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).

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).

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).

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).

Taal volcano is in a caldera system located in southern Luzon island and is one of the most active volcanoes in the Philippines. It has produced around 35 recorded eruptions since 3,580 BCE, ranging from VEI 1 to 6, with the majority of eruptions being a VEI 2. The caldera contains a lake with an island that also contains a lake within the Main Crater (figure 12). Prior to 2020 the most recent eruption was in 1977, on the south flank near Mt. Tambaro. The United Nations Office for the Coordination of Humanitarian Affairs in the Philippines reports that over 450,000 people live within 40 km of the caldera (figure 13). This report covers activity during January through February 2020 including the 12 to 22 January eruption, and is based on reports by Philippine Institute of Volcanology and Seismology (PHIVOLCS), satellite data, geophysical data, and media reports.

Figure 12. Annotated satellite images showing the Taal caldera, Volcano Island in the caldera lake, and features on the island including Main Crater. Imagery courtesy of Planet Inc.

Figure 13. Map showing population totals within 14 and 17 km of Volcano Island at Taal. Courtesy of the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).

The hazard status at Taal was raised to Alert Level 1 (abnormal, on a scale of 0-5) on 28 March 2019. From that date through to 1 December there were 4,857 earthquakes registered, with some felt nearby. Inflation was detected during 21-29 November and an increase in CO₂ emission within the Main Crater was observed. Seismicity increased beginning at 1100 on 12 January. At 1300 there were phreatic (steam) explosions from several points inside Main Crater and the Alert Level was raised to 2 (increasing unrest). Booming sounds were heard in Talisay, Batangas, at 1400; by 1402 the plume had reached 1 km above the crater, after which the Alert Level was raised to 3 (magmatic unrest).

Phreatic eruption on 12 January 2020. A seismic swarm began at 1100 on 12 January 2020 followed by a phreatic eruption at 1300. The initial activity consisted of steaming from at least five vents in Main Crater and phreatic explosions that generated 100-m-high plumes. PHIVOLCS raised the Alert Level to 2. The Earth Observatory of Singapore reported that the International Data Center (IDC) for the Comprehensive test Ban Treaty (CTBT) in Vienna noted initial infrasound detections at 1450 that day.

Booming sounds were heard at 1400 in Talisay, Batangas (4 km NNE from the Main Crater), and at 1404 volcanic tremor and earthquakes felt locally were accompanied by an eruption plume that rose 1 km; ash fell to the SSW. The Alert Level was raised to 3 and the evacuation of high-risk barangays was recommended. Activity again intensified around 1730, prompting PHIVOLCS to raise the Alert Level to 4 and recommend a total evacuation of the island and high-risk areas within a 14-km radius. The eruption plume of steam, gas, and tephra significantly intensified, rising to 10-15 km altitude and producing frequent lightning (figures 14 and 15). Wet ash fell as far away as Quezon City (75 km N). According to news articles schools and government offices were ordered to close and the Ninoy Aquino International Airport (56 km N) in Manila suspended flights. About 6,000 people had been evacuated. Residents described heavy ashfall, low visibility, and fallen trees.

Figure 14. Lightning produced during the eruption of Taal during 1500 on 12 January to 0500 on 13 January 2020 local time (0700-2100 UTC on 12 January). Courtesy of Chris Vagasky, Vaisala.

Figure 15. Lightning strokes produced during the first days of the Taal January 2020 eruption. Courtesy of Domcar C Lagto/SIPA/REX/Shutterstock via The Guardian.

In a statement issued at 0320 on 13 January, PHIVOLCS noted that ashfall had been reported across a broad area to the north in Tanauan (18 km NE), Batangas; Escala (11 km NW), Tagaytay; Sta. Rosa (32 km NNW), Laguna; Dasmariñas (32 km N), Bacoor (44 km N), and Silang (22 km N), Cavite; Malolos (93 km N), San Jose Del Monte (87 km N), and Meycauayan (80 km N), Bulacan; Antipolo (68 km NNE), Rizal; Muntinlupa (43 km N), Las Piñas (47 km N), Marikina (70 km NNE), Parañaque (51 km N), Pasig (62 km NNE), Quezon City, Mandaluyong (62 km N), San Juan (64 km N), Manila; Makati City (59 km N) and Taguig City (55 km N). Lapilli (2-64 mm in diameter) fell in Tanauan and Talisay; Tagaytay City (12 km N); Nuvali (25 km NNE) and Sta (figure 16). Rosa, Laguna. Felt earthquakes (Intensities II-V) continued to be recorded in local areas.

Figure 16. Ashfall from the Taal January 2020 eruption in Lemery (top) and in the Batangas province (bottom). Photos posted on 13 January, courtesy of Ezra Acayan/Getty Images, Aaron Favila/AP, and Ted Aljibe/AFP via Getty Images via The Guardian.

Magmatic eruption on 13 January 2020. A magmatic eruption began during 0249-0428 on 13 January, characterized by weak lava fountaining accompanied by thunder and flashes of lightning. Activity briefly waned then resumed with sporadic weak fountaining and explosions that generated 2-km-high, dark gray, steam-laden ash plumes (figure 17). New lateral vents opened on the N flank, producing 500-m-tall lava fountains. Heavy ashfall impacted areas to the SW, including in Cuenca (15 km SSW), Lemery (16 km SW), Talisay, and Taal (15 km SSW), Batangas (figure 18).

Figure 17. Ash plumes seen from various points around Taal in the initial days of the January 2020 eruption, posted on 13 January. Courtesy of Eloisa Lopez/Reuters, Kester Ragaza/Pacific Press/Shutterstock, Ted Aljibe/AFP via Getty Images, via The Guardian.

Figure 18. Map indicating areas impacted by ashfall from the 12 January eruption through to 0800 on the 13th. Small yellow circles (to the N) are ashfall report locations; blue circles (at the island and to the S) are heavy ashfall; large green circles are lapilli (particles measuring 2-64 mm in diameter). Modified from a map courtesy of Lauriane Chardot, Earth Observatory of Singapore; data taken from PHIVOLCS.

News articles noted that more than 300 domestic and 230 international flights were cancelled as the Manila Ninoy Aquino International Airport was closed during 12-13 January. Some roads from Talisay to Lemery and Agoncillo were impassible and electricity and water services were intermittent. Ashfall in several provinces caused power outages. Authorities continued to evacuate high-risk areas, and by 13 January more than 24,500 people had moved to 75 shelters out of a total number of 460,000 people within 14 km.

A PHIVOLCS report for 0800 on the 13th through 0800 on 14 January noted that lava fountaining had continued, with steam-rich ash plumes reaching around 2 km above the volcano and dispersing ash SE and W of Main Crater. Volcanic lighting continued at the base of the plumes. Fissures on the N flank produced 500-m-tall lava fountains. Heavy ashfall continued in the Lemery, Talisay, Taal, and Cuenca, Batangas Municipalities. By 1300 on the 13th lava fountaining generated 800-m-tall, dark gray, steam-laden ash plumes that drifted SW. Sulfur dioxide emissions averaged 5,299 metric tons/day (t/d) on 13 January and dispersed NNE (figure 19).

Figure 19. Compilation of sulfur dioxide plumes from TROPOMI overlaid in Google Earth for 13 January from 0313-1641 UT. Courtesy of NASA Global Sulfur Dioxide Monitoring Page and Google Earth.

Explosions and ash emission through 22 January 2020. At 0800 on 15 January PHIVOLCS stated that activity was generally weaker; dark gray, steam-laden ash plumes rose about 1 km and drifted SW. Satellite images showed that the Main Crater lake was gone and new craters had formed inside Main Crater and on the N side of Volcano Island.

PHIVOLCS reported that activity during 15-16 January was characterized by dark gray, steam-laden plumes that rose as high as 1 km above the vents in Main Crater and drifted S and SW. Sulfur dioxide emissions were 4,186 t/d on 15 January. Eruptive events at 0617 and 0621 on 16 January generated short-lived, dark gray ash plumes that rose 500 and 800 m, respectively, and drifted SW. Weak steam plumes rose 800 m and drifted SW during 1100-1700, and nine weak explosions were recorded by the seismic network.

Steady steam emissions were visible during 17-21 January. Infrequent weak explosions generated ash plumes that rose as high as 1 km and drifted SW. Sulfur dioxide emissions fluctuated and were as high as 4,353 t/d on 20 January and as low as 344 t/d on 21 January. PHIVOLCS reported that white steam-laden plumes rose as high as 800 m above main vent during 22-28 January and drifted SW and NE; ash emissions ceased around 0500 on 22 January. Remobilized ash drifted SW on 22 January due to strong low winds, affecting the towns of Lemery (16 km SW) and Agoncillo, and rose as high as 5.8 km altitude as reported by pilots. Sulfur dioxide emissions were low at 140 t/d.

Steam plumes through mid-April 2020. The Alert Level was lowered to 3 on 26 January and PHIVOLCS recommended no entry onto Volcano Island and Taal Lake, nor into towns on the western side of the island within a 7-km radius. PHIVOLCS reported that whitish steam plumes rose as high as 800 m during 29 January-4 February and drifted SW (figure 20). The observed steam plumes rose as high as 300 m during 5-11 February and drifted SW.

Sulfur dioxide emissions averaged around 250 t/d during 22-26 January; emissions were 87 t/d on 27 January and below detectable limits the next day. During 29 January-4 February sulfur dioxide emissions ranged to a high of 231 t/d (on 3 February). The following week sulfur dioxide emissions ranged from values below detectable limits to a high of 116 t/d (on 8 February).

Figure 20. Taal Volcano Island producing gas-and-steam plumes on 15-16 January 2020. Courtesy of James Reynolds, Earth Uncut.

On 14 February PHIVOLCS lowered the Alert Level to 2, noting a decline in the number of volcanic earthquakes, stabilizing ground deformation of the caldera and Volcano Island, and diffuse steam-and-gas emission that continued to rise no higher than 300 m above the main vent during the past three weeks. During 14-18 February sulfur dioxide emissions ranged from values below detectable limits to a high of 58 tonnes per day (on 16 February). Sulfur dioxide emissions were below detectable limits during 19-20 February. During 26 February-2 March steam plumes rose 50-300 m above the vent and drifted SW and NE. PHIVOLCS reported that during 4-10 March weak steam plumes rose 50-100 m and drifted SW and NE; moderate steam plumes rose 300-500 m and drifted SW during 8-9 March. During 11-17 March weak steam plumes again rose only 50-100 m and drifted SW and NE.

PHIVOLCS lowered the Alert Level to 1 on 19 March and recommended no entry onto Volcano Island, the area defined as the Permanent Danger Zone. During 8-9 April steam plumes rose 100-300 m and drifted SW. As of 1-2 May 2020 only weak steaming and fumarolic activity from fissure vents along the Daang Kastila trail was observed.

Evacuations. According to the Disaster Response Operations Monitoring and Information Center (DROMIC) there were a total of 53,832 people dispersed to 244 evacuation centers by 1800 on 15 January. By 21 January there were 148,987 people in 493 evacuation. The number of residents in evacuation centers dropped over the next week to 125,178 people in 497 locations on 28 January. However, many residents remained displaced as of 3 February, with DROMIC reporting 23,915 people in 152 evacuation centers, but an additional 224,188 people staying at other locations.

By 10 February there were 17,088 people in 110 evacuation centers, and an additional 211,729 staying at other locations. According to the DROMIC there were a total of 5,321 people in 21 evacuation centers, and an additional 195,987 people were staying at other locations as of 19 February.

The number of displaced residents continued to drop, and by 3 March there were 4,314 people in 12 evacuation centers, and an additional 132,931 people at other locations. As of 11 March there were still 4,131 people in 11 evacuation centers, but only 17,563 staying at other locations.

Deformation and ground cracks. New ground cracks were observed on 13 January in Sinisian (18 km SW), Mahabang Dahilig (14 km SW), Dayapan (15 km SW), Palanas (17 km SW), Sangalang (17 km SW), and Poblacion (19 km SW) Lemery; Pansipit (11 km SW), Agoncillo; Poblacion 1, Poblacion 2, Poblacion 3, Poblacion 5 (all around 17 km SW), Talisay, and Poblacion (11 km SW), San Nicolas (figure 21). A fissure opened across the road connecting Agoncillo to Laurel, Batangas. New ground cracking was reported the next day in Sambal Ibaba (17 km SW), and portions of the Pansipit River (SW) had dried up.

Figure 21. Video screenshots showing ground cracks that formed during the Taal unrest and captured on 15 and 16 January 2020. Courtesy of James Reynolds, Earth Uncut.

Dropping water levels of Taal Lake were first observed in some areas on 16 January but reported to be lake-wide the next day. The known ground cracks in the barangays of Lemery, Agoncillo, Talisay, and San Nicolas in Batangas Province widened a few centimeters by 17 January, and a new steaming fissure was identified on the N flank of the island.

GPS data had recorded a sudden widening of the caldera by ~1 m, uplift of the NW sector by ~20 cm, and subsidence of the SW part of Volcano Island by ~1 m just after the main eruption phase. The rate of deformation was smaller during 15-22 January, and generally corroborated by field observations; Taal Lake had receded about 30 cm by 25 January but about 2.5 m of the change (due to uplift) was observed around the SW portion of the lake, near the Pansipit River Valley where ground cracking had been reported.

Weak steaming (plumes 10-20 m high) from ground cracks was visible during 5-11 February along the Daang Kastila trail which connects the N part of Volcano Island to the N part of the main crater. PHIVOLCS reported that during 19-24 February steam plumes rose 50-100 m above the vent and drifted SW. Weak steaming (plumes up to 20 m high) from ground cracks was visible during 8-14 April along the Daang Kastila trail which connects the N part of Volcano Island to the N part of the main crater.

Seismicity. Between 1300 on 12 January and 0800 on 21 January the Philippine Seismic Network (PSN) had recorded a total of 718 volcanic earthquakes; 176 of those had magnitudes ranging from 1.2-4.1 and were felt with Intensities of I-V. During 20-21 January there were five volcanic earthquakes with magnitudes of 1.6-2.5; the Taal Volcano network (which can detect smaller events not detectable by the PSN) recorded 448 volcanic earthquakes, including 17 low-frequency events. PHIVOLCS stated that by 21 January hybrid earthquakes had ceased and both the number and magnitude of low-frequency events had diminished.

Geologic Background. Taal is one of the most active volcanoes in the Philippines and has produced some of its most powerful historical eruptions. Though not topographically prominent, its prehistorical eruptions have greatly changed the landscape of SW Luzon. The 15 x 20 km Talisay (Taal) caldera is largely filled by Lake Taal, whose 267 km2 surface lies only 3 m above sea level. The maximum depth of the lake is 160 m, and several eruptive centers lie submerged beneath the lake. The 5-km-wide Volcano Island in north-central Lake Taal is the location of all historical eruptions. The island is composed of coalescing small stratovolcanoes, tuff rings, and scoria cones that have grown about 25% in area during historical time. Powerful pyroclastic flows and surges from historical eruptions have caused many fatalities.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Disaster Response Operations Monitoring and Information Center (DROMIC) (URL: https://dromic.dswd.gov.ph/); United Nations Office for the Coordination of Humanitarian Affairs, Philippines (URL: https://www.unocha.org/philippines); James Reynolds, Earth Uncut TV (Twitter: @EarthUncutTV, URL: https://www.earthuncut.tv/, YouTube: https://www.youtube.com/user/TyphoonHunter); Chris Vagasky, Vaisala Inc., Louisville, Colorado, USA (URL: https://www.vaisala.com/en?type=1, Twitter: @COweatherman, URL: https://twitter.com/COweatherman); Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, Singapore (URL: https://www.earthobservatory.sg/); 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/); Relief Web, Flash Update No. 1 - Philippines: Taal Volcano eruption (As of 13 January 2020, 2 p.m. local time) (URL: https://reliefweb.int/report/philippines/flash-update-no-1-philippines-taal-volcano-eruption-13-january-2020-2-pm-local); Bloomberg, Philippines Braces for Hazardous Volcano Eruption (URL: https://www.bloomberg.com/news/articles/2020-01-12/philippines-raises-alert-level-in-taal-as-volcano-spews-ash); National Public Radio (NPR), Volcanic Eruption In Philippines Causes Thousands To Flee (URL: npr.org/2020/01/13/795815351/volcanic-eruption-in-philippines-causes-thousands-to-flee); Reuters (http://www.reuters.com/); Agence France-Presse (URL: http://www.afp.com/); Pacific Press (URL: http://www.pacificpress.com/); Shutterstock (URL: https://www.shutterstock.com/); Getty Images (URL: http://www.gettyimages.com/); Google Earth (URL: https://www.google.com/earth/).

In the northern Tonga region, approximately 80 km NW of Vava’u, large areas of floating pumice, termed rafts, were observed starting as early as 7 August 2019. The area of these andesitic pumice rafts was initially 195 km² with the layers measuring 15-30 cm thick and were produced 200 m below sea level (Jutzeler et al. 2020). The previous report (BGVN 44:11) described the morphology of the clasts and the rafts, and their general westward path from 9 August to 9 October 2019, with the first sighting occurring on 9 August NW of Vava’u in Tonga. This report updates details regarding the submarine pumice raft eruption in early August 2019 using new observations and data from Brandl et al. (2019) and Jutzeler et al. (2020).

The NoToVE-2004 (Northern Tonga Vents Expedition) research cruise on the RV Southern Surveyor (SS11/2004) from the Australian CSIRO Marine National Facility traveled to the northern Tonga Arc and discovered several submarine basalt-to-rhyolite volcanic centers (Arculus, 2004). One of these volcanic centers 50 km NW of Vava’u was the unnamed seamount (volcano number 243091) that had erupted in 2001 and again in 2019, unofficially designated “Volcano F” for reference purposes by Arculus (2004) and also used by Brandl et al. (2019). It is a volcanic complex that rises more than 1 km from the seafloor with a central 6 x 8.7 km caldera and a volcanic apron measuring over 50 km in diameter (figures 19 and 20). Arculus (2004) described some of the dredged material as “fresh, black, plagioclase-bearing lava with well-formed, glassy crusts up to 2cm thick” from cones by the eastern wall of the caldera; a number of apparent flows, lava or debris, were observed draping over the northern wall of the caldera.

Figure 19. Visualization of the unnamed submarine Tongan volcano (marked “Volcano F”) using bathymetric data to show the site of the 6-8 August 2020 eruption and the rest of the cone complex. Courtesy of Philipp Brandl via GEOMAR.

Figure 20. Map of the unnamed submarine Tongan volcano using satellite imagery, bathymetric data, with shading from the NW. The yellow circle indicates the location of the August 2019 activity. Young volcanic cones are marked “C” and those with pit craters at the top are marked with “P.” Courtesy of Brandl et al. (2019).

The International Seismological Centre (ISC) Preliminary Bulletin listed a particularly strong (5.7 Mw) earthquake at 2201 local time on 5 August, 15 km SSW of the volcano at a depth of 10 km (Brandl et al. 2019). This event was followed by six slightly lower magnitude earthquakes over the next two days.

Sentinel-2 satellite imagery showed two concentric rings originating from a point source (18.307°S 174.395°W) on 6 August (figure 21), which could be interpreted as small weak submarine plumes or possibly a series of small volcanic cones, according to Brandl et al. (2019). The larger ring is about 1.2 km in diameter and the smaller one measures 250 m. By 8 August volcanic activity had decreased, but the pumice rafts that were produced remained visible through at least early October (BGVN 44:11). Brandl et al. (2019) states that, due to the lack of continued observed activity rising from this location, the eruption was likely a 2-day-long event during 6-8 August.

Figure 21. Sentinel-2 satellite image of possible gas/vapor emissions (streaks) on 6 August 2019 drifting NW, which is the interpreted site for the unnamed Tongan seamount. The larger ring is about 1.2 km in diameter and the smaller one measures 250 m. Image using False Color (urban) rendering (bands 12, 11, 4); courtesy of Sentinel Hub Playground.

The pumice was first observed on 9 August occurred up to 56 km from the point of origin, according to Jutzeler et al. (2020). By calculating the velocity (14 km/day) of the raft using three satellites, Jutzeler et al. (2020) determined the pumice was erupted immediately after the satellite image of the submarine plumes on 6 August (UTC time). Minor activity at the vent may have continued on 8 and 11 August (UTC time) with pale blue-green water discoloration (figure 22) and a small (less than 1 km²) diffuse pumice raft 2-5 km from the vent.

Figure 22. Sentinel-2 satellite image of the last visible activity occurring W of the unnamed submarine Tongan volcano on 8 August 2019, represented by slightly discolored blue-green water. Image using Natural Color rendering (bands 4, 3, 2) and enhanced with color correction; courtesy of Sentinel Hub Playground.

Continuous observations using various satellite data and observations aboard the catamaran ROAM tracked the movement and extent of the pumice raft that was produced during the submarine eruption in early August (figure 23). The first visible pumice raft was observed on 8 August 2019, covering more than 136.7 km² between the volcanic islands of Fonualei and Late and drifting W for 60 km until 9 August (Brandl et al. 2019; Jutzeler 2020). The next day, the raft increased to 167.2-195 km² while drifting SW for 74 km until 14 August. Over the next three days (10-12 August) the size of the raft briefly decreased in size to less than 100 km² before increasing again to 157.4 km² on 14 August; at least nine individual rafts were mapped and identified on satellite imagery (Brandl et al. 2019). On 15 August sailing vessels observed a large pumice raft about 75 km W of Late Island (see details in BGVN 44:11), which was the same one as seen in satellite imagery on 8 August.

Figure 23. Map of the extent of discolored water and the pumice raft from the unnamed submarine Tongan volcano between 8 and 14 August 2019 using imagery from NASA’s MODIS, ESA’s Sentinel-2 satellite, and observations from aboard the catamaran ROAM (BGVN 44:11). Back-tracing the path of the pumice raft points to a source location at the unnamed submarine Tongan volcano. Courtesy of Brandl et al. (2019).

By 17 August high-resolution satellite images showed an area of large and small rafts measuring 222 km² and were found within a field of smaller rafts for a total extent of 1,350 km², which drifted 73 km NNW through 22 August before moving counterclockwise for three days (figure f; Jutzeler et al., 2020). Small pumice ribbons encountered the Oneata Lagoon on 30 August, the first island that the raft came into contact (Jutzeler et al. 2020). By 2 September, the main raft intersected with Lakeba Island (460 km from the source) (figure 24), breaking into smaller ribbons that started to drift W on 8 September. On 19 September the small rafts (less than 100 m x less than 2 km) entered the strait between Viti Levu and Vanua Levu, the two main islands of Fiji, while most of the others were stranded 60 km W in the Yasawa Islands for more than two months (Jutzeler et al., 2020).

Figure 24. Time-series map of the raft dispersal from the unnamed submarine Tongan volcano using multiple satellite images. A) Map showing the first days of the raft dispersal starting on 7 August 2019 and drifting SW from the vent (marked with a red triangle). Precursory seismicity that began on 5 August is marked with a white star. By 15-17 August the raft was entrained in an ocean loop or eddy. The dashed lines represent the path of the sailing vessels. B) Map of the raft dispersal using high-resolution Sentinel-2 and -3 imagery. Two dispersal trails (red and blue dashed lines) show the daily dispersal of two parts of the raft that were separated on 17 August 2019. Courtesy of Jutzeler et al. (2020).

References: Arculus, R J, SS2004/11 shipboard scientists, 2004. SS11/2004 Voyage Summary: NoToVE-2004 (Northern Tonga Vents Expedition): submarine hydrothermal plume activity and petrology of the northern Tofua Arc, Tonga. https://www.cmar.csiro.au/data/reporting/get file.cfm?eovpub id=901.

Brandl P A, Schmid F, Augustin N, Grevemeyer I, Arculus R J, Devey C W, Petersen S, Stewart M , Kopp K, Hannington M D, 2019. The 6-8 Aug 2019 eruption of ‘Volcano F’ in the Tofua Arc, Tonga. Journal of Volcanology and Geothermal Research: https://doi.org/10.1016/j.jvolgeores.2019.106695

Jutzeler M, Marsh R, van Sebille E, Mittal T, Carey R, Fauria K, Manga M, McPhie J, 2020. Ongoing Dispersal of the 7 August 2019 Pumice Raft From the Tonga Arc in the Southwestern Pacific Ocean. AGU Geophysical Research Letters: https://doi.orh/10.1029/2019GL086768.

Geologic Background. A submarine volcano along the Tofua volcanic arc was first observed in September 2001. The newly discovered volcano lies NW of the island of Vava'u about 35 km S of Fonualei and 60 km NE of Late volcano. The site of the eruption is along a NNE-SSW-trending submarine plateau with an approximate bathymetric depth of 300 m. T-phase waves were recorded on 27-28 September 2001, and on the 27th local fishermen observed an ash-rich eruption column that rose above the sea surface. No eruptive activity was reported after the 28th, but water discoloration was documented during the following month. In early November rafts and strandings of dacitic pumice were reported along the coast of Kadavu and Viti Levu in the Fiji Islands. The depth of the summit of the submarine cone following the eruption determined to be 40 m during a 2007 survey; the crater of the 2001 eruption was breached to the E.

Information Contacts: Jan Steffen, Communication and Media, GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany; Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Klyuchevskoy is part of the Klyuchevskaya volcanic group in northern Kamchatka and is one of the most frequently active volcanoes of the region. Eruptions produce lava flows, ashfall, and lahars originating from summit and flank activity. This report summarizes activity during October 2019 through May 2020, and is based on reports by the Kamchatkan Volcanic Eruption Response Team (KVERT) and satellite data.

There were no activity reports from 1 to 22 October, but gas emissions were visible in satellite images. At 1020 on 24 October (2220 on 23 October UTC) KVERT noted that there was a small ash component in the ash plume from erosion of the conduit, with the plume reaching 130 km ENE. The Aviation Colour Code was raised from Green to Yellow, then to Orange the following day. An ash plume continued on the 25th to 5-7 km altitude and extending 15 km SE and 70 km SW and reached 30 km ESE on the 26th. Similar activity continued through to the end of the month.

Moderate gas emissions continued during 1-19 November, but the summit was obscured by clouds. Strong nighttime incandescence was visible at the crater during the 10-11 November and thermal anomalies were detected on 8 and 10-13 November. Explosions produced ash plumes up to 6 km altitude on the 20-21st and Strombolian activity was reported during 20-22 November. Degassing continued from 23 November through 12 December, and a thermal anomaly was visible on the days when the summit was not covered by clouds. An ash plume was reported moving to the NW on the 13th, and degassing with a thermal anomaly and intermittent Strombolian activity then resumed, continuing through to the end of December with an ash plume reported on the 30th.

Gas-and-steam plumes continued into January 2020 with incandescence noted when the summit was clear (figure 33). Strombolian activity was reported again starting on the 3rd. A weak ash plume produced on the 6th extended 55 km E, and on the 21st an ash plume reached 5-5.5 km altitude and extended 190 km NE (figure 34). Another ash plume the next day rose to the same altitude and extended 388 km NE. During 23-29 Strombolian activity continued, and Vulcanian activity produced ash plumes up to 5.5 altitude, extending to 282 km E on the 30th, and 145 km E on the 31st.

Figure 33. Incandescence and degassing were visible at Klyuchevskoy through January 2020, seen here on the 11th. Courtesy of KVERT.

Figure 34. A low ash plume at Klyuchevskoy on 21 January 2020 extended 190 km NE. Courtesy of KVERT.

Strombolian activity continued throughout February with occasional explosions producing ash plumes up to 5.5 km altitude, as well as gas-and-steam plumes and a persistent thermal anomaly with incandescence visible at night. Starting in late February thermal anomalies were detected much more frequently, and with higher energy output compared to the previous year (figure 35). A lava fountain was reported on 1 March with the material falling back into the summit crater. Strombolian activity continued through early March. Lava fountaining was reported again on the 8th with ejecta landing in the crater and down the flanks (figure 36). A strong persistent gas-and-steam plume containing some ash continued along with Strombolian activity through 25 March (figure 37), with Vulcanian activity noted on the 20th and 25th. Strombolian and Vulcanian activity was reported through the end of March.

Figure 35. This MIROVA thermal energy plot for Klyuchevskoy for the year ending 29 April 2020 (log radiative power) shows intermittent thermal anomalies leading up to more sustained energy detected from February through March, then steadily increasing energy through April 2020. Courtesy of MIROVA.

Figure 36. Strombolian explosions at Klyuchevskoy eject incandescent ash and gas, and blocks and bombs onto the upper flanks on 8 and 10 March 2020. Courtesy of IVS FEB RAS, KVERT.

Figure 37. Weak ash emission from the Klyuchevskoy summit crater are dispersed by wind on 19 and 29 March 2020, with ash depositing on the flanks. Courtesy of IVS FEB RAS, KVERT.

Activity was dominantly Strombolian during 1-5 April and included intermittent Vulcanian explosions from the 6th onwards, with ash plumes reaching 6 km altitude. On 18 April a lava flow began moving down the SE flank (figures 38). A report on the 26th reported explosions from lava-water interactions with avalanches from the active lava flow, which continued to move down the SE flank and into the Apakhonchich chute (figures 39 and 40). This continued throughout April and May with sustained Strombolian and intermittent Vulcanian activity at the summit (figures 41 and 42).

Figure 38. Strombolian activity produced ash plumes and a lava flow down the SE flank of Klyuchevskoy on 18 April 2020. Courtesy of IVS FEB RAS, KVERT.

Figure 39. A lava flow descends the SW flank of Klyuchevskoy and a gas plume is dispersed by winds on 21 April 2020. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Figure 40. Sentinel-2 thermal satellite images show the progression of the Klyuchevskoy lava flow from the summit crater down the SE flank from 19-29 April 2020. Associated gas plumes are dispersed in various directions. Courtesy of Sentinel Hub Playground.

Figure 41. Strombolian activity at Klyuchevskoy ejects incandescent ejecta, gas, and ash above the summit on 27 April 2020. Courtesy of D. Bud'kov, IVS FEB RAS, KVERT.

Figure 42. Sentinel-2 thermal satellite images of Klyuchevskoy show the progression of the SE flank lava flow through May 2020, with associated gas plumes being dispersed in multiple directions. Courtesy of Sentinel Hub Playground.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); 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).

Nyamuragira (also known as Nyamulagira) is located in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo and consists of a lava lake that reappeared in the summit crater in mid-April 2018. Volcanism has been characterized by lava emissions, thermal anomalies, seismicity, and gas-and-steam emissions. This report summarizes activity during December 2019 through May 2020 using information from monthly reports by the Observatoire Volcanologique de Goma (OVG) and satellite data.

According to OVG, intermittent eruptive activity was detected in the lava lake of the central crater during December 2019 and January-April 2020, which also resulted in few seismic events. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data shows thermal anomalies within the summit crater that varied in both frequency and power between August 2019 and mid-March 2020, but very few were recorded afterward through late May (figure 88). Thermal hotspots identified by MODVOLC from 15 December 2019 through March 2020 were mainly located in the active central crater, with only three hotspots just outside the SW crater rim (figure 89). Sentinel-2 thermal satellite imagery also showed activity within the summit crater during January-May 2020, but by mid-March the thermal anomaly had visibly decreased in power (figure 90).

Figure 88. The MIROVA graph of thermal activity (log radiative power) at Nyamuragira during 27 July through May 2020 shows variably strong, intermittent thermal anomalies with a variation in power and frequency from August 2019 to mid-March 2020. Courtesy of MIROVA.

Figure 89. Map showing the number of MODVOLC hotspot pixels at Nyamuragira from 1 December 2019 t0 31 May 2020. 37 pixels were registered within the summit crater while 3 were detected just outside the SW crater rim. Courtesy of HIGP-MODVOLC Thermal Alerts System.

Figure 90. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) confirmed ongoing thermal activity (bright yellow-orange) at Nyamuragira from February into April 2020. The strength of the thermal anomaly in the summit crater decreased by late March 2020, but was still visible. Courtesy of Sentinel Hub Playground.

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: Information contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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/exp.

Nyiragongo is located in the Virunga Volcanic Province (VVP) in the Democratic Republic of the Congo, part of the western branch of the East African Rift System and contains a 1.2 km-wide summit crater with a lava lake that has been active since at least 1971. Volcanism has been characterized by strong and frequent thermal anomalies, incandescence, gas-and-steam emissions, and seismicity. This report summarizes activity during December 2019 through May 2020 using information from monthly reports by the Observatoire Volcanologique de Goma (OVG) and satellite data.

In the December 2019 monthly report, OVG stated that the level of the lava lake had increased. This level of the lava lake was maintained for the duration of the reporting period, according to later OVG monthly reports. Seismicity increased starting in November 2019 and was detected in the NE part of the crater, but it decreased by mid-April 2020. SO₂ emissions increased in January 2020 to roughly 7,000 tons/day but decreased again near the end of the month. OVG reported that SO₂ emissions rose again in February to roughly 8,500 tons/day before declining to about 6,000 tons/day. Unlike in the previous report (BGVN 44:12), incandescence was visible during the day in the active lava lake and activity at the small eruptive cone within the 1.2-km-wide summit crater has since increased, consisting of incandescence and some lava fountaining (figure 72). A field survey was conducted on 3-4 March where an OVG team observed active lava fountains and ejecta that produced Pele’s hair from the small eruptive cone (figure 73). During this survey, OVG reported that the level of the lava lake had reached the second terrace, which was formed on 17 January 2002 and represents remnants of the lava lake at different eruption stages. There, the open surface lava lake was observed; gas-and-steam emissions accompanied both the active lava lake and the small eruptive cone (figures 72 and 73).

Figure 72. Webcam image of Nyiragongo in February 2020 showing an open lava lake surface and incandescence from the active crater cone within the 1.2 km-wide summit crater visible during the day, accompanied by white gas-and-steam emissions. Courtesy of OVG (Rapport OVG February 2020).

Figure 73. Webcam image of Nyiragongo on 4 March 2020 showing an open lava lake surface and incandescence from the active crater cone within the 1.2 km-wide summit crater visible during the day, accompanied by white gas-and-steam emissions. Courtesy of OVG (Rapport OVG Mars 2020).

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data continued to show frequent strong thermal anomalies within 5 km of the summit crater through May 2020 (figure 74). Similarly, the MODVOLC algorithm reported multiple thermal hotspots almost daily within the summit crater between December 2019 and May 2020. These thermal signatures were also observed in Sentinel-2 thermal satellite imagery within the summit crater (figure 75).

Figure 74. Thermal anomalies at Nyiragongo from 27 July through May 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and strong. Courtesy of MIROVA.

Figure 75. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed ongoing thermal activity (bright yellow-orange) in the summit crater at Nyiragongo during January through April 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. One of Africa's most notable volcanoes, Nyiragongo contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes of a stratovolcano contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; 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).

Kavachi is a submarine volcano located in the Solomon Islands south of Gatokae and Vangunu islands. Volcanism is frequently active, but rarely observed. The most recent eruptions took place during 2014, which consisted of an ash eruption, and during 2016, which included phreatomagmatic explosions (BGVN 42:03). This reporting period covers December 2016-April 2020 primarily using satellite data.

Activity at Kavachi is often only observed through satellite images, and frequently consists of discolored submarine plumes for which the cause is uncertain. On 1 January 2018 a slight yellow discoloration in the water is seen extending to the E from a specific point (figure 20). Similar faint plumes were observed on 16 January, 25 February, 2 March, 26 April, 6 May, and 25 June 2018. No similar water discoloration was noted during 2019, though clouds may have obscured views.

Figure 20. Satellite images from Sentinel-2 revealed intermittent faint water discoloration (yellow) at Kavachi during the first half of 2018, as seen here on 1 January (top left), 25 February (top right), 26 April (bottom left), and 25 June (bottom right). Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

Activity resumed in 2020, showing more discolored water in satellite imagery. The first instance occurred on 16 March, where a distinct plume extended from a specific point to the SE. On 25 April a satellite image showed a larger discolored plume in the water that spread over about 30 km², encompassing the area around Kavachi (figure 21). Another image on 30 April showed a thin ribbon of discolored water extending about 50 km W of the vent.

Figure 21. Sentinel-2 satellite images of a discolored plume (yellow) at Kavachi beginning on 16 March (top left) with a significant large plume on 25 April (right), which remained until 30 April (bottom left). Images with “Natural color” rendering (bands 4, 3, 2); courtesy of Sentinel Hub Playground.

Geologic Background. Named for a sea-god of the Gatokae and Vangunu peoples, Kavachi is one of the most active submarine volcanoes in the SW Pacific, located in the Solomon Islands south of Vangunu Island. Sometimes referred to as Rejo te Kvachi ("Kavachi's Oven"), this shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 1 km long many times since its first recorded eruption during 1939. Residents of the nearby islands of Vanguna and Nggatokae (Gatokae) reported "fire on the water" prior to 1939, a possible reference to earlier eruptions. The roughly conical edifice rises from water depths of 1.1-1.2 km on the north and greater depths to the SE. Frequent shallow submarine and occasional subaerial eruptions produce phreatomagmatic explosions that eject steam, ash, and incandescent bombs. On a number of occasions lava flows were observed on the ephemeral islands.

Information Contacts: Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Explosive activity decreased with only five explosions in January . . .; no damage was caused. The highest ash plume of the month rose to 3,000 m in the late morning hours of 27 January. There were no volcanic earthquake swarms recorded in January.

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.

Relatively quiet conditions prevailed at Arenal in December 1993 and most of January 1994. Starting on 28 January, both Strombolian activity and pyroclastic output increased. These outbursts were noted in La Palma, a village 4 km N of Crater C, where explosions vibrated the glass windows of houses.

Arenal continued its ongoing emission of gases and lava from its active vent, Crater C, situated just W of the summit. Some basic observations were made in the interval January 1993 through January 1994. Crater C emitted lava that flowed toward the S, SW, W, and NW. Strombolian eruptions produced columns that reached as high as several kilometers above the crater. Ash chiefly fell toward the NW, W, and SW, and noticeable amounts fell as far away as 25 km WNW (Tronadora), and 28 km W (Tilarán). Pyroclastic flows mainly traveled towards the NW, W, SW, S, and SE. The most noteworthy pyroclastic flow of the interval took place on 28 August; the same eruption also produced an accretionary lapilli fall deposit.

Figure 67 shows seismicity and tremor at Arenal for January 1993 through January 1994. The OVSICORI seismic station functioned for 19 days in January. During this time it registered an average of 15.8 earthquakes/day. Assuming the 19 days suitably reflect the average seismicity for the whole month, the estimated number of January events is 491 (figure 67). This estimate is in rough accord with an unbroken record of January seismicity at the ICE station ("La Fortuna," located 3.5 km E of Crater C). A similar averaging and readjustment procedure was also applied for tremor, and again this is in rough accord with the record seen at the ICE station.

Figure 67. Number of seismic events and tremor duration at Arenal for January 1993 through January 1994 (received at station "VACR," 2.7 km NW of the active crater). The intervals phase 1 and 2 are discussed in the text. Courtesy of OVSICORI.

The seismicity for January 1993 through January 1994 was divided into two phases. The interval January-April 1993, termed Phase 1, had the most explosive behavior. Phase 1 was characterized as follows: a) seismicity typically in the range of 27-44 events/day, b) seismic signals predominantly in the high-to-moderate frequency range, c) high sound levels, and d) explosions, sometimes accompanied by pyroclastic flows.

The interval May 1993-January 1994 was termed Phase 2. It was characterized as follows: a) an average of 7-26 seismic events/day, b) seismic signals of low-frequency, short duration, and low amplitude, and c) intermediate sound levels described as like those of a jet or locomotive. Phase 2 also had relatively low numbers of seismic events and the most pronounced highs in tremor duration, the latter associated with lava emission.

Scientists at OVSICORI described the eruptive behavior of Arenal based on the following model of the magma conduit. Phase 1 behavior, when major explosions took place, resulted from a more restricted conduit. Phase 2 behavior, characterized by jet and locomotive sounds, high tremor, and common lava emissions, resulted from a less restricted conduit.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, Univ de Costa Rica.

Seismicity was at background levels from mid-December into early January. During 4-6 January, a slight increase in seismic activity (16 events) was recorded at the volcano, but seismicity had returned to background levels (2-3 events/day) by mid-month. Seismicity was again above background in late January through mid-February. Strong fumarolic activity continued from the summit lava flow in the central crater. Similar activity was noted in May 1992, and in April, August, September, and October 1993.

Geologic Background. Avachinsky, one of Kamchatka's most active volcanoes, rises above Petropavlovsk, Kamchatka's largest city. It began to form during the middle or late Pleistocene, and is flanked to the SE by the parasitic volcano Kozelsky, which has a large crater breached to the NE. A large horseshoe-shaped caldera, breached to the SW, was created when a major debris avalanche about 30,000-40,000 years ago buried an area of about 500 km2 to the south underlying the city of Petropavlovsk. Reconstruction of the volcano took place in two stages, the first of which began about 18,000 years before present (BP), and the second 7000 years BP. Most eruptive products have been explosive, with pyroclastic flows and hot lahars being directed primarily to the SW by the breached caldera, although relatively short lava flows have been emitted. The frequent historical eruptions have been similar in style and magnitude to previous Holocene eruptions.

Information Contacts: V. Kirianov, IVGG.

Seismicity was at background levels from mid-December 1993 through mid-February 1994. On 3 January a large steam-and-gas plume was seen to extend 40 km NE. The next day a large ash-and-gas plume stretched 60 km NE from the crater. One shallow earthquake was recorded on 18 January centered beneath the volcano. Volcanic tremor lasting 30 minutes on 21 January may have been caused by a small explosion from the extrusive summit lava dome. A gas-and-steam plume with a small amount of ash was again observed in early February extending 60 km SE.

Geologic Background. Prior to its noted 1955-56 eruption, Bezymianny had been considered extinct. The modern volcano, much smaller in size than its massive neighbors Kamen and Kliuchevskoi, was formed about 4700 years ago over a late-Pleistocene lava-dome complex and an ancestral edifice built about 11,000-7000 years ago. Three periods of intensified activity have occurred during the past 3000 years. The latest period, which was preceded by a 1000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large horseshoe-shaped crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: V. Kirianov, IVGG.

Following the earthquake swarm in late November and early December 1993, seismic activity returned to the low levels of previous months. During January, activity consisted primarily of rock-fracture events located at superficial levels and others at depths of <3 km around the active crater. The largest event (M 2.0), on 3 January, was located at a depth of 2.9 km. The deepest were at 2.5-8 km, centered ~ 3.5 km NNE of the active crater where the November and December 1993 swarms were located. One event was felt on 17 January (M 2.6), centered on the NNE source at a depth of 5.2 km. High-frequency butterfly-type events showed a periodicity similar to tidal patterns.

Long-period events associated with fluid movements, though few in number (14), were a new type of event detected in January. These events had durations of 7-64 seconds, with maximum amplitudes between 5.2 and 27.8 mm, and frequencies of 1-10 Hz. Variations in background tremor continued to be registered at the Crater-2 station located 1.6 km from the active crater.

Deformation data did not indicate variations on the surface of the volcano. However, on 18 January there were increases in both the tangential and radial components at the Peladitos tiltmeter (1.6 km E of the active crater) of 14.2 µrad, which may have been caused by electronic problems.

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, Pasto.

Over the past year a lake covered the bottom of the main crater at Irazú, and in January 1994 fumaroles within it and on the NW flank grew in number, size, and vigor. Overall, reported lake temperatures for January were in the range 18-25°C (ICE); some subaqueous fumarole temperatures reached 40°C, up from near 24°C during previous visits (OVSICORI). In January the lake had a minimum pH of 5, and a color of green to turquoise-green. ICE reported that in late January the temperature and pH of hot and cold springs around the volcano were unchanged. OVSICORI reported Irazú's hottest fumarole temperature, 91°C, on the NW flank.

During 1993, seismicity took the form of sporadic, locally detected earthquakes. The earthquakes typically had magnitudes in the range 1.7-2.2. They were thought to originate along the Irazú fault, which lies within 5 km of the crater. Irazú inflated at a rapid rate during 1991, but it has only deformed a limited amount since then (18:1 & 3). In December and January, dry-tilt measurements, together with leveling and distance surveys, found insignificant changes.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, Guillerma E. Alvarado, and Francisco (Chico) Arias, ICE; Héctor (Chopo) Flores, Escuela Centroamericana de Geologia, Univ de Costa Rica.

Low-level eruptive activity began in late December 1993. Unusual amounts of steam, ashfall on the flanks of the volcano, and sulfur smell in Adak . . . were reported through mid-January 1994. Pilots and observers in Adak also reported continued low-level emission of steam, and possibly minor ash, from Kanaga during the third week of January. Photographs taken by the U.S. Navy show two steam vents near the summit. Observers in Adak were unable to view Kanaga from 27 January through 18 February. However, the smell of sulfur was detected in early February.

Pilot reports indicated another eruption on 21 February with plume heights of 3.6-4.5 km, rising at times to 7.5 km altitude; ground observers reported a steam plume rising to 600-900 m altitude. Satellite data were inconclusive in detecting the plume.

Geologic Background. Symmetrical Kanaga stratovolcano is situated within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. The volume of subaerial dacitic tuff is smaller than would typically be associated with caldera collapse, and deposits of a massive submarine debris avalanche associated with edifice collapse extend nearly 30 km to the NNW. Several fresh lava flows from historical or late prehistorical time descend the flanks of Kanaga, in some cases to the sea. Historical eruptions, most of which are poorly documented, have been recorded since 1763. Kanaga is also noted petrologically for ultramafic inclusions within an outcrop of alkaline basalt SW of the volcano. Fumarolic activity occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east.

Information Contacts: AVO; J. Lynch, SAB.

The . . . eruption continued as lava . . . traveled through tubes to the ocean. There was no change at the vent area, with the exception of small pahoehoe overflows within the crater of the E-53 vent; collapse pits around the E-51 vent continued to widen. The level of Pu`u `O`o lava pond was 84 m below the N spillway rim. A large, new skylight was spotted above 705 m elevation.

On 6 January, an active pahoehoe flow fanned out behind its aa terminus at the base of a fault scarp (Pulama pali) and descended to 60 m elevation before stagnating. A small flow oozed out of the Kamoamoa tube, at the base of another fault scarp (Paliuli). Another flow broke out at 120 m elevation on 8 January. This channelized aa and pahoehoe flow cascaded over the Paliuli fault scarp the next day before it stagnated.

Another new skylight opened above 60 m elevation. Lava in the skylight was almost 7 m below the ground surface and an underground lava fall was visible. The volume of lava in the skylight was high on 19 January, but decreased on the 27th.

Lava flows continued to enter the ocean on the W side of the Kamoamoa delta. The E bench stopped building into the ocean and began to erode during this interval while the W bench continued to grow. Prominent littoral cones formed as pieces of the bench collapsed, exposing lava tubes. There were several small collapses along the length of the bench but no major collapses were observed. The bench area extended >50 m into the ocean.

Eruption tremor continued during the first half of the month along the east rift zone. Low amplitude tremor fluctuated from near background to ~3x background levels. Higher amplitudes ranged from a half hour to ~2 hours duration. Shallow, long-period microearthquakes were moderate in rate. The number of short-period events was low beneath the summit and moderate to high along the east flank.

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.

Weak volcanic tremor (5-9 hours/day) and shallow volcanic earthquakes (5-12 events/day) were recorded during the third week of January. Volcanic tremor increased in late January to nearly continuous levels (19-21 hours/day), although the number of shallow volcanic earthquakes decreased (1-6 events/day). High volcanic tremor levels continued into early February (16-22 hours/day), but had declined by 10 February to 0.3 hour/day. Tremor increased again slightly the following week (10-16 February) to 0.6-1.3 hours/day; 1-3 volcanic earthquakes/day were registered during that period. Weak fumarolic activity continued in the central crater throughout January and early February.

Geologic Background. Klyuchevskoy (also spelled Kliuchevskoi) is Kamchatka's highest and most active volcano. Since its origin about 6000 years ago, the beautifully symmetrical, 4835-m-high basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of sharp-peaked Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during the past roughly 3000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 m and 3600 m elevation. The morphology of the 700-m-wide summit crater has been frequently modified by historical eruptions, which have been recorded since the late-17th century. Historical eruptions have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: V. Kirianov, IVGG.

"Both craters continued their spasmodic Vulcanian eruptions throughout January, generating moderate volumes of ash. Activity was stronger at Crater 3; explosions were heard intermittently and produced ash clouds rising a few hundred meters above the crater before being dispersed. In contrast, Crater 2 explosions were muffled and ash emission less frequent. Being the rainy season, the craters were covered by atmospheric clouds on most nights, but incandescent projections were seen at Crater 3 on 3 January. A steady red glow above Crater 2 was seen on 3 and 30 January. Seismic activity was relatively high, with up to 76 explosion events/day . . . ."

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: D. Lolok and P. de Saint-Ours, RVO.

"At 0835 on 17 December 1993, Lascar had a new short-lived explosive eruption. Ibar Torrejón, the teacher in Talabre (17 km WNW) who maintains a log of Lascar's activity, reported that a strong explosion was followed by the rise of a dark gray plume up to 8-10 km above rim of the active crater. The eruption was accompanied by seismic activity felt in Talabre with an intensity of MM 3. Another small explosion was recorded at 0930, with a dark plume rising 2 km. Although the dark coloration of the plume suggests significant ash content, no ashfall was recorded in the few populated areas E or SE of Lascar, where prevailing winds normally transport the plumes (El Laco, 60 km SSE; Catua, 80 km SE; and Salta, 285 km SE). There were no witnesses closer to the S or E flanks of the volcano at the moment of the eruption. Also, no ballistic blocks were observed, suggesting that it was a small, mainly phreatic, eruption. Since then Lascar has returned to normal fumarolic activity, with columns rising 200-400 m above the active crater rim.

"[The domes] progressive collapse since May 1993 suggested that it was following the pattern shown since 1985, so local authorities were warned in November of the possibility of a new eruption within the next months. Local authorities have again been warned that similar or larger eruptions can take place in the near future."

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: M. Gardeweg, SERNAGEOMIN, Santiago.

"Reports of possible eruptive activity . . . 22-23 January reflected a combination of intense lightning near the volcano and a strong sulfur odor detected in Dutch Harbor, 25 km E. The lightning was apparently associated with the passing of a strong storm front, and winds were blowing directly toward Dutch Harbor.

Geologic Background. The ice-covered Makushin volcano on northern Unalaska Island west of the town of Dutch Harbor is capped by a 2.5-km-wide caldera. Its broad, dome-like structure contrasts with the steep-sided profiles of most other Aleutian stratovolcanoes. Much of the volcano was formed during the Pleistocene, but the caldera (which formed about 8,000 years ago), Sugarloaf cone on the ENE flank, and a cluster of about a dozen explosion pits and cinder cones at Point Kadin on the WNW flank, are of Holocene age. A broad band of NE-SW-trending satellitic vents cuts across the volcano. The composite Pakushin cone, with multiple summit craters, lies 8 km to the SW. Frequent explosive eruptions have occurred during the past 4,000 years, sometimes accompanied by pyroclastic flows and surges. Geothermal areas are found in the summit caldera and on the SE and E flanks. Small-to-moderate explosive eruptions have been recorded at Makushin since 1786.

Information Contacts: AVO.

"A brief eruption occurred from Southern Crater on 5 January. The intensity was similar to the stronger phases of activity in the prolonged eruption of 1992 and to the brief episodes in July and October 1993. An increase in activity started on 3 January with occasional deep roaring sounds accompanying thin grey emissions and night-time incandescent lava projections as high as 150 m. On 4 January, emissions were of moderate volume and incandescent projections reached 400 m. Simultaneously, there was a progressive increase in seismicity (amplitude, number of events, and tremor).

"The peak of the eruption occurred during the night of 5-6 January, with strong incandescent projections to ~1 km (estimated in cloudy weather conditions), and ash much higher. The pilot of an aircraft passing over the volcano at about 0230 on the 6th reported a lava flow in SE Valley. Another pilot, at 0815, estimated that the ash cloud was rising to 4-8 km asl and drifting SW. Scoria and ash fell on the SSW flank of the volcano. Seismicity during that night reached 9x the usual inter-eruptive background level.

"Lava effusion into SE Valley was voluminous, and the foot of the flow almost reached the coast, ~ 5 km from the summit crater. A small pyroclastic flow descended into SW Valley, but stopped at 750 m elev, ~1 km from the source. The volume of ash emission, and the seismicity, decreased from 6-8 January. Although a weak night glow prevailed for the remainder of the month, emissions from Southern Crater were reduced to weak white vapour release.

"Paradoxically, the eruption occurred after two months of steady deflationary tilting (-3 µrad at the observatory . . .). The eruption itself caused an additional 3 µrad of deflation, but rapid recovery took place throughout the remainder of January. Main Crater activity was unaffected throughout the month, consisting of gentle release of small volumes of white vapour."

A volcanic ash advisory was issued to aircraft following pilot reports on 6 January of ash clouds drifting SW and lava flows. The Bureau of Meteorology in Darwin, Australia, was unable to detect any ash clouds on satellite imagery.

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: D. Lolok and P. de Saint-Ours, RVO; BOM, Darwin.

Seismic and visual activity have decreased since the beginning of 1993. In the interval from May-Nov VSI staff reported only 3-10 avalanches/month, as compared to 10 avalanches/day during dome growth in 1992 and early 1993. Some specific cases of tremor, and tremor with avalanche, are depicted on figures 7 and 8.

Figure 7. Three-component seismic record for Merapi during a large tremor (top). Recorded at Klathakan on 3 June 1993, starting at 1542. Three-component maximum entropy spectral analysis for the seismic record above (bottom). Courtesy of A. Brodscholl and K. Brotopuspito, GMU.

Figure 8. Three-component seismic record for Merapi during a tremor and coincident avalanche down the W flank (top). Recorded at Klathakan on 3 June 1993, starting at 1430. Three-component maximum entropy spectral analysis for the seismic record above (bottom). Courtesy of A. Brodscholl and K.S. Brotopuspito, GMU.

About a week after the rainy season began at the end of November 1993, both the tremor rate and the volume of released gas increased. In early December the amplitudes of volcanic tremor rose to 2-3x higher than in previous months.

As of November the VSI supported seven seismic receiver stations at Merapi and the Geophysical Laboratory of GMU supported three. Most of the stations consisted of a vertical-component seismograph with a 1.0 Hz natural frequency. Starting in April 1993 GMU also studied Merapi using a portable 3-component system with a 0.2 Hz cut-off frequency. The portable system collected good quality data at two sites on Merapi: a) Klathakan, 1.8 km WNW of the summit between 1,200-1,300 m elevation (in the interval 1240-1600, 3 June 1993) and b) At Puncak, 0.5 km N of the summit (in the interval 0730-1530, 14 October 1993).

At Puncak, particle analysis on multiple tremor records revealed azimuths that ranged WSW-SW, occasionally swinging to ESE. From these results researchers suggested two possible tremor sources may lie at depth beneath the summit region: one offset slightly toward the W, another toward the E.

At Klathan the seismic record included intervals with tremor and little other noise (figure 7, top). The seismic record also included an interval of tremor coincident with an avalanche, which tumbled and bounced down the W flanks (figure 8, top). These two different signal sources were characterized using spectral response (figures 7 and 8, bottom); both produced spectra with a broad peak centered near 6 Hz. The broad 6 Hz peak is largest for the case of the tremor. But the case of the coincident tremor and avalanche yielded substantial peaks in the low-frequency range (centered near 0.25 Hz and at or below 0.1 Hz).

On the broad 6-Hz peaks the three components differ slightly, particularly with respect to individual spikes on the peaks. For the case of the tremor without the avalanche (figure 7, bottom), the spikes often correspond between the vertical and N-S horizontal (approximately radial), but the spikes of these components often differ from those on the E-W horizontal (approximately tangential). For the case of the tremor and avalanche, the largest spikes occur mainly to the low-frequency side of the broad peak (figure 8, bottom).

These results suggest that for some cases at Merapi an avalanche accompanied by tremor can be discriminated from tremor alone by looking at the low-frequency content of the signal. Other cases, such as typical earthquakes at Merapi, are not discussed here, but it is conceivable that advancing avalanches can be detected seismically as one means of early warning.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: A. Brodscholl and K. Brotopuspito, GMU; S. Bronto, MVO.

Seismic activity significantly increased starting on 8 February and peaked at >200 high-frequency volcanic earthquakes on 11 February. The number of explosion earthquakes and duration of low-frequency harmonic tremor also increased during this period. Swarms of shallow volcanic earthquakes were recorded at the CRAZ station on the N caldera rim. Some of these events were also detected by the FNGZ station located 9 km SE of the caldera. Aerial surveys on 25 January and 8 February revealed phreatic cones and explosion pits on the NW shore of the lake and on the caldera floor near the N wall. The W half of the caldera lake also appeared muddy, but there was no sign of dome growth. These events are similar to those seen before the dome-building activity in July 1992, and may represent a continuation of that activity.

PHIVOLCS interpreted these observations as indicative of renewed movement of magma beneath the volcano that could result in lava-dome growth and small to moderate eruptions with columns < 6 km high. Because of this possibility, the hazard status was raised to Alert Level 3, indicating that eruptive activity as described above could occur within a period of weeks. COSPEC measurements of SO₂ flux were attempted on 15 February, but were unsuccessful because the steam plume dissipated before rising above the crater rim.

Starting on 18 February, seismicity began to decrease. Explosion earthquakes and tremor gradually dropped to 0-2 events/day by 21 February, with no shallow earthquakes recorded at the CRAZ station. On 22 February the CRAZ station recorded 10 high-frequency volcanic earthquakes; the FNGZ station recorded 4. Low-frequency harmonic tremor was also recorded for 77 minutes. Thick cloud cover at the summit prevented visual observations of steaming activity. If seismicity continues to decrease through the first week of March, the hazard status will be lowered back to Alert Level 2. As a precaution against possible secondary explosions from the still-hot 1991 pyroclastic-flow deposits, the public continues to be advised to observe the 10-km-radius danger zone.

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.

OVSICORI reported in January 1994 that two dry-tilt inclinometers installed in 1987 have thus far showed insignificant changes.

Geologic Background. The Platanar volcanic center is the NW-most volcano in the Cordillera Central of Costa Rica. The massive complex covers about 900 km2 and is dominated by two largely Pleistocene stratovolcanoes, Platanar and Porvenir. These volcanoes were constructed within the Pleistocene Chocosuela caldera, which may have formed during a major slope failure. The Cerro Platanar volcano (known locally as Volcán Congo) on the N side of the complex has prehistorical lava flows on its W flanks and is the youngest volcanic center. The highest peak is Porvenir, whose summit crater lies 3 km S of Platanar. A thin layer of phreatic ash suggested that an eruption from Platanar occurred within the past few thousand years (Stine and Banks, 1991). The Aguas Zarcas group of nine basaltic cinder cones, located on the N flank of the Platanar-Porvenir complex to as low as 160 m altitude is, in part, Holocene in age.

Information Contacts: E. Fernández, J. Barquero, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI.

During January the 200-m-diameter, northerly crater lake continued to emit steam in a column <100 m tall. The lake level descended 50 cm compared to December 1993; it still remained pale-green to turquoise-green with suspended clouds of sulfur, and in the NW part of the lake, it contained isolated subaqueous fumaroles. OVSICORI reported lake temperature as 60°C, down slightly from the 64°C they measured in September. ICE reported that in January fumarole temperatures on the dome reached up to 81°C.

The number of earthquakes dropped about 20-30% in December and January compared to October and November (figure 47). For the interval January 1993 through January 1994, insignificant tilt and deformation took place, although a slow deflation was noted over the course of 1993.

Figure 47. Seismic events at Poás received at station POA2, 2.7 km SW of the active crater, January 1993-January 1994. Courtesy of OVSICORI.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI; G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, UCR.

In 1993 steam vents in the summit crater grew in number and output, leading to off-white plumes that extended 50 km downwind in mid-November (18:6 & 11). A visit to the summit crater on 30 January revealed conditions similar to those in June 1993, but the small intermittent lake had changed from clear green to a milky greenish color. This change was confirmed by photographic comparisons, and is thought to reflect the increased absorption of acid gases and steam.

SO₂ output was measured by ultraviolet absorption correlation spectrometry (COSPEC) from a fixed-wing airplane on 1 February 1994. Wind speed, an important parameter in the SO₂ flux calculation, was estimated at 36 km/hr by two means: 1) knowledge of the wind speed at the crater rim on the previous day, and 2) aircraft navigation instrumentation (Global Positioning System).

Fischer, Williams, Delgado Granados, and Siebe described their data and its significance. "Nine traverses of the plume were recorded and measurements averaged 1,200 ± 400 metric tons/ day SO₂. The weather was good and the plume could be observed very well, favoring the quality of the measurements. This level of SO₂ emissions puts Popocatépetl into a small league of half a dozen volcanoes worldwide that emit such high levels of magmatic gases. It is a symptom of reawakening and is usually associated with significant danger of magmatic activity. However, with no previous records from Popocatépetl, one must be careful about reaching conclusions before a good baseline is available."

Seismic events at the volcano are defined by Guillermo González-Pomposo, Valdés González, and Martin del Pozzo as follows. Type-A events have frequencies above 5 Hz, with identifiable and impulsive P and S phases, and S minus P times of 1.5 to 3 seconds. Coda duration (the "tail" of the event) is <40 seconds. Type-B events have frequencies in the 1.0 to 1.6 Hz range. Type-B events characteristically exhibit an emergent P phase arrival, that is the P phase increases in amplitude with time. They also have an S phase arrival that is hard to identify. Coda duration is <125 seconds. Type-AB events are made up of both low and high frequencies. The high frequency phases include impulsive P and S waves. These high frequency phases arrive first and extend for <5.5 seconds; the S - P times are 1.5 to 2 seconds. The amplitudes of P waves and S waves for type AB are smaller than for type A. The characteristic period of the low-frequency phases is 0.8 seconds, and these signals may last for 150 seconds.

During December type-A events took place 12 times, type-B events 62 times, and type-AB events 12 times. For each type, the frequency of events was roughly evenly distributed across the month. For type A there were 5 events on 8 December and 1-2 events on 4 other days. For type B there were 7 events/day on 3 December and 0-5 events on other days. For type AB there were 3 events on 15 December and 0-2 events on other days.

Geologic Background. Volcán Popocatépetl, whose name is the Aztec word for smoking mountain, rises 70 km SE of Mexico City to form North America's 2nd-highest volcano. The glacier-clad stratovolcano contains a steep-walled, 400 x 600 m wide crater. The generally symmetrical volcano is modified by the sharp-peaked Ventorrillo on the NW, a remnant of an earlier volcano. At least three previous major cones were destroyed by gravitational failure during the Pleistocene, producing massive debris-avalanche deposits covering broad areas to the south. The modern volcano was constructed south of the late-Pleistocene to Holocene El Fraile cone. Three major Plinian eruptions, the most recent of which took place about 800 CE, have occurred since the mid-Holocene, accompanied by pyroclastic flows and voluminous lahars that swept basins below the volcano. Frequent historical eruptions, first recorded in Aztec codices, have occurred since Pre-Columbian time.

Information Contacts: Guillermo González-Pomposo, Carlos Valdés González, and Ana Lillian Martin del Pozzo, Departamento de Sismología y Volcanología, Instituto de Geofísica, UNAM; Tobias P. Fisher and Stanley N. Williams, Arizona State Univ, USA; Claus Siebe and Hugo Delgado Granados, Instituto de Geofisíca, UNAM.

"Seismic activity declined further in January, with 591 small earthquakes recorded . . . . A number of these earthquakes occurred in small swarms of 30-80 events at intervals of 1-2 weeks. Twenty-three of these earthquakes were located. As usual, they originated from the annular caldera seismic zone, mainly to the NE (Greet Harbour area) and W (Vulcan headland). Routine levelling on 4 January showed that the greatest change since the previous survey (6 December 1993) was only 4 mm uplift at the S end of Matupit Island. Other ground deformation measurements were inconclusive.

"A review of . . . 1993 shows that earthquakes occurred in all parts of the caldera seismic zone, but the W and NE parts were the most active (figure 13). Rates of seismicity and ground deformation were variable (figure 14). Seismicity showed two periods of higher level activity, March-May and October-November. Rates of uplift were low early in the year, but accelerated at the time of a seismic swarm in May. Higher rates of uplift then continued through several months of low seismicity (June-October), until the second period of increased seismicity ended in November. At the end of the year there were indications that the rate of uplift had declined. The greatest measured elevation change in 1993 (~12 cm) is consistent with the long-term rate of uplift between 1973 and 1983."

Figure 13. Map showing seismicity in Rabaul Caldera during 1993. Earthquakes with location error (horizontal and vertical) of

Figure 14. Seismicity and uplift at Rabaul Caldera during 1993. Lines show cumulative uplift (left scale) at deformation stations (inset map); solid bars show daily number of earthquakes (right scale). Courtesy of RVO.

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: Rabaul Volcano Observatory (RVO).

During 1993 Rincón de la Vieja continued fumarolic venting in Rincón crater. Gas columns rose to 500 m high and contained sufficient gas concentrations to irritate the eyes and sting the skin of observers on the crater margin. The crater lake was clear gray in color, with clouds of suspended sulfur and several areas of discontinuous bubbling. It had a temperature of 35°C.

When seismically active, as in January and September 1993 (figure 8), both high- and low-frequency signals were common. A swarm of 25 high-frequency events took place on 26-29 January. On 26 March, 10 small-amplitude high-frequency events registered, their S - P (S minus P) times were

Figure 8. Seismic events at Rincón de la Vieja received at station RIN3, 5 km SW of the active crater, January 1993-January 1994 (October-December 1993 data presumed unrecorded). Courtesy of OVSICORI.

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, R. Van der Laat, F. de Obaldia, T. Marino, V. Barboza, and R. Sáenz, OVSICORI.

On 3 February, VSI observers and local residents near Semeru heard a sharp thunder-clap at 0350 and later saw falling ash. Lava avalanches and pyroclastic flows from the summit crater descended SE along the Kembar and Kobokan rivers, reaching 7.5 km and 11.5 km from the summit, respectively. Volcanic materials also entered Sumbersari village (8 km SE of the summit), located between the rivers. Four people were killed; another three were injured, and two of those later died. Following the lava avalanches and pyroclastic flows, 275 people were evacuated from Sumbersari, where many houses and 1.5 km² of plantation land were destroyed. A seismic station maintained by VSI was also destroyed.

Visual and seismic activity had increased in late 1993, with frequent collapses from the summit lava flow SE into the area between the Kembar and Kobokan rivers. Volcanic earthquakes had been recorded since October, and peaked on 25 December 1993. Continuous tremor with a maximum amplitude of 26 mm was recorded from 26 January until 3 February 1994 (figure 4).

Figure 4. Diagram showing seismicity at Semeru, 7 January-14 February 1994, with A- and B-type events along with tremor (top), and avalanche and explosion events (bottom). Courtesy of VSI.

Eruptive and seismic activity continued after 3 February, but the intensity and energy had decreased. The daily number of explosion earthquakes increased to 360 on 5 February, then declined again, but remained >70/day through mid-month (figure 4). Volcanic tremor was recorded again on 9-14 February with a maximum amplitude of 13 mm. Ash eruptions generated clouds reaching heights of up to 700 m above the summit. Pyroclastic flows and lava avalanches traveled as far as 3 km down the Kembar and Kobokan rivers. Lava extruded from the crater over the S rim formed a 750-m-long lobe moving down the S flank toward the Kembar river.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: W. Tjetjep, VSI; A. Brodsholl, GMU.

Fumarolic activity from the extrusive dome was observed throughout January and into mid-February. The SE part of the extrusive dome continued to grow in mid-February. The persistent gas-and-steam plume rose 400-1,000 m above the crater rim in January. Weak volcanic tremor was recorded for ~0.3 hours/day in late January, a significant decrease from earlier in the month when tremor was recorded for 7-9 hours/day (18:12). Late January temperatures in Kliuchi, 8 km S, were as low as -40°C.

The height of the gas-and-steam plume above the crater rim was estimated to be 800-2,000 m in the first half of February, but only 400 m at mid-month. Weak volcanic tremor (1-5 hours/day) was recorded in late January to early February, and some shallow volcanic earthquakes (average of 1 event/day) were registered. Weak volcanic tremor increased to 3-5.5 hours/day in the second week of February when 1-2 shallow volcanic earthquakes/day were detected. Seismicity increased the following week to 8-12 hours/day of tremor and 1-3 earthquakes/day.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1300 km3 volcano is one of Kamchatka's largest and most active volcanic structures. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes dot its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large horseshoe-shaped caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. At least 60 large eruptions have occurred during the Holocene, making it the most vigorous andesitic volcano of the Kuril-Kamchatka arc. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: V. Kirianov, IVGG.

A sudden decrease in seismicity followed the explosive episodes of mid-October 1993. Both number of recorded events and tremor level declined to below "normal" levels (figure 32). This pattern is similar to that observed after other explosive episodes in February and May 1993. Although problems with solar panel efficiency resulted in data loss during December, an increase in seismicity was still evident through December 1993 and January 1994.

Figure 32. Seismicity recorded at Stromboli, October 1993-January 1994. Open bars show the number of recorded events/day, the solid bars those with ground velocities >100 microns/second. The lines show daily tremor energy computed by averaging hourly 60-second samples. Courtesy of R. Carniel.

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: R. Carniel, Univ di Udine.

Seismic monitoring . . . by the KVERT began in late January 1994 when two stations were installed. Since then seismicity was considered to be at background levels. One earthquake was registered on 25 January. During the period from 28 January to 2 February, shallow volcanic earthquakes were registered at a rate of 5-13/day. The following week (3-8 February) there were 1-4 shallow earthquakes recorded each day.

Geologic Background. The massive Tolbachik basaltic volcano is located at the southern end of the dominantly andesitic Kliuchevskaya volcano group. The massif is composed of two overlapping, but morphologically dissimilar volcanoes. The flat-topped Plosky Tolbachik shield volcano with its nested Holocene Hawaiian-type calderas up to 3 km in diameter is located east of the older and higher sharp-topped Ostry Tolbachik stratovolcano. The summit caldera at Plosky Tolbachik was formed in association with major lava effusion about 6500 years ago and simultaneously with a major southward-directed sector collapse of Ostry Tolbachik volcano. Lengthy rift zones extending NE and SSW of the volcano have erupted voluminous basaltic lava flows during the Holocene, with activity during the past two thousand years being confined to the narrow axial zone of the rifts. The 1975-76 eruption originating from the SSW-flank fissure system and the summit was the largest historical basaltic eruption in Kamchatka.

Information Contacts: V. Kirianov, IVGG.

Microearthquakes beneath the lava dome, which increased in mid-December, suddenly decreased to 20-50/day after 6 January 1994. The ground deformation that began in early November ceased in early January, based on EDM measurements by the Geological Survey of Japan, at the same time that seismicity decreased. A new lobe (lobe 12) appeared in a graben at the top of the dome on 15 January (figure 66). The peak of the lobe, where lava was extruding, was at 1,420 m elevation. During endogenous growth the graben deepened and lobe 12 swelled to the SW, allowing lava extrusion from the base. The new lobe grew at a rate of ~0.1 x 10⁶ m³/day during the first week, reaching dimensions of ~140 m long, 110 m wide, and 30 m high. The lobe was composed of lava blocks, reflecting the low eruption rate. Lava samples of lobe 12 have SiO₂ contents of about 64 wt.%, almost the same as previous lobes.

Figure 66. Sketch map of the lava dome at Unzen, early February 1994. Arrows indicate the main direction of pyroclastic flows and rockfalls from mid-January to mid-February. Courtesy of Setsuya Nakada.

A pyroclastic flow on 20 January traveled 1.5 km SW, reaching the small Akamatsu valley. The pyroclastic flow was due to partial collapse of both lava blocks S of lobe 12 and talus deposits from previous collapses that had filled the crater floor S of the dome. The flow descended a gully, incorporating prehistoric deposits from the gully walls. The gully had developed during rainy seasons since the start of the 1991 eruption, and was the source for debris-flow material. The pyroclastic flow burned trees in the valley and produced ashfall a few kilometers away from the dome. Tremor caused by this event lasted 910 seconds, much longer than most previous pyroclastic flows. This long tremor duration is thought to have been caused by successive small-scale collapses from the dome. The estimated volume of the pyroclastic-flow deposits is several hundred thousand cubic meters.

Collapses generated a total of 75 pyroclastic flows in January, the most since October 1993. The flows eroded deeply into the basement rocks (pyroclastic-flow deposits of ~4 Ka) on the SE margin of the talus apron. A pyroclastic flow on 31 January, from lobe 12, traveled 2.5 km SE, the longest distance in January. Fresh, gray-colored lava from lobe 12 appeared within the horseshoe-shaped crater formed by the collapses. By 3 February, the total volume of lobe 12 and adjacent talus that had collapsed was ~1 x 10⁶ m³. Rockfalls occurred mainly to the NW.

Pyroclastic flows on 3 February traveled about 3.5 km from their source. A field inspection the next day showed that the resulting deposits contained larger amounts of cooled oxidized blocks and mud-coated blocks, derived from older lobes and prehistoric basement deposits, respectively. The amount of juvenile materials in these deposits (fresh, hot blocks and fragments) was as low as 10%. On 6 February, pyroclastic flows descended NNW beyond the notch of the caldera rim, from which andesitic lava flowed N in 1663. Previously, talus and pyroclastic-flows that moved N had remained within the caldera.

The growth rate of lobe 12 declined to3/day in early February, and no fresh lava appeared on the lobe surface. Endogenous growth then started; upward swelling produced NW-SE trending cracks on the surface of the lobe. The entire lava dome also started to expand NW at a rate of ~1 m/day in early February. Field inspection by staff of the SEVO showed pressure ridges and low-angle reverse faults in the NW crater floor near the dome, implying pressure from the dome. These phenomena resembled those during the endogenous events of March-April and November-December 1993.

The monthly total of microearthquakes in January was 1,863, a significant decrease from December (25,340) and November (2,662) (18:11-12). There have not been any earthquakes felt at the UWS in early February. Only 27 earthquakes were felt in January, compared to 170 in December (18:12).

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: JMA; S. Nakada, Kyushu Univ.