hybrid-type earthquake in the early morning of 10 June was followed by a series of explosions that ejected gas and particulate matter from the active crater. The vent where the emissions occurred was located within the Nicanor crater close to the Arrau crater; it had been degassing since 30 May.

Figure 61. A tremor-type seismic signal was first recorded on the afternoon of 9 June 2020 at Nevados de Chillan. It was associated with the continuous emission of gas and dark gray ash that drifted SE, and incandescent ejecta visible after dark. View is to the S, courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

After the explosions on the afternoon of 9 June, a number of other nearby vents became active. In particular, the vent located between the Nuevo and Nicanor craters began emitting material for the first time during this eruptive cycle. The explosion also generated pyroclastic flows that traveled less than 50 m in multiple directions away from the vent. Abundant incandescent material was reported during the explosion early on 10 June. Deformation measurements showed inflation over the previous 12 days.

SERNAGEOMIN identified a surface feature in satellite imagery on 11 June 2020 that they interpreted as a new effusive lava dome. It was elliptical with dimensions of about 85 x 120 m. In addition to a thermal anomaly attributed to the dome, they noted three other thermal anomalies between the Nuevo, Arrau, and Nicanor craters. They reported that within four days the base of the active crater was filled with effusive material. Seismometers recorded tremor activity after 11 June that was interpreted as associated with lava effusion. Incandescent emissions were visible at night around the active crater. Sentinel-2 satellite imagery recorded a bright thermal anomaly inside the Nicanor crater on 14 June (figure 62).

Figure 62. A bright thermal anomaly was recorded inside the Nicanor crater at Nevados de Chillan on 14 June 2020. SERNAGEOMIN scientists attributed it to the growth of a new lava dome within the crater. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

A special report from SERNAGEOMIN on 24 June 2020 noted that vertical inflation had increased during the previous few weeks. After 20 June the inflation rate reached 2.49 cm/month, which was considered high. The accumulated inflation measured since July 2019 was 22.5 cm. Satellite imagery continued to show the growth of the dome, and SERNAGEOMIN scientists estimated that it reached the E edge of the Nicanor crater on 23 June. Based on these images, they estimated an eruptive rate of 0.1-0.3 m³/s, about two orders of magnitude faster than the Gil-Cruz dome that emerged between December 2018 and early 2019.

Webcams revealed continued low-level explosive activity and incandescence visible both during the day and at night. By the end of June, webcams recorded a lava flow that extended 94 m down the N flank from the Nicanor crater and continued to advance. Small explosions with abundant pyroclastic debris produced recurring incandescence at night. Satellite infrared imagery indicated thermal radiance from effusive material that covered an area of 37,000 m², largely filling the crater. DEM analysis suggested that the size of the crater had tripled in volume since December 2019 due largely to erosion from explosive activity since May 2020. Sentinel-2 satellite imagery showed a bright thermal anomaly inside the crater on 27 June.

Geologic Background. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The largest stratovolcano, dominantly andesitic, Cerro Blanco (Volcán Nevado), is located at the NW end of the group. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The new Volcán Nuevo lava-dome complex formed between 1906 and 1945 between the two volcanoes and grew to exceed Volcán Viejo in elevation. The Volcán Arrau dome complex was constructed SE of Volcán Nuevo between 1973 and 1986 and eventually exceeded its height.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); #EmergenciasÑuble (URL: https://twitter.com/urgenciasnuble/status/1222943399185207296); T13, Channel 13 Press Department (URL: https://twitter.com/T13/status/1222951071443771394); Chillanonlinenoticia (URL: https://twitter.com/ChillanOnline/status/1240754211932995595); Noticias Valpo Express (URL: https://twitter.com/NoticiasValpoEx/status/1252715033131388928); Sismo Alerta Mexicana (URL: https://twitter.com/Sismoalertamex/status/1269351579095691265); Volcanology Chile (URL: https://twitter.com/volcanologiachl/status/1270548008191643651).

Bagana lies in a nearly inaccessible mountainous tropical rainforest area of Bougainville Island in Papua New Guinea and is primarily monitored by satellite imagery of ash plumes and thermal anomalies. After a state of elevated activity that lasted through December 2018 (BGVN 43:05, 44:06, 44:12), the volcano entered a quieter period that persisted through at least May 2020. This report focuses on activity between December 2019 and May 2020.

Atmospheric clouds often obscured satellite views of the volcano during the reporting period. When the volcano could be observed, light-colored gas plumes were often observed (figure 43). Based on satellite and wind model data, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that during 29 February-2 March ash plumes rose to an altitude of 1.8-2.1 km and drifted SW and N. On 1 May an ash plume rose to an altitude of 3 km and drifted NW and W. According to both Darwin VAAC volcanic ash advisories, the Aviation Color Code was Orange (second highest of four hazard levels).

Figure 43. Sentinel-2 image of Bagana, showing a gas plume drifting SE on 13 March 2020, during a period when the Darwin VAAC had not reported any ash explosions (Natural Color rendering, bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system recorded only intermittent thermal anomalies, all of which were of low radiative power. Sulfur dioxide emissions detected by satellite-based instruments over this reporting period were at low levels.

Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/).

Kerinci is a stratovolcano located in Sumatra, Indonesia that has been characterized by explosive eruptions with ash plumes and gas-and-steam emissions. The most recent eruptive episode began in April 2018 which has included intermittent explosions and ash plumes. The previous report (BGVN 44:12) described more recent activity consisting of intermittent gas-and-steam and ash plumes which occurred during June through early November 2019. This volcanism continued through May 2020, though little to no activity was reported during December 2019. The primary source of information for this report comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

Activity during December 2019 consisted of white gas-and-steam emissions rising 100-500 m above the summit. White and brown emissions continued intermittently through May 2020, rising to a maximum altitude of 1 km above the summit on 14 April. During 3-6 and 8-9 January 2020, the Darwin VAAC and PVMBG issued notices reporting brown volcanic ash rising 150-600 m above the summit drifting S and ESE (figure 19). PVMBG published a VONA notice on 24 January at 0828 reporting ash rising 400 m above the summit. Brown emissions continued intermittently throughout the reporting period. On 1 February, volcanic ash was observed rising 300-960 m above the summit and drifting NE; PVMBG reported continuing brown emissions during 1-3 February. During 16-17 February, two VONA notices reported that brown ash plumes rose 150-400 m above the summit and drifted SW accompanied by consistent white gas-and-steam emissions (figure 20).

Figure 19. Brown ash plume rose 500-600 m above Kerinci on 4 January 2020. Courtesy of MAGMA Indonesia via Øystein Lund Andersen.

Figure 20. White gas-and-steam emissions rose 400 m above Kerinci on 19 February 2020. Courtesy of MAGMA Indonesia via Øystein Lund Andersen.

During 1-16 and 25-26 March 2020 brown ash emissions were frequently observed rising 100-500 m above the summit drifting in multiple directions. During 6-8 and 10-15, April brown ash emissions were reported 50-1,000 m above the summit. The most recent Darwin VAAC and VONA notices were published on 14 April, reporting volcanic ash rising 400 and 600 m above the summit, respectively; however, PVMBG reported brown emissions rising up to 1,000 m. By 25-27 April brown ash emissions rose 50-300 m above the summit. Intermittent white gas-and-steam emissions continued through May. The last brown emissions seen in May were reported on the 7th rising 50-100 m above the summit.

Geologic Background. Gunung Kerinci in central Sumatra forms Indonesia's highest volcano and is one of the most active in Sumatra. It is capped by an unvegetated young summit cone that was constructed NE of an older crater remnant. There is a deep 600-m-wide summit crater often partially filled by a small crater lake that lies on the NE crater floor, opposite the SW-rim summit. The massive 13 x 25 km wide volcano towers 2400-3300 m above surrounding plains and is elongated in a N-S direction. Frequently active, Kerinci has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com, images at https://twitter.com/OysteinLAnderse/status/1213658331564269569/photo/1 and https://twitter.com/OysteinLAnderse/status/1230419965209018369/photo/1).

Tinakula is a remote stratovolcano located 100 km NE of the Solomon Trench at the N end of the Santa Cruz. In 1971, an eruption with lava flows and ash explosions caused the small population to evacuate the island. Volcanism has previously been characterized by an ash explosion in October 2017 and the most recent eruptive period that began in December 2018 with renewed thermal activity. Activity since then has consisted of intermittent thermal activity and dense gas-and-steam plumes (BGVN 45:01), which continues into the current reporting period. This report updates information from January-June 2020 using primary source information from various satellite data, as ground observations are rarely available.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed weak, intermittent, but ongoing thermal activity during January-June 2020 (figure 41). A small cluster of slightly stronger thermal signatures was detected in late February to early March, which is correlated to MODVOLC thermal alert data; four thermal hotspots were recorded on 20, 27, and 29 February and 1 March. However, observations using Sentinel-2 satellite imagery were often obscured by clouds. In addition to the weak thermal signatures, dense gas-and-steam plumes were observed in Sentinel-2 satellite imagery rising from the summit during this reporting period (figure 42).

Figure 41. Weak thermal anomalies at Tinakula from 26 June 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were intermittent and clustered more strongly in late February to early March.

Figure 42. Sentinel-2 satellite imagery shows ongoing gas-and-steam plumes rising from Tinakula during January through May 2020. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

Three distinct thermal anomalies were observed in Sentinel-2 thermal satellite imagery on 22 January, 11 April, and 6 May 2020, accompanied by some gas-and-steam emissions (figure 43). The hotspot on 22 January was slightly weaker than the other two days, and was seen on the W flank, compared to the other two that were observed in the summit crater. According to MODVOLC thermal alerts, a hotspot was recorded on 6 May, which corresponded to a Sentinel-2 thermal satellite image with a notable anomaly in the summit crater (figure 43). On 10 June no thermal anomaly was seen in Sentinel-2 satellite imagery due to the presence of clouds; however, what appeared to be a dense gas-and-steam plume was extending W from the summit.

Figure 43. Sentinel-2 thermal satellite images showing a weak thermal activity (bright yellow-orange) on 22 January 2020 on the W flank of Tinakula (top) and slightly stronger thermal hotspots on 11 April (middle) and 6 May (bottom) in at the summit, which are accompanied by gas-and-steam emissions. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. Similar to Stromboli, it has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The satellitic cone of Mendana is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Frequent historical eruptions have originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Ibu is an active stratovolcano located along the NW coast of Halmahera Island in Indonesia. Volcanism has recently been characterized by frequent ash explosions, ash plumes, and small lava flows within the crater throughout 2019 (BGVN 45:01). Activity continues, consisting of frequent white-and-gray emissions, ash explosions, ash plumes, and lava flows. This report updates activity through June 2020, using data from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellites.

Volcanism during the entire reporting period dominantly consisted of white-and-gray emissions that rose 200-800 m above the summit drifting in multiple directions. The ash plume with the maximum altitude of 13.7 km altitude occurred on 16 May 2020. Sentinel-2 thermal satellite imagery detected multiple smaller hotspots within the crater throughout the reporting period.

Continuous ash emissions were reported on 6 February rising to 2.1 km altitude drifting E, accompanied by a hotspot visible in infrared satellite imagery. On 16 February, a ground observer reported an eruption that produced an ash plume rising 800 m above the summit drifting W, according to a Darwin VAAC notice. Ash plumes continued through the month, drifting in multiple directions and rising up to 2.1 km altitude. During 8-10 March, video footage captured multiple Strombolian explosions that ejected incandescent material and produced ash plumes from the summit (figures 21 and 22). Occasionally volcanic lightning was observed within the ash column, as recorded in video footage by Martin Rietze. This event was also documented by a Darwin VAAC notice, which stated that multiple ash emissions rose 2.1 km altitude drifting SE. PVMBG published a VONA notice on 10 March at 1044 reporting ash plumes rising 400 m above the summit. PVMBG and Darwin VAAC notices described intermittent eruptions on 26, 28, and 29 March, all of which produced ash plumes rising 300-800 m above the summit.

Figure 21. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and a dense ash plume. Video footage copyright by Martin Rietze, used with permission.

Figure 22. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and ash. Frequent volcanic lightning was also observed. Video footage copyright by Martin Rietze, used with permission.

A majority of days in April included white-and-gray emissions rising up to 800 m above the summit. A ground observer reported an eruption on 9 April, according to a Darwin VAAC report, and a hotspot was observed in HIMAWARI-8 satellite imagery. Minor eruptions were reported intermittently during mid-April and early to mid-May. On 12 May at 1052 a VONA from PVMBG reported an ash plume 800-1,100 m above the summit. A large short-lived eruption on 16 May produced an ash plume that rose to a maximum of 13.7 km altitude and drifted S, according to the Darwin VAAC report. By June, volcanism consisted predominantly of white-and-gray emissions rising 800 m above the summit, with an ash eruption on 15 June. This eruptive event resulted in an ash plume that rose 1.8 km altitude drifting WNW and was accompanied by a hotspot detected in HIMAWARI-8 satellite imagery, according to a Darwin VAAC notice.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected frequent hotspots during July 2019 through June 2020 (figure 23). In comparison, the MODVOLC thermal alerts recorded a total of 24 thermal signatures over the course of 19 different days between January and June. Many thermal signatures were captured as small thermal hotspots in Sentinel-2 thermal satellite imagery within the crater (figure 24).

Figure 23. Thermal anomalies recorded at Ibu from 2 July 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and consistent in power. Courtesy of MIROVA.

Figure 24. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed occasional thermal hotspots (bright orange) in the Ibu summit crater during January through June 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, contained several small crater lakes through much of historical time. The outer crater, 1.2 km wide, is breached on the north side, creating a steep-walled valley. A large parasitic cone is located ENE of the summit. A smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. Only a few eruptions have been recorded in historical time, the first a small explosive eruption from the summit crater in 1911. An eruption producing a lava dome that eventually covered much of the floor of the inner summit crater began in December 1998.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos, video at https://www.youtube.com/watch?v=qMkfT1e4HQQ).

Suwanosejima is an active stratovolcano located in the northern Ryukyu Islands. Volcanism has previously been characterized by Strombolian explosions, ash plumes, and summit incandescence (BGVN 45:01), which continues to occur intermittently. A majority of this activity originates from vents within the large Otake summit crater. This report updates information during January through June 2020 using monthly reports from the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and various satellite data.

During 3-10 January 2020, 13 explosions were detected from the Otake crater rising to 1.4 km altitude; material was ejected as far as 600 m away and ashfall was reported in areas 4 km SSW, according to JMA. Occasional small eruptive events continued during 12-17 January, which resulted in ash plumes that rose 1 km above the crater rim and ashfall was again reported 4 km SSW. Crater incandescence was visible nightly during 17-24 January, while white plumes rose as high as 700 m above the crater rim.

Nightly incandescence during 7-29 February, and 1-6 March, was accompanied by intermittent explosions that produced ash plumes rising up to 1.2 km above the crater rim (figure 44); activity during early February resulted in ashfall 4 km SSW. On 19 February an eruption produced a gray-white ash plume that rose 1.6 km above the crater (figure 45), resulting in ashfall in Toshima village (4 km SSW), according to JMA. Explosive events during 23-24 February ejected blocks onto the flanks. Two explosions were recorded during 1-6 March, which sent ash plumes as high as 900-1,000 m above the crater rim and ejected large blocks 300 m from the crater.

Figure 44. Surveillance camera images of summit incandescence at Suwanosejima on 29 January (top left), 21 (middle left) and 23 (top right) February, and 25 March (bottom left and right) 2020. Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Figure 45. Surveillance camera images of which and white-and-gray gas-and-steam emissions rising from Suwanosejima on 5 January (top), 19 February (middle), and 24 March 2020 (bottom). Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Nightly incandescence continued to be visible during 13-31 March, 1-10 and 17-24 April, 1-8, 15-31 May, 1-5 and 12-30 June 2020; activity during the latter part of March was relatively low and consisted of few explosive events. In contrast, incandescence was frequently accompanied by explosions in April and May. On 28 April at 0432 an eruption produced an ash plume that rose 1.6 km above the crater rim and drifted SE and E, and ejected blocks as far as 800 m from the crater. The MODVOLC thermal alerts algorithm also detected four thermal signatures during this eruption within the summit crater. An explosion at 1214 on 29 April caused glass in windows to vibrate up to 4 km SSW away while ash emissions continued to be observed following the explosion the previous day, according to the Tokyo VAAC.

During 1-8 May explosions occurred twice a day, producing ash plumes that rose as high as 1 km above the crater rim and ejecting material 400 m from the crater. An explosion on 29 May at 0210 produced an off-white plume that rose as high as 500 m above the crater rim and ejected large blocks up to 200 m above the rim. On 5 June an explosion produced gray-white plumes rising 1 km above the crater. Small eruptive events continued in late June, producing ash plumes that rose as high as 900 m above the crater rim.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed relatively stronger thermal anomalies in late February and late April 2020 with an additional six weaker thermal anomalies detected in early January (2), early February (1), mid-April (2), and mid-May (1) (figure 46). Sentinel-2 thermal satellite imagery in late January through mid-April showed two distinct thermal hotspots within the summit crater (figure 47).

Figure 46. Prominent thermal anomalies at Suwanosejima during July-June 2020 as recorded by the MIROVA system (Log Radiative Power) occurred in late February and late April. Courtesy of MIROVA.

Figure 47. Sentinel-2 thermal satellite images showing small thermal anomalies (bright yellow-orange) from two locations within the Otake summit crater at Suwanosejima. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed forming a large debris avalanche and creating the horseshoe-shaped Sakuchi caldera, which extends to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

The steeply sloped 1.4-km-diameter Kadovar Island is located in the Bismark Sea offshore from the mainland of Papua New Guinea about 25 km NNE from the mouth of the Sepik River. Its first confirmed observed eruption began in early January 2018, with ash plumes and lava extrusion resulting in the evacuation of around 600 residents from the N side of the island (BGVN 43:03). A dome appeared at the base of the E flank during March-May 2018 (Planka et al., 2019); by November activity had migrated to a new dome growing near the summit on the E flank. Pulsating steam plumes, thermal anomalies, and periodic ash emissions continued throughout 2019 (BGVN 44:05, 45:01), and from January-June 2020, the period covered in this report. Information was provided by the Rabaul Volcano Observatory (RVO), the Darwin Volcanic Ash Advisory Center (VAAC), satellite sources, and photographs from visitors.

Activity during January-June 2020. Intermittent ash plumes, pulsating gas and steam plumes, and thermal anomalies continued at Kadovar during January-June 2020. MIROVA thermal data suggested persistent low-level anomalies throughout the period (figure 45). Sentinel-2 satellite data confirmed thermal anomalies at the summit on 5 and 25 January 2020, and an ash emission on 20 January (figure 46). Persistent pulsating steam plumes were visible whenever the skies were clear enough to see the volcano.

Figure 45. Persistent low-level thermal activity at Kadovar was recorded in the MIROVA graph of radiative power from 2 July 2019 through June 2020. The island location is mislocated in the MIROVA system by about 5.5 km SE due to older mis-registered imagery; the anomalies are all on the island. Courtesy of MIROVA.

Figure 46. Sentinel-2 satellite data confirmed thermal anomalies at the summit of Kadovar on 5 (left) and 25 January 2020, and an ash emission and steam plume that drifted SE on 20 January (center). Pulsating steam-and-gas emissions left a trail in the atmosphere drifting SE for several kilometers on 25 January (right). Left image uses Atmospheric penetration rendering (bands 12, 11, 8a), center and right images use Natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

On 2 February 2020 the Darwin VAAC reported a minor eruption plume that rose to 1.5 km altitude and drifted ESE for a few hours. Another plume was clearly discernible in satellite imagery on 5 February at 2.1 km altitude moving SE. RVO issued an information bulletin on 7 February reporting that, since the beginning of January, the eruption had continued with frequent Vulcanian explosions from the Main Vent with a recurrence interval of hours to days. Rocks and ash were ejected 300-400 m above the vent. Rumbling could be heard from Blupblup (Rubrub) island, 15 km E, and residents there also observed incandescence at night. On clear days the plume was sometimes visible from Wewak, on the mainland 100 km W. Additional vents produced variable amounts of steam. The Darwin VAAC reported continuous volcanic ash rising to 1.5 km on 22 February that extended ESE until it was obscured by a meteoric cloud; it dissipated early the next day. A small double ash plume and two strong thermal anomalies at the summit were visible in satellite imagery on 24 February (figure 47).

Figure 47. Ash emissions and thermal anomalies continued at Kadovar during February 2020. Two small plumes of ash or dense steam rose from the summit on 24 February 2020, seen in this Natural color rendering (bands 4, 3, 2) on the left. The same image rendered in Atmospheric penetration (bands 12, 11, 8a) on the right shows two thermal anomalies in the same locations as the ash plumes. Courtesy of Sentinel Hub Playground.

The Darwin VAAC reported continuous ash emissions beginning on 13 March 2020 that rose to 1.5 km altitude and drifted SE. The plume was visible intermittently in satellite imagery for about 36 hours before dissipating. During April, pulsating steam plumes rose from two vents at the summit, and thermal anomalies appeared at both vents in satellite data (figure 48). Small but distinct SO₂ anomalies were visible in satellite data on 15 and 16 April (figure 49).

Figure 48. Steam plumes and thermal anomalies continued at Kadovar during April 2020. Top: A thermal anomaly at the summit accompanied pulsating steam plumes that drifted several kilometers SE before dissipating on 4 April 2020. Bottom left: Two gas-and-steam plumes drifted E from the summit on 9 April. Bottom right: Two adjacent thermal anomalies were present near the summit on 19 April. Top and bottom right images use Atmospheric penetration rendering (bands 12, 11, 8a), bottom left image uses Natural color rendering (bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

Figure 49. Small but distinct SO2 anomalies were detected at Kadovar on 15 and 16 April 2020 with the TROPOMI instrument on the Sentinel-5P satellite. Nearby Manam often produces larger SO2 plumes that obscure evidence of activity at Kadovar. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Two summit vents remained active throughout May and June 2020, producing pulsating steam plumes that were visible for tens of kilometers and thermal anomalies visible in satellite data (figure 50). A strong thermal anomaly was visible beneath meteoric clouds on 8 June.

Figure 50. During May and June 2020 thermal and plume activity continued at Kadovar. Top: Gas-and-steam plumes drifted NW from two sources at the summit of Kadovar on 19 May 2020. Bottom left: Two thermal anomalies marked the E rim of the summit crater on 28 June 2020. Bottom right: A zoomed out view of the same 28 June image shows pulsating steam plumes drifting 10 km NW from Kadovar. Top image is Natural color rendering (bands 4, 3, 2). Bottom images are Atmospheric penetration rendering (bands 12, 11, 8a) of Sentinel-2 images. Courtesy of Sentinel Hub Playground.

Visitor observations on 21 October 2019. Claudio Jung visited Kadovar on 21 October 2019. Shortly before arriving on the island an ash plume rose tens of meters above the summit and drifted W (figure 51). From the NW side of the summit crater rim, Jung saw the actively growing dome on the side of a larger dome, and steam and gas issuing from the growing dome (figure 52). The crater rim was covered with dead vegetation, ash, and large bombs from recent explosions (figure 53). The summit dome had minor fumarolic activity around the summit area and dead vegetation halfway up the flank (figure 54) while the fresh blocky lava of the actively growing dome on the E side of the summit produced significant steam and gas emissions. The growing dome produced periodic pulses of dense steam during his visit (figure 55).

Figure 51. Views looking S show the shoreline dome at the base of the E flank of Kadovar that was active during March-May 2018 (left), and an ash plume drifting W from the summit dome located on the E side of the summit crater (right) on 21 October 2019. Copyrighted photos courtesy of Claudio Jung, used with permission.

Figure 52. A panorama looking SE from the crater rim of Kadovar on 21 October 2019 shows the actively growing dome on the far left with a narrow plume of steam and gas being emitted. A large dome fills the summit crater; the crater rim is visible on the right. Copyrighted photo courtesy of Claudio Jung, used with permission.

Figure 53. The crater rim of Kadovar on 21 October 2019 was covered with dead vegetation, ash, and large bombs from recent explosions. Person is sitting on a large bomb; weak fumarolic activity is visible along the rim. Copyrighted photo courtesy of Claudio Jung, used with permission.

Figure 54. The summit dome of Kadovar on 21 October 2019 had minor fumarolic activity around most of its summit and dead vegetation half-way up the flank (left). The dead tree stumps suggest that vegetation covered the lower half of the dome prior to the eruption that began in January 2018. The fresh blocky lava of the actively growing dome on the E side of the summit dome produced significant steam and gas emissions (right). Copyrighted photos courtesy of Claudio Jung, used with permission.

Figure 55. Dense steam from the growing dome on the E side of the summit drifted W from Kadovar on 21 October 2019. Copyrighted photo courtesy of Claudio Jung, used with permission.

Reference: Planka S, Walter T R, Martinis S, Cescab S, 2019, Growth and collapse of a littoral lava dome during the 2018/19 eruption of Kadovar Volcano, Papua New Guinea, analyzed by multi-sensor satellite imagery, Journal of Volcanology and Geothermal Research, v. 388, 15 December 2019, 106704, https://doi.org/10.1016/j.jvolgeores.2019.106704.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Claudio Jung (URL: https://www.instagram.com/jung.claudio/).

Frequent activity at Ecuador's Sangay has included pyroclastic flows, lava flows, ash plumes, and lahars reported since 1628. Its remoteness on the east side of the Andean crest make ground observations difficult; remote cameras and satellites provide important information on activity. The current eruption began in March 2019 and continued through December 2019 with activity focused on the Cráter Central and the Ñuñurco (southeast) vent; they produced explosions with ash plumes, lava flows, and pyroclastic flows and block avalanches. In addition, volcanic debris was remobilized in the Volcan river causing significant damming downstream. This report covers ongoing similar activity from January through June 2020. Information is provided by Ecuador's Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), and a number of sources of remote data including the Washington Volcanic Ash Advisory Center (VAAC), the Italian MIROVA Volcano HotSpot Detection System, and Sentinel-2 satellite imagery. Visitors also provided excellent ground and drone-based images and information.

Throughout January-June 2020, multiple daily reports from the Washington Volcanic Ash Advisory Center (VAAC) indicated ash plumes rising from the summit, generally 500-1,100 m. Each month one or more plumes rose over 2,000 m. The plumes usually drifted SW or W, and ashfall was reported in communities 25-90 km away several times during January-March and again in June. In addition to explosions with ash plumes, pyroclastic flows and incandescent blocks frequently descended a large, deep ravine on the SE flank. Ash from the pyroclastic flows rose a few hundred meters and drifted away from the volcano. Incandescence was visible on clear nights at the summit and in the ravine. The MIROVA log radiative power graph showed continued moderate and high levels of thermal energy throughout the period (figure 57). Sangay also had small but persistent daily SO₂ signatures during January-June 2020 with larger pulses one or more days each month (figure 58). IG-EPN published data in June 2020 about the overall activity since May 2019, indicating increases throughout the period in seismic event frequency, SO₂ emissions, ash plume frequency, and thermal energy (figure 59).

Figure 57. This graph of log radiative power at Sangay for 18 Aug 2018 through June 2020 shows the moderate levels of thermal energy through the end of the previous eruption in late 2018 and the beginning of the current one in early 2019. Data is from Sentinel-2, courtesy of MIROVA.

Figure 58. Small but persistent daily SO2 signatures were typical of Sangay during January-June 2020. A few times each month the plume was the same or larger than the plume from Columbia’s Nevado del Ruiz, located over 800 km NE. Image dates are shown in the header over each image. Courtesy of NASA’s Global Sulfur Dioxide Monitoring Page.

Figure 59. A multi-parameter graph of activity at Sangay from May 2019 to 12 June 2020 showed increases in many types of activity. a) seismic activity (number of events per day) detected at the PUYO station (source: IG-EPN). b) SO2 emissions (tons per day) detected by the Sentinel-5P satellite sensor (TROPOMI: red squares; source: MOUNTS) and by the IG-EPN (DOAS: green bars). c) height of the ash plumes (meters above crater) detected by the GOES-16 satellite sensor (source: Washington VAAC). d) thermal emission power (megawatt) detected by the MODIS satellite sensor (source: MODVOLC) and estimate of the accumulated lava volume (million M3, thin lines represent the error range). Courtesy of IG-EPN (Informe Especial del Volcán Sangay - 2020 - N°3, “Actualización de la actividad eruptiva”, Quito, 12 de junio del 2020).

Activity during January-March 2020. IG-EPN and the Washington VAAC reported multiple daily ash emissions throughout January 2020. Gas and ash emissions generally rose 500-1,500 m above the summit, most often drifting W or SW. Ashfall was reported on 8 January in the communities of Sevilla (90 km SSW), Pumallacta and Achupallas (60 km SW) and Cebadas (35 km WNW). On 16 January ash fell in the Chimborazo province in the communities of Atillo, Ichobamba, and Palmira (45 km W). Ash on 28 January drifted NW, with minor ashfall reported in Púngala (25 km NW) and other nearby communities. The town of Alao (20 km NW) reported on 30 January that all of the vegetation in the region was covered with fine white ash; Cebadas and Palmira also noted minor ashfall (figure 60).

Figure 60. Daily ash plumes and repeated ashfall were reported from Sangay during January 2020. Top left: 1 January 2020 (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-2, JUEVES, 2 ENERO 2020). Top right: 20 January 2020 (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-21, MARTES, 21 ENERO 2020). Bottom left: 26 January-1 February 2020 expedition (Martes, 18 Febrero 2020 12:21, EXPEDICIÓN AL VOLCÁN SANGAY). Bottom right: 30 January 2020, minor ashfall was reported in the Province of Chimborazo (#IGAlInstante Informativo VOLCÁN SANGAY No. 006, JUEVES, 30 ENERO 2020). Courtesy of IG-EPN.

A major ravine on the SE flank has been the site of ongoing block avalanches and pyroclastic flows since the latest eruption began in March 2019. The pyroclastic flows down the ravine appeared incandescent at night; during the day they created ash clouds that drifted SW. Satellite imagery recorded incandescence and dense ash from pyroclastic flows in the ravine on 7 January (figure 61). They were also reported by IG on the 9th, 13th, 26th, and 28th. Incandescent blocks were reported in the ravine several times during the month. The webcam captured images on 31 January of large incandescent blocks descending the entire length of the ravine to the base of the mountain (figure 62). Large amounts of ash and debris were remobilized as lahars during heavy rains on the 25th and 28th.

Figure 61. Sentinel-2 satellite imagery of Sangay from 7 January 2020 clearly showed a dense ash plume drifting W and ash and incandescent material from pyroclastic flows descending the SE-flank ravine. Left image uses natural color (bands 4, 3, 2) rendering and right images uses atmospheric penetration (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 62. Pyroclastic flows at Sangay produced large trails of ash down the SE ravine many times during January 2020 that rose and drifted SW. Top left: 9 January (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-9, JUEVES, 9 ENERO 2020). Top right: 13 January (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-14, MARTES, 14 ENERO 2020). On clear nights, incandescent blocks of lava and pyroclastic flows were visible in the ravine. Bottom left: 16 January (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-17, VIERNES, 17 ENERO 2020). Bottom right: 31 January (#IGAlInstante Informativo VOLCÁN SANGAY No. 007, VIERNES, 31 ENERO 2020). Courtesy of IG-EPN.

Observations by visitors to the volcano during 9-17 January 2020 included pyroclastic flows, ash emissions, and incandescent debris descending the SE flank ravine during the brief periods when skies were not completely overcast (figure 63 and 64). More often there was ash-filled rain and explosions heard as far as 16 km from the volcano, along with the sounds of lahars generated from the frequent rainfall mobilizing debris from the pyroclastic flows. The confluence of the Rio Upano and Rio Volcan is 23 km SE of the summit and debris from the lahars has created a natural dam on the Rio Upano that periodically backs up water and inundates the adjacent forest (figure 65). A different expedition to Sangay during 26 January-1 February 2020 by IG personnel to repair and maintain the remote monitoring station and collect samples was successful, after which the station was once again transmitting data to IG-EPN in Quito (figure 66).

Figure 63. Hikers near Sangay during 9-17 January 2020 witnessed pyroclastic flows and incandescent explosions and debris descending the SE ravine. Left: The view from 40 km SE near Macas showed ash rising from pyroclastic flows in the SE ravine. Right: Even though the summit was shrouded with a cap cloud, incandescence from the summit crater and from pyroclastic flows on the SE flank were visible on clear nights. Courtesy of Arnold Binas, used with permission.

Figure 64. The steep ravine on the SE flank of Sangay was hundreds of meters deep in January 2020 when these drone images were taken by members of a hiking trip during 9-17 January 2020 (left). Pyroclastic flows descended the ravine often (right), coating the sides of the ravine with fine, white ash and sending ash billowing up from the surface of the flow which resulted in ashfall in adjacent communities several times. Courtesy of Arnold Binas, used with permission.

Figure 65. Debris from pyroclastic flows that descended the SE Ravine at Sangay was carried down the Volcan River (left) during frequent rains and caused repeated damming at the confluence with the Rio Upano (right), located 23 km SE of the summit. These images show the conditions along the riverbeds during 9-17 January 2020. Courtesy of Arnold Binas, used with permission.

Figure 66. An expedition by scientists from IG-EPN to one of the remote monitoring stations at Sangay during 26 January-1 February 2020 was successful in restoring communication to Quito. The remote location and constant volcanic activity makes access and maintenance a challenge. Courtesy of IG-EPN (Martes, 18 Febrero 2020 12:21, EXPEDICIÓN AL VOLCÁN SANGAY).

During February 2020, multiple daily VAAC reports of ash emissions continued (figure 67). Plumes generally rose 500-1,100 m above the summit and drifted W, although on 26 February emissions were reported to 1,770 m. Ashfall was reported in Macas (40 km SE) on 1 February, and in the communities of Pistishi (65 km SW), Chunchi (70 km SW), Pumallacta (60 k. SW), Alausí (60 km SW), Guamote (40 km WNW) and adjacent areas of the Chimborazo province on 5 February. The Ecuadorian Red Cross reported ash from Sangay in the provinces of Cañar and Azuay (60-100 km SW) on 25 February. Cebadas and Guamote reported moderate ashfall the following day. The communities of Cacha (50 km NW) and Punín (45 km NW) reported trace amounts of ashfall on 29 February. Incandescent blocks were seen on the SE flank multiples times throughout the month. A pyroclastic flow was recorded on the SE flank early on 6 February; additional pyroclastic flows were observed later that day on the SW flank. On 23 February a seismic station on the flank recorded a high-frequency signal typical of lahars.

Figure 67. Steam and ash could be seen drifting SW from the summit of Sangay on 11 February 2020 even though the summit was hidden by a large cap cloud. Ash was also visible in the ravine on the SE flank. Courtesy of Sentinel Hub Playground, natural color (bands 4, 3, 2) rendering.

A significant ash emission on 1 March 2020 was reported about 2 km above the summit, drifting SW. Multiple ash emissions continued daily during the month, generally rising 570-1,170 m high. An emission on 12 March also rose 2 km above the summit. Trace ashfall was reported in Cebadas (35 km WNW) on 12 March. The community of Huamboya, located 40 km ENE of Sangay in the province of Morona-Santiago reported ashfall on 17 March. On 19 and 21 March ashfall was seen on the surface of cars in Macas to the SE. (figure 68). Ash was also reported on the 21st in de Santa María De Tunants (Sinaí) located E of Sangay. Ash fell again in Macas on 23 March and was also reported in General Proaño (40 km SE). The wind changed direction the next day and caused ashfall on 24 March to the SW in Cuenca and Azogues (100 km SW).

Figure 68. Ashfall from Sangay was reported on cars in Huamboya on 17 March 2020 (left) and in Macas on 19 March (right). Courtesy IG-EPN, (#IGAlInstante Informativo VOLCÁN SANGAY No. 024, MARTES, 17 MARZO 2020 and #IGAlInstante Informativo VOLCÁN SANGAY No. 025, JUEVES, 19 MARZO 2020).

Incandescence from the dome at the crater and on the SE flank was noted by IG on 3, 4, and 13 March. Remobilized ash from a pyroclastic flow was reported drifting SW on 13 March. The incandescent path of the flow was still visible that evening. Numerous lahars were recorded seismically during the month, including on days 5, 6, 8, 11, 15, 30 and 31. Images from the Rio Upano on 11 March confirmed an increase from the normal flow rate (figure 69) inferred to be from volcanic debris. Morona-Santiago province officials reported on 14 March that a new dam had formed at the confluence of the Upano and Volcano rivers that decreased the flow downstream; by 16 March it had given way and flow had returned to normal levels.

Figure 69. Images from the Rio Upano on 11 March 2020 (left) confirmed an increase from the normal flow rate related to lahars from Sangay descending the Rio Volcan. By 16 March (right), the flow rate had returned to normal, although the large blocks in the river were evidence of substantial activity in the past. Courtesy of IG (#IGAlInstante Informativo VOLCÁN SANGAY No. 018, MIÉRCOLES, 11 MARZO 2020 and #IGAlInstante Informativo VOLCÁN SANGAY No. 023, LUNES, 16 MARZO 2020).

Activity during April-June 2020. Lahar activity continued during April 2020; they were reported seven times on 2, 5, 7, 11, 12, 19, and 30 April. A significant reduction in the flow of the Upano River at the entrance bridge to the city of Macas was reported 9 April, likely due to a new dam on the river upstream from where the Volcan river joins it caused by lahars related to ash emissions and pyroclastic flows (figure 70). The flow rate returned to normal the following day. Ash emissions were reported most days of the month, commonly rising 500-1,100 m above the summit and drifting W. Incandescent blocks or flows were visible on the SE flank on 4, 10, 12, 15-16, and 20-23 April (figure 71).

Figure 70. A significant reduction in the flow of the Upano River at the entrance bridge to the city of Macas was reported on 9 April 2020, likely due to a new dam upstream from lahars related to ash emissions and pyroclastic flows from Sangay. Courtesy of IG-EPN (#IGAlInstante Informativo VOLCÁN SANGAY No. 032, JUEVES, 9 ABRIL 2020).

Figure 71. Incandescent blocks rolled down the SE ravine at Sangay multiple times during April 2020, including on 4 April (left). Pyroclastic flows left two continuous incandescent trails in the ravine on 23 April (right). Courtesy of IG-EPN (INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-95, SÁBADO, 4 ABRIL 2020 and INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-114, JUEVES, 23 ABRIL 2020).

Activity during May 2020 included multiple daily ash emissions that drifted W and numerous lahars from plentiful rain carrying ash and debris downstream. Although there were only a few visible observations of ash plumes due to clouds, the Washington VAAC reported plumes visible in satellite imagery throughout the month. Plumes rose 570-1,170 m above the summit most days; the highest reported rose to 2,000 m above the summit on 14 May. Two lahars occurred in the early morning on 1 May and one the next day. A lahar signal lasted for three hours on 4 May. Two lahar signals were recorded on the 7th, and three on the 9th. Lahars were also recorded on 16-17, 20-22, 26-27, and 30 May. Incandescence on the SE flank was only noted three times, but it was cloudy nearly every day.

An increase in thermal and overall eruptive activity was reported during June 2020. On 1 and 2 June the webcam captured lava flows and remobilization of the deposits on the SE flank in the early morning and late at night. Incandescence was visible multiple days each week. Lahars were reported on 4 and 5 June. The frequent daily ash emissions during June generally rose to 570-1,200 m above the summit and drifted usually SW or W. The number of explosions and ash emissions increased during the evening of 7 June. IG interpreted the seismic signals from the explosions as an indication of the rise of a new pulse of magma (figure 72). The infrasound sensor log from 8 June also recorded longer duration tremor signals that were interpreted as resulting from the descent of pyroclastic flows in the SE ravine.

Figure 72. Seismic and infrasound signals indicated increased explosive and pyroclastic flow activity at Sangay on 7-8 June 2020. Left: SAGA station (seismic component) of 7 and 8 June. The signals correspond to explosions without VT or tremor signals, suggesting the rise of a new magma pulse. Right: SAGA station infrasound sensor log from 8 June. The sharp explosion signals are followed a few minutes later (examples highlighted in red) by emergent signals of longer duration, possibly associated with the descent of pyroclastic material in the SE flank ravine. Courtesy if IG-EPN (Informe Especial del Volcán Sangay - 2020 - N°3, “Actualización de la actividad eruptiva”, Quito, 12 de junio del 2020).

On the evening of 8 June ashfall was reported in the parish of Cebadas and in the Alausí Canton to the W and SW of Sangay. There were several reports of gas and ash emissions to 1,770 m above the summit the next morning on 9 June, followed by reports of ashfall in the provinces of Guayas, Santa Elena, Los Ríos, Morona Santiago, and Chimborazo. Ashfall continued in the afternoon and was reported in Alausí, Chunchi, Guamote, and Chillanes. That night, which was clear, the webcam captured images of pyroclastic flows down the SE-flank ravine; IG attributed the increase in activity to the collapse of one or more lava fronts. On the evening of 10 June additional ashfall was reported in the towns of Alausí, Chunchi, and Guamote (figure 73); satellite imagery indicated an ash plume drifting W and incandescence from pyroclastic flows in the SE-flank ravine the same day (figure 74).

Figure 73. Ashfall from Sangay was reported in Alausí (top left), Chunchi (top right) and Guamote (bottom) on 10 June 2020. Courtesy of IG-EPN (#IGAlInstante Informativo VOLCÁN SANGAY No. 049, MIÉRCOLES, 10 JUNIO 2020).

Figure 74. Incandescent pyroclastic flows (left) and ash plumes that drifted W (right) were recorded on 10 June 2020 at Sangay in Sentinel-2 satellite imagery. Courtesy of Sentinel Hub Playground.

Ashfall continued on 11 June and was reported in Guayaquil, Guamote, Chunchi, Riobamba, Guaranda, Chimbo, Echandía, and Chillanes. The highest ash plume of the report period rose to 2,800 m above the summit that day and drifted SW. That evening the SNGRE (Servicio Nacional de Gestion de Riesgos y Emergencias) reported ash fall in the Alausí canton. IG noted the increase in intensity of activity and reported that the ash plume of 11 June drifted more than 600 km W (figure 75). Ash emissions on 12 and 13 June drifted SW and NW and resulted in ashfall in the provinces of Chimborazo, Cotopaxi, Tungurahua, and Bolívar. On 14 June, the accumulation of ash interfered with the transmission of information from the seismic station. Lahars were reported each day during 15-17 and 19-21 June. Trace amounts of ashfall were reported in Macas to the SE on 25 June.

Figure 75. The ash plume at Sangay reported on 11 June 2020 rose 2.8 km above the summit and drifted W according to the Washington VAAC and IG (left). Explosions and high levels of incandescence on the SE flank were captured by the Don Bosco webcam (right). Courtesy of IG-EPN (#IGAlInstante Informativo VOLCÁN SANGAY No. 055, JUEVES, 11 JUNIO 2020 and INFORME DIARIO DEL ESTADO DEL VOLCÁN SANGAY No. 2020-164, VIERNES, 12 JUNIO 2020).

During an overflight of Sangay on 24 June IG personnel observed that activity was characterized by small explosions from the summit vent and pyroclastic flows down the SE-flank ravine. The explosions produced small gas plumes with a high ash content that did not rise more than 500 m above the summit and drifted W (figure 76). The pyroclastic flows were restricted to the ravine on the SE flank, although the ash from the flows rose rapidly and reached about 200 m above the surface of the ravine and also drifted W (figure 77).

Figure 76. A dense ash plume rose 500 m from the summit of Sangay on 24 June 2020 and drifted W during an overflight by IG-EPN personnel. The aerial photograph is taken from the SE; snow-covered Chimborazo is visible behind and to the right of Sangay. Photo by M Almeida, courtesy of IG EPN (Jueves, 02 Julio 2020 10:29, INFORME DEL SOBREVUELO AL VOLCÁN SANGAY EL 24 DE JUNIO DE 2020).

Figure 77. Pyroclastic flows descended the SE flank ravine at Sangay during an overflight by IG-EPN personnel on 24 June 2020. Ash from the pyroclastic flow rose 200 m and drifted W, and infrared imagery identified the thermal signature of the pyroclastic flow in the ravine. Photo by M Almeida, IR Image by S Vallejo, courtesy of IG EPN (Jueves, 25 Junio 2020 12:24, SOBREVUELO AL VOLCÁN SANGAY).

Infrared imagery taken during the overflight on 24 June identified three significant thermal anomalies in the large ravine on the SE flank (figure 78). Analysis by IG scientists suggested that the upper anomaly 1 (125°C) was associated with explosive activity that was observed during the flight. Anomaly 2 (147°C), a short distance below Anomaly 1, was possibly related to effusive activity of a small flow, and Anomaly 3 (165°C) near the base of the ravine that was associated with pyroclastic flow deposits. The extent of the changes at the summit of Sangay and along the SE flank since the beginning of the eruption that started in March 2019 were clearly visible when images from May 2019 were compared with images from the 24 June 2020 overflight (figure 79). The upper part of the ravine was nearly 400 m wide by the end of June.

Figure 78. A thermal image of the SE flank of Sangay taken on 24 June 2020 indicated three thermal anomalies. Anomaly 1 was associated with explosive activity, Anomaly 2 was associated with effusive activity, and Anomaly 3 was related to pyroclastic-flow deposits. Image prepared by S Vallejo Vargas, courtesy of IG EPN (Jueves, 02 Julio 2020 10:29, INFORME DEL SOBREVUELO AL VOLCÁN SANGAY EL 24 DE JUNIO DE 2020).

Figure 79. Aerial and thermal photographs of the southern flank of the Sangay volcano on 17 May 2019 (left: visible image) and 24 June 2020 (middle: visible image, right: visible-thermal overlay) show the morphological changes on the SE flank, associated with the formation of a deep ravine and the modification of the summit. Photos and thermal image by M Almeida, courtesy of IG EPN (Jueves, 02 Julio 2020 10:29, INFORME DEL SOBREVUELO AL VOLCÁN SANGAY EL 24 DE JUNIO DE 2020).

Geologic Background. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within horseshoe-shaped calderas of two previous edifices, which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been sculpted by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of a historical eruption was in 1628. More or less continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Arnold Binas (URL: https://www.doroadventures.com).

The Karangetang andesitic-basaltic stratovolcano (also referred to as Api Siau) at the northern end of the island of Siau, north of Sulawesi, Indonesia, has had more than 50 observed eruptions since 1675. Frequent explosive activity is accompanied by pyroclastic flows and lahars, and lava-dome growth has created two active summit craters (Main to the S and Second Crater to the N). Rock avalanches, observed incandescence, and satellite thermal anomalies at the summit confirmed continuing volcanic activity since the latest eruption started in November 2018 (BGVN 44:05). This report covers activity from December 2019 through May 2020. Activity is monitored by Indonesia's Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), and ash plumes are monitored by the Darwin VAAC (Volcanic Ash Advisory Center). Information is also available from MODIS thermal anomaly satellite data through both the University of Hawaii's MODVOLC system and the Italian MIROVA project.

Increased activity that included daily incandescent avalanche blocks traveling down the W and NW flanks lasted from mid-July 2019 (BGVN 44:12) through mid-January 2020 according to multiple sources. The MIROVA data showed increased number and intensity of thermal anomalies during this period, with a sharp drop during the second half of January (figure 40). The MODVOLC thermal alert data reported 29 alerts in December and ten alerts in January, ending on 14 January, with no further alerts through May 2020. During December and the first half of January incandescent blocks traveled 1,000-1,500 m down multiple drainages on the W and NW flanks (figure 41). After this, thermal anomalies were still present at the summit craters, but no additional activity down the flanks was identified in remote satellite data or direct daily observations from PVMBG.

Figure 40. An episode of increased activity at Karangetang from mid-July 2019 through mid-January 2020 included incandescent avalanche blocks traveling down multiple flanks of the volcano. This was reflected in increased thermal activity seen during that interval in the MIROVA graph covering 5 June 2019 through May 2020. Courtesy of MIROVA.

Figure 41. An episode of increased activity at Karangetang from mid-July 2019 through mid-January 2020 included incandescent avalanche blocks traveling up to 1,500 m down drainages on the W and NW flanks of the volcano. Top left: large thermal anomalies trend NW from Main Crater on 5 December 2019; about 500 m N a thermal anomaly glows from Second Crater. Top center: on 15 December plumes of steam and gas drifted W and SW from both summit craters as seen in Natural Color rendering (bands 4,3,2). Top right: the same image as at top center with Atmospheric penetration rendering (bands 12, 11, 8a) shows hot zones extending WNW from Main Crater and a thermal anomaly at Second Crater. Bottom left: thermal activity seen on 14 January 2020 extended about 800 m WNW from Main Crater along with an anomaly at Second Crater and a hot spot about 1 km W. Bottom center: by 19 January the anomaly from Second Crater appeared slightly stronger than at Main Crater, and only small anomalies appeared on the NW flank. Bottom right: an image from 14 March shows only thermal anomalies at the two summit craters. Courtesy of Sentinel Hub Playground.

A single VAAC report in early April noted a short-lived ash plume that drifted SW. Intermittent low-level activity continued through May 2020. Small SO₂ plumes appeared in satellite data multiple times in December 2019 and January 2020; they decreased in size and frequency after that but were still intermittently recorded into May 2020 (figure 42).

Figure 42. Small plumes of sulfur dioxide were measured at Karangetang with the TROPOMI instrument on the Sentinel-5P satellite multiple times during December 2019 (top row). They were less frequent but still appeared during January-May 2020 (bottom row). Larger plumes were also detected from Dukono, located 300 km ESE at the N end of North Maluku. Courtesy of Global Sulfur Dioxide Monitoring Page.

PVMBG reported in their daily summaries that steam plumes rose 50-150 m above the Main Crater and 25-50 m above Second Crater on most days in December. The incandescent avalanche activity that began in mid-July 2019 also continued throughout December 2019 and January 2020 (figure 43). Incandescent blocks from the Main Crater descended river drainages (Kali) on the W and NW flanks throughout December. They were reported nearly every day in the Nanitu, Sense, and Pangi drainages, traveling 1,000-1,500 m. Incandescence from both craters was visible 10-25 m above the crater rim most nights.

Figure 43. Incandescent block avalanches descended the NW flank of Karangetang as far as 1,500 m frequently during December 2019 and January 2020. Left image taken 13 December 2019, right image taken 6 January 2020 by PVMBG webcam. Courtesy of PVMBG, Oystein Anderson, and Bobyson Lamanepa.

A few blocks were noted traveling 800 m down Kali Beha Barat on 1 December. Incandescence above the Main crater reached 50-75 m during 4-6 December. During 4-7 December incandescent blocks appeared in Kali Sesepe, traveling 1,000-1,500 m down from the summit. They were also reported in Kali Batang and Beha Barat during 4-14 December, usually moving 800-1,000 m downslope. Between 5 and 14 December, gray and white plumes from Second Crater reached 300 m multiple times. During 12-15 December steam plumes rose 300-500 m above the Main crater. Activity decreased during 18-26 December but increased again during the last few days of the month. On 28 December, incandescent blocks were reported 1,500 m down Kali Pangi and Nanitu, and 1,750 m down Kali Sense.

Incandescent blocks were reported in Kali Sesepi during 4-6 January and in Kali Batang and Beha Barat during 4-8 and 12-15 January (figure 44); they often traveled 800-1,200 m downslope. Activity tapered off in those drainages and incandescent blocks were last reported in Kali Beha Barat on 15 January traveling 800 m from the summit. Incandescent blocks were also reported traveling usually 1,000-1,500 m down the Nanitu, Sense, and Pangi drainages during 4-19 January. Blocks continued to occasionally descend up to 1,000 m down Kali Nanitu through 24 January. Pulses of activity occurred at the summit of Second Crater a few times in January. Steam plumes rose 25-50 m during 8-9 January and again during 16-31 January, with plumes rising 300-400 m on 20, 29, and 31 January. Incandescence was noted 10-25 m above the summit of Second Crater during 27-30 January.

Figure 44. Incandescent material descends the Beha Barat, Sense, Nanitu, and Pangi drainages on the NW flank of Karangetang in early January 2020. Courtesy of Bobyson Lamanepa; posted on Twitter on 6 January 2020.

Activity diminished significantly after mid-January 2020. Steam plumes at the Main Crater rose 50-100 m on the few days where the summit was not obscured by fog during February. Faint incandescence occurred at the Main Crater on 7 February, and steam plumes rising 25-50 m from Second Crater that day were the only events reported there in February. During March, steam plumes persisted from the Main Crater, with heights of over 100 m during short periods from 8-16 March and 25-30 March. Weak incandescence was reported from the Main Crater only once, on 25 March. Very little activity occurred at Second Crater during March, with only steam plumes reported rising 25-300 m from the 22nd to the 28th (figure 45).

Figure 45. Steam plumes at Karangetang rose over 100 m above both summit craters multiple times during March, including on 26 March 2020. Courtesy of PVMBG and Oystein Anderson.

The Darwin VAAC reported a continuous ash emission on 4 April 2020 that rose to 2.1 km altitude and drifted SW for a few hours before dissipating. Incandescence visible 25 m above both craters on 13 April was the only April activity reported by PVMBG other than steam plumes from the Main Crater that rose 50-500 m on most days. Steam plumes of 50-100 m were reported from Second Crater during 11-13 April. Activity remained sporadic throughout May 2020. Steam plumes from the Main Crater rose 50-300 m each day. Satellite imagery identified steam plumes and incandescence from both summit craters on 3 May (figure 46). Faint incandescence was observed at the Main Crater on 12 and 27 May. Steam plumes rose 25-50 m from Second Crater on a few days; a 200-m-high plume was reported on 27 May. Bluish emissions were observed on the S and SW flanks on 28 May.

Figure 46. Dense steam plumes and thermal anomalies were present at both summit craters of Karangetang on 3 May 2020. Sentinel 2 satellite image with Natural Color (bands 4, 3, 2) (left) and Atmospheric Penetration rendering (bands 12, 11, 8a) (right); courtesy of Sentinel Hub Playground.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi island. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com); Bobyson Lamanepa, Yogyakarta, Indonesia, (URL: https://twitter.com/BobyLamanepa/status/1214165637028728832).

Shishaldin is located near the center of Unimak Island in Alaska, with the current eruption phase beginning in July 2019 and characterized by ash plumes, lava flows, lava fountaining, pyroclastic flows, and lahars. More recently, in late 2019 and into January 2020, activity consisted of multiple lava flows, pyroclastic flows, lahars, and ashfall events (BGVN 45:02). This report summarizes activity from February through May 2020, including gas-and-steam emissions, brief thermal activity in mid-March, and a possible new cone within the summit crater. The primary source of information comes from the Alaska Volcano Observatory (AVO) reports and various satellite data.

Volcanism during February 2020 was relatively low, consisting of weakly to moderately elevated surface temperatures during 1-4 February and occasional small gas-and-steam plumes (figure 37). By 6 February both seismicity and surface temperatures had decreased. Seismicity and surface temperatures increased slightly again on 8 March and remained elevated through the rest of the reporting period. Intermittent gas-and-steam emissions were also visible from mid-March (figure 38) through May. Minor ash deposits visible on the upper SE flank may have been due to ash resuspension or a small collapse event at the summit, according to AVO.

Figure 37. Photo of a gas-and-steam plume rising from the summit crater at Shishaldin on 22 February 2020. Photo courtesy of Ben David Jacob via AVO.

Figure 38. A Worldview-2 panchromatic satellite image on 11 March 2020 showing a gas-and-steam plume rising from the summit of Shishaldin and minor ash deposits on the SE flank (left). Aerial photo showing minor gas-and-steam emissions rising from the summit crater on 11 March (right). Some erosion of the snow and ice on the upper flanks is a result of the lava flows from the activity in late 2019 and early 2020. Photo courtesy of Matt Loewen (left) and Ed Fischer (right) via AVO.

On 14 March, lava and a possible new cone were visible in the summit crater using satellite imagery, accompanied by small explosion signals. Strong thermal signatures due to the lava were also seen in Sentinel-2 satellite data and continued strongly through the month (figure 39). The lava reported by AVO in the summit crater was also reflected in satellite-based MODIS thermal anomalies recorded by the MIROVA system (figure 40). Seismic and infrasound data identified small explosions signals within the summit crater during 14-19 March.

Figure 39. Sentinel-2 thermal satellite images (bands 12, 11, 8A) show a bright hotspot (yellow-orange) at the summit crater of Shishaldin during mid-March 2020 that decreases in intensity by late March. Courtesy of Sentinel Hub Playground.

Figure 40. MIROVA thermal data showing a brief increase in thermal anomalies during late March 2020 and on two days in late April between periods of little to no activity. Courtesy of MIROVA.

AVO released a Volcano Observatory Notice for Aviation (VONA) stating that seismicity had decreased by 16 April and that satellite data no longer showed lava or additional changes in the crater since the start of April. Sentinel-2 thermal satellite imagery continued to show a weak hotspot in the crater summit through May (figure 41), which was also detected by the MIROVA system on two days. A daily report on 6 May reported a visible ash deposit extending a short distance SE from the summit, which had likely been present since 29 April. AVO noted that the timing of the deposit corresponds to an increase in the summit crater diameter and depth, further supporting a possible small collapse. Small gas-and-steam emissions continued intermittently and were accompanied by weak tremors and occasional low-frequency earthquakes through May (figure 42). Minor amounts of sulfur dioxide were detected in the gas-and-steam emissions during 20 and 29 April, and 2, 16, and 28 May.

Figure 41. Sentinel-2 thermal satellite images (bands 12, 11, 8A) show occasional gas-and-steam emissions rising from Shishaldin on 26 February (top left) and 24 April 2020 (bottom left) and a weak hotspot (yellow-orange) persisting at the summit crater during April and early May 2020. Courtesy of Sentinel Hub Playground.

Figure 42. A Worldview-1 panchromatic satellite image showing gas-and-steam emissions rising from the summit of Shishaldin on 1 May 2020 (local time) (left). Aerial photo of the N flank of Shishaldin with minor gas-and-steam emissions rising from the summit on 8 May (right). Photo courtesy of Matt Loewen (left) and Levi Musselwhite (right) via AVO.

Geologic Background. The beautifully symmetrical Shishaldin is the highest and one of the most active volcanoes of the Aleutian Islands. The glacier-covered volcano is the westernmost of three large stratovolcanoes along an E-W line in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." A steam plume often rises from its small summit crater. Constructed atop an older glacially dissected volcano, it is largely basaltic in composition. Remnants of an older ancestral volcano are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is blanketed by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Masaya, which is about 20 km NW of the Nicaragua’s capital of Managua, is one of the most active volcanoes in that country and has a caldera that contains a number of craters (BGVN 43:11). The Santiago crater is the one most currently active and it contains a small lava lake that emits weak gas plumes (figure 85). This report summarizes activity during February through May 2020 and is based on Instituto Nicaragüense de Estudios Territoriales (INETER) monthly reports and satellite data. During the reporting period, the volcano was relatively calm, with only weak gas plumes.

Figure 85. Satellite images of Masaya from Sentinel-2 on 18 April 2020, showing and a small gas plume drifting SW (top, natural color bands 4, 3, 2) and the lava lake (bottom, false color bands 12, 11, 4). Courtesy of Sentinel Hub Playground.

According to INETER, thermal images of the lava lake and temperature data in the fumaroles were taken using an Omega infrared gun and a forward-looking infrared (FLIR) SC620 thermal camera. The temperatures above the lava lake have decreased since November 2019, when the temperature was 287°C, dropping to 96°C when measured on 14 May 2020. INETER attributed this decrease to subsidence in the level of the lava lake by 5 m which obstructed part of the lake and concentrated the gas emissions in the weak plume. Convection continued in the lava lake, which in May had decreased to a diameter of 3 m. Many landslides had occurred in the E, NE, and S walls of the crater rim due to rock fracturing caused by the high heat and acidity of the emissions.

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system recorded numerous thermal anomalies from the lava lake based on MODIS data (figure 86). Infrared satellite images from Sentinel-2 regularly showed a strong signature from the lava lake through 18 May, after which the volcano was covered by clouds.

Figure 86. Thermal anomalies at Masaya during February through May 2020. The larger anomalies with black lines are more distant and not related to the volcano. Courtesy of MIROVA.

Measurements of sulfur dioxide (SO2) made by INETER in the section of the Ticuantepe - La Concepción highway (just W of the volcano) with a mobile DOAS system varied between a low of just over 1,000 metric tons/day in mid-November 2019 to a high of almost 2,500 tons/day in late May. Temperatures of fumaroles in the Cerro El Comalito area, just ENE of Santiago crater, ranged from 58 to 76°C during February-May 2020, with most values in the 69-72°C range.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of historical eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Historical lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Masaya has been frequently active since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold." Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Krakatau, located in the Sunda Strait between Indonesia’s Java and Sumatra Islands, experienced a major caldera collapse around 535 CE, forming a 7-km-wide caldera ringed by three islands. On 22 December 2018, a large explosion and flank collapse destroyed most of the 338-m-high island of Anak Krakatau (Child of Krakatau) and generated a deadly tsunami (BGVN 44:03). The near-sea level crater lake inside the remnant of Anak Krakatau was the site of numerous small steam and tephra explosions. A larger explosion in December 2019 produced the beginnings of a new cone above the surface of crater lake (BGVN 45:02). Recently, volcanism has been characterized by occasional Strombolian explosions, dense ash plumes, and crater incandescence. This report covers activity from February through May 2020 using information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, also known as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), the Darwin Volcanic Ash Advisory Center (VAAC), and various satellite data.

Activity during February 2020 consisted of dominantly white gas-and-steam emissions rising 300 m above the crater, according to PVMBG. According to the Darwin VAAC, a ground observer reported an eruption on 7 and 8 February, but no volcanic ash was observed. During 10-11 February, a short-lived eruption was detected by seismograms which produced an ash plume up to 1 km above the crater drifting E. MAGMA Indonesia reported two eruptions on 18 March, both of which rose to 300 m above the crater. White gas-and-steam emissions were observed for the rest of the month and early April.

On 10 April PVMBG reported two eruptions, at 2158 and 2235, both of which produced dark ash plumes rising 2 km above the crater followed by Strombolian explosions ejecting incandescent material that landed on the crater floor (figures 108 and 109). The Darwin VAAC issued a notice at 0145 on 11 April reporting an ash plume to 14.3 km altitude drifting WNW, however this was noted with low confidence due to the possible mixing of clouds. During the same day, an intense thermal hotspot was detected in the HIMAWARI thermal satellite imagery and the NASA Global Sulfur Dioxide page showed a strong SO₂ plume at 11.3 km altitude drifting W (figure 110). The CCTV Lava93 webcam showed new lava flows and lava fountaining from the 10-11 April eruptions. This activity was evident in the MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data (figure 111).

Figure 108. Webcam (Lava93) images of Krakatau on 10 April 2020 showing Strombolian explosions, strong incandescence, and ash plumes rising from the crater. Courtesy of PVMBG and MAGMA Indonesia.

Figure 109. Webcam image of incandescent Strombolian explosions at Krakatau on 10 April 2020. Courtesy of PVMBG and MAGMA Indonesia.

Figure 110. Strong sulfur dioxide emissions rising from Krakatau and drifting W were detected using the TROPOMI instrument on the Sentinel-5P satellite on 11 April 2020 (top row). Smaller volumes of SO2 were visible in Sentinel-5P/TROPOMI maps on 13 (bottom left) and 19 April (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 111. Thermal activity at Anak Krakatau from 29 June-May 2020 shown on a MIROVA Log Radiative Power graph. The power and frequency of the thermal anomalies sharply increased in mid-April. After the larger eruptive event in mid-April the thermal anomalies declined slightly in strength but continued to be detected intermittently through May. Courtesy of MIROVA.

Strombolian activity rising up to 500 m continued into 12 April and was accompanied by SO₂ emissions that rose 3 km altitude, drifting NW according to a VAAC notice. PVMBG reported an eruption on 13 April at 2054 that resulted in incandescence as high as 25 m above the crater. Volcanic ash, accompanied by white gas-and-steam emissions, continued intermittently through 18 April, many of which were observed by the CCTV webcam. After 18 April only gas-and-steam plumes were reported, rising up to 100 m above the crater; Sentinel-2 satellite imagery showed faint thermal anomalies in the crater (figure 112). SO₂ emissions continued intermittently throughout April, though at lower volumes and altitudes compared to the 11th. MODIS satellite data seen in MIROVA showed intermittent thermal anomalies through May.

Figure 112. Sentinel-2 thermal satellite images showing the cool crater lake on 20 March (top left) followed by minor heating of the crater during April and May 2020. Sentinel-2 satellite images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The renowned volcano Krakatau (frequently misstated as Krakatoa) lies in the Sunda Strait between Java and Sumatra. Collapse of the ancestral Krakatau edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of this ancestral volcano are preserved in Verlaten and Lang Islands; subsequently Rakata, Danan, and Perbuwatan volcanoes were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption, the 2nd largest in Indonesia during historical time, caused more than 36,000 fatalities, most as a result of devastating tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former cones of Danan and Perbuwatan. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Remeasurement of five EDM lines on 15-16 May yielded no significant changes (>1 cm) since the network was established in September 1990. Two seismometers temporarily operated on the caldera floor recorded no local shallow seismicity. The temperature of the boiling spring in the caldera was 98°C, the same as in 1990. The volume of water issuing from the hot spring was less than in 1990, maybe because of seasonal rainfall variations. The highest measured fumarole temperature was 102°C, 4° higher than in 1990, perhaps related to a drop in the water table.

Geologic Background. The highest of the Marianas arc volcanoes, Agrigan contains a 500-m-deep, flat-floored caldera. The elliptical island is 8 km long; its summit is the top of a massive 4000-m-high submarine volcano. Deep radial valleys dissect the flanks of the thickly vegetated stratovolcano. The elongated caldera is 1 x 2 km wide and is breached to the NW, from where a prominent lava flow extends to the coast and forms a lava delta. The caldera floor is surfaced by fresh-looking lava flows and also contains two cones that may have formed during the only historical eruption in 1917. This eruption deposited large blocks and 3 m of ash and lapilli on a village on the SE coast, prompting its evacuation.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Only two explosions occurred . . . in June, causing no damage. The month's highest ash clouds rose 2,000 m on 9 and 18 June. Two 9-hour swarms of volcanic earthquakes were recorded, a relatively low level of seismicity for the volcano.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.

Information Contacts: JMA.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating.

[At Alamagan] the team measured a temperature of 72°C at one fumarole. No shallow earthquakes or volcanic tremor have been recorded on the Alamagan seismic station since it was installed in September 1990. Charcoal was collected that should date the youngest and one of the oldest eruptions.

Geologic Background. Alamagan is the emergent summit of a large stratovolcano in the central Mariana Islands with a roughly 350-m-deep summit crater east of the center of the island. The exposed cone is largely Holocene in age. A 1.6 x 1 km graben cuts the SW flank. An extensive basaltic-andesite lava flow has extended the northern coast of the island, and a lava platform also occurs on the S flank. Pyroclastic-flow deposits erupted about 1000 years ago have been dated, but reports of historical eruptions were considered invalid (Moore and Trusdell, 1993).

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Remeasurement of the EDM network on 22 May showed no significant changes, consistent with the lack of shallow seismicity since September 1990. Boiling hot springs on the eastern crater floor and solfataras at the base of the nearby crater wall had maximum temperatures of 98°C.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Lava production, tephra ejection, and fumarolic activity continued through mid-July. Most of the W-flank lava moved down a channel feeding the flow's S lobe, which moved into young forest on the WSW flank, an area that had been affected by the 1968 pyroclastic flows. Since mid-May, the S lobe's front had advanced almost 300 m, reaching 665 m elevation on 10 June and 650 m elevation by the 24th. As it advanced, the lava flow continued to start fires that burned well over a hectare of the surrounding woodland. Between 12 and 22 July, the flow front advanced at an average rate of ~20 m/day, reaching ~2.5 km from the new summit crater (C). The lava supply to the N lobe had dwindled, and its front had halted at 830 m elevation.

Explosions were stronger and more numerous in June than in May. Some caused rumbling that vibrated house windows in La Palma, 4 km N of the volcano. An impact crater 1 m in diameter and 30 cm deep was found at 780 m elevation on the W flank, and large blocks frequently reached slightly >1 km from the new summit crater (C) 12-22 July. Some ash columns rose >1 km above Crater C. The rate of explosions varied; during observations on 12 June, an explosion was heard every hour. Ashfall on the observation point at 780 m elevation, 1.8 km W of the active crater, accumulated more rapidly in the 4 weeks ending 10 June than in the succeeding 2 weeks (see table 5). Vegetation on the NE, E, and SE flanks continues to be affected by acid rain and tephra fall, as it has for more than 20 years. Fumarolic activity occurred from the remnants of the old summit crater (D).

Volcanic seismicity recorded at a station (Fortuna) 4 km E of the active crater averaged 30 events/day, with a maximum of 51 on 18 June (figure 48). Conspicuous tremor episodes occurred on 4, 6, 10, 17, and 30 June. The level of both seismic and pyroclastic activity decreased 12-22 July, as did the number of avalanches from the advancing lava flow front.

Figure 48. Daily number of seismic events recorded at a station (Fortuna) 4 km E of Arenal's active crater, June 1992. Courtesy of the Instituto Costarricense de Electricidad.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto, ICE; M. Fernández, Univ de Costa Rica.

Eruptions that occurred from Crater 1 during the night of 30 June-1 July were the first [strong explosions] since . . . December 1990. The daily number of isolated volcanic tremor episodes began to increase in October 1991, and had reached ~100/day by the end of May. Isolated tremor episodes rapidly became more frequent in late June, and the amplitude of continuous tremor also increased through the month.

Ejections of mud and water from the lake in Crater 1 were first noted on 23 April and were sporadically observed later in April and in May. The ejections became more vigorous in late June, increasing in height from 5 m on 24 June to 20 m on the 26th, 50 m on the 29th, and 150 m on the 30th. Surface temperatures of the lake water increased from around 20°C in May 1991 to 78°C in June 1992. Steam plumes also grew to 1,000 m height in late June.

Strong tremor episodes were recorded during the night of 30 June-1 July. During fieldwork at noon on 1 July, the crater was quiet, but many blocks to 0.8 m across had been scattered to 100 m from the crater's NE rim. The eruptions were not seen or heard, but seismic and air-vibration records suggested that they may have occurred at 2349 on 30 June and 0316 on 1 July.

Tremor decreased in early July, but remained at higher levels than in mid-June. Ejections of mud and water to heights of a few tens of meters occurred sporadically through early July, but no additional strong mud/water ejections or eruptions were reported.

Because of the increasing activity, the area within 1 km of the crater was closed to tourists on 24 June, and remained closed as of mid-July.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Vigorous steaming was occurring from several locations in the summit crater [of Asuncion] during observations from a helicopter on 18 May.

Geologic Background. A single large asymmetrical stratovolcano, steeper on the NE side, forms 3-km-wide Asuncion Island. The steep NE flank terminates in high sea cliffs. The gentler SW flanks have low-angle slopes bounded by sea cliffs only a few meters high. The southern flank is cut by a large landslide scar. The southern flanks and western flanks are mantled by ash deposits that may have originated during eruptions in historical time. An explosive eruption in 1906 also produced lava flows that descended about half way down the western and SE flanks, but several other historical eruption reports are of uncertain validity. Few invesitgations have been done on the Cheref and Poyo seamounts, 30 and 50 km SE, respectively.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

A eruption . . . had begun by 6 July, when airplane pilots first reported steam and ash rising through low clouds. Similar activity was seen through the week, when satellite images revealed repeated plumes from Bogoslof. Pilots reported a cloud to ~3 km altitude on 14 July at 1815. Satellite images showed the plume extending roughly 100 km SE, to the S side of Unalaska Island. An image from 16 July at 1140 showed another plume extending ~100 km E to Unalaska. That day, a pilot saw a white plume rising to ~4 km altitude. An episode of vigorous steam and ash ejection began on 20 July at about 1700, and material had reached nearly 8 km asl by 1725, drifting NNE. A dark gray cloud that was ~15 km wide at 3 km altitude was moving NW from the volcano several hours later. Poor weather prevented subsequent observations, but satellite images showed no volcanic plumes rising above weather-cloud tops at ~6 km elevation. There have been no reports of ashfall. Cloudy weather has prevented direct observation of the island . . . .

Geologic Background. Bogoslof is the emergent summit of a submarine volcano that lies 40 km north of the main Aleutian arc. It rises 1500 m above the Bering Sea floor. Repeated construction and destruction of lava domes at different locations during historical time has greatly modified the appearance of this "Jack-in-the-Box" volcano and has introduced a confusing nomenclature applied during frequent visits of exploring expeditions. The present triangular-shaped, 0.75 x 2 km island consists of remnants of lava domes emplaced from 1796 to 1992. Castle Rock (Old Bogoslof) is a steep-sided pinnacle that is a remnant of a spine from the 1796 eruption. Fire Island (New Bogoslof), a small island located about 600 m NW of Bogoslof Island, is a remnant of a lava dome that was formed in 1883.

Information Contacts: AVO; SAB.

The following, from José Luís Macías, Arturo Macías, Jean-Christophe Komorowski, Claus Siebe, and Robert Tilling, describes observations during fieldwork 18 April-21 May 1992, ten years after the major 1982 eruption.

Geology. We made several visits to the crater. The very significant erosion that has occurred in the last 10 years allowed us to descend relatively easily into the crater through its SE wall, where the rim's altitude is 1,060 m. The crater floor is at 900 m elevation.

The only changes that we noticed during our visits were caused by frequent rockfalls from the crater walls. Between the first and second visits, on 19 April and 3 May, new crater-floor rockfall deposits had originated from the SE crater wall. Recently exhumed fault planes veneered by secondary mineralization in the crater wall were also quite common. On the SE part of the rim, a fracture system 90 m long, 6-9 cm wide at its SE end, and 0.2-8 cm wide at the NE end, trended N 65°E, and was associated with mild fumarolic activity. The fracture cuts through bedded domal talus breccia mapped by Rose and others (1984) and might evolve to produce rockfalls in the near future. Several other curviplanar slump fractures encompass apparent areas of several hundred square meters on the crater wall. Thus, more vigorous rockfall activity might be expected, particularly during the coming rainy season or periods of heightened regional seismic activity.

People living near the volcano reported an eruption in late March or early April that produced light ashfall near the volcano, and was accompanied by loud, thunder-like noises. We think that the ashfall most likely was dust produced during large rockfalls from the crater walls, and the noise was the sound of the rockfalls. Eruption-like dust clouds produced by rockfall activity have been described at Kilauea by Tilling (1974) and Tilling and others (1975).

To try to reduce local alarm, J.L. Macías and J.-C. Komorowski described the current activity and their interpretations of it during an informal conference on 19 May with residents of Chapultenango (11 km ESE of the crater), local authorities, and a group of elementary school teachers. Rumors in El Volcán (5 km E of the crater) that the volcano would erupt on its 10th anniversary caused many women and children to leave their homes.

Crater lake. Temperature and acidity of the crater lake were measured three times at two different sites (table 2). Lake temperature had increased from 28.6°C in 1986 to more than 40° in May 1992, nearing the 42° of October 1983 and February 1984. The pH values of 1.8 and 1.9 measured in 1983 and 1984, respectively, were similar to the April 1992 value. Although no heavy rainfall occurred between 18 April and 8 May, brief rains were common at night and may have diluted the lake with meteoric water, raising its pH. Water samples collected on the lake's N shore are being studied by M.A. Armienta and S. de la Cruz-Reyna at the Instituto de Geofísica, UNAM.

Table 2. Temperature and acidity of the crater lake at El Chichón, measured at sites on the SE and N shores.

Fumarolic activity. Gas emission from the crater fed a low-altitude plume visible on clear days. Fumarolic activity was observed throughout the crater but was much more extensive and vigorous in its NNE sector (steaming ground zone of Casadevall and others, 1984). Almost all of the fumaroles showed a steady, audible release of overpressured gas, except for one just N of the crater lake, where frequent noise changes showed that output was distinctly discontinuous. At times, vapor formed only within about 1 m above this vent, suggesting that the gas is initially superheated. All of the fumaroles produced sublimates, primarily native sulfur. A high-temperature fumarole NE of the crater lake contains molten orange sulfur within the orifice of a 1-m-high feature otherwise covered with needle-like amorphous yellow sulfur. Numerous mildly steaming areas were found in the NW and NE parts of the crater, and small fumaroles were active several tens of meters above the crater floor along the path descending from the SE crater wall. Relict portions of altered brecciated trachyandesite described by Rose and others (1984) as remnants of the pre-1982 dome and shown on the map of Casadevall and others (1984) as "altered areas" are still actively steaming.

A few fumaroles on the NE side of the crater are characterized by vigorous geyser activity, sending a constant flux of boiling water to 2-3 m height. In the same area, several boiling springs about 2-3 m above the present crater-lake surface produce boiling streams with a significant discharge into the lake, 50 m away. A similar situation was evident near a boiling mud pit in the NW part of the crater. These boiling streams are sites of mineral precipitation, and active red, brown, and green algae growth. Ferns and grasses have returned to some of these hydrothermal areas. Ponds 1 m in diameter on the NW side of the lake contained vigorously boiling mud (rising <1 m) and water.

The crater lake, which had recovered to November 1982 levels by November 1990, was turquoise-blue and had at least two large zones of intense surface effervescence as described by Casadevall and others (1984).

Although an acrid smell was noted at active hydrothermal areas, H₂S concentrations must have decreased below the 2-6 ppm that forced geologists to take special precautions in 1983 and to leave the crater in 1984. During several 4-hour periods in the crater, we never needed gas masks, even in the most active areas.

Other observations. In the Río Magdalena near Xochimilco (8 km NW of the crater), vegetation has made a strong comeback on pyroclastic-flow deposits, which are now covered by tall grasses and acacia trees up to 2 m high with trunks several centimeters in diameter. In all other areas within 2-3 km of the crater, the 1982 deposits are covered only by moss, lichen, and tall grass. Where pyroclastic flows and surges did not surmount topographic barriers or deposited only a thin veneer of material, vegetation is much more lush, with trees, ferns, and other broad-leafed tropical plants. Trees that were charred but not totally blown down >5 km away have begun to grow again from their stumps. The river that now passes through El Volcán was formed after the pyroclastic flows changed the former drainage pattern. An abundant, rusty colored precipitate (Fe oxides) was sampled for analysis.

Future work. More extensive field observations within the crater are planned for November or December. We will measure temperature and pH, and sample sites of hydrothermal activity. An attempt will be made to overfly the crater with a COSPEC, to bring portable seismometers into the crater and somma flanks, and to make bathymetric measurements.

References. Casadevall, T., de la Cruz-Reyna, S., Rose, W., Bagley, S., Finnegan, D., and Zoller, W., 1984, Crater lake and post-eruption hydrothermal activity, El Chichón Volcano, México: Journal of Volcanology and Geothermal Research, v. 23, p. 169-191.

Rose, W., Bornhorst, T., Halsor, S., Capaul, W., Plumley, P., de la Cruz-Reyna, S., Mena, M., and Mota, R., 1984, Volcán el Chichón, México: pre-1982 S-rich eruptive activity: Journal of Volcanology and Geothermal Research, v. 23, p. 147-167.

Tilling, R., 1974, Rockfall activity in pit craters, Kilauea Volcano, Hawaii: Proceedings of the Symposium on "Andean and Antarctic Volcanology Problems", IAVCEI, Santiago, Chile, September 1974, p. 518-528.

Tilling, R., Koyanagi, R., and Holcomb, R., 1975, Rockfall seismicity-correlation with field observations, Makaopuhi Crater, Kilauea Volcano, Hawaii: Journal of Research, U.S. Geological Survey, v. 3, p. 345-361.

Geologic Background. El Chichón is a small, but powerful trachyandesitic tuff cone and lava dome complex that occupies an isolated part of the Chiapas region in SE México far from other Holocene volcanoes. Prior to 1982, this relatively unknown volcano was heavily forested and of no greater height than adjacent nonvolcanic peaks. The largest dome, the former summit of the volcano, was constructed within a 1.6 x 2 km summit crater created about 220,000 years ago. Two other large craters are located on the SW and SE flanks; a lava dome fills the SW crater, and an older dome is located on the NW flank. More than ten large explosive eruptions have occurred since the mid-Holocene. The powerful 1982 explosive eruptions of high-sulfur, anhydrite-bearing magma destroyed the summit lava dome and were accompanied by pyroclastic flows and surges that devastated an area extending about 8 km around the volcano. The eruptions created a new 1-km-wide, 300-m-deep crater that now contains an acidic crater lake.

Information Contacts: José Luís Macías V. and Michael Sheridan, State Univ of New York, Buffalo, NY; Jean-Christophe Komorowski and Claus Siebe, Instituto de Geofísica, UNAM; Robert Tilling, USGS.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. Clark submarine volcano lies near the southern end of the Southern Kermadec arc. This basaltic and dacitic stratovolcano consists of a basal substrate of massive lava flows, pillow lavas, and pillow tubes overlain by volcaniclastic sediments. Craters occupy the complex crest of the volcano. Clark is the southernmost volcano of the submarine chain that displays hydrothermal activity. Diffuse hydrothermal venting and sulfide chimneys were observed near the summit of Clark volcano during a New Zealand-American NOAA Vents Program expedition in 2006.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Geysers geothermal area report. Film records showed 50 small events in the 24 hours following the M 7.5 earthquake, 46 of which had coda durations

Geologic Background. The late-Pliocene to early Holocene Clear Lake volcanic field in the northern Coast Ranges, contains lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. The westernmost site of Quaternary volcanism in California, the Clear Lake field is located far to the west of the Cascade Range in a complex geologic setting within the San Andreas transform fault system. Mount Konocti, a composite dacitic lava dome on the south shore of Clear Lake, is the largest volcanic feature. Volcanism has been largely non-explosive, with only one major airfall tuff and no ash flows. The latest eruptive activity, forming maars and cinder cones along the shores of Clear Lake, continued until about 10,000 years ago. A large silicic magma chamber provides the heat source for the Geysers, the world's largest producing geothermal field.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

The following . . . covers activity between 10 April and 30 June 1992, and describes the 25 June 1991 lahar deposits.

Seismicity and rockfall activity. After a brief seismic crisis 4-10 March, activity at Colima remained near background levels. Starting 10 April, seismicity became more frequent. Nine B-type earthquakes were detected by the Red Sismológica de Colima (RESCO) and up to 60 events were recorded 10-20 May at the SW-flank Yerbabuena station (figure 17). Subsequent seismic activity remained near background, with only four B-type earthquakes recorded by RESCO 20-31 May, and three between 1 and 20 June. Seismic activity increased slightly 21-30 June, when 22 B-type earthquakes were recorded and the number of associated seismically detected rockfalls reached 55. Other rockfalls were also noted, probably associated with small diurnal changes in the volcano's hydrothermally altered summit region, which might be related to changes in rock temperature and surface water content. Extraordinary out-of-season precipitation in January, related to the El Niño/Southern Oscillation event of 1991-92, exceeded 700% of the monthly mean of the past 30 years and must have saturated the volcano's upper porous regions.

Figure 17. Sketch map of the summit area and SW flank of Colima, showing major canyons and recent volcanic deposits. Modified from Rodríguez-Elizarrarás, and others, 1991.

Current thermal activity. Fumarolic activity has been steady, with an impressive white plume that can rise several hundred meters above the summit before dissipating. This represents the systematic release of meteoric water accumulated in the upper part of the volcano, not an increase in the magmatic component of the fumarolic activity. Further avalanching of the most precarious hydrothermally altered regions of the summit area is expected during the rainy season, which has just started.

25 June 1991 lahar deposit. Block-and-ash flows emplaced about 1 x 10⁶ m³ of loose pyroclastic debris in the upper Barranca El Cordobán during collapse of the crater dome and rim on 16-17 April 1991, just before the 1991 lava flow began to move down the SW flank (figure 17) (Rodríguez-Elizarrarás and others, 1991). Despite heavy rains in May-September 1991, geologists from the CICT reported that most of the pyroclastic deposits had been washed away without producing sizeable mudflows (Rodríguez-Elizarrarás, and others, 1991). Nevertheless, on 28 March 1992, S. de la Cruz-Reyna and CICT geologists observed a significant laharic mass-flow deposit near El Jabalí, which was studied 5-7 June by J.-C. Komorowski and CICT geologists. A more thorough field and laboratory investigation of this deposit is in progress.

The lahar reached the settlements of La Becerrera and San Antonio, ~12 km SW of the summit (figure 17). Unequivocal non-reworked lahar material was seen at 1,280 m elevation, ~500 m above the confluence of the barrancas El Zarco and El Cordobán. The total thickness was 2 m with a channel width of 30 m. Deposits from this lahar have been identified up to ~1,900 m above sea level, at the bottom of a 20-30-m vertical lava wall in the barranca El Cordobán. The barranca's slope flattens drastically after the lava wall, so deposition probably began below this point. The most distant block-and-ash flow deposits in this barranca reached down to 2,100 m elevation. Upstream, the barranca was significantly eroded by water and debris from a maximum elevation of 2,600 m. Although there is no clear evidence of lahar deposits at San Antonio and La Becerrera, one person reported that the water crossing on the San Antonio-Laguna Verde road was obstructed for two days by lahar material, until machines cleared the debris. Such occurrences are frequent in the rainy season, because several large barrancas draining the upper slopes join there to form a channel 30 m wide.

We estimate the total lahar path at 9.9 km. Based on several measurements at different sites, the lahar deposit averages 25 m wide and 2 m thick. Maximum width was 38 m and maximum thickness 2.9 m at 1,640 m elevation (star on figure 17). Volume was estimated at approximately 0.5 x 10⁶ m³, or about 50% of the material estimated to have been emplaced by the 16-17 April 1991 pyroclastic activity. Field evidence and testimony (see below) unequivocally show that all of the lahar deposit was emplaced during one event. April 1992 field studies of barrancas at higher altitude revealed tremendous erosion since April 1991, leaving ravines incised deeply (to 15 m) into the pre-1991 pyroclastic deposits. A significant volume of loose 1991 debris remains on the mountain, ready to be incorporated into lahars during the rainy season.

Preliminary field investigations showed that the lahar deposit is characterized by a very flat surface, with suspended lava blocks to 1-2 m in maximum dimension protruding through the surface, and abundant pumiceous clasts from eroded 1913 deposits. The deposit is massive, non-stratified, non-graded, poorly sorted, and matrix supported. Its typical massive lowermost zone (0.6 m thick), locally well-sorted, has a concentration of blocks (to 0.5 m size) and wood fragments at the base, a prominent clast-supported medial zone (0.7 m thick) with imbricated sub-rounded boulders (to 0.3 m), and an uppermost massive unit (0.8 m) with a tendency toward reverse grading of lithic cobbles, supported in a sandy matrix. The deposit is typically semi-indurated. Inter-unit contacts are sharply defined in several places, most likely reflecting shear between rheologically different portions of the mass flow. Given the large suspended blocks, the very flat surface, the constant thickness over 9 km of travel distance, the presence of marginal levees, and overturned logs that came to rest vertically, the mass flow clearly had a significant yield strength. However, it must have been relatively swift, as it was able to flow around topographic barriers in the channel, and in some places to leave an elevated deposit on the outside wall when it rounded a sharp curve.

Few people witnessed the lahar. The best testimony came from a farmer (Ramón Aguirre Valencia) who went to Barranca El Cordobán on 26 June 1991 to check his cattle. At 1,600 m altitude, the barranca was filled by a gravel- and boulder-rich deposit with a flat surface. Rocks on the surface were coated with a thin layer of light-colored fine ash. Of the 20 cows killed by the lahar, several could be seen, with horns, heads, and feet protruding from the deposit. Numerous tree trunks several meters long and as much as 30 cm in diameter were also on the lahar's surface. Heavy rains had occurred the previous day, and the lahar apparently began to form after about 2 hours of heavy precipitation, accompanied by loud thunder. The nearest meteorological station (Cofradía de Suchitlán), about 12 km from the lahar's most likely source area, recorded 50 mm of rain on 25 June. By 3 July, a ravine had developed in the new lahar that was as deep (4.6 m) but not as wide as the present channel, which now spans 10.6 m of the 38-m-wide deposit. Five kilometers downstream, the lahar overran and destroyed a 2-m-high stone wall at El Jabalí and clogged the existing channel, but 2 km farther downslope, residents of La Becerrera noticed nothing unusual. Larger sediment flows reported at La Becerrera in January may have been related to breaching of a small earthen dam.

Warnings of future lahar flows and the hazards within Barranca El Cordobán were reiterated to authorities in 1992, as abundant loose material remains from the 1991 eruption and recently exposed 1913 pyroclastic units. The El Jabalí basin is filled with old mass-flow deposits that have traveled down several steep, deeply incised barrancas. On 12 June, CICT organized a meeting that included civil protection authorities to discuss these hazards.

Reference. Rodríguez-Elizarrarás, C., Siebe, C., Komorowski, J.-C., Espindola, J.M., and Saucedo, R., 1991, Field observations of pristine block-and-ash flow deposits emplaced April 16-17, 1991 at Volcán de Colima, México: Journal of Volcanology and Geothermal Research, v. 48, no. 3/4, p. 399-412.

Geologic Background. The Colima volcanic complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide caldera, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent historical eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Carlos Navarro, Abel Cortés, I. Galindo, José J. Hernández, and Ricardo Saucedo, CICT, Universidad de Colima; Jean-Christophe Komorowski and Claus Siebe, Instituto de Geofísica, UNAM.

Lava production continued from the fissure that opened in the W wall of the Valle del Bove on 15 December. Gas emission from 4 vents in the upper part of the fissure (2,215-2,235 m altitude; figure 52) fluctuated daily, probably with changes in weather conditions. However, gas emission has diminished since the eruption's initial months.

Figure 52. Sketch map of the fissure system and the upper part of the lava field at Etna, June 1992. Contour interval, 50 m. Courtesy of Romolo Romano.

No variation was evident in the movement of lava visible through a skylight high in the main channel, at 2,205 m altitude. Lava was also seen flowing through a skylight in lava tubes that formed in June along the channel into which lava was artificially diverted on 27 May (~ 1,980 m elevation) (17:05). From there, lava advanced through a complex series of tubes past the field that had formed in recent months. Lava again reached the surface around 1,800 m altitude from a changing number (generally 3-4) of ephemeral vents at varying locations representing tube bases. Lava flows extruded from these vents have generally been modest, have remained in the center of the lava field, and have not advanced beyond 1,600 m asl. As of the morning of 9 July, only one flow was active within the Valle del Bove, near the center at around 1,670 m altitude, with a fairly well-fed front. The volume of lava produced during ~7 months of eruption is estimated to be around 165 x 10⁶ m³.

Seismic activity during the period was characterized by low energy release. Significant increases were observed 8-9 July, when events of 2-4 Hz were recorded. The most significant perturbations were detected on 8 July at 1554, for 180 seconds, and at 1601 for 130 seconds. Tremor was almost nonexistent, obscured by seismic noise that characterizes periods of low activity at the volcano.

More or less voluminous gas emissions occurred from two vents at the bottom (~100 m from the rim) of the two central craters (Bocca Nuova and La Voragine). Incandescence caused by superheated gases (>1,000°C) from the vent in La Voragine was sometimes visible. Gas also emerged from a vent that has opened in Southeast Crater. Northeast Crater appeared to have been completely obstructed by internal collapse. COSPEC measurements of SO₂ flux from the summit crater showed relatively high values of ~ 8,000 t/d.

Geologic Background. Mount Etna, towering above Catania, Sicily's second largest city, has one of the world's longest documented records of historical volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km horseshoe-shaped caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: R. Romano and T. Caltabiano, IIV; P. Carveni, M. Grasso, and C. Monaco, Univ di Catania; G. Luongo, OV.

When observed from an airplane on 13 May, the volcano continued to fume vigorously, but no active lava was seen.

Geologic Background. The small 2-km-wide island of Farallon de Pajaros (also known as Uracas) is the northernmost and most active volcano of the Mariana Islands. Its relatively frequent historical eruptions dating back to the mid-19th century have caused the andesitic volcano to be referred to as the "Lighthouse of the western Pacific." The symmetrical, sparsely vegetated summit is the central cone within a small caldera cutting an older edifice, remnants of which are seen on the SE and southern sides near the coast. Flank fissures have fed lava flows during historical time that form platforms along the coast. Both summit and flank vents have been active during historical time. Eruptions have also been observed from nearby submarine vents, and Makhahnas seamount, which rises to within 640 m of the sea surface, lies about 10 km to the SW.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

An explosion on 16 July, the largest since activity began in 1989, ejected large tephra and may have generated a small pyroclastic flow. Partial collapse of the summit crater's lava dome occurred in June, and minor seismicity had been recorded a few days before the explosion.

June activity. The NW portion of the 1991 lava dome collapsed during June, and explosions and ash emissions occurred from the collapsed area. Las Portillas fumarole, formerly just NW of the dome, was larger after the collapse, and a line of new vents had opened nearby. The fracture on the NW crater wall remained active, and it and Las Portillas appeared to be the highest temperature vents in the crater. Gas columns were generally small, and were dispersed to the N and W. The number and energy release of long-period events (figure 55) and high-frequency earthquakes were low. Ten high-frequency earthquakes occurred in the NW part of the crater, with magnitudes of 0.3-1.7. The amplitude and period of background tremor showed small variations on 15 and 30 June. The maximum rate of SO₂ emission measured by COSPEC was ~5,500 t/d.

Figure 55. Daily number of long-period seismic events at Galeras, 1 January 1991-30 June 1992. The first observation of the summit lava dome is marked by an arrow. Courtesy of INGEOMINAS.

Precursory seismicity and tilt. Banded tremor episodes of moderate to high energy occurred 11-12 July, accompanied by a small inflationary tilt event recorded on both instruments near the summit. Between 14 and 16 July, six monochromatic long-period events were recorded, with durations on the order of 80 seconds. On 15 July, there was a small swarm of high-frequency events with magnitudes of 0-0.5.

16 July explosion. The explosion began at 1740 with a strong shock felt in Pasto . . . . More than 90% of the summit lava dome was destroyed as at least 120,000 m³ of blocks were ejected, falling primarily on the E and NE flanks. Blocks 30 cm in diameter fell 2.3 km from the crater, and impact craters to 3.5 m across were found 400 m away. Incandescent blocks started fires 2 km from the crater on the NE flank. The tephra severely damaged a small military facility on the crater rim, and dropped 40-cm blocks on telephone and television facilities near the summit. Roughly 30,000 m³ of ash were dispersed in a narrow band to the W, with the 1-mm isopach extending ~10 km. The dark-gray cauliflower-shaped eruption column reached ~4 km altitude. Reports from observers 10 km WSW of the crater (in Consacá) suggested that small pyroclastic flows may have descended the W flank. A powerful sonic wave generated by the explosion broke windows in Pasto, and reportedly in Consacá.

A seismic signal lasting ~8 minutes accompanied the explosion, saturating instruments for the first 37 seconds. Two distinct signals were recognized, one with a frequency of 1 Hz and a duration magnitude of 3, the other a 1.3-Hz tremor episode that lasted 4 minutes. A high-frequency, M 3.2-3.5 event occurred 26 hours after the explosion, in the S part of the volcano at ~5 km depth.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large horseshoe-shaped caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate historical eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS-Observatorio Vulcanológico del Sur.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Observations [of Guguan] from an airplane on 13 May and a helicopter on 21 May revealed no gas emission.

Geologic Background. The small island of Guguan, only 2.8 km wide, is composed of an eroded volcano on the south, a caldera with a post-caldera cone, and a northern volcano. The latter has three coalescing cones and a breached summit crater that fed lava flows to the west and NW. The 287-m high point of the island is the south rim of the caldera. Freycinet misidentifed Guguan with Alamagan; reported eruptions in 1819 and 1901 (Catalog of Active Volcanoes of the World) actually refer to solfataric activity on Alamagan (Corwin, 1971). The only known historical eruption of Guguan took place between 1882 and 1884 and produced the northern volcano and lava flows that reached the coast.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Fumarolic activity continued in the main crater. Its lime-green lake had a mean temperature of 28°C and a minimum pH of 4.9 on 3 June. Fumaroles persisted in the area NE of the lake, with temperatures of 84-90°C. Areas of bubbling to the NE remained vigorous, with strong emission of cold gas, perhaps CO₂. Hot bubbling areas were stable at temperatures <=91°C. Fumarolic vents in the sedimentary fan N of the lake were buried by new sedimentation triggered by heavy rains. The resulting zone of steaming ground had surface temperatures of up to 90°C.

Seismicity continued, with 48 events recorded during June at a station (ICR) 2.2 km E of the active crater and 36 low-frequency microseisms registered 5 km WSW of the crater (at station IRZ2). The largest daily earthquake count was 7 on 2 June (at ICR). On 30 June, a M 1.9 event occurred 6.7 km SW of the main crater, at 3 km depth.

Geologic Background. Irazú, one of Costa Rica's most active volcanoes, rises immediately E of the capital city of San José. The massive volcano covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad flat-topped summit crater complex. At least 10 satellitic cones are located on its S flank. No lava flows have been identified since the eruption of the massive Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the historically active crater, which contains a small lake of variable size and color. Although eruptions may have occurred around the time of the Spanish conquest, the first well-documented historical eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G.J. Soto, ICE; Mario Fernández, Escuela Centroamericana de Geología, Univ de Costa Rica.

Activity began to increase in February 1992. Glowing rockfalls on 18 May filled the upper Keting river valley to 4 km from the crater. The volume of the deposit was estimated at 1.2 x 10⁶ m³, ~ 20% of the dome (17:04). Since then, the eruption has fluctuated, but a general decrease in intensity was indicated by declines in the height of the ash plume, the behavior of the glowing lava flow, and the vigor of incandescent tephra ejection. In July, glowing rockfalls advanced down the Keting river to 1,500 m from the crater. The number of volcanic and local tectonic earthquakes decreased in June and July compared to previous months. June-July seismicity was dominated by surface activity, such as explosion earthquakes and rockfalls (figure 2).

Figure 2. Tectonic seismicity (top) and volcanic earthquakes (bottom) at Karangetang, June-July 1992. Courtesy of VSI.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi island. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented in the historical record (Catalog of Active Volcanoes of the World: Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: W. Modjo, VSI.

Lava production continued through early July from the E-51 vent . . . (figure 85), but was interrupted by several brief pauses. With each resumption in activity, lava reoccupied tubes on the S flank of the E-51 shield. Flows emerged from the tubes under some pressure, creating small, meter-high dome fountains at their heads. The lava pond at the top of the E-51 shield drained and refilled with changing lava supply, sustaining frequent overflows that did not advance far. Some lava also ponded at the base of the shield before flows advanced S and E. The small lava lake in Pu`u `O`o crater remained active, fluctuating between 38 and 55 m below the crater rim in June. The lake surface rose during pauses in activity at the episode-51 vent and dropped when lava production resumed there. By early July, it had dropped farther, to 65 m below the rim.

Activity resumed on 2 June, after a 3-day pause (17:5), while harmonic tremor began a gradual increase to about twice background levels at 0000. Large flows advanced N along the W flank of Pu`u `O`o cinder cone. These shelly pahoehoe flows formed shallow tubes and stagnated within a few days. The eruption stopped briefly on 5 June, as tremor dropped to near background at 1800, resumed the next day accompanied by a tremor increase at about 0700, and halted again ~24 hours later on the 7th, when lava drained slowly from the pond atop the shield.

Another increase in tremor began early on 9 June, reaching about twice background levels by noon on the 10th. Shallow, long-period microearthquakes (LPC-A, 3-5 Hz) were frequent on 9 June, as were upper east rift events on 9-10 June. Lava started to emerge from the E-51 vent at 1325 on 10 June, re-entering the tube system on the S flank of the E-51 shield. The lava lake in Pu`u `O`o crater had been nearly level with the crater floor when E-51 activity resumed, but had dropped ~9 m by the next day.

A small spatter cone formed 3-11 June over a weak point in the tube on the N flank of the E-51 shield. This tube had fed numerous aa ooze-outs that spread out around the shield's N flank in past months. On 13 June, an aa flow was active on the shield's N flank, appearing to originate from the new spatter cone.

Lava production stopped again on 16 June, the pond at the top of the shield drained, and flows slowed their advance. The eruption restarted during the morning of 21 June, continuing through the end of the month. Pahoehoe flows extended N and SE from the vent. Through 25 June, the shield's pond was full and intermittently overflowing, but by 1 July it had drained to ~15 m depth with a solid crust at the bottom. However, lava continued to ooze into the S-flank tube system and to break out at the base of the shield. Tremor amplitudes gradually declined to near background by 2000 on 29 June, and remained at low levels into early July.

Geologic Background. Kilauea, which overlaps the E flank of the massive Mauna Loa shield volcano, has been Hawaii's most active volcano during historical time. Eruptions are prominent in Polynesian legends; written documentation extending back to only 1820 records frequent summit and flank lava flow eruptions that were interspersed with periods of long-term lava lake activity that lasted until 1924 at Halemaumau crater, within the summit caldera. The 3 x 5 km caldera was formed in several stages about 1500 years ago and during the 18th century; eruptions have also originated from the lengthy East and SW rift zones, which extend to the sea on both sides of the volcano. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1100 years old; 70% of the volcano's surface is younger than 600 years. A long-term eruption from the East rift zone that began in 1983 has produced lava flows covering more than 100 km2, destroying nearly 200 houses and adding new coastline to the island.

Information Contacts: T. Mattox and P. Okubo, HVO.

A M 5.2 earthquake, centered in the sea 8 km SW of the volcano at 9 km depth, occurred on 15 June at 1046. Island residents felt the shock at intensity 5 on the JMA scale of 0-7. Data from 30 stations of the Worldwide Standardized Seismic Network yielded magnitudes of 4.9 (m_b) and 4.7 (Ms). One person was slightly injured by a rockfall, and wallrock collapse at 10 sites closed 5 roads to traffic. Aftershocks continued until 17 June off the island's SW coast. The event was the second largest since . . . April 1991 (figure 1). No surface anomalies were observed on the island or on the sea-surface nearby.

Geologic Background. A cluster of rhyolitic lava domes and associated pyroclastic deposits form the small 4 x 6 km island of Kozushima in the northern Izu Islands. Kozushima lies along the Zenisu Ridge, one of several en-echelon ridges oriented NE-SW, transverse to the trend of the northern Izu arc. The youngest and largest of the 18 lava domes, 574-m-high Tenjoyama, occupies the central portion of the island. Most of the older domes, some of which are Holocene in age, flank Tenjoyama to the north, although late-Pleistocene domes are also found at the southern end of the island. Only two possible historical eruptions, from the 9th century, are known. A lava flow may have reached the sea during an eruption in 832 CE. Tenjosan lava dome was formed during a major eruption in 838 CE that also produced pyroclastic flows and surges. Earthquake swarms took place during the 20th century.

Information Contacts: JMA; NEIC.

"A new phase of eruptive activity that started on 30 May lasted until 8 June. From 1 to 4 June, both Crater 2 and Crater 3 produced ash-rich Strombolian explosions to 500-700 m height. A new, short lava flow was emplaced on the NW flank of Crater 3. Emissions from Crater 2 became markedly ash-laden 4-7 June, with a plume rising a few kilometers above the crater and ashfalls on coastal areas 10 km NW. After the 7th, only weak to moderate vapour emissions and occasional Vulcanian explosions were noted from Crater 2.

"Activity at Crater 3 also waned after the first week in June, although more progressively. On the night of 7 June, intermittent explosions projected incandescent lava fragments to 250 m above the crater, while on 8 June there was weak steady glow over the crater. Intermittent explosions still occurred daily until the 24th, producing dark convoluting ash clouds that rose a few hundred meters above the crater.

"Seismic monitoring resumed on 11 June and showed only low-level activity throughout the rest of the month."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: P. de Saint-Ours, D. Lolok, and C. McKee, RVO.

"A Landsat TM image recorded the night of 15 April 1992 shows the most intense thermal anomaly of a dataset extending back to December 1984. The thermal signature, in the short-wavelength infrared bands 5 (1.55-1.75 mm) and 7 (2.08-2.35 mm), represents the active lava dome in the central crater. Comparison with the previous image (night of 7 January 1991) shows a marked increase in the anomaly's area (figure 11). In the April 1992 scene, the core of the anomaly occupies an irregular area of ~7 x 6 pixels (equivalent to 210 x 180 m). These dimensions correspond closely with the 180-190 m dome diameter estimated from 20 March airphotos (17:5). The increase in area of the TM anomaly may be explained, at least in part, by the growth of a subsidiary lava dome first sighted on 4 March. The summed thermal radiance from the whole hot spot shows a corresponding increase in the April Landsat image (figure 12).

Figure 11. 15 x 15 pixel maps (equivalent to 450 x 450 m) of the signal recorded in band 7 of the Landsat TM over Lascar at night on 7 January 1991 (left) and 15 April 1992 (right). The vertical axis represents the number between 0 and 255 proportional to the spectral radiance. In each case, several pixels are saturated. Courtesy of C. Oppenheimer.

Figure 12. Summed spectral radiance in bands 5 and 7 for fifteen images acquired over Lascar since December 1984. The dataset includes several processing formats, and images acquired during the day and night. Only pixels with a thermal signal >=10 were included. The total was then converted to spectral radiance using calibration coefficients supplied with the digital data. Arrows mark the explosive eruptions of September 1986 and February 1990 (12:4-5 and 15:2-3). Courtesy of C. Oppenheimer.

"An interesting feature of the two most recent TM acquisitions is the persistence of a discrete hot site ~200 m W of the centre of the main anomaly (figure 11). This is very likely the expression of incandescent fumarole vent(s) beyond the steep margin of the extruded lava."

Reference. Oppenheimer, C., Francis, P.W., Rothery, D.A., Carlton, R.W., and Glaze, L.S., Analysis of Volcanic Thermal Features in Infrared Images: Lascar Volcano, Chile, 1984-1992; Journal of Geophysical Research, in press.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: C. Oppenheimer, D. Rothery, P. Francis, and R. Carlton, Open Univ.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Lassen Report. Of the three major Holocene volcanoes in the California Cascades, Lassen (~800 km NNW of the epicenter) had the strongest response to the 28 June earthquake (figure 1). About 10 minutes after the S-wave's arrival and while surface waves were still being recorded, a M 2.8 event occurred south of Lassen Peak. Film records showed 9 more earthquakes in the first hour, and 22 events were identified during the first 24 hours. Although most were M 1 or smaller, at least two and perhaps as many as four were of magnitude greater than or equal to 2. Nine were detected by the RTP system. The best preliminary locations were concentrated ~3 km SW of Lassen Peak at

Figure 1. Seismic events in the Lassen area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of S. Walter.

Geologic Background. The Lassen volcanic center consists of the andesitic Brokeoff stratovolcano SW of Lassen Peak, a dacitic lava dome field, and peripheral small andesitic shield volcanoes and large lava flows, primarily on the Central Plateau NE of Lassen Peak. A series of eruptions from Lassen Peak from 1914 to 1917 marks the most recent eruptive activity in the southern Cascade Range. Activity spanning about 825,000 years began with eruptions of the Rockland caldera complex and was followed beginning about 590,000 years ago by construction of Brokeoff stratovolcano. Beginning about 310,000 years ago activity shifted to the north flank of Brokeoff, where episodic, more silicic eruptions produced the Lassen dome field, a group of 30 dacitic lava domes including Bumpass Mountain, Mount Helen, Ski Heil Peak, and Reading Peak. At least 12 eruptive episodes took place during the past 100,000 years, with Lassen Peak being constructed about 27,000 years ago. The Chaos Crags dome complex was constructed about 1100-1000 years ago north of Lassen Peak. The Cinder Cone complex NE of Lassen Peak was erupted in a single episode several hundred years before present and is considered part of the Lassen volcanic center (Clynne et al., 2000). The 1914-1917 eruptions of Lassen Peak began with phreatic eruptions and included emplacement of a small summit lava dome, subplinian explosions, mudflows, and pyroclastic flows.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

During a previously unreported 26 February climb by David Peterson, Howard Brown, and students from St. Lawrence Univ, activity was continuing from one cone (T20) . . . . Periodic gurgling and rumbling noises from the cone were audible from the crater rim. As Peterson and several students approached the active cone, lava fragments were ejected, one of which struck a student on the leg, causing a small burn. Crater photographs show a small dark vent at the summit of T20, but no dark (fresh) lava was evident on its flanks. However, by . . . 12 March, T20 had extruded a lava flow that covered much of the W part of the crater floor (17:03).

Brown's 26 February photographs show . . . T5/T9 as tall but pale gray, with no fresh, dark patches of lava. T15 was composed of jagged dark-gray pinnacles with medium-brown lower slopes and no sign of fresh lava. T8 and T8A seemed little changed from recent photographs, with slight yellow coloring at T8's summit. T14 appeared to have been surrounded by younger lava, which had turned pale gray to white. Some dark patches were visible around its summit vent. No dark fresh flows were evident on the crater floor.

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, St. Lawrence Univ; D. Peterson, Arusha; H. Brown, Nairobi, Kenya.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers. No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Long Valley Report. Within eight minutes of the major earthquake's origin time, seismic activity within Long Valley caldera (400 km NNW of the epicenter) increased abruptly (figure 15). Of the >260 events located by the RTP system during the next three days, three were of M 3 or greater. The first event within the caldera located by the RTP system was a M 1.4 earthquake at 1207, but develocorder film from caldera stations provides evidence of local earthquakes beginning at least a minute earlier within the strong coda waves from the M 7.5 event. The P-wave travel-time from the epicenter is just over 1 minute, and the S-wave travel-time just under two minutes, so it appears that local earthquake activity began no later than six minutes after the S-wave arrival.

Figure 15. Earthquakes >M 1.5 in the Long Valley area, 25 June-1 July 1992. Larger events are identified by numbered triangular labels beside earthquake symbols: (1) 25 June, 2143 GMT, M 2.4; (2) 28 June, 1214, 1230, 1232, M 2.6, 3.0, 2.5; (3) 29 June, 0103, M 3.1; (4) 29 June, 0537, 0638, M 3.7, 2.3; (5) 29 June, 0758, M 3.4; (6) 29 June, 0834, 0838, 0839, M 2.0, 2.1, 2.0. Courtesy of D. Hill.

Earthquake activity within Long Valley caldera had persisted, but at relatively low levels, through the first half of 1992, averaging

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: D. Hill, USGS Menlo Park.

"The eruption . . . ended on 15 June after another paroxysmal phase from Main Crater (on 7 June). Following the paroxysmal phase of 31 May from Southern Crater, the level of activity was moderate in the first days of June. Both craters were emitting white and blue vapours in weak to moderate amounts, with occasional explosions of ash-laden vapour rising a few hundred meters above the craters, weak roaring noises, and weak fluctuating glow at night.

"On the afternoon of 5 June, Southern Crater entered a phase of intermittent Strombolian activity that sprayed incandescent spatter to as much as 300 m above the crater at intervals of 30-40 minutes. At 1600, Main Crater emitted a dark ash column to ~1,000 m above the crater. Strombolian explosions within the crater must have started soon afterwards, as suggested by fluctuating night glow and roaring sounds. On the 6th, the level of activity remained moderate at Southern Crater while it strengthened at Main Crater. The forceful emissions of grey-brown ash from the latter were identified as Strombolian projections at night. From 0025 until about 1830 on 7 June, this crater produced continuous incandescent projections to 600 m above the rim in an ash column that rose 2-3 km. New lava flows were erupted into the NE Valley and followed the path of the previous flows (4-6 May) on the southern side of the valley, down to 110 m asl.

"Pyroclastic flows were also produced, scorching vegetation and some garden areas on the southern side of the NE Valley to about 1 km from Bokure Village. Downwind from the crater, on the NW side of the island, the sustained dark ash cloud overhead, the fall of ash and lapilli, and roaring sounds of the eruption caused some concern to the population.

"This paroxysmal eruption phase ended with loud explosions from 1817 to 1830 on 7 June. In the following days there was hardly any visible activity from either crater, apart from weak-to-moderate vapour emission. However, the seismicity, which had increased dramatically during the eruptive phase of 6-7 June, remained moderately high. On 12 June, occasional dull explosion sounds were heard again from Main Crater with occasional brown ash clouds and incandescent projections at night. This activity lasted until the 14th, becoming more and more intermittent. The last significant event from Main Crater observed in this eruption was a moderately strong Vulcanian explosion at 0800 on 14 June, which projected a convoluting cloud to 2-3 km above the crater. Likewise, Southern Crater was somewhat reactivated 13-15 June, with occasional weak explosions, a fluctuating night glow, and incandescent projections to 250 m above the crater rim. From 16 June onward, the seismicity dropped markedly and neither crater showed further signs of activity apart from weak, fumarolic emission. The Stage 2 volcanic alert that had applied since 13 April was dropped to Stage 1 (i.e. non-threatening, background level) on 25 June.

"This eruption of Manam is among the most significant since 1958, and can be compared with the eruption of 1974 (Palfreyman and Cooke, 1976; Cooke et al., 1976) as it involved both craters, produced pyroclastic flows and lava flows of significant volume, and affected all but one of the main valleys. However, the 1992 eruption appears to have been larger than the 1974 event. A preliminary estimate of the 1992 lava-flow volume is 17 x 10⁶ m³, compared with only 3 x 10⁶ m³ of lava flows in 1974."

References. Cooke, R.J.S., McKee, C.O., Dent, V.F., and Wallace, D.A., 1976, Striking Sequence of Volcanic Eruptions in the Bismarck Volcanic Arc, Papua New Guinea, in 1972-75; in Johnson, R.W, ed., Volcanism in Australasia, Elsevier, p. 149-172.

Palfreyman, W.D. and Cooke, R.J.S., 1976, Eruptive History of Manam Volcano, Papua New Guinea; Ibid., p. 117-131.

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: P. de Saint-Ours, D. Lolok, and C. McKee, RVO.

An explosion on 5 July killed one person and injured five others. Marapi has been erupting since 1987, with explosions typically occurring about once every 1-7 days. Material ejected by the smaller explosions rises 100-800 m, whereas ejecta from larger explosions reach 800-2,000 m above the summit. The recent explosions, which produce ash and lapilli, have originated from Verbeek Crater in the summit complex. Ashfalls have been frequent NW of the volcano in Bukittinggi (roughly 15 km NW of the summit), Sungai Puar (30 km NW), and the Agam district (>30 km NW), depending on wind direction. Fluctuations in Marapi's explosions seem to parallel shallow volcanic earthquakes (figure 2), suggesting that the activity is primarily caused by degassing from a relatively shallow source through an open vent.

Figure 2. Number of explosion, A-, and B-type earthquakes at Marapi, January 1991-June 1992. Courtesy of VSI.

Activity in June began with an explosion on the 1st. Continuous tremor followed, and on 6 June at 0227 another explosion occurred. Repeated explosions then deposited ~0.5 mm of ash on Bukittinggi. On 25 June, witnesses 2 km from the volcano (at the Batu Palano Volcano Observatory) heard a detonation and saw glow. A brownish-black cauliflower-shaped plume rose 1,800 m above the summit. During June, 45 deep and 312 shallow volcanic earthquakes, 108 volcanic tremor episodes, and 2,104 explosion earthquakes were recorded.

The strongest explosion occurred on 5 July at 0912. Bukittinggi and vicinity were covered by 0.5-1.5 mm of ash several hours later, with ash in some areas reaching 2 mm thickness. Ash also extended to Padang, ~10 km SW of the crater. Bombs killed one person, seriously injured three, and caused minor injuries to two others. The victims had climbed to the summit without consultation with the Mt. Marapi Volcano Observatory or local authorities, although a hazard warning had been in effect since 1987.

Geologic Background. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in historical time.

Information Contacts: W. Modjo, VSI.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Aerial observations [of Maug] on 13 May revealed no signs of steaming or other evidence of recent volcanic activity.

Geologic Background. Three small elongated islands up to 2.3 km long mark the northern, western, and eastern rims of a largely submerged 2.5-km-wide caldera. The highest point of the Maug Islands reaches only 227 m above sea level; the submerged southern notch on the caldera rim lies about 140 m below sea level. The caldera has an average submarine depth of about 200 m and contains a twin-peaked central lava dome that rises to within about 20 m of the sea surface. The Maug Islands form a twin volcanic massif with Supply Reef, about 11 km N. The truncated inner walls of the caldera on all three islands expose lava flows and pyroclastic deposits that are cut by radial dikes; bedded ash deposits overlie the outer flanks of the islands. No eruptions are known since the discovery of the islands by Espinosa in 1522. The presence of poorly developed coral reefs and coral on the central lava dome suggests a long period of general quiescence, although it does not exclude mild eruptions (Corwin, 1971). A 2003 NOAA expedition detected possible evidence of submarine geothermal activity.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Medicine Lake Report. Twelve events were detected in the Medicine Lake area (~900 km NNW of the epicenter) in the 30 minutes after the M 7.5 earthquake. All had coda durations less than or equal to 10 seconds. The lack of any S-P separation indicated that they were centered very close to the single seismic station, near the center of the caldera. All known historical seismicity had occurred in the central caldera as part of a mainshock/aftershock sequence during the fall and winter of 1988-89.

Geologic Background. Medicine Lake is a large Pleistocene-to-Holocene, basaltic-to-rhyolitic shield volcano east of the main axis of the Cascade Range. Volcanism, similar in style to that of Newberry volcano in Oregon, began less than one million years ago. A roughly 7 x 12 km caldera truncating the summit contains a lake that gives the volcano its name. A series of young eruptions lasting a few hundred years began about 10,500 years before present (BP) and produced 5 km3 of basaltic lava. Nine Holocene eruptions clustered during three eruptive episodes at about 5000, 3000, and 1000 years ago produced a chemically varied group of basaltic lava flows from flank vents and silicic obsidian flows from vents within the caldera and on the upper flanks. The last eruption produced the massive Glass Mountain obsidian flow on the E flank about 900 years BP. Lava Beds National Monument on the N flank of Medicine Lake shield volcano contains hundreds of lava-tube caves displaying a variety of spectacular lava-flow features, most of which are found in the voluminous Mammoth Crater lava flow, which extends in several lobes up to 24 km from the vent.

Information Contacts: S. Walter and D. Hill, USGS Menlo Park.

Vigorous lava production continued through June . . . . The eruption has built 23 cinder cones along a 2.5-km zone that trends generally NE, ~15 km NE of Nyamuragira caldera and 5 km ENE of the 1957 Kitsimbanyi vent (figure 12 and table 1). The eruption's early phases produced substantial lava flows, but since 20 November activity has been characterized by vigorous ejection of bombs, lava fragments, and ash, with lava flows of only limited extent.

Figure 12. Schematic map of cones built by the 1991-92 eruption of Nyamuragira, in a zone ~15 km NE of the caldera. Vent 20, shown in black, opened on 14 July, and remained active in August 1992. Courtesy of N. Zana.

Table 1. Sequence of activity at Nyamuragira's 1991-92 eruption vents. Locations are shown on figure 12. Some small, short-lived vents removed by subsequent lava flows are not listed.

From 20 September until 5 February, activity was confined to a N32-34°E fissure (cones 1-8). The most persistent activity at a single vent, 25 October-3 February, has made Cone 3 the largest of the eruption, rising ~80 m above the surrounding lava plain. Three new cones developed in February, nos. 9 (6 February), 10a and 10b (26 February). In March, activity resumed at the S end of the fissure along a branch that trended E from the initial vent, successively building cones 11, 12, and 14. Vent 13, 1 km to the N, erupted during the same period.

In early May, activity moved to the N end of the fissure, as a NE branch developed and formed vents 15-17. These vents remained active at the end of May, as did no. 14 at the S end of the fissure, producing intermittent lava fountains. Vent 18, near the middle of the fissure, began to erupt at about 1100 on 24 May. By 8 June it had grown to ~25 m height and its lava flows had extended ~3 km N, eroding away cones 10a and 10b. Activity at the new vent was preceded by an increase in microtremor amplitude recorded at a seismic station (Katale) 12 km E. Amplitude increased significantly from 8 June, indicating movement of new magma from a deeper source. As of 1 July, there was no indication that the eruption was nearing its end. Lava production remained vigorous, with high lava fountains, and strong emission of bombs and other tephra.

Geologic Background. Africa's most active volcano, Nyamuragira, is a massive high-potassium basaltic shield about 25 km N of Lake Kivu. Also known as Nyamulagira, it has generated extensive lava flows that cover 1500 km2 of the western branch of the East African Rift. The broad low-angle shield volcano contrasts dramatically with the adjacent steep-sided Nyiragongo to the SW. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Historical eruptions have occurred within the summit caldera, as well as from the numerous fissures and cinder cones on the flanks. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Historical lava flows extend down the flanks more than 30 km from the summit, reaching as far as Lake Kivu.

Information Contacts: N. Zana, CRSN, Bukavu.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Reports from brief visits to Pagan indicate that the most recent small ash eruption occurred on 13 April. Continuing seismicity was dominated by short bursts of long-period earthquakes and volcanic tremor. The highest measured steam temperature was 76°C; solfataras that are probably hotter are inaccessible deep within the crater. Episodic fuming, marked by periods of relatively high SO2 outgassing followed by quiescence, was observed continuously 13-21 May. EDM lines from the coast to reflectors on the flanks had shortened by as much as 11.3 cm since September 1990. These lines had shown no significant changes between 1983 and 1990, a period characterized by frequent small ash eruptions following the large Plinian eruption of 15 May 1981 (Banks and others, 1984). After the first remeasurement on 17 May, no large changes in line lengths were detected during the next 3 days.

The team collected three charcoal samples on Pagan. Two of the units to be dated are relatively old, and their ages should help to constrain the age of the caldera.

South Pagan . . . has several steaming fumaroles, but no temperatures were measured. No shallow earthquake swarms have been recorded since the installation of the seismic station in 1990.

Reference. Banks, N.G., Koyanagi, R.Y., Sinton, J.M., and Honma, K.T., 1984, The eruption of Mount Pagan volcano, Mariana Islands, 15 May 1981: JVGR, v. 22, p. 225-269.

Geologic Background. Pagan Island, the largest and one of the most active of the Mariana Islands volcanoes, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7 and 4 km in diameter, respectively. North Pagan at the NE end of the island rises above the flat floor of the northern caldera, which may have formed less than 1,000 years ago. South Pagan is a stratovolcano with an elongated summit containing four distinct craters. Almost all of the recorded eruptions, which date back to the 17th century, have originated from North Pagan. The largest eruption during historical time took place in 1981 and prompted the evacuation of the sparsely populated island.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Increased seismicity preceded the emergence of a lava dome into the center of the caldera lake. Moderate steam-and-ash emission was associated with the lava extrusion.

Long-period earthquakes and tremor began to be recorded on 6 July. An aerial survey during the morning of 7 July showed no visible change in steaming from crater vents, although the caldera lake was convecting and somewhat muddier than normal. A small island was reported in the caldera lake early on 9 July. An overflight that day at 1500 revealed a mud cone about 100 m in diameter near the center of the lake, protruding about 5 m above the lake surface. Small phreatic explosions to about 100 m height occurred near the side of the island. PHIVOLCS raised the official alert level to 3, indicating the possibility of an eruption within weeks. The announcement described possible activity as quiet extrusion of a lava dome or moderately explosive phreatomagmatic eruptions. A danger zone of 10-km radius was being enforced.

The cone had reportedly reached 200-300 m in diameter by 12 July. A lava dome 100-150 m in diameter was visible near the center of the island during an aerial survey on 14 July at 0900-1000. The island had grown to around 250-300 m across and was 8-10 m above lake level. A continuous dirty white steam column that included some ash was emerging from the dome and drifting SW during the overflight. Ashfall was reported on two towns ~30 km SW of the summit (San Marcelino and Castillejos) at about 0600 and 1300. The alert level was raised to 5 (eruption in progress).

On the flanks of the volcano, monsoon rains triggered secondary explosions and lahars that forced the evacuation of thousands of people living along rivers. Two people were reported killed by lahars on 12 July. The Department of Social Welfare said that about 70,000 people remained in evacuation centers and resettlement sites in the aftermath of the June 1991 eruption.

Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Information Contacts: PHIVOLCS; UPI; Reuters; AP.

Water level in the crater lake had dropped at least 3 m since April, shrinking it substantially by early June (figure 41). Its color was lime green to sky blue, and the temperature in accessible areas reached 85.8°C. Numerous cones and miniature mud volcanoes were visible within the lake. The nine main fumaroles emitted water vapor with yellowish and bluish gases (sulfur and SO₂). Bluish gases and orange flames, probably caused by combustion of sulfur, emerged from the northernmost fumarole. The fumaroles to the SE occurred among collapsed sulfur-and-mud cones, as in the past 3 years.

Figure 41. Sketch map of the crater at Poás, 10 June 1992. Courtesy of the Instituto Costarricense de Electricidad.

As the rainy season began, fumaroles exposed by the shrinkage of the crater lake were covered by water. The resulting continuous phreatic activity produced plumes 1-2 m high. As the lake rose, it cooled to 64-73°C, with a pH of 1.1. Weak fumarolic activity continued on the 1953-55 dome, with a maximum measured temperature of 89°C and a condensate pH of 4.4.

A daily average of 200 low-frequency events and 24 A-B-type (medium-frequency) events were recorded 2.7 km SW of the summit (by station POA2) in June (figure 42). Highest seismicity was on 2 June.

Figure 42. Daily number of seismic events recorded at a station (POA2) 2.7 km SW of the summit of Poás, June 1992. Courtesy of the Univ Nacional.

Geologic Background. The broad, well-vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano, which is one of Costa Rica's most prominent natural landmarks, are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the 2708-m-high complex stratovolcano extends to the lower northern flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, is cold and clear and last erupted about 7500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since the first historical eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSCIORI; G. Soto, ICE; M. Fernández, UCR.

"Seismic activity . . . has shown a slight increase over the last 2 months (June: 410 caldera earthquakes, May: 425) compared with activity over the last 2.5 years (100-300 events/month). Less than 1% of the recorded earthquakes in June could be located. Most were from the NW part of the caldera seismic zone. Similarly, levelling measurements showed a slight uplift of the central part of the caldera during the last two months (20 mm, 11 May-4 June; and an additional 13 mm by 8 July)."

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the 688-m-high asymmetrical pyroclastic shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1400 years ago. An earlier caldera-forming eruption about 7100 years ago is now considered to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the northern and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and western caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: P. de Saint-Ours, D. Lolok, and C. McKee, RVO.

Fumarolic activity continued through June in the active crater, where it had fed a plume more than 100 m high during May fieldwork. Chemical analyses of water collected 13 May showed pH values of less than 3 in two of the three N-flank rivers sampled, and some enhancement in sulfate and chloride concentrations (table 2). A seismographic station 5 km SW of the crater (RIN3) registered seven low-frequency earthquakes in June.

Table 2. Chemistry of water collected 13 May 1992 from three rivers on the N flank of Rincón de la Vieja. Data courtesy of the Univ. de Costa Rica.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto, ICE; Mario Fernández, Univ. de Costa Rica.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. The Rumble III seamount, the largest of the Rumbles group of submarine volcanoes along the South Kermadec Ridge, rises 2300 m from the sea floor to within about 200 m of the sea surface. Collapse of the edifice produced a horseshoe-shaped caldera breached to the west and a large debris-avalanche deposit. Fresh-looking andesitic rocks have been dredged from the summit and basaltic lava from its flanks. Rumble III has been the source of several submarine eruptions detected by hydrophone signals.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. The submarine volcano Rumble IV was thought to have been active from April to December 1966, based on hydrophone signals (Kibblewhite, 1967), but later evidence indicates that the hydrophone array had been damaged and that the signals originated from Rumble III (Hall, 1985). Fresh, glassy andesitic lava was dredged from the summit in 1992 during a New Zealand Oceanographic Institute cruise, and gas bubbles were acoustically detected rising from Rumble IV.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. A previously unknown submarine volcano, Rumble V was discovered in 1992 at the southernmost of a group of seamounts on the southern Kermadec Ridge, known as the Rumbles. It rises more than 2,000 m to nearly 400 m below the sea surface and shows a pristine morphology. Andesitic and basaltic-andesite rocks have been dredged from Rumble V, which lies 17 km ESE of Rumble IV. A large plume of gas bubbles was acoustically detected rising from the summit of Rumble V in 1992, and subsequent expeditions detected evidence of vigorous hydrothermal activity.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Gas emission [from Sarigan] was not evident during overflights in an airplane on 13 May and a helicopter on 21 May.

Geologic Background. Sarigan volcano forms a 3-km-long, roughly triangular island. A low truncated cone with a 750-m-wide summit crater contains a small ash cone. The youngest eruptions produced two lava domes from vents above and near the south crater rim. Lava flows from each dome reached the coast and extended out to sea, forming irregular shorelines. The northern flow overtopped the crater rim on the north and NW sides. The sparse vegetation on the flows indicates they are of Holocene age (Meijer and Reagan, 1981).

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Shasta report. The film record showed no earthquake activity beneath Shasta (~900 km NNW of the epicenter), although telemetry problems limited the ability to detect events below M 2. Of the six earthquakes in the 24 hours following the M 7.5 shock, two were large enough to be recorded by the RTP system. These were centered about 60 km SE of Shasta and about equidistant from Lassen (figure 1). Because the arrival times and S-P sequences of the other four events were similar to those of the two located shocks, it is likely that all had similar epicenters. Occasional M 2 earthquakes have previously occurred in this area, which includes several mapped N-trending normal faults with Quaternary movement. Three days after the M 7.5 earthquake, a M 2.0 shock occurred beneath Shasta's SE flank, followed by a M 2.7 event the next day. Both were centered at about 15 km depth, similar to most earthquakes beneath Shasta in the last decade.

Figure 1. Seismic events in the Shasta/Medicine Lake area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of Stephen Walter.

Geologic Background. The most voluminous of the Cascade volcanoes, northern California's Mount Shasta is a massive compound stratovolcano composed of at least four main edifices constructed over a period of at least 590,000 years. An ancestral edifice was destroyed by one of Earth's largest known Quaternary subaerial debris avalanches, which filled the Shasta River valley NW of the volcano. The Hotlum cone, forming the present summit, the Shastina lava dome complex, and the SW flank Black Butte lava dome, were constructed during the early Holocene. Eruptions from these vents have produced pyroclastic flows and mudflows that affected areas as far as 20 km from the summit. Eruptions from Hotlum cone continued throughout the Holocene.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

Increased seismicity preceded a brief eruption of Spurr that began on 27 June at 0704, producing an eruption cloud that was carried rapidly NNE. Seismic data suggested that the eruption ended at about 1100, after apparent eruptive pulses at 0814 and 0904. By 1049, shortly before feeding of the plume stopped, data from the Nimbus-7 satellite's TOMS showed its leading edge roughly 500 km from the volcano, near Fairbanks (figure 3), with an apparent SO₂ content of 35 kilotons. The next day, the cloud was detached from the volcano but still clearly visible on weather satellite imagery, extending in a 2,000-km arc E and SE over NE Alaska and NW Canada (figures 3 and 4). As the plume elongated, SO₂ detected by the TOMS instrument increased to a maximum of 185 kilotons on 28 June at 1125, then decreased slightly to 160 kilotons as it started to dissipate on 29 June. The cloud remained visible on both TOMS data and weather satellite imagery for several more days.

Figure 3. Three overlain images of the SO2 cloud from Spurr, as detected by the Total Ozone Mapping Spectrometer on the Nimbus-7 satellite. Values of SO2 in each 50 x 50-km pixel are shown on a relative scale of 0-9, then upward through alphabetic characters with increasing concentration. The cloud slowly dispersed until 3 July, when it could no longer be distinguished above background. Courtesy of Gregg Bluth.

Figure 4. Image from the NOAA 11 polar-orbiting weather satellite on 29 June at about 0600, showing the plume from Spurr over the Beaufort Sea and western Canada. Courtesy of NOAA/NESDIS.

The maximum eruption cloud altitude reported by pilots was about 12 km. However, radar installed on the Kenai Peninsula after the Redoubt eruption, to monitor nearby volcanic activity, measured higher altitudes. At 0803, radar detected a vertical cloud to about 9 km altitude; at 0840, strong returns to 9 km and some material to 14.5 km; at 0950 and 1004, columns to 16 km altitude; and at 1018, to 18 km (figure 5).

Figure 5. One of several radar images of the eruption column from Spurr on 27 June. This image, at 1018, shows echoes from the plume to about 18 km altitude. The instrument, an Enterprise Electronics WSR74C, 5-cm radar, is at Kenai, Alaska, about 80 km away. Vertical scans were used to maximize detection of the vertical cloud; the plume extending downwind is not visible. Courtesy of Joel Curtis and Dale Eubanks.

Because the plume was carried northward, major air routes to Asia that extend along the Aleutian chain from Anchorage were not affected. A Notice to Airmen warned aircraft to avoid the immediate vicinity of the volcano. No routes were officially closed, but airlines avoided using routes N and NW of the volcano (J501, 111, 133, 120, and 122; and V319, 444, and 480) during the eruption. Flights arriving in Anchorage, 120 km E of Spurr, were routed along normal approaches from the south.

Geologic Background. The summit of Mount Spurr, the highest volcano of the Aleutian arc, is a large lava dome constructed at the center of a roughly 5-km-wide horseshoe-shaped caldera open to the south. The volcano lies 130 km W of Anchorage and NE of Chakachamna Lake. The caldera was formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an ancestral edifice. The debris avalanche traveled more than 25 km SE, and the resulting deposit contains blocks as large as 100 m in diameter. Several ice-carved post-caldera cones or lava domes lie in the center of the caldera. The youngest vent, Crater Peak, formed at the breached southern end of the caldera and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash on the city of Anchorage.

Information Contacts: AVO; G. Bluth, NASA GSFC; SAB, NOAA/NESDIS; Joel Curtis and Dale Eubanks, NWS Alaska Region, Anchorage; Darla Gerlach, Air Traffic Division, FAA, Anchorage.

Fieldwork during the first week in June revealed that eruptive activity was mainly concentrated in craters C1 (vent 1) and C3 (vent 4), which fed black plumes no more than 100 m high. Seismicity remained high in June (figure 26), near the 180 events/day reached in the last third of May. A minimum of 108 events was recorded on 24 June. After declining rapidly about 20 May, tremor energy returned to levels characteristic of the period since November 1991.

Figure 26. Seismicity at Stromboli, June 1992. Open bars show the number of recorded events per day, black bars those with ground velocities exceeding 100 mm/s. The curve represents the each day's average of tremor energies on hourly 60-second samples. Courtesy of M. Riuscetti.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent horseshoe-shaped scarp formed about 5,000 years ago due to a series of slope failures that extend to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: M. Riuscetti, Univ di Udine.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. Tangaroa submarine volcano in the southern Kermadec arc rises to within 600 m of the sea surface. The volcano is elongated in a NW-SE direction and contains smaller cones on its SE to eastern flanks. A larger edifice lies further to the SE. Tangaroa lies between Clark and Rumble V submarine volcanoes near the southern end of the Kermadec arc and is one of more than a half dozen volcanoes in this part of the arc showing evidence for active hydrothermal vent fields.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

A telemetering seismic station (VTU) 0.5 km E of the active crater recorded 17 events in June. The maximum daily number, 4, occurred on 13 June.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

Growth of the lava dome continued through early July. Partial collapses of the dome complex frequently generated pyroclastic flows. Dome 7, which had begun to emerge in late March, grew exogenously against dome 6 (figure 43), which was buried and eroded by dome 7's lava blocks. Frequent rockfalls from the front and margins of dome 7 reduced its length (to ~ 200 m) and height (to ~ 50 m). Petal or peel structures, which had always appeared on the dome's surface during periods of rapid lava extrusion, were not evident, perhaps indicating a declining magma supply rate. The cryptodome, including dome 5, grew endogenously, frequently generating small rockfalls that were probably triggered by earthquakes within or beneath the dome complex.

Figure 43. Sketch of the dome complex at the summit of Unzen, 8 July 1992. Courtesy of Setsuya Nakada.

Volcanic gas was emitted continuously from the E part of dome 3, as well as from the depression between domes 3 and 7. The depression divides the cryptodome area into a conical NE section that includes the dome's summit, and a lower SW section with a flat top.

Deposits of the pyroclastic flows that cascade down the SE flank continue to bury the Akamatsu valley. The lowest saddle of the valley's southern cliff remains ~ 10 m high. On 23 June, the ash-cloud surge from a pyroclastic flow struck the saddle, but the main flow did not reach the cliff. The surge toppled brush on the saddle and to ~ 100 m distance, but small cedar trees remained standing. Bark and leaves were not burned, but leaves in the area died. About 10 cm of ash was deposited on the saddle. Thin lead foil, set in a stainless-steel hole to detect the pressure of the ash-cloud surge, was hollowed, and aluminum foil was broken.

Debris flows that have occasionally occurred during the current rainy season eroded pyroclastic flow deposits in the valley. Pyroclastic-flow material was deposited along the valley's N side and in its upper reaches. This deposition pattern, erosion by debris flows, and the declining magma-supply rate delayed the overflow of the lowest part of the saddle by southern-cliff pyroclastic flow deposits. In early July, the Nagasaki prefectural government began to construct a steel fence, 35 m wide and 10 m high, in a stream originating from the saddle, hoping to prevent ash-cloud surges from entering the stream.

JMA reported that the daily number of seismically detected pyroclastic flows ranged from 6 to 21 in June. The total of 373 in June was almost unchanged from previous months. The longest June flow extended 3 km SE from the dome. Most ash clouds generated by the flows rose about 1,000 m, with the highest, to 1,200 m, on 13 and 17 June.

Small earthquakes continued to occur within and beneath the dome complex, at rates of 50-200/day through mid-July. The June total, 3,671 recorded earthquakes, was similar to previous months.

Evacuated areas . . . were somewhat reduced on 11 July, decreasing the number of evacuees from 6,746 to 6,064.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: S. Nakada, Kyushu Univ; JMA.