Volcanism at Sheveluch has been ongoing for the past 20 years. Previous activity consisted of pyroclastic flows, explosions, moderate gas-and-steam emissions, and lava dome growth, according to the Kamchatka Volcanic Eruptions Response Team (KVERT). Between May 2018 and mid-December 2018 activity levels were low, with intermittent low-power thermal anomalies and gas-and-steam emissions. Activity increased in the second half of December 2018, remaining high through at least April 2019.

Activity intensified beginning in late December through April 2019, which included increased and more frequent thermal anomalies, according to KVERT and the MIROVA system (figure 50). On 30 December 2018, video data from KVERT showed explosions producing an ash cloud that rose up to 11 km altitude and drifted 244 km WSW and 35 km NE. Eruptive activity included incandescent lava flows and hot avalanches. The ash cloud that drifted WSW resulted in ashfall over Klyuchi Village (50 km SW) and Kozyrevsk (100 km SW).

Figure 50. Thermal anomalies at Sheveluch increased in late December 2018, as seen on this MIROVA Log Radiative Power graph for the year ending 5 April 2019. The elevated thermal activity continued through March 2019. Courtesy of MIROVA.

Beginning in early January and going through April 2019, the lava dome at the northern part of the volcano continued to grow, extruding incandescent, viscous lava blocks (figure 51). Throughout these months, KVERT reported that satellite imagery and video data showed strong fumarolic activity, as well as strong gas-and-steam plumes containing some amount of ash; gas-and-steam plumes rose as high as 7 km. According to the KVERT Daily Reports on 3 and 4 January 2019, a gas-and-steam plume containing ash drifted NE up to about 600 and 400 km, respectively. Gas-and-steam plumes noted in the KVERT Daily Report, Weekly Releases, and Volcano Observatory Notice for Aviation (VONA), drifted 50-263 km in different directions. On 9 November 2018, the KVERT Daily Report recorded an ash plume drifting 461 km E from the volcano and on 26 December 2018, the KVERT Weekly Information Release recorded an ash cloud drifting 300 km NW. The KVERT Weekly Information Release reported that on 10 April 2019 an ash cloud drifted up to 1,300 km SE.

Figure 51. Incandescent avalanches from the lava dome and an ash plume can be seen in this photo of Sheveluch on 22 February 2019. Photo by Yu. Demyanchuk; courtesy of the Institute of Volcanology and Seismology FEB RAS, KVERT.

Thermal anomalies based on MODIS satellite instruments analyzed using the MODVOLC algorithm were frequent beginning on 28 December 2018. In just three days in late December (28-31 December 2018) there were 34 thermal alerts. Hotspots were detected 21-27 days each month between January-April 2019. A majority of these hotspot pixels occurred within the summit crater.

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: 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/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); 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/); 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/).

Steep-sloped and symmetrical Mayon has recorded historical eruptions back to 1616 that range from Strombolian fountaining to basaltic and andesitic flows, as well as large ash plumes, and devastating pyroclastic flows and lahars. A phreatic explosion with an ash plume in mid-January 2018 began the latest eruptive episode which included the growth of a lava dome with pyroclastic flows down the flanks and lava fountaining (BGVN 43:04). Activity tapered off during March; occasional ash emissions continued through August 2018. Minor ash emissions and summit incandescence were intermittent from October 2018-April 2019, the period covered in this report. Information is provided primarily by the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

Pyroclastic density currents were reported in early November 2018; ash plumes were produced from phreatic events a few times during both November and December 2018. Emissions produced SO₂ anomalies during January-March 2019; a series of events in early March generated several small ash plumes. Satellite images showing a thermal anomaly at the summit were recorded multiple times each month from October 2018-April 2019 (figure 44).

Figure 44. Small but distinct persistent thermal anomalies were recorded in satellite imagery from the summit of Mayon during October 2018-April 2019. Top left: 12 October 2018. Top right: 26 November 2018. Middle left: 11 December 2018. Middle right: 30 January 2019. Bottom left: 14 February 2019. Bottom right: 25 April 2019. All images are using the "Atmospheric penetration" filter (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

Very little activity was reported at Mayon during October 2018. Steam plumes rose daily from 250-750 m above the summit before drifting with the prevailing winds and dissipating. Incandescence was observed at the summit most nights during the month, and seismicity remained low with only a few earthquakes reported. Leveling data obtained during 30 August-3 September indicated significant short-term deflation of the volcano relative to 17-24 July 2018. New leveling data obtained on 22-31 October indicated inflation of the SE quadrant and short-term deflation on the N flank relative to the 30 August-3 September data. The volcano remained inflated compared with 2010 baseline data. Electronic tilt data showed pronounced inflation of the mid-slopes beginning 25 June 2018.

Activity increased during November 2018. In addition to steam plumes rising to 750 m and an incandescent glow at the summit most nights, pyroclastic density currents and ash plumes were reported. The seismic monitoring network recorded pyroclastic density currents on 5 and 6 November. On 8 November around noontime, a small, short-lived brownish ash plume, associated with degassing, drifted WSW from the summit. A seismic event on the morning of 11 November was associated with a short-lived fountaining event that produced a brownish-gray ash plume that drifted SW. Another similar plume was reported on the morning of 12 November, also drifting SW before dissipating. Two phreatic events were observed on the morning of 26 November. They produced grayish to grayish-white ash plumes that rose 300-500 m above the summit before drifting SW. The following morning, another event produced a grayish ash plume 500 m above the summit that drifted SW. On 30 November a 1-minute-long ash emission event produced a grayish white plume that also drifted SW.

Steam plume emissions and incandescence at night continued at Mayon during December 2018. The seismic network recorded a four-minute-long event shortly after noon on 9 December that produced a grayish-brown ash plume which drifted W. Precise leveling data obtained on 8-13 December 2018 indicated a slight inflation of the volcano relative to 22-31 October 2018. A 30-second-long ash emission event in the afternoon on 18 December produced a brownish ash plume. Two phreatic events were observed on the morning of 27 December. They produced grayish to grayish-white ash plumes that rose 600 and 200 m above the summit, before drifting SW (figure 45).

Figure 45. Ash plumes rose a few hundred m from the summit of Mayon on 27 December 2018. Courtesy of Twitter users "k i t" (left) and "georgianne" (right).

Very little surface activity except for white steam-laden plumes that crept downslope and drifted NW or SW was noted during January 2019. Incandescence at the summit, visible with the naked eye, became more frequent during February 2019, along with continued steam plumes. Precise leveling data obtained on 25 January-3 February 2019 indicated a slight deflation relative to 8-13 December 2018. However, continuous GPS and electronic tilt data showed inflation of the mid-slopes since June 2018. Small SO₂ plumes were detected by the TROPOMI satellite instrument a few times during January-March 2019 (figure 46).

Figure 46. Emissions of SO2 that exceeded 2 DU (Dobson Units) occurred a few times at Mayon during January-March 2019. Top left: 25 January. Top right: 16 February. Lower left: 4 March. Lower right: 15 March. Courtesy of NASA Goddard Space Flight Center.

Steam plumes rose 250-500 m above the summit and drifted generally W in early March 2019; incandescence continued daily at the summit. Phreatic events occurred on 7 and 8 March, producing ash plumes that rose 500 and 300 m from the summit before drifting SW (figure 47). Three more phreatic events occurred on the afternoon of 12 March; they produced light brown to grayish ash plumes that rose 500, 1,000, and 500 m, respectively, and drifted SW. Six phreatic events occurred throughout the day on 13 March, producing ash plumes that rose 200-700 m above the summit and drifted W. A single explosion the next day produced a 500-m-tall ash plume. The Tokyo VAAC reported an ash plume visible for several hours in satellite imagery drifting W at 3.7 km altitude on 13 March (UTC). An increase in the daily number of rockfall events from 1-2 per day to 5-10 per day was noted during the second half of March. Precise leveling data obtained on 20-26 March 2019 indicated a slight inflation relative to 25 January-3 February 2019.

Figure 47. A small ash emission at Mayon was reported by PHIVOLCS on 8 March 3019; the plume rose 300 m from the summit and drifted SW. Courtesy of PHOVOLCS.

Steam plumes drifted SW or NW throughout April, rising 200-400 m from the summit. Incandescence could be observed at night for the first half of the month. Leveling data obtained during 9-17 April 2019 indicated a slight inflation relative to 20-26 March 2019. Seismicity remained low during the month with only occasional volcanic earthquakes and rockfall events. Lenticular clouds around the summit were observed (figure 48), but these are an unusual meteorological occurrence caused by weather conditions not related to volcanic activity.

Figure 48. A double lenticular cloud surrounded the summit of Mayon early in the morning on 23 April 2019 and was captured by a local observer; it was not related to volcanic activity. Courtesy of Twitter user Ivan.

Geologic Background. Beautifully symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the Philippines' most active volcano. The structurally simple edifice has steep upper slopes averaging 35-40 degrees that are capped by a small summit crater. Historical eruptions date back to 1616 and range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer term andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic flows and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often devastated populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

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/); 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Twitter user "Ivan", Naga City, Philippines (URL: https://twitter.com/ivanxlcsn); Twitter user "k i t", Legazpi City, Philippines (URL: https://twitter.com/jddmgc); Twitter user "georgianne", Costa Leona, Philippines (URL: https://twitter.com/xolovesgia_).

Remote Tinakula lies 100 km NE of the Solomon Trench at the N end of the Santa Cruz Islands, which are part of the country of the Solomon Islands located 400 km to the W. It has been uninhabited since an eruption with lava flows and ash explosions in 1971 when the small population was evacuated (CSLP 87-71). The nearest communities live on Te Motu (Trevanion) Island (about 30 km S), Nupani (40 km N), and the Reef Islands (60 km E); residents occasionally report noises from explosions at Tinakula. Ashfall from larger explosions has historically reached these islands. The most recent eruptive episode was a large ash explosion and substantial SO₂ plume during 21-26 October 2017; satellite imagery suggested that a flow of some type traveled down the scarp on the W flank. Renewed thermal activity that was recognized in satellite imagery beginning in December 2018 continued intermittently through June 2019 and is covered in this report. Satellite imagery and thermal data are the primary sources of information for this volcano. It is occasionally visited by members of the National Disaster Management Office (NDMO) of the Solomon Islands Government, tourists, and research vessels who are able to capture ground-based information.

Satellite images from December 2018 to February 2019 show thermal anomalies at the summit vent. Excellent ship-based photographs of the island on 24-25 January 2019 provided by a crewmember from the R/V Petrel identify numerous volcanic features and show a steam-and-gas plume at the vent. Satellite images from April and May 2019 show thermal anomalies at both the summit vent and along the W flank scarp suggesting flow activity during that time.

A stream of incandescence on the NW flank of Tinakula in a Sentinel 2 satellite image on 24 October 2017 confirmed that some type of high-temperature flow accompanied the explosions and eruptive activity of 21-25 October 2017 (BGVN 43:02). Satellite imagery during most of 2018 recorded steam plumes drifting in several directions from the summit, but no thermal activity (figure 24). There was no further evidence of activity in satellite visible or thermal data until almost exactly one year later when the MIROVA project recorded two thermal alerts in the third week of October 2018 (figure 25). Satellite images from that week were cloudy and did not confirm any surface activity.

Figure 24. Sentinel-2 satellite imagery of Tinakula provides valuable information about activity at this remote volcano in the South Pacific. A large explosion with ash plumes and flows occurred during 21-26 October 2017. Top left: a strong E-W linear thermal anomaly suggesting a flow event from the summit was evident on the NW flank on 24 October 2017. Top right: a small steam plume rose from the summit vent on a cloudless 11 February 2018. Bottom left: a dense steam plume drifted SE from the summit vent on 4 September 2018. Bottom right: clouds and dense steam obscure the summit on 24 October 2018, about the same time that MIROVA reported a thermal anomaly. Top left image uses bands 12, 11, 8A, others use 12, 4, 2. Courtesy of Sentinel Hub Playground.

Figure 25. The MIROVA project recorded the first thermal anomaly in a year from Tinakula during the third week of October 2018. Courtesy of MIROVA.

The first satellite imagery confirming renewed thermal activity appeared on 8 December 2018, around the same time as a small MIROVA anomaly. After that, several images during January and February 2019 confirmed moderately strong thermal activity at the summit (figure 26). Whether the anomalies were the result of active lava effusion or strong incandescent gases from the summit vent is uncertain.

Figure 26. Thermal anomalies at the summit vent of Tinakula were recorded six times between early December 2018 and early February 2019 with Sentinel-2 satellite images. Top row: 8 December 2018 and 2 January 2019. Middle row: 12 (anomaly is just below date) and 27 January 2019. Bottom row: 1 and 6 February 2019. All images are bands 12, 4, 2. Courtesy of Sentinel Hub Playground.

Visual confirmation of activity at Tinakula is rare, but the research vessel R/V Petrel sailed past the volcano on 24 and 25 January 2019 and a crewmember provided detailed images of the W flank and vent area. The summit vent is located at the top of a W facing scarp, and steam is frequently observed rising from the vent (figures 27). Recent flows and volcaniclastic deposits were visible in the ravine on the W flank (figures 28 and 29). Fresh-looking lava was also visible near the summit vent on top of older deposits (figure 30). Eroded volcaniclastic deposits near the base of the scarp on the W flank were visible on top of older veined and layered volcanic rocks (figure 31). Crewmembers on the vessel R/V Petrel could clearly see an incandescent glow from the summit crater at night (figure 32).

Figure 27. A view from the SW of the W flank of Tinakula on 24-25 January 2019. The summit vent is at the top of a W facing scarp, the steam plume drifted E. Used with permission from Paul G Allen's Vulcan Inc.

Figure 28. The W flank of Tinakula as seen from the W on 24-25 January 2019. The steam plume drifted E. Recent flows and volcaniclastic deposits appeared dark in the steep ravine on the W face (left side). Used with permission from Paul G Allen's Vulcan Inc.

Figure 29. Steam and gas rose from the summit vent at Tinakula on 24-25 January 2019. Recent lava deposits are visible in front of the plume and in the ravine on the left (the W flank). Used with permission from Paul G Allen's Vulcan Inc.

Figure 30. The edge of the summit vent of Tinakula on 24-25 January 2019 had recent lava on older deposits; steam and gas is rising from the vent in the background. Used with permission from Paul G Allen's Vulcan Inc.

Figure 31. The W flank of Tinakula on 24-25 January 2019. Eroded volcaniclastic deposits overlie older veined and layered volcanic rocks. Used with permission from Paul G Allen's Vulcan Inc.

Figure 32. Incandescence was clearly visible from the summit vent at Tinakula on 24-25 January 2019. Used with permission from Paul G Allen's Vulcan Inc.

During April and May 2019, both the MIROVA project and MODVOLC measured a number of thermal anomalies (figure 33) using MODIS satellite data. MODVOLC alerts were issued on 4 and 20 April, and 11, 18, and 27 May. Sentinel-2 satellite images during the period confirmed that a flow on the W flank was a likely source of the thermal energy in addition to the summit vent (figure 34). Thermal anomalies appeared again at the end of June in MIROVA data, but no satellite images showed anomalies at that time.

Figure 33. The number and intensity of MIROVA thermal anomalies increased at Tinakula during April and May 2019. After a short pause, they returned at the end of June. Courtesy of MIROVA.

Figure 34. Sentinel-2 satellite images captured thermal anomalies at the summit and on the W flank of Tinakula during April and May 2019 suggesting the presence of an incandescent flow down the W scarp. Top row: 7 and 22 April 2019 (bands 12, 8, 4). Bottom row: 27 April and 12 May 2019 (bands 12, 11, 8A). Courtesy of Sentinel Hub Playground.

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

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

Short pulses of intermittent eruptive activity have characterized Piton de la Fournaise, the large basaltic shield volcano on La Réunion Island in the western Indian Ocean, for several thousand years. For the last 20 years, frequent effusive basaltic eruptions have occurred on average twice per year. The activity is characterized by lava fountains and lava flows, and occasional explosive eruptions that shower blocks over the summit area and produce ash plumes. Almost all of the recent activity has occurred within the Enclos Fouqué caldera, although past eruptions in 1977, 1986, and 1998 have occurred at vents outside of the caldera. Four separate eruptive episodes were reported during 2018; from 3-4 April, 27 April-1 June, 13 July, and 15 September-1 November (BGVN 43:12, 43:09). Two episodes from 2019 during February-March and June are covered in this report, with information provided primarily by the Observatoire Volcanologique du Piton de la Fournaise (OVPF) as well as satellite instruments.

Piton de la Fournaise experienced two eruptions during November 2018-June 2019. The first lasted from 18 February to 10 March 2019, and the second episode was 11-13 June. The episode in February-March started consisted of multiple fissures opening on the E flank of the Dolomieu crater on 18 February with lava flows that traveled several hundred meters. After a brief pause, one new fissure opened nearby on 19 February and produced up to 3 million m³ of lava in a little over four days. Although the flow rate then declined, the eruption continued until 10 March. During the last three days, 7-10 March, two new fissures opened nearby and produced large volumes of lava, bringing the total eruptive volume to about 14.5 million m³. After little activity during April and May, a small eruption occurred on the SSE outer slope of Dolomieu crater that lasted for about 48 hours on 11-13 June; multiple small flows traveled about 1,000 m down the steep flank before ceasing. The MIROVA thermal anomaly graph of log radiative power clearly showed the abruptness of the beginning and ends of the last three eruptive episodes at Piton de la Fournaise from August 2018 through June 2019 (figure 165).

Figure 165. The MIROVA graph of thermal energy from Piton de la Fournaise from 30 July 2018 through June 2019 shows the last three eruptive episodes at the volcano. From 15 September through 1 November 2018 fissures and flows were active on the SW flank of Dolomieu crater near Rivals crater (BGVN 43:12). Fissures opened on the E flank of the crater on 18 February 2019, and after a brief pause resumed on 19 February at the foot of Piton Madoré. Lava flows remained active until 10 March 2019. A short episode of lava effusion occurred on 11-12 June 2019 on the SSE outer slope of Dolomieu crater. Courtesy of MIROVA.

Activity during November 2018-March 2019. Following the end of the 15 September-1 November 2018 eruption, seismic activity immediately below the summit remained low (with only 20 shallow and two deep earthquakes during November). The inflationary signal recorded since the beginning of September stopped, and the OVPF deformation networks did not record any significant deformation. There were 35 shallow earthquakes (0-2 km depth) below the summit crater during December, and one deep earthquake. Only 12 shallow earthquakes and one deep earthquake (greater than 2 km below the surface) were reported in January.

OVPF reported an increase in CO₂ concentrations beginning in December 2018, and noted the beginning of inflation on 13 February 2019. A seismic swarm of 379 earthquakes accompanied by minor but rapid deformation (less than 1 cm) was reported on 16 February 2019. A new seismic swarm of 208 earthquakes began early on 18 February with a much larger ground deformation (10 cm of elongation of the summit zone). A volcanic tremor indicative of the arrival of magma near the surface began at 0948 that morning. Webcams indicated that eruptive fissures had opened in the NE part of the Enclos Fouqué caldera. The onset of the eruption was marked by a sudden drop in CO₂ flux which then stabilized. The eruptive sites were confirmed visually around 1130. Three fissures with actively flowing lava opened on the E flank of Dolomieu Crater; the fountains of lava were less than 30 m high. The front of the longest flow had reached 1,900 m elevation after one hour. The eruption lasted a little over 12 hours and was over by 2200 that evening; it covered about 150-200 m of the hiking trail to the summit.

Seismicity remained high after the event ended, and at 1500 on 19 February 2019 another seismic swarm of 511 deep earthquakes located under the E flank at about 2.5 km depth occurred. It was not accompanied by a significant amount of deformation. At 1710 tremor signals appeared on the observatory seismographs and the first gas plumes and lava ejection were observed at 1750 and 1912, respectively. During an overflight the next day (20 February), OVPF team members observed the new eruptive site at an elevation of 1,800 m at the foot of Piton Madoré. One fissure and one fountain were active at 0620 on 20 February and the flow front was at 1,300 m elevation (figure 166). During the night of 20-21 February the flow front crossed over the "Grandes Pentes" area in the eastern half of the Enclos Fouque (figure 167).

Figure 166. The eruption which began on 19 February 2019 on the E flank of Dolomieu crater at Piton de la Fournaise produced a lava fountain and flow which traveled down at least 500 m of elevation by the next morning when this photo was taken at 0620 local time. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du mercredi 20 février 2019 à 11h00, Heure locale).

Figure 167. The active fissure at Piton de la Fournaise was producing lava fountains and an active flow during the evening of 20 February 2019. Overnight the flow crossed over the "Grandes Pentes" area of the caldera. Photo courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du jeudi 21 février 2019 à 14H00, Heure locale).

OVPF reported on 22 February 2019 that 22 shallow earthquakes had been reported since the eruption began on 19 February. Surface flow rates estimated from satellite data, via the HOTVOLC system (OPGC - University of Auvergne), were between 2.5 and 15 m³/s. The quantity of lava emitted between 19 and 22 February was between 1 and 3 million m³. OVPF observed the growth of an eruptive cone that was filled with a small lava lake producing ejecta during a morning overflight on 22 February. A channelized flow moved downstream from the cone and split into two lobes about 1 km from (and 200 m below) the cone (figure 168). The split in the flow occurred near the Guyanin crater. The N flowing lobe, about 50 m wide, had an actively flowing front located at 1,320 m elevation; the incandescent flow was travelling over a recent flow (likely from the previous night). The S-flowing lobe spread to 200 m wide and split into two tongues 300 m SE of Guyanin crater.

Figure 168. During an overflight on the morning of 22 February 2019 scientists from OVPF observed a growing spatter cone with a small lava lake at Piton de la Fournaise. A channelized flow moved downstream from the fissure and split into two flows. Photo courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du vendredi 22 février 2019 à 13h30, Heure locale).

Incandescent ejecta from the cone was captured in a webcam image overnight on 22-23 February 2019 (figure 169). The rate of advance of the flow slowed significantly by 24 February, but the intensity of the eruptive tremor remained relatively constant. Mapping of the lava flow on 28 February carried out by the OI2 platform (OPGC - University Clermont Auvergne) from satellite data confirmed the slow progress of the flow after 24 February (300 m in 5 days) (figure 170). The flow front was located at 1,200 m elevation, and only the N arm was active; the lava had traveled about 2.2 km from the vent by 28 February.

Figure 169. Incandescent ejecta from the eruptive cone at Piton de la Fournaise was captured in the webcam in the early hours of 23 February 2019. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du samedi 23 février 2019 à 13h30, Heure locale).

Figure 170. Contours of the lava flows at Piton de la Fournaise from 18-28 February 2019 were determined from satellite data by the OI2 platform (Université Clermont Auvergne), dated 18 (red) and 19 (blue) February (top image); 20 (green), 21 (red), 22 (blue), 27 (turquoise), and 28 (pink) February (bottom image). Courtesy of and copyright by OVPF/IPGP. Top: Bulletin d'activité du vendredi 22 février 2019 à 13h30 (Heure locale); bottom: Bulletin d'activité du jeudi 28 février 2019 à 16h30 (Heure locale).

Between 28 February and 1 March 2019 a third lobe of lava appeared flowing NE from the vent on the N side of the new flow area; it split into two lobes sometime on 1 March. Very little new lava was recorded on the other lobes. By 4 March the flow rate estimated by satellite data was about 7.5 m³/s. During a site visit on the morning of 5 March OVPF scientists sampled the N lobe of the flow and bombs and tephra near the cone, and acquired infrared and visible images. They noted the continued growth of the cone which still had an open vent at the summit and a base 100 m in diameter. It was 25 m high with a 50-m-wide eruptive vent at the top (figure 171). High-temperature gas emissions and strong Strombolian activity issued from the vent. Steam emissions were present around the base of the cone, suggesting the presence of lava tunnels. A single lobe of lava flowed N from the cone.

Figure 171. The eruptive cone at Piton de la Fournaise on 5 March 2019 had a 100-m-diameter base, 25 m of vertical height, and 50-m-wide vent at the summit. Courtesy of and copyright by OVPF/IPGP, (Bulletin d'activité du mardi 5 mars 2019 à 17h30, Heure locale).

A new fissure that opened about 150 m from the main vent on the NW flank of Piton Madoré was first observed on the morning of 6 March (figure 172); OVPF concluded that it had opened late on 5 March. A small cone was forming and a new flow traveled N from the main eruptive site. At least six new emission points were noted the following morning (7 March) around the Piton Madoré. Poor weather prevented confirmation by aerial reconnaissance that day, but in a site visit on 8 March OVPF scientists determined that the new fissure from 5 March remained active; a small cone about 10 m high had two flow lobes on the W and N sides (figure 173). A fissure that opened on 7 March was located 300 m S of the 19 February vent and oriented E-W. It was very active on the morning of 8 March with two 50-m-high lava fountains (figure 174). Samples collected by OVPF indicated that the vents of 5 and 7 March produced lava of different compositions.

Figure 172. A new fissure that opened about 150 m from the main vent on the NW flank of Piton Madoré at Piton de la Fournaise was first observed on the morning of 6 March 2019; OVPF concluded that it had opened late on 5 March. A small cone was forming on the flank of an old one and a new flow traveled N from the main eruptive site. Courtesy of OVPF/IPGP, copyright by Helicopter Coral (Bulletin d'activité du jeudi 7 mars 2019 à 15h00 Heure locale).

Figure 173. The 5 March 2019 fissure at Piton de la Fournaise on the NW flank of Piton Madoré still had two active flow lobes emerging from it and heading N and W on 8 March 2019. Courtesy of and copyright by OVPF/IPGP (Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, March 2019).

Figure 174. A fissure that opened on 7 March 2019 at Piton de la Fournaise was located 300 m S of the 19 February vent and oriented E-W. It was very active on the morning of 8 March 2019 with two 50-m-high lava fountains. Courtesy of and copyright by OVPF/IPGP (Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, March 2019).

There was a strong increase in the eruptive tremor intensity on 7 March, related to the opening of the two new fissures on 5 and 7 March (figure 175). As a result, the surface flow estimates made from satellite data increased significantly to high values greater than 50 m³/s, with the average values on 7-8 March of around 20-25 m³/s. The increased flow rates resulted in the flows traveling much greater distances. By the morning of 9 March the active flow had reached 650-700 m above sea level. The flow front had traveled about 1 km in 24 hours. Strong seismicity had been increasing under the summit zone for the previous 48 hours. After a phase of very strong surface activity observed overnight on 9-10 March that included lava fountains 50-100 m high (figure 176), surface activity ceased around 0630 on 10 March, and seismic activity decreased significantly. OVPF noted that sudden increases in seismicity and flow rates near the end of an eruption have occurred at about half of the eruptions at Piton de la Fournaise in recent years. Lava volumes emitted on the surface between 18 February and 10 March 2019 were estimated at about 14.5 million m³ (figure 177).

Figure 175. An infrared view of the eruptive site on the E flank of Dolomieu crater at Piton de la Fournaise on 8 March 2019 clearly showed the original fissure from 19 February (bottom right of center), the fissure on Piton Madore that opened on 5 March (right) and the fissures that opened on 7 March (upper, right of center). The combined activity produced significant thermal and seismic activity at the volcano. Courtesy of and copyright by OVPF/IPGP (Bulletin d'activité du vendredi 8 mars 2019 à 17h00, Heure locale).

Figure 176. Lava fountains 50-100 m high were the result of very strong surface activity observed overnight on 9-10 March 2019 at Piton de la Fournaise. Surface activity ceased around 0630 on 10 March, and seismic activity decreased significantly. Photo taken on 9 March 2019 around midnight from the RN2. Courtesy of OVPF/IPGP, copyright by A. Finizola LGSR/IPGP (Bulletin d'activité du dimanche 10 mars 2019 à 19h30 Heure locale).

Figure 177. A sudden increase in the flow rate at the end of the 18 February-10 March 2019 eruption at Piton de la Fournaise was recorded by researchers at the Université Clermont Auvergne. OVPF noted this was typical of about half of the eruptions at Piton de la Fournaise. Courtesy of OVPF/IPGP, copyright by HOTVOLC, Université Clermont Auvergne (OVPF Monthly bulletin of the Piton de la Fournaise Volcanological Observatory, March 2019).

Significant SO₂ plumes were captured by the TROPOMI instrument on the Sentinel 5-P satellite throughout the 18 February-10 March eruption (figure 178). After the surface eruption ceased, shallow seismicity continued at a lower rate of about 12 earthquakes per day. The end of the eruption (7-10 March) was accompanied by a marked deflation, interpreted by OVPF as the rapid emptying of the magma reservoir. Following the end of the eruption, inflation resumed for the rest of March but then ceased. Seismicity continued at a lower level during April with an average of six shallow earthquakes per day.

Figure 178. Multiple days of high DU value SO2 plumes were recorded by the TROPOMI instrument on the Sentinel 5-P satellite during the 18 February-10 March 2019 eruption at Piton de la Fournaise. Top row: during 18, 21, and 22 February SO2 plumes drifted SE. Middle row: during 23, 24, and 25 February the wind direction changed from SE through S to SW and left a curling trail of SO2. Bottom row: 5, 7, and 8 March showed an increase in SO2 emissions that corresponded with increased seismicity and lava flow output before the eruption ceased.

Activity during May-June 2019. OVPF reported slight inflation near the summit beginning in early May, and an increase in CO₂ concentration in the soil near Plaine des Cafres and Plaine des Palmistes. Strong shallow seismicity reappeared on 27 May 2019 and recurred on 30 and 31 May. Two small seismic swarms were measured on 31 May in the early morning. A new seismic swarm beginning at 0603 on 11 June accompanied by rapid deformation suggested a new eruption was imminent. A tremor near the summit area was first noted at 0635 local time; the webcams indicated a plume of gas, but poor visibility prevented evidence of fresh lava. Around 0930 that morning OVPF confirmed that five fissures had opened on the outer SSE slope of Dolomieu crater at elevations ranging from 2480 to 2025 m (figure 179). The flow fronts were not visible due to weather. Lava fountains under 30 m in height and lava flows were present in the three lowest fissures. The flows traveled rapidly down the steep flank of the crater (figure 180).

Figure 179. Around 0930 on the morning of 11 June 2019 OVPF confirmed that five fissures had opened on the outer SSE slope of Dolomieu crater at Piton de la Fournaise at elevations ranging from 2480 to 2025 m. Courtesy of and copyright by OVPF-IPGP and Imazpress (Bulletin d'activité du mardi 11 juin 2019 à 11h00).

Figure 180. Thermal imaging of the 11-12 June 2019 eruptive site at Piton de la Fournaise showed multiple streams of lava traveling rapidly down the steep flank from several fissures on 11 June 2019. Courtesy of and copyright by OVPF-IPGP (Bulletin d'activité du mardi 11 juin 2019 à 11h00).

The intensity of the eruptive tremor decreased throughout the day, and by 1530 only the lowest elevation fissure was still active (figure 181). The next afternoon (12 June) images in the OVPF webcam located in Piton des Cascades indicated the flow front was at about 1,200-1,300 m elevation. Seismographs indicated that the eruption stopped around 1200 on 13 June. Poor weather obscured visibility of the flow activity. Seismic activity decreased following the eruption, but appeared to increase again beginning on 21 June, with 10 events detected on 30 June. SO₂ plumes were recorded in satellite data on 11 and 12 June 2019.

Figure 181. The intensity of the eruptive activity at Piton de la Fournaise on 11 June 2019 decreased throughout the day, and by 1530 only the lowest elevation fissure was still active. Courtesy of and copyright by OVPF-IPGP (Bulletin d'activité du mardi 11 juin 2019 à 17h45 Heure locale).

Geologic Background. The massive Piton de la Fournaise basaltic shield volcano on the French island of Réunion in the western Indian Ocean is one of the world's most active volcanoes. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three calderas formed at about 250,000, 65,000, and less than 5000 years ago by progressive eastward slumping of the volcano. Numerous pyroclastic cones dot the floor of the calderas and their outer flanks. Most historical eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest caldera, which is 8 km wide and breached to below sea level on the eastern side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures on the outer flanks of the caldera. The Piton de la Fournaise Volcano Observatory, one of several operated by the Institut de Physique du Globe de Paris, monitors this very active volcano.

Information Contacts: Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, 14 route nationale 3, 27 ème km, 97418 La Plaine des Cafres, La Réunion, France (URL: http://www.ipgp.fr/fr); 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/).

Heard Island, in the Southern Indian Ocean, includes the large Big Ben stratovolcano and the smaller, apparently inactive, Mt. Dixon. Because of the island's remoteness, satellites are the primary monitoring tool. Big Ben has been active intermittently since 1910, and was active during October 2017-September 2018 (BGVN 43:10). Activity continued during October 2018-March 2019.

Satellite photos using Sentinel Hub showed hotspots every month between October 2018 and March 2019. Because the area was frequently covered by a heavy cloud layer, most of the hotspot signals were partially obscured. Though thermal anomalies are usually seen at summit vents, on 18 October 2018 an anomaly was present about 300 m down the E flank. Similarly, on 1 January 2019, a weak anomaly beginning about 200 m down the NW flank was about 300 m long (figure 40).

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected three hotspots, two in October and one in early November 2018, all of low radiative power. There were no MODVOLC alert pixels during this period.

Figure 40. Sentinel-2 L1C image of Heard Island's Big Ben volcano on 1 January 2019 one summit hotspot and an elongated thermal anomaly to the NW. Scale bar (bottom right) is 200 m. The photo was taken in atmospheric penetration view (bands 12, 11, and 8A), courtesy of Sentinel Hub Playground.

Geologic Background. Heard Island on the Kerguelen Plateau in the southern Indian Ocean consists primarily of the emergent portion of two volcanic structures. The large glacier-covered composite basaltic-to-trachytic cone of Big Ben comprises most of the island, and the smaller Mt. Dixon volcano lies at the NW tip of the island across a narrow isthmus. Little is known about the structure of Big Ben volcano because of its extensive ice cover. The historically active Mawson Peak forms the island's 2745-m high point and lies within a 5-6 km wide caldera breached to the SW side of Big Ben. Small satellitic scoria cones are mostly located on the northern coast. Several subglacial eruptions have been reported in historical time at this isolated volcano, but observations are infrequent and additional activity may have occurred.

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

The ongoing eruption at Semeru has been characterized by numerous ash explosions and thermal anomalies, but activity apparently diminished in 2018 (BGVN 43:01 and 43:09); this decreased activity continued through at least February 2019. The current report summarizes activity from 24 August 2018 to 28 February 2019.

The Indonesian volcano monitoring agency, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), reported ongoing daily seismicity, dominated by explosion earthquakes and emission-related events from late November through February (figure 35). Ash plumes resulting in aviation advisories by the Darwin Volcanic Ash Advisory Centre (VAAC) were reported on 4, 6-7, and 19 September, and 12 October 2018. The next significant ash plume reported by the VAAC wasn't until 24 February 2019 (table 23).

Figure 35. Seismicity recorded at Semeru during 28 November 2018-26 February 2019. Plot shows explosion earthquakes ('Letusan'), emission-related events ('Hembusan'), felt earthquakes ('Gempa Terasa'), local tectonic events ('Tektonic Lokal'), and distant tectonic events ('Tektonic Jauh'). Courtesy of PVMBG and MAGMA Indonesia.

Table 23. Summary of ash plumes at Semeru during 25 August 2018 through February 2019. The summit is at 3,657 m elevation. Data courtesy of Darwin VAAC.

Thermal anomalies using MODIS satellite instruments processed by the MODVOLC algorithm were only recorded on 26, 28, and 30 August 2018, and 22 and 31 October 2018. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected numerous hotspots within 5 km of the volcano during August and early September, with a significant decrease in frequency through October (figure 36); only a few scattered hotspots were recorded from November 2018 through February 2019.

Figure 36. MIROVA plot of thermal anomalies (Log Radiative Power) at Semeru during July 2018-February 2019. Courtesy of MIROVA.

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: 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/); 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/); 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/).

The eruption at Dukono that began in 1933 has showered the area with ash from frequent explosions (BGVN 43:04, 43:12). The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), also known as the Center for Volcanology and Geological Hazard Mitigation (CVGHM), is responsible for monitoring this volcano.

This long-term pattern of intermittent ash explosions continued during October 2018-March 2019, with ash plumes rising to between 1.5 and 2.7 km altitude, or about 300-1,500 m above the summit (table 19). Although meteorological clouds often obscured views, satellite imagery captured typical ash plumes on 28 September 2018 (figure 10) and 5 February 2019 (figure 11). Instruments aboard NASA satellites (TROPOMI and OMPS) detected high levels of sulfur dioxide near or directly above the volcano on multiple days during January-March 2019. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to remain outside of the 2-km exclusion zone.

Table 19. Monthly summary of reported ash plumes from Dukono for October 2018-March 2019. The direction of drift for the ash plume through each month was highly variable. Data courtesy of the Darwin VAAC and PVMBG.

Figure 10. Satellite image from Sentinel-2 (LC1 natural color) of an ash plume at Dukono on 28 September 2018 with the plume blowing towards the NE. Courtesy of Sentinel Hub Playground.

Figure 11. Satellite image from Sentinel-2 (LC1 natural color) of an ash plume at Dukono on 5 February 2019, with the plume blowing SW. Courtesy of Sentinel Hub Playground.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, occurred from 1933 until at least the mid-1990s, when routine observations were curtailed. During a major eruption in 1550, a lava flow filled in the strait between Halmahera and the north-flank cone of Gunung Mamuya. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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).

Intermittent small phreatic explosions from the acid lake of Rincón de la Vieja's active crater has most recently occurred since 2011 (BGVN 42:08, 43:03, and 43:09). This activity continued through at least February 2019. The volcano is monitored by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), and the information below comes from its weekly bulletins between 18 August 2018 and 28 February 2019. Weather conditions often prevented webcam views and estimates of plume heights. The volcano was in Activity Level 3 throughout the reporting period (volcano erupting, steady state).

According to OVSICORI-UNA, two distinct, 2-minute-long explosions occurred on 31 August 2018 beginning at 0434 and 1305. Several hours after the eruption tremor became continuous but low-frequency long-period (LP) earthquakes ceased. OVSICORI-UNA reported a gas emission late on 7 September. An unconfirmed small phreatic explosion occurred on 11 September at 0634, and another on 17 September at 1014. The seismic record showed continuous background tremor and very sporadic LP earthquakes.

Intermittent background tremor was recorded during the first half of October, along with a few emissions and phreatic explosions. Deformation measurements during October showed a contraction between the N and S of the volcano, with subsidence. On 17 October there was another phreatic explosion, and thereafter tremor disappeared and seismicity decreased. On 23 and 27 October seismic stations signaled additional possible phreatic explosions.

OVSICORI-UNA reported that a series of explosions began at 1945 on 4 November and consisted of at least three 2-minute-long episodes. The next day at 1511 a plume of water vapor and diffuse gas, recorded by a webcam and visible to residents to the N, rose about 100 m above the crater rim and drifted W. On 9 November a 2-minute-long explosion began at 1703. Another explosion on 27 November at 0237 produced a plume of water vapor and gas that rose 600 m above the crater rim and drifted SW. A short 1-minute explosion began at 1054 on 3 December.

Based on OVSICORI-UNA weekly bulletins, activity remained stable in January 2019 with small-amplitude phreatic explosions on 11, 12, and 14 January. More energetic phreatomagmatic explosions on 17 and 20 January produced lahars. Several small-amplitude explosions were detected at the end of the month. During January, a few LPs, no VTs, and intermittent tremor were recorded.

OVSICORI-UNA reported that two small-scale explosions occurred on 1 February, along with possible events at 1906 and 1950 on 5 February and at 0120 on 6 February. An event at 0000 on 6 February was also recorded; the report noted that poor weather conditions prevented visual observations of the crater. On 16 and 17 February strong degassing was observed. No LPs were recorded, but two significant VTs were detected on 17 and 22 February near or under the crater.

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 that was 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 1916-m-high 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 3500 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: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/).

This report summarizes activity at Turrialba during September 2018-February 2019. During this period there was similar activity as described earlier in 2018 (BGVN 43:09), with occasional ash explosions and numerous, sometimes continuous, periods of gas-and-ash emissions (table 8). Data were provided by the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Table 8. Ash emissions at Turrialba, September 2018-February 2019. Cloudy weather sometimes obscured observations. Maximum plume height is above the crater rim. Information courtesy of OVSICORI-UNA.

According to OVSICORI-UNA's annual summary for 2018, a slow decline in activity occurred after the volcano reached its highest emission rate during 2016. Activity during 2018 was consistent with an open system, generating frequent passive ash emissions. The volcano emitted ash on 58% of the days during the year. Some explosions were large enough to eject ballistics more than 400 m around the crater. Typical activity can be seen in a photo from 11 September 2018 (figure 50) and satellite imagery on 7 November 2018 (figure 51).

Figure 50. Photo of an ash explosion at Turrialba taken on 11 September 2018. Courtesy of Red Sismologica Nacional (RSN: UCR-ICE), Universidad de Costa Rica.

Figure 51. Sentinel-2 satellite image of an ash emission from Turrialba on 7 November 2018, taken in natural color (gamma adjusted). Courtesy of Sentinel Hub Playground.

During January into early February 2019, passive ash emissions continued irregularly and with less intensity and duration. Emissions sometimes lacked ash. In their report of 4 February 2019, OVSICORI-UNA indicated that passive ash emissions were weak and slow. For the rest of February, they characterized ash emissions as frequent, but of low intensity.

Seismic activity. On 1 November 2018 OVSICORI-UNA reported that seismicity remained high, and involved low-amplitude banded volcanic tremor along with long-period (LP) and volcano-tectonic (VT) earthquakes. In late January-early February 2019, OVSICORI-UNA reported that seismicity remained relatively stable, although a small increase was associated with the hydrothermal system. VT earthquakes were absent, and tremors had decreased in both energy and duration. The number of low-frequency LP volcanic earthquakes remained stable, although they had decreasing amplitudes. No explosions were documented, and emissions were weak and had short durations and very dilute ash content.

Thermal anomalies. No thermal anomalies were recorded during the reporting period using MODIS satellite instruments processed by MODVOLC algorithm. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected five scattered hotspots during September-October 2018, none during November-December 2018, and two during January-February 2019. All were within 2 km of the volcano and of low radiative power.

Gas measurements. Significant sulfur dioxide levels near the volcano were recorded by NASA's satellite-borne ozone instruments only on 29 September 2018 (both NPP/OMPS and Aura/OMI instruments) and on 11 February 2019 (Sentinel 5P/TROPOMI instrument). OVSICORI-UNA's gas measuring instruments were compromised in September 2018 through January 2019 due to vandalism. In early February, however, they detected hydrogen sulfide for the first time since 2016.

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

Information Contacts: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Red Sismologica Nacional (RSN) a collaboration between a) the Sección de Sismología, Vulcanología y Exploración Geofísica de la Escuela Centroamericana de Geología de la Universidad de Costa Rica (UCR), and b) the Área de Amenazas y Auscultación Sismológica y Volcánica del Instituto Costarricense de Electricidad (ICE), Costa Rica (URL: https://rsn.ucr.ac.cr/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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://hotspot.higp.hawaii.edu/); 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/); Costa Rica Star (URL: https://news.co.cr); The Tico Times (URL: https://ticotimes.net).

San Cristóbal has produced occasional weak explosions since 1999, with intermittent gas-and-ash emissions. The only reported explosion during the first half of 2018 was on 22 April, the first since November 2017 (BGVN 43:03). The current report covers activity between 1 August 2018 and 1 May 2019. The volcano is monitored by the Instituto Nicaragüense de Estudios Territoriales (INETER).

According to INETER, a series of explosions occurred on 9 January 2019 that lasted several hours. INETER stated that one explosion occurred at 1643; the Washington VAAC's first advisory stated that an explosion occurred at 1145 (local time). The weak explosions, which occurred after a period of heightened seismic activity, generated an ash plume that reached 200 m above the edge of the crater and drifted W. The Washington VAAC reported volcanic ash plumes on 10-11 January extending about 92 km SW, and on 24-25 January extending about 185 km WSW. A low-energy explosion was detected by the seismic network at 1550 on 4 March 2019. The event produced a gas-and-ash plume that rose 400 m above the crater rim and drifted SW.

Monitoring data reported by INETER (table 6) showed elevated levels of seismicity during October 2018 through January 2019. Sulfur dioxide was also measured at higher levels in January 2019.

Table 6. Monthly sulfur dioxide measurements and seismicity reported at San Cristóbal during August 2018-March 2019. "Most" indicates that type of seismicity was dominant that month. Data courtesy of INETER.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://webserver2.ineter.gob.ni/vol/dep-vol.html); 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).

The remote Semisopochnoi comprises the uninhabited volcanic island of the same name, ~20 km in diameter, in the Rat Islands group of the western Aleutians (figure 1). Plumes had been reported several times in the 18th and 19th centuries, and most recently observed in April 1987 from Sugarloaf Peak (SEAN 12:04). The volcano is dominated by an 8-km diameter caldera that contains a small lake (Fenner Lake) and a number of post-caldera cones and craters. Monitoring is done by the Alaska Volcano Observatory (AVO) using an on-island seismic network along with satellite observations and lightning sensors. An infrasound array on Adak Island, about 200 km E, may detect explosive emissions with a 13 minute delay if atmospheric conditions permit.

On 16 September 2018 increased seismicity was detected at 0831, prompting AVO to raise the Aviation Color Code (ACC) to Yellow and Volcano Alert Level (VAL) to Advisory. Retrospective analysis of satellite data acquired on 10 September revealed small ash deposits on the N flank of Mount Cerberus, possibly associated with two bursts of tremor recorded on 8 September (figure 5). This new information, coupled with intensifying seismicity and a strong tremor signal recorded at 1249 on 17 September, resulted in AVO raising the ACC to Orange and the VAL to Watch. Seismicity remained elevated on 18 September with nearly constant tremor recorded by local sensors. At the same time, no ash emissions were observed in cloudy satellite images and no eruptive activity was recorded on regional pressure sensors at Adak.

Figure 1. Minor ash deposits can be seen on the south and west flanks of the N cone of Mount Cerberus, Semisopochnoi Island, in this ESA Sentinel-2 image from 1200 on 10 September 2018. Also note probable minor steam emissions obscuring the crater of the N cone. Image courtesy of AVO.

During 19-25 September 2018 seismicity remained elevated, alternating between periods of continuous and intermittent bursts of tremor. Tremor bursts at 1319 on 21 September and at 1034 on 22 September produced airwaves detected on a regional infrasound array on Adak Island; no ash emissions were identified above the low cloud deck in satellite data, and the infrasound detections likely reflected an atmospheric change instead of volcanic activity.

Seismicity remained elevated during 3-9 October 2018, with intermittent bursts of tremor. No volcanic activity was detected in infrasound or satellite data. On 11 October satellite data indicated partial erosion of a tephra cone in the crater of Cerberus's N cone. A crater lake about 90 m in diameter filled the vent. The data also suggested that the vent had not erupted since 1 October. Seismicity remained elevated and above background levels. The next day AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory, noting the recent satellite data results and lack of tremor recorded during the previous week. AVO reported that unrest continued during 11-24 October.

An eruptive event began at 2047 on 25 October 2018, identified based on seismic data; strong volcanic tremor lasted about 20 minutes and was followed by 40 minutes of weak tremor pulses. A weak infrasound signal was detected by instruments on Adak Island. The Aviation Color Code was raised to Orange (the second highest level on a four-color scale) and Volcano Alert Level was raised to Watch (the second highest level on a four-level scale). A dense meteorological cloud deck prevented observations below 3 km, but a diffuse cloud was observed in satellite data rising briefly above the cloud deck, though it was unclear if it was related to eruptive activity. Tremor ended after the event, and seismicity returned to low levels.

Small explosions were detected by the seismic network at 2110 and 2246 on 26 October 2018, and 0057 and 0603 on 27 October. No ash clouds were identified in satellite data, but the volcano was obscured by high meteorological clouds. Additional small explosions were detected in seismic and infrasound data during 28-29 October; no ash clouds were observed in partly-cloudy-to-cloudy satellite images.

AVO reported on 31 October 2018 that unrest continued. Two small explosions were detected, one just before 0400 and the other around 1000. Satellite views were obscured by clouds at the time, and no ash clouds were observed. Unrest continued through 1 November, at which time the satellite link and the seismic line failed. On 21 November the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is 1221-m-high Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked 774-m-high Mount Cerberus volcano was constructed during the Holocene within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side. Other post-caldera volcanoes include the symmetrical 855-m-high Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time.

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/).

Japan's 24-km-wide Asosan caldera on the island of Kyushu has been active throughout the Holocene. Nakadake has been the most active of 17 central cones within the caldera for 2,000 years. Historical eruptions have been primarily basaltic to basaltic-andesitic ash eruptions, with periodic Strombolian activity, all from Nakadake Crater 1. The most recent major eruptive episode began in late November 2014 and continued through 1 May 2016. Another eruption, with the largest ash plume in 20 years, occurred on 8 October 2016. Asosan remained quiet until renewed activity from Crater 1 began in mid-April 2019; it is covered in this report, through the end of June 2019. The Japan Meteorological Agency (JMA) provides monthly reports of activity; the Tokyo Volcanic Ash Advisory Center (VAAC) issues aviation alerts reporting on possible ash plumes.

Asosan remained quiet during 2017 and 2018 with steam plumes rising a few hundred meters from Crater 1 and low levels of SO₂ emissions; a warm acidic lake was present within the crater. Fumarolic activity from two areas on the S and SW wall of the crater rim generated occasional thermal anomalies in satellite data and incandescence at night. A brief period of increased seismicity was reported in mid-March 2019. An increase in seismic amplitude on 14 April 2019 preceded a small explosion on 16 April; it produced an ash plume which rose 200 m above the crater rim and drifted NW. It was followed by additional small explosions on 19 April. A new explosion on 3 May produced minor ashfall in adjacent communities; ash emissions were reported multiple times during May with plumes reaching 1,400 m above the crater rim. No additional ash emissions were reported in June.

Activity during 2017 and 2018. JMA reported that no eruptions occurred during 2017. Amplitudes of volcanic tremor increased somewhat during March but were generally low for the rest of the year. The earthquake hypocenters were mostly located near the active crater at around sea level. SO2 emissions were slightly less than 1,000 tons per day (t/d) from January through April; for the rest of the year they ranged from 600 to 2,500 t/d. The Alert Level had been lowered from 2 to 1 on 7 February 2017 where it remained throughout the year. Steam plumes generally rose no more than 600 m above the active crater rim (figure 42). JMA noted that from January to June they often observed crater incandescence at night with a high-sensitivity surveillance camera; Sentinel-2 satellite images also captured thermal anomalies a few times (figure 43). The green lake inside the crater persisted throughout the year with water temperatures of 50-60°C. Two fumaroles were present with high-temperature gas emissions on the SW and S crater walls. Temperatures at the S crater wall were over 600°C from February to May; they decreased to 320-560°C during the rest of the year (figure 44). Sulfur deposits were visible around the SW crater wall fumarole during July.

Figure 42. Steam plumes that rose around 600 m above Nakadake Crater 1 at Asosan were typical activity throughout 2017. Images taken with JMA webcam on 9 June (top left), 22 August (top right), 12 November (bottom left), and 20 December (bottom right) 2017. Courtesy of JMA (Aso volcano monthly activity reports, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 43. Sentinel-2 images captured thermal anomalies at the S rim of the green lake at Asosan's Nakadake Crater 1 on 16 February (left) and 27 May 2017 (right). JMA reported that incandescence was occasionally visible during the night from January-June from the same area. Courtesy of Sentinel Hub Playground.

Figure 44. High-temperature gas and steam from fumaroles on the S wall of the Nakadake Crater 1 at Asosan on 24 August (top) and 17 November 2017 (bottom) were persistent all year, with temperatures ranging from 300 to over 600°C. The green lake inside the crater persisted throughout the year as well with water temperatures of 50-60°C. Courtesy of JMA (Aso volcano monthly activity reports, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

The Alert Level did not change at Asosan during 2018, and no eruptions were reported. Sulfur dioxide emissions fluctuated between 400 and 1,800 t/d throughout the year. Steam plumes generally rose less than 500 m above the active crater (figure 45); incandescence was observed at night during May-October and sometimes observed in satellite imagery as thermal anomalies (figure 46). The temperature of the green lake inside the crater ranged from 58 to 75°C throughout the year. The thermal anomaly on the S wall of the crater was consistently in the 300-500°C range, and had a high temperature in April of 580°C; in December the high temperature had risen to 738°C (figure 47). A brief increase in the number of isolated tremors occurred during March, with 1,044 reported on 4 March, exceeding the previous maximum of 1,000 on 27 October 2014. Seismicity also increased briefly during June, with more than 400 events reported each day on 8, 18, and 20 June. The Minami Aso village Yoshioka fumarole zone, located about 5 km W of Nakadake Crater 1, continued to produce modest steam plumes throughout 2017 and 2018 (figure 48).

Figure 45. Typical steam plumes at Asosan during 2018 rose around 500 m above the Nakadake Crater 1. Images are from 4 March (top left), 22 July (top right), 17 August (lower left), and 13 September 2018 (lower right). Courtesy of JMA (Aso volcano monthly activity reports, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 46. Nighttime incandescence was reported by JMA during May-October 2018 from the S rim of Nakadake Crater 1 at Asosan; Sentinel-2 satellite images (bands 12, 4, 2) captured thermal anomalies from the same area numerous times during 2018 including on 16 June (top left), 26 July and 19 September (middle row), and 18 and 23 November (bottom row). JMA photographed incandescence at night on 17 July 2018 at the S fumarole area (top right). Courtesy of Sentinel Hub Playground and JMA (Aso volcano Monthly Report for July 2018).

Figure 47. The "Green Tea Pond" inside Nakadake Crater 1 at Asosan had temperatures that ranged from 58 to 75°C during 2018 (top row, 26 March 2018); the thermal anomaly on the S wall of the crater consistently had temperatures measured in the 300-500°C range and the SW fumarole area had somewhat lower temperatures (bottom row, 22 June 2018). Courtesy of JMA (monthly Asosan reports for March, May, and June 2018).

Figure 48. The Minami Aso village Yoshioka fumarole zone, located about 5 km W of Nakadake Crater 1 at Asosan, continued to produce modest steam plumes throughout 2017 and 2018. It is shown here on 20 December 2017 (top) and 12 March 2018 (bottom). Courtesy of JMA (December 2017 and March 2018 monthly volcano reports).

Activity during 2019. Steam plumes rose to 800 m above the crater rim during January 2019. Overall activity increased slightly during February; SO₂ emissions peaked at 2,200 t/d early in the month; they ranged from 800 to 1,800 t/d for most of the month. The amplitude of volcanic tremor also increased slightly during February. A further increase in tremor amplitude on 11 March 2019 prompted JMA to raise the Alert Level from 1 to 2 the following morning. Volcanic tremor amplitude decreased on 15 March; JMA determined that activity had decreased, and the Alert Level was lowered back to 1 on 29 March 2019. The amount of water in the crater decreased significantly between 27 February and 20 March, exposing part of the crater floor.

The surface temperature of the lake rose during the first part of 2019; it was 78°C in February and 84°C in March. Steam plumes rose to 1,200 m above the crater rim during March and April. SO₂ emissions rose to 4,500 t/d on 12 March but dropped to a lower range of 1,300-2,400 for the rest of the month. Another surge in SO₂ emissions on 12 April 2019 to 3,600 t/d prompted a special report from JMA the following day. SO₂ emissions varied from about 1,700 to 4,100 t/d during the month; values remained high during the second half of the month. JMA noted that the color of the water in the lake inside Nakadake Crater 1 changed from green to gray after 4 April. Fountains of muddy water were periodically observed; they reached 15 m high on 9 April. The temperatures of both the lake (82°C) and around the two fumarole areas (S area about 530°C, SW area about 310°C) remained constant during April and similar to March.

A large increase in the amplitude of volcanic tremor early on 14 April 2019 prompted JMA to raise the Alert Level from 1 to 2 later in the day. The epicenters of the earthquakes were very shallow, located within 1 km beneath the crater. A small eruption occurred at 1828 on 16 April at Nakadake Crater 1; it produced a gray and white plume that rose 200 m above the crater rim and was the first eruption since 8 October 2016 (figure 49). Incandescence was observed inside the crater on 3 and 17 April. The amplitude of seismic tremors decreased on 18 April. Three very small eruptions on 19 April produced ash and steam plumes that rose 500 m above the crater rim. During a site visit that day JMA measured a high-temperature area that produced incandescence from the bottom of the crater at night (figure 50).

Figure 49. The first eruption since October 2016 at Nakadake Crater 1 at Asosan on 16 April 2019 sent an ash plume 200 m above the crater rim (top). Incandescent gas appeared on the crater floor the next day (bottom). Courtesy of JMA (Aso volcano monthly activity reports, April 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 50. Three small explosions on 19 April 2019 at Asosan's Nakadake Crater 1 produced small ash emissions that rose 500 m above the crater rim (top). A strong thermal signal also appeared from the bottom of the crater. Courtesy of JMA (Aso volcano monthly activity reports, April 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

A new eruption began at 1540 on 3 May that lasted until 0620 on 5 May (figure 51). Initially the ash plume rose 600 m above the crater rim, but a few hours later the volume of ash increased, and the plume reached 2 km above the crater rim for a brief period. Incandescence was visible from the webcam. The Tokyo VAAC reported the ash plume at 3 km altitude drifting SE on 3 May. Later in the day it rose to 3.7 km altitude and drifted SW. During a field survey the following day (4 May) JMA reported a steam and ash plume rising from the center of the active crater. The infrared thermal imaging camera recorded the temperature of the plume at about 500°C (figure 52).

Figure 51. An explosion at Asosan's Nakadake Crater 1 on 3 May 2019 produced an ash plume that reached 2 km above the crater rim (top) and incandescence visible from the webcam (bottom). Courtesy of JMA (Aso volcano monthly activity reports, April 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 52. During a site visit on 4 May 2019, staff from JMA witnessed an ash and steam plume rising from the bottom of Nakadake Crater 1 at Asosan (top). The infrared thermal imaging camera recorded the temperature of the plume at about 500°C (bottom). Courtesy of JMA (Aso volcano monthly activity reports, May 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Ash fell on the S flank, and a small amount of ashfall on 4 May was confirmed by evidence on a car windshield in Takamori Town (6 km S), Kumamoto Prefecture (figure 53). Ashfall was also reported in Takamori-machi, Minami Aso village (9 km SW), and part of Yamato-cho (25 km SW), also in the Kumamoto Prefecture. SO₂ emissions were measured as high as 4,000 t/d on 4 May. Additional explosions with ash plumes were reported from Asosan on 9, 12-16, 29, and 31 May; the plumes rose from 200 to 1,400 m above the crater rim but were not visible in satellite imagery. The TROPOMI instrument on the Sentinel-5 satellite captured SO₂ plumes on 3 and 26 May 2019 (figure 54).

Figure 53. Ashfall was reported on 4 May 2019 in Takamori Town, Kumamoto Prefecture, from the eruption at Asosan's Nakadake Crater 1 on 3 May 2019. Courtesy of JMA (Aso volcano monthly activity reports, May 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 54. Plumes of SO2 from Asosan were recorded by the TROPOMI instrument on the Sentinel-5P satellite on 3 (left) and 26 (right) May 2019. Courtesy of NASA Goddard Space Flight Center.

Steam plumes rose to 1,700 m above the crater rim during June 2019 (figure 55). During field visits on 6 and 25 June diffuse ash emissions were observed rising from the center of the active crater, but they did not extend significantly above the crater rim (figure 56). The maximum temperature of the plume was measured at about 340°C with a thermal imaging camera. Almost all of the water in the crater bottom had evaporated since early May; incandescence continued to be observed within the crater at night with the high-resolution webcam (figure 57).

Figure 55. Steam plumes rose to 1,700 m above the crater rim at Asosan's Nakadake Crater 1 on 10 June 2019. Courtesy of JMA (Aso volcano monthly activity reports, June 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 56. Plumes of gas and minor ash were visible at Asosan's Nakadake Crater 1 during site visits by JMA on 6 (left) and 25 (right) June 2019. Courtesy of JMA (Aso volcano monthly activity reports, June 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

Figure 57. Incandescent gas was visible from the vent at Asosan's Nakadake Crater 1 on 18 (left) and 25 (right) June 2019. Courtesy of JMA (Aso volcano monthly activity reports, June 2019, Fukuoka District Meteorological Observatory, Regional volcano monitoring and warning center).

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

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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/).

The previous report on Chaitén volcano, Chile (BGVN 35:12) noted that a second new lava dome was reported on 4 November 2008. The Alert Level remained at Red (the highest of the alert level system) from the onset of the eruption on 2 May 2008 through April 2010. This report will chronologically summarize the growth of the new lava domes and major eruptive events. In general, a gradual decline in activity occurred during November 2008-September 2011. The Alert Level stood at Yellow during May 2010-May 2011, and was then lowered to Green, where it remained through the end of 2011. Most of the information in this report comes from weekly to monthly reports from the Observatorio Volcanológico de los Andes del Sur-Servicio Nacional de Geología y Minería (OVDAS-SERNAGEOMIN) with collaboration from the Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI). Finally, the current status of the town of Chaitén (which was abandoned by all but a handful of residents who continued to live there without electricity or running water) will be discussed. The potential hazards from dome-collapse-generated block-and-ash flows and lahars from remobilized volcanic deposits were persistent throughout the reporting period.

Portions of this report were initially synthesized and edited by Nick Legg (covering November 2008-March 2009) and Eduardo Guerrero (covering April 2009-July 2009), as part of a graduate student writing assignment in a volcanology class at Oregon State University under the guidance of professor Shan de Silva.

New lava domes observed. Initially occupying the caldera of Chaitén was the lava dome ('old dome') that existed prior to the 2 May 2008 eruption (and for the ~ 9,400 years since the previous eruption). A lava dome ('Dome 1') first observed on 21 May 2008 and a new lava dome ('Dome 2') confirmed on 4 November 2008 during an overflight (BGVN 35:12) were extruded on top of the old dome (figures 21 and 22). A third phase of dome extrusion ('Dome 3') was observed on 29 September 2009.

Figure 21. December 2008 aerial photograph of the Chaitén dome complex taken looking approximately SE into the caldera. The domes were emplaced on top of the old dome, emplaced ~9,400 years ago. Courtesy of OVDAS-SERNAGEOMIN.

Figure 22. Satellite photograph of Chaitén and surroundings, acquired on 30 September 2009. (a) Chaitén volcano (top right corner), Chaitén (Blanco) River, and their proximity to the town of Chaitén (bottom left). Note the significant lahar deposits at the mouth of the Chaitén river and cutting through the town of Chaitén. Inset index map shows Chaitén's location in Southern Chile. (b) Enlarged, annotated view of Chaitén volcano. Dome 1 is in the W side of the caldera and Dome 2 is in the NNE part. Dome 3 is not visible in this photograph. Satellite photograph courtesy of NASA Earth Observatory; index map modified from MapsOf.net.

November 2008-April 2010 (Red Alert). Following the confirmation of the existence of Dome 2 on 4 November 2008, a lateral explosion occurred on 17 November, directed WSW. The explosion was not constrained to either Dome 1 or 2 alone, but was associated with a collapse resulting from continued dome extrusion. By 6 December 2008, both active domes had exceed the height of the caldera rim (Dome 1 by ~ 250 m and Dome 2 by ~ 350 m). SERNAGEOMIN reported an increase in the growth of Dome 2, corresponding to a temporally slight increase in both hybrid (HB) and volcano-tectonic (VT) earthquakes.

Plumes continued to vary in color, indicating varying contributions of steam and ash from both new domes. By 9 January 2009, an observational overflight revealed that the inner caldera had been filled by Domes 1 and 2, and growth of sharp spines or pinnacles was reported.

On 19 January, a major collapse of the Dome 2 summit spines occurred, producing block-and-ash flows that traveled down the SE and E flanks of Chaitén. After the collapse, observers noted a decrease in seismicity and slowed dome extrusion.

During a flyby facilitated by ONEMI and the Chilean Air Force on 21 January 2009, researchers from the University at Buffalo-State University of New York (UB-SUNY) and the University of Chile acquired thermal images of the dome complex (figure 23). The thermal images indicated that, while Dome 1 still had areas of elevated temperature, the highest temperatures (greater than 150°C) corresponded to the pinnacles at the summit of Dome 2 (figure 23a). Images of pyroclastic flow and rockfall deposits resting within the E part of the caldera disclosed elevated temperatures there as well (higher than 80°C in places, figure 23b). Their research on thermal imaging of Chaitén will be showcased in an upcoming publication.

Figure 23. Thermal images of the dome complex of Chaitén acquired on 21 January 2009. (a) The summit of Dome 2, where the highest temperatures were recorded; the same area corresponded to the pinnacles and the area of the most rapid growth of Dome 2. (b) Pyroclastic flow and rockfall deposits within the E part of the caldera, emplaced just prior to 21 January, that were still elevated in temperature. The colors are scaled uniquely for both images with the temperature scales shown at right. The flyby was facilitated by ONEMI and the Chilean Air Force, the images were acquired by Patrick Whelley (UB) and Andrés Paves (University of Chile), and processing was performed by Marc Bernstein (UB); use of the thermal camera was courtesy of Eliza Calder under the UB-SUNY Research Foundation.

By 9 February 2009, a new high-standing pinnacle atop Dome 2 indicated the return of rapid dome growth. This did not coincide with significant increases in seismicity.

On 19 February, a large partial collapse generated pyroclastic flows that traveled down the Chaitén River towards the town of Chaitén (see annotated photo below). This event produced a plume reaching 9.1 km altitude according to the Buenos Aires Volcanic Ash Advisory Center (VAAC). The plume was white (indicating high water vapor content) at the top, and contained abundant ash at the base. The collapse created a scarp measuring approximately 500 m by 500 m. Increased seismicity occurred on the same day; background volcanic tremor occurred from 1028 until 1346, and swarms of earthquakes ranging from M 3.6-4.2 originated at depths of 3-5 km. Due to the amount of material deposited from the collapse (~ 10 x 10⁶ m³), SERNAGEOMIN reported a significantly higher than normal danger of lahars.

On 25 February 2009, dome growth focused on Dome 1, although seismicity had gradually decreased in frequency (excluding the outstanding events of mid-February discussed above) compared to November-December 2008 values. Despite the decrease, on the afternoon of 3 March, dome collapses occurred every ~ 40 minutes. Within the next few days, SERNAGEOMIN reported that "VT's almost disappeared." A gradual decrease in seismicity continued until 24 March, when the frequency of HB earthquakes increased briefly, but they decreased again by early April.

On 6 April, a prominent spine of lava was observed on the S part of Dome 1 (figure 24). It indicated that dome growth was still concentrated in the W part of the caldera (Dome 1). The next week, the same spine was reported to have a wider base, and the crater reportedly glowed at night. In early May, an aerial photograph captured a close view of a very fractured central lava spine of the dome complex (figure 25).

Figure 24. Images taken by Dirección General de Aeronáutica Civil (DGAC) and a Channel 13 cameraman on 6 April 2009 showing a prominent spine of lava that had grown in the S area of Dome 1. Courtesy of OVDAS-SERNAGEOMIN.

Figure 25. Aerial view of the dome complex in early May 2009 showing a very fractured central pinnacle. Courtesy of Javier Romero.

In the later parts of May 2009, dome growth and seismicity continued to be focused on the W part of the caldera, associated with Dome 1. Although there was no significant increase in seismicity until a slight increase in June, witnesses in the town of Chaitén reporting feeling tremors in May. Otherwise, activity through August consisted of continuous ash-and-steam emissions, small collapses of unstable portions of the domes, and resulting block-and-ash flows. Seasonal precipitation remobilized previously erupted material and lahars reached the town in July.

Seismicity remained slightly elevated (relative to April and May) through September. On 14 September, a surveillance camera captured a plume as wide as the caldera reaching ~ 1.5 km high. This plume was significantly wider than plumes in the previous months. On 29 September, witness reports prompted SERNAGEOMIN to fly past the caldera, and obervers saw evidence of a significant recent collapse.

SERNAGEOMIN's 29 September observation flight provided stunning views of the dome complex leading to the detection of a new dome, Dome 3, that had filled the 19 February 2009 collapse scar (figures 26 and 27). Dome 3 had a depression in its N sector and small central pinnacles (figure 27). The central pinnacle of the whole complex had disappeared, and a large active depression, elongated to the NNW, had formed E of Dome 3. This depression reportedly resulted from either a lateral explosion or a relatively slow and structurally controlled internal collapse.

Figure 26. Aerial views of Chaitén's dome complex taken following the partial collapses of (a) 19 February and (b) 29 September 2009. (a) The dome complex following the 19 February partial col lapse. Dimensions of the central pinnacle (*) and the col lapse scarp (**) are given at the lower left. (b) The dome complex following the 29 Sep tem ber partial col lapse, showing the first ob servation of Dome 3. Labels in all capital letters indicate structures or deposits, and labels in all lowercase indicate relative amounts of water vapor and ash in emit ted plumes (>, greater than;

Figure 27. Aerial view of the center of Chaitén's dome complex on 29 September 2009. The then-newly ob served central depression is the most active area in the photograph, and a small new pinnacle, Proto-pinnacle 2, is seen near this area. Labels in all capital letters indicate structures or de pos its, and labels in all low er case indicate relative amounts of water vapor and ash in emitted plumes. Courtesy of OVDAS-SERNAGEOMIN; photo and interpretation by Jorge Muñoz.

Following the events of September, no major outstanding activity was reported through the rest of 2009. In late January 2010, two new telemetered seismic stations were added to the monitoring network, increasing the number of seismic stations around Chaitén to ten. A new observation camera was also installed ~ 800 m from the dome complex.

During the end of 2009 and January 2010, the growth rate of the dome complex slowed, and seismicity declined significantly, with larger earthquakes (stronger than M 3.5) being absent through at least March (figure 28). Following a few months of relatively calmer activity at Chaitén, and after at least a month without emissions of ash, the Alert Level was lowered to Yellow on 1 May 2010.

Figure 28. Daily earthquakes between M 3.5 and M 4.5 at Chaitén, from the onset of the eruption on 2 May 2008 to 1 April 2010 (a month prior to the lowering of the Alert Level to Yellow). Courtesy of OVDAS-SERNAGEOMIN.

1 May 2010-2 May 2011 (Yellow Alert). During the period of Yellow Alert, reported plume heights remained below 2.1 km altitude (Buenos Aires VAAC); compared to the prior, more active period when plumes regularly reached 3-4 km in height, this was a significant decline. Over this period, the Buenos Aires VAAC reported occasional emissions that included ash (table 2). OVDAS-SERNAGEOMIN began recording mainly VT events (interpreted as relating to rock fracturing) and long period (LP) events (interpreted as related to fluid dynamics in and beneath the volcanic edifice). Both types of seismicity remained low throughout the remainder of 2010 and into 2011. However, incandescence of the lava dome surface was observed at night in late January 2011.

Table 2. Emissions from Chaitén's lava dome complex during the period of Yellow Alert (1 May 2010-2 May 2011); '--' indicates information that was not reported. Courtesy of the Buenos Aires VAAC.

3 May 2011-September 2011 (Green Alert). On 2 May 2011, the Alert Level was lowered to Green due to lower levels of activity since January 2011, including (1) seismicity remaining at low levels of occurrence and magnitude; (2) no significant emissions of ash; (3) a lack of dome growth and associated partial collapses; and (4) a lack of visual observations suggesting restlessness.

Since 3 May 2011, ash-free plumes rose no higher than 0.5 km; the exception was one plume in late May or early June. Seismicity remained low, with daily counts averaging fewer than 2 for LP events, less than 10 for VT events, and less than 1 for HB events.

Magma ascent prior to 2 May 2008 eruption. Wicks and others (2011) stated that "Because of the historically rare and explosive nature of rhyolite eruptions and because of the surprisingly short warning before the eruption of the Chaitén volcano, any information about the workings of the magmatic system at Chaitén, and rhyolitic systems in general, is important from both the scientific and hazard perspectives." There were only about 24 hours between the first felt seismicity in the town of Chaitén and the onset of the eruption. Such a short precursory period has been recorded for basaltic eruptions (e.g. Hekla volcano, Iceland; Soosalu and Einarsson, 2002), but not for silicic eruptions such as Chaitén (Castro and Dingwell, 2009).

Castro and Dingwell (2009) used petrological experiments to constrain the temperatures and decompression rates of the magma erupted explosively at Chaitén on 2 May 2008. Their results suggested that the magma rose from depths of at least 5 km in about four hours, shorter than the roughly 1-day period of seismicity that was felt in the town of Chaitén.

Wicks and others (2011) interpreted radar interferometry observations to indicate that the rapid ascent (as reported by Castro and Dingwell, 2009) of the Chaitén magma was controlled by pre-existing faults in the crust beneath Chile (figure 29). Specifically, they modeled a large, dipping, sill-like body (their "reservoir") residing under Minchinmávida volcano (~ 20 km E of Chaitén, "M" on figure 29) and rising towards the surface to the W of Chaitén (Morro Vilcún, "MV" on figure 29). The magma followed a path that intersected an inferred vertical conduit feeding Chaitén. They interpreted the rhyolitic reservoir as originating from either 1) a combination of tectonic stresses and magmatic overpressure draining rhyolitic magma from a mafic reservoir beneath Minchinmávida, or 2) an event similar to the M 9.5, 1960 Chilean earthquake creating permeability and a pressure gradient, allowing the overpressured magma to migrate to beneath Chaitén.

Figure 29. Cross-sectional model of the magmatic system that ultimately erupted at Chaitén (Wicks and others, 2011). The profile A-A' is approximately W-E; Morro Vilcún (MV), Chaitén (Ch), and Minchinmávida (M) are plotted at their relative positions on the surface. Stages a, b, and c are illustrated by a series of model cartoons. (a) The sill-like body (Reservoir) extends to the W, towards the surface, from the magma chamber beneath Minchinmávida; the dike (red body) begins propagation upwards, but at this stage has not intersected Chaitén's conduit. (b) Diking leads to an injection-caused earthquake (inflational, shown by the moment tensor solution "beachball diagram" where black indicates compression and white indicates tension) which occurred 2 hours before the 2 May 2008 eruption began. (c) As the eruption progressed and drained the shallow storage beneath Chaitén, the sill-like reservoir collapsed (shown by the moment tensor solution diagram). In all frames, the Liquiñe-Ofqui Fault Zone (LOFZ) is indicated beneath Minchinmávida volcano. CMT 1 refers to the centroid mean solution (CMT) of a moment magnitude 5.2 earthquake that occurred 2 hours prior to the main eruption on 2 May 2008. CMT 2 refers to an earthquake of magnitude 5.0 that occurred 19 hours after the onset of eruption. From Wicks and others (2011).

Restoring the town and damaged infrastructure. On 9 April 2011, the Chilean government reported that President Sebastián Piñera had announced the "North Chaitén Solution" plan. After restoring basic services (e.g. electricity) to the town of Chaitén in the first months of 2011, President Piñera stated that "everything will be definitively restored, electricity and light has already returned and ... all of the electricity poles are new." He also announced plans to dredge the harbor a second time (necessary due to the amount of remobilized volcanic material deposited in the bay) to allow boats to dock, a plan to install a floating dock, ongoing surveying for new paved roads to other surrounding cities, and plans for the town's school and aerodome.

References. Castro, J.M., and Dingwell, D.B., 2009, Rapid ascent of rhyolitic magma at Chaitén volcano, Chile: Nature, v. 461, p. 780-784 (DOI:10.1038/nature08458).

Soosalu, H., and Einarsson, P., 2002, Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland: Bulletin of Volcanology, v. 63, p. 536-544.

Wicks, C., de la Llera, J.C., Lara., L.E., and Lowenstern, J., 2011, The role of dyking and fault control in the rapid onset of eruption at Chaitén volcano, Chile, Nature, v. 478, pp. 374-377 (DOI:10.1038/nature10541).

Geologic Background. Chaitén is a small, glacier-free caldera with a compound Holocene lava dome located 10 km NE of the town of Chaitén on the Gulf of Corcovado. Early work had identified only a single explosive eruption during the early Holocene prior to the major 2008 eruption, but later work has identified multiple explosive eruptions throughout the Holocene. A rhyolitic obsidian lava dome occupies much of the caldera floor. Obsidian cobbles from this dome found in the Blanco River are the source of prehistorical artifacts from archaeological sites along the Pacific coast as far as 400 km from the volcano to the N and S. The caldera is breached on the SW side by a river that drains to the bay of Chaitén. The first historical eruption, beginning in 2008, produced major rhyolitic explosive activity and growth of a lava dome that filled much of the caldera.

Information Contacts: Observatorio Volcanológico de los Andes del Sur-Servico Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Oficina Nacional de Emergencia - Ministerio del Interior (ONEMI), Beaucheff 1637 / 1671, Santiago, Chile (URL: http://www.onemi.cl/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); MapsOf.net (URL: http://mapsof.net/); Marc Bernstien, Eliza Calder, and Patrick Whelley, University at Buffalo, State University of New York, 411 Cooke Hall, Buffalo, NY 14260; Andrés Paves, University of Chile, 2002 Blanco Encalada, Santiago, Chile; Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php); Javier Romero, Dirección de Vialidad - Ministerio de Obras Públicas, Puerto Montt; Gobierno de Chile, Santiago, Chile (URL: http://www.gob.cl/ or http://www.gob.cl/english/).

Microseismicity preceded and accompanied a jökulhlaup (a glacier-outburst flood) on 9 July 2011, as reported by the Iceland Met Office (IMO). The jökulhlaup escaped from under Mýrdalsjökull, the glacier that rests above Iceland's Katla volcano, its 10 x 14 km caldera, and environs (figure 4). IMO reported that microseismicity was registered near several ice cauldrons in the caldera for a few weeks prior to the event (figure 5). Peak harmonic tremor on 8 July coincided with rising water levels and increased water conductivity, as measured by the main flood gauge (figure 6; gauge is at red triangle on figure 4).

Figure 4. A map of road closures and restricted areas of Mýrdalsjökull glacier resulting from the 9 July 2011 jökulhlaup at Katla (see key, lower left). The town of Vík is shown near the bottom (in black), and the main road through the area is shown in red; the trace of Katla caldera is shown in black and labeled. The main flood gauge was on the bridge across the Múlakvísl river; both were destroyed in the jökulhlaup event (red triangle). Inset shows the geographic location of Katla and Mýrdalsjökull in the S of Iceland. Restricted areas map modified from ágúst Gunnar Gylfason of the National Commissioner of the Icelandic Police-Department of Civil Protection and Emergency Management; index map modified from Ginkgo Maps.

Figure 5. Map (top) and plot (bottom) of the seismicity recorded during 8-9 July 2011 at Katla. Colors indicate the timing of epicenters and their respective plotted magnitudes, recorded as late as 2250 on 9 July 2011, according to the scheme shown below the map. Black triangles indicate seismic monitoring stations. Courtesy of Iceland Met Office (IMO).

Figure 6. Running plots of (a) water level, (b) water temperature, and (c) water conductivity at the main flood gauge of the Múlakvísl river during 3-9 July 2011. The plots show rising water level and conductivity that were coincident with peak harmonic tremor. The plots stop abruptly (red vertical line) when the gauge was destroyed along with the bridge crossing the Múlakvísl river. Courtesy of Iceland Met Office (IMO).

IMO reported that, on the same day, the main flood gauge was damaged when flood waters reached the instrument near midnight; another station, normally not in the water, started recording rising water around 0400 on 9 July, and the water level there rose 5 m within 5 minutes (figure 7). When the flood reached the main road approximately one hour later, the main bridge over the Múlakvísl river was destroyed and the road was closed (red triangle, figure 41).

Figure 7. A running plot of water level at the second flood gauge (normally not submerged). The plot shows a significant rise in water level (5 m within 5 minutes). Courtesy of Iceland Met Office (IMO).

According to the news source Morgunblaðið, 200 people were safely evacuated, and allowed to return to their homes by that afternoon. Morgunblaðið reported that analysis of the flood waters indicated that the flood was caused by geothermal water, but that a sub-glacial eruption at Katla could not be ruled out. IMO stated that the harmonic tremor declined on 9 July, following the jökulhlaup event. After observational flights, new cracks and cauldrons were reported in the ice of Mýrdalsjökull glacier (figure 8).

Figure 8. Cracking and subsidence of the Mýrdalsjökull glacier around an ice cauldron above the Katla caldera. Widespread gray tephra deposited on the ice surface is due to the 2010 Eyjafjallajökull eruption (BGVN 35:03, 35:04). Courtesy of the Icelandic Coast Guard.

By 16 July, the National Commissioner of Icelandic Police in the Department of Civil Protection and Emergency Management reported that a new bridge had been built to replace the bridge destroyed in the jökulhlaup (figure 9).

Figure 9. Photograph of the remains of the bridge crossing of the Múlakvísl river, destroyed in the jökulhlaup event on 9 July 2011. The new bridge, constructed by the 16 July 2011, can be seen in the background. Courtesy of John A. Stevenson.

August-December seismicity. IMO reported increased seismicity under Mýrdalsjökull in October (figure 10). They reported that 512 earthquakes occurred, with ~ 380 originating within the Katla caldera; a large portion (nearly 100) of those 512 earthquakes occurred on one day near the beginning of October (figure 11). The largest reported earthquake was M 4, with seven being larger than M 3. On 8 November, an M 3.2 earthquake that originated in the S most part of the caldera was felt by residents in the town of Vík.

Overall, following the July 2011 jökulhlaup event, seismicity has increased above background levels of the past year. The seismic peak is noticeable with respect to the number of earthquakes, their largest magnitudes, and the clustering under Katla (figures 10 and 11). The largest earthquakes were as large, or slightly larger, than the other earthquakes of M 3 or greater in earlier episodes of unrest (i.e., 1999 and 2002-2004, figure 10). The bulk of the 2011 seismic increase occurred over a shallow depth range (within 4 km of the surface, figure 12).

Figure 10. Plots of seismicity (greater than M 0.6) at Katla since 1999, showing the October 2011 seismicity in comparison with past episodes of non-eruptive unrest, such as in 1999 (sub-glacial eruption is uncertain in the GVP database) and 2002-2004. Plots (from the top) show: the monthly number of earthquakes (log scale); the magnitudes of earthquakes; cumulative number of earthquakes (red) and cumulative seismic moment (blue); and the focal depths of the located earthquakes. Courtesy of Iceland Met Office (IMO).

Figure 11. Seismic events (stronger than M 0.5) per day at Katla during December 2010-December 2011. Raw data is shown in blue, the 5 day moving average is shown in red, and events stronger than M 3.0 are indicated by gold stars. These trends highlight the increased seismicity of August-December 2011. Courtesy of the University of Edinburgh School of Geosciences.

Figure 12. Cumulative number of seismic events (stronger than M 0.5) at Katla since 23 November 2010. All events are shown in yellow, and events originating at depths greater than 4 and 10 km are shown in orange and red, respectively. During the August-December 2011 increase in seismicity, the majority of the recorded events originated from shallow depths (less than 4 km). Courtesy of the University of Edinburgh School of Geosciences.

Television filming sparks eruption fears. The Iceland Review reported that, in the early morning of 9 December, the Icelandic emergency hotline received calls from residents reporting bright lights on the slopes of Mýrdalsjökull. Callers feared that an eruption had started at Katla. The bright lights had also been noticed on a webcam by observers in Norway, who also enquired if there was an eruption. When the glacial slopes were inspected to find the cause of the lights, it was discovered that they were from film crews for the HBO series "Game of Thrones", who were filming in the early morning to capture the desired light conditions.

Geologic Background. Katla volcano, located near the southern end of Iceland's eastern volcanic zone, is hidden beneath the Myrdalsjökull icecap. The subglacial basaltic-to-rhyolitic volcano is one of Iceland's most active and is a frequent producer of damaging jökulhlaups, or glacier-outburst floods. A large 10 x 14 km subglacial caldera with a long axis in a NW-SE direction is up to 750 m deep. Its high point reaches 1380 m, and three major outlet glaciers have breached its rim. Although most historical eruptions have taken place from fissures inside the caldera, the Eldgjá fissure system, which extends about 60 km to the NE from the current ice margin towards Grímsvötn volcano, has been the source of major Holocene eruptions. An eruption from the Eldgjá fissure system about 934 CE produced a voluminous lava flow of about 18 km3, one of the world's largest known Holocene lava flows. Katla has been the source of frequent subglacial basaltic explosive eruptions that have been among the largest tephra-producers in Iceland during historical time and has also produced numerous dacitic explosive eruptions during the Holocene.

Information Contacts: Einar Kjartansson, Iceland Met Office (IMO), Bústaðavegi 9, 150 Reykjavík, Iceland (URL: http://en.vedur.is/); National Commissioner of the Icelandic Police-Department of Civil Protection and Emergency Management, Skúlagata 21, 101 Reykjavík, Iceland (URL: http://www.almannavarnir.is/); Ginkgo Maps (URL: http://ginkgomaps.com/); Morgunblaðið, Hádegismóum 2, 110 Reykjavík, Iceland (URL: http://mbl.is/); Icelandic Coast Guard, Skógarhlíð 14, 105 Reykjavík, Iceland (URL: http://www.lhg.is/); John A. Stevenson (URL: http://all-geo.org/volcan01010/); The University of Edinburgh School of Geosciences (URL: http://www.ed.ac.uk/schools-departments/geosciences); The Iceland Review, Borgartúni 23, 105 Reykjavík, Iceland (URL: http://www.icelandreview.com/).

Lokon-Empung has been in a state of unrest since 2007 (BGVN 33:02). Between mid-February through mid-July 2011, occasional phreatic eruptions, modest ash plumes and elevated seismicity occurred, with a larger ash plume in July 2011 (BGVN 36:06). This report addresses seismic events from mid-July through 1 December 2011.

According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), during 20-21 July 2011, seismicity and visual observations of Tompaluan crater in the saddle between the twin peaks of Lokon and Empung indicated that activity continued to be high. On 20 July plumes rose 100-500 m above Tompaluan crater, and during 21-24 July 2011 white plumes again rose 100-300 m. CVGHM noted that, since an eruption on 18 July, most data showed a decline in activity and therefore on 24 July the Alert Level was lowered to 3 (on a scale of 1-4). Residents and tourists were not permitted within 3 km of the crater. A news article (Straits Times) stated that on that same day about 5,000 residents that had evacuated returned home, and about 200 people remained in shelters.

CVGHM reported that during 24 July-8 August 2011 seismicity decreased at Tompaluan crater, with a drastic reduction on 26 July. According to a news article (BNO News, accessed on Daijiworld News), during 27 July-8 August white plumes rose 100-400 m above the crater. The article stated that at the end of August, Tompaluan crater erupted several times (12 times on 28 August). One explosion on 29 August 2011 ejected material 250 m above the crater. According to the article, activity decreased after 29 August. The article also noted that 222 people remained at temporary refugee camps because their homes were located within 3 km of the crater.

CVGHM reported that on 10 October 2011 white and gray plumes rose 100-300 m above Tompaluan crater. Based on information from CVGHM, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that on 11 October an ash plume rose to an altitude of 2.1 km.

According to a news article (Kompas.com), a gray plume rose 1.2 km above Tompaluan crater and drifted SW on 26 October, followed by an explosion that sent incandescent material as far as 800 m away from Tompaluan crater. A second eruption produced a plume that rose 500 m above the crater.

Geologic Background. The twin volcanoes Lokon and Empung, rising about 800 m above the plain of Tondano, are among the most active volcanoes of Sulawesi. Lokon, the higher of the two peaks (whose summits are only 2 km apart), has a flat, craterless top. The morphologically younger Empung volcano to the NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century, but all subsequent eruptions have originated from Tompaluan, a 150 x 250 m wide double crater situated in the saddle between the two peaks. Historical eruptions have primarily produced small-to-moderate ash plumes that have occasionally damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred. A ridge extending WNW from Lokon includes Tatawiran and Tetempangan peak, 3 km away.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Straits Times (URL: http://www.straitstimes.com); BNO News (URL: http://www.bnonews.com/); Kompas.com (URL: http://www.kompas.com/); Antara News (URL: http://www.antaranews.com/en/); Daijiworld News (URL: http://www.daijiworld.com/).

This report on Masaya presents a summary of activity through mid-2011. Our last report was issued in March 2009 (BGVN 34:03) and highlighted the intermittent plumes and explosions of 2006 and 2008.

From 2008-2010 activity generally consisted of degassing with sulfur dioxide (SO₂) fluxes typically under 1,200 tons per day. Instability of the S andW crater walls was a concern for the National Park and monitored by the agency INETER (Instituto Nicaragüense de Estudios Territoriales). Mass wasting, frequently triggered by heavy rain, occurred within the crater with debris occasionally blocking the active vents.

Throughout this 3-year period, fumarole temperatures ranged from 58 to 84°C and regular monitoring of the El Comalito cinder cone showed that degassing continued. Tremor sources shallowed during 2008-2010, rising from a 2008 depth of 26 km to a 2010 depth of ~ 1 km.

On 12 October 2010 incandescence occurred in the intra-crater area's largest opening (figure 24). Temperature at the points of incandescence reached 207°C. Differential optical-absorption spectroscopy (DOAS) measurements from vents registered SO₂ fluxes of 465 tons per day. SO₂ emissions increased throughout October 2010, reaching 586 tons per day. INETER reports contain plots with more detailed SO₂ data.

Figure 24. Incandescence seen in Masaya's Santiago crater on 12 October 2010. Note the crater's vertical walls and depth. Courtesy of INETER.

SO₂ fluxes in 2011. In January 2011, INETER's team measured SO₂ fluxes while in transit along the easternmost route on figure 25 (between the town of Ticuantepe and the community of San Juan). Those SO₂ measurements averaged 642 tons per day, an increase over 2010 that was attributed to increased gas and magma output.

Figure 25. Vehicle routes (heavy lines) used while recording Masaya's SO2 fluxes. The scale at bottom shows distance in meters. The topographic margin of Masaya's main caldera sits ~2 km E of the easternmost vehicle route. Courtesy of INETER.

During 7-30 March 2011 collaborators from the University of East Anglia, Heidelberg University, and Oxford University measured Santiago crater's SO₂ and other gas emissions. A Mini-DOAS mobile was one of the many instruments used to monitor the atmosphere and SO₂ fluxes (figures 26-28).

Figure 26. SO2 measurements underway at Masaya on 20 January 2011. The vehicle passed beneath Masaya's gas plume on the Southern Pan-American Highway. The laptop displays a well-defined red histogram representing SO2 measured along the plume transect. Courtesy of INETER.

Figure 27. Instruments used during the 7-30 March 2011 campaign to measure SO2 on a continuous basis from a viewing platform overlooking Masaya. Courtesy of INETER.

Figure 28. A compact, automatic meteorological station used to measure wind velocity, air humidity, and other parameters that could refine and enable comparisons with the gas measurements. Courtesy of INETER.

In addition to mobile DOAS and fixed gas monitoring systems, a small dirigible (Zeppelin) represented a novel monitoring approach. One potential use for the dirigible was as a platform from which to measure gas concentrations inside the volcanic plume at altitude. Unfortunately, when deployed on its trial launch, heavy winds quickly blew it out of control (figure 29).

Figure 29. The dirigible (Zeppelin) deployed at Masaya during 7-30 March 2011. The dirigible undergoing instrumental work (top), and floating above Santiago crater moments before being blown away by heavy winds (bottom). Courtesy of INETER.

Crater-wall collapse leads to 6 August 2011 Park closure. More than a dozen crater-wall collapses occurred at Santiago crater during June and July 2011. INETER geologist Marisol Echaverry López noted that the SW and W sides of the crater wall had severely eroded. Echaverry recommended that, should the situation worsen, nearby residents be evacuated since debris-covered vents could pressurize the system and lead to explosions. On 14 July, geologist Martha Ibarra found that debris shed from the steep walls was accumulating and the recent collapses had blocked two gas vents. The deep, steep wall of Santiago crater frequently collapsed along fracture zones.

On 6 August 2011, Masaya National Park officials alerted INETER that significant portions of the SW crater rim had collapsed and completely covered the active vent. The park closed for the day during inspections by INETER. The SW rim was the site of frequent failures and field investigators noted that gas emissions were blocked for ~ 10 minutes. No additional failures were observed and activity did not escalate.

During field investigations in September and October 2011, INETER described and measured temperatures from three new fumaroles within Santiago crater. These sites were located at the edges of debris fill within the crater, along the S and E walls and were degassing with temperatures from 48 to 74°C. SO₂ measurements from Mini-DOAS indicated decreasing emissions during this time period, from 518 tons per day in September to 153 tons per day in October 2011.

Geologic Background. Masaya is one of Nicaragua's most unusual and most active volcanoes. It lies within the massive Pleistocene Las Sierras pyroclastic shield volcano and is 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 twin volcanoes of Nindirí and Masaya, 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 6500 years ago. Historical lava flows cover much of the caldera floor and have confined a lake to the far eastern end of the caldera. 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 cause health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

This report summarizes a news article by Lourdes Solidum-Montevirgen of the Philippines' Department of Science and Technology (Solidum-Montevirgen, 2011). The article noted that 20 years after one of the last century's biggest volcanic eruption (April-September 1991, BGVN 16:03-16:10), hunger and lahars continue to threaten Aeta communities around Pinatubo's foothills. The Aetas (an indigenous people who live in scattered, isolated mountainous parts of Luzon, Philippines) resided, in part, before the eruption in the towns of San Marcelino and Botolan, settlements almost destroyed by the 1991 eruption. The rainy season has resulted in lahar flows that continue to threaten these and nearby towns, displacing thousands of people. Agriculture continues to suffer badly, as hundreds of square kilometers of formerly arable land remain unproductive. Pinatubo is located NW of the capital of Manila (figure 39).

Figure 39. A map of major Philippine volcanoes, including Pinatubo. Courtesy of Lyn Topinka, US Geologic Survey.

Aetas were hardest hit because they were both uprooted from their homes and their way of life. Many remain in government resettlement areas, huddled in makeshift homes, tents, and evacuation dwellings. Many of them are recent refugees after part of a protective dike along the Bucao River collapsed during Typhoon Kiko in August 2009, flooding Botolan and ten villages, resulting in death and hunger. Typhoons that followed two months later (October 2009) broke down an additional 1-km portion of the dike, causing lahars and floodwaters to rise more than 1.5 m, displacing over 20,000 people in nine villages. Over 9,000 of these recent refugees remained in evacuation centers as they awaited dike repair. They have joined thousands of evacuees still huddled in the ten evacuation centers inside three resettlement sites that were created following Pinatubo's eruption.

There is still widespread devastation in Botolan and nearby towns where several square kilometers of lakes and farm lands were "desertified". It is doubtful whether a new bridge and the dike, when repaired, will hold lahar floods because the Bucao river is heavily silted. Botolan (population 51,675), the largest town in Zambales and closest to Pinatubo, also has the largest Aeta population in the province. In 2010, 160 km² (16,000 hectares) of the area nearest Pinatubo was declared as Aeta ancestral domain by the National Commission on Indigenous Peoples.

Farming has not yet resumed in many rice paddies and vegetable farms damaged by flash floods and lahars. Farm lands were covered with thick ash and reworked tephra, irrigation equipment ruined, roads and bridges destroyed, properties lost, trade and business centers collapsed. Overall, 364 communities and 2.1 million people were affected by the eruption, and more than 80,000 houses were lost. Roads and communications were damaged by pyroclastic flows and lahars. Some 800 km² of rice lands and almost 800,000 farm animals were lost. The cost to agriculture was estimated at P1.5 billion (~ $25 million US) and the cost of repairs to damaged infrastructure was P3.8 billion (~ $62 million US).

Reference. Solidum-Montevirgen, L., 2011, Hunger, lahar haunt homeless Aetas 20 years after Pinatubo, Malay Business Insight, 29 July.

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: Lourdes Solidum-Montevirgen, Industrial Technology Development Institute-Food Processing Division, Department of Science and Technology, Phillippines; Malaya Business Insight (URL: http://www.malaya.com.ph).

Popocatépetl continued to be active during September 2010 to 13 December 2011 with explosions, tremor, and frequent gas-and-steam emissions occasionally containing ash (figure 59). This report continues the table in the previous report (BGVN 35:08) that tallies the seismic activity and ash emissions (table 21). Figure 60 shows the proximity of the volcano to population centers.

Figure 59. Photograph of the lava dome at Popocatépetl taken on 8 September 2011. Courtesy of CENAPRED.

Table 21. Reported plumes above Popocatépetl's summit crater that contained some ash between 5 October 2010 and 13 December 2011. Data provided by the MWO (Mexico City Meteorological Watch Office), Washington Volcanic Ash Advisory Center (VAAC), and Centro Nacional de Prevención de Desastres (CENAPRED), abbreviated CEN.

Figure 60. Map of Popocatépetl in relation to Mexico City and other large communities. Circular areas are approximate; they show some hazard zones, including the innermost "Red" zone, which is within 5 km of the summit and excludes the public from entry. Courtesy of CENAPRED.

During the reporting interval, there were a large number of MODVOLC thermal alerts for Popocatépetl.

Arana-Salinas and others (2010) discuss the Ochre Pumice Sequence, a major (VEI 6) event that occurred ~ 5,000 years ago. That unit contained a sequence of pyroclastic flow and fall deposits that covered ~ 300 km² directed NNE. The erupted magma amounts to a (dense rock equivalent) volume of ~2 km³. The authors stated that, depending on the wind direction, an equivalent event today would impact 15 million residents of Mexico City (the Capital), Puebla, Atlixco, and Cuautla and elsewhere, and it would severely damage infrastructure.

Reference. Arana-Salinas, L., Siebe, C., Macías, J.L, 2010, Dynamics of the ca. 4,965 yr 14C BP "Ochre Pumice" Plinian eruption of Popocatépetl volcano, México: Journal of Volcanology and Geothermal Research v. 192. p. 212-231.

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: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: https://www.gob.mx/cenapred/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); MODVOLC, 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/).

Soputan volcano, Sulawesi, Indonesia (figure 4) was relatively quiet for more than two years following our last report in September 2008 (BGVN 33:09). Thermal anomalies appeared in late May 2011 and in late June 2011, Soputan re-commenced eruptive activity. This report covers activity at Soputan during 2011 (through 2 October). Unless otherwise noted, data was reported by the Center for Volcanology and Geological Hazard Mitigation (CVGHM).

Figure 4. A photograph of Soputan volcano, taken 6 March 2011 by Flickr account user Akhal-Téké. Index maps at left show the location of Soputan volcano on the island of Sulawesi (close-up, bottom) in Indonesia (top). Index maps modified from MapsOf.net (top) and Ginkgo Maps (bottom).

The first signs of the June-October eruption at Soputan occurred with some diffuse white plumes in June reaching 25-150 m above the crater. After an increase in seismicity during 21 June-2 July, CVGHM raised the Alert Level from 2 to 3 (on a scale from 1-4); climbing the slopes of the volcano was prohibited, and residents were discouraged from going within 6 km of Soputan's crater.

A Strombolian eruption, reported at 0603 on 3 July, generated an ash plume that rose 6 km altitude and drifted W. The eruption plume was captured in a NASA Earth Observatory satellite image (figure 5). A pyroclastic flow traveled up to 4 km W. A 10 pixel MODVOLC thermal alert was triggered at 0225 (UTC) on the same day (figure 5, table 8).

Figure 5. A natural color satellite image of the eruptive plume generated at Soputan on 3 July 2011. The plume is seen here drifting E over Laut Maluku (Molucca Sea); the brownish color of the plume indicates that it consisted of both gas and ash. The red outline highlights the area of the 10-pixel MODVOLC thermal alert (table 2) of the same day. Courtesy of NASA Earth Observatory.

Table 8. MODVOLC thermal anomalies recorded at Soputan in 2011. A 58 pixel thermal anomaly was recorded on 2 October 2011, but was omitted due to the sun-glint angle being below 12°. The University of Hawaii states that "If a pixel has a sun-glint angle of less than 12° it is potentially contaminated by sunglint and should not be trusted." Courtesy of HIGP Thermal Alerts System, University of Hawaii. [Note that the 21 May 2011 pixel originally reported below (deleted) was actually located at some distance from the volcano in the ocean, and was most likely due to sunglint.]

The Jakarta Globe reported that, due to ash fall, the Indonesian Red Cross (Pa Merah Indonesia - PMI) distributed ~ 31,000 face masks to residents (figure 6). It also reported that Sutopo Purwo Nugroho, spokesman for the National Board for Disaster Managment (BNPB), said that "there is no need for evacuation because the nearest residents are living some 8 km from the mountain." Sam Ratui International Airport was closed for 3 hours (during 1200-1500) that afternoon, according to The Jakarta Globe. Following the eruption of 3 July, seismicity decreased, and the only reported activity was dense white plumes rising to 75 m above the crater on 18 July. The Alert Level was lowered to 2 on 19 July, allowing residents to come within 4 km of the crater.

Figure 6. Residents near Soputan with face masks they received from the Indonesian Red Cross (Pa Merah Indonesia - PMI). Courtesy of the Jakarta Globe.

Seismicity continued to decrease until 10 August. On 14 August, a plume containing ash rose to 1 km above the crater, and two other plumes rose to 1.3 km above the crater later in the day (figure 7). The Darwin Volcanic Ash Advisory Centre (VAAC) reported an ash plume that drifted more than 100 km W. The Alert Level was again raised to 3 on 14 August, once again prohibiting residents within 6 km of the crater.

Figure 7. An ash plume from Soputan rising to greater than 1 km above the crater on 14 August 2011. In the foreground, a resident of one of the local towns is working in their field. Courtesy of Andreas/AFP-Getty Images.

Following the eruptions of 14 August, seismicity decreased significantly, and small white plumes rose above the crater. The plumes steadily decreased from 200 m high above the crater (14-18 August) to, at most, 100 m above the crater (29 August-7 September). An early morning photograph captured an eruption on 15 August, showing a small plume and lava flows down the flank of Soputan (figure 8). On 8 September, the Alert Level was lowered to 2, allowing residents to come no closer than 4 km to the crater.

Figure 8. An early morning photograph of Soputan erupting on 15 August 2011. Lava flows down the flank of Soputan brightened the small eruptive plume billowing overhead. Courtesy of Andreas/AFP-Getty Images.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is located SW of Riendengan-Sempu, which some workers have included with Soputan and Manimporok (3.5 km ESE) as a volcanic complex. It was constructed at the southern end of a SSW-NNE trending line of vents. During historical time the locus of eruptions has included both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM) Diponegoro 57, Bandung, Jawa Barat 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Akhal-Téké, Flickr photostream (URL: http://www.flickr.com/photos/51873088@N04/); MapsOf.net (URL: http://mapsof.net/); Ginkgo Maps (URL: http://www.ginkgomaps.com/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Hawai'i Institute of Geophysics and Planetology (HIGP) 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/); The Jakarta Globe, Citra Graha Building 11th Floor, Suite 1102, Jakarta 12950, Indonesia (URL: http://www.thejakartaglobe.com/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Andreas/AFP - Getty Images (URL: http://www.gettyimages.com/).

Our last report discussed Telica volcano's intermittent gas emissions from 2009 through early 2010 as well as installation of an early warning system (Sistema de Alerta Temprana, SAT) in March 2010 (BGVN 35:03). New information has been released by INETER (the Instituto Nicaragüense de Estudios Territoriales) detailing the escalation of activity that culminated in a major eruption in May 2011. This report also covers the field investigations from April 2010 through October 2011, seismic data from January 2010 through October 2011, SO₂ monitoring from 25 May 2011 through 13 September, and regular thermal measurements from the crater and flank fumaroles.

Volcanic activity at Telica during 2010 was characterized by diffuse degassing. Persistent gas emissions from this volcano have caused a legacy of hazards for local communities and have been linked to acute respiratory infections (Bellos and others, 2010; Freundt and others, 2006; Malilay and others, 1996). Field investigations in 2010 conducted by INETER focused on measuring temperatures from the crater as well as fumaroles located near the flanks (figure 20). Heavy rain and inaccessible roads limited visual and thermal monitoring to short field excursions in April, July, and August 2010.

Figure 20. (A) Telica's 700-m-wide double crater (gray to white central area) and E-flank fumaroles (diamond) were sites of thermal monitoring for 3 months in 2010. The seismic station TELN is located 30 m N of the orange diamond. (B) Typical diffuse degassing from the summit photographed on 14 January 2011. Courtesy of INETER.

Using a thermal camera, INETER determined the maximum temperatures within Telica's crater on 28 July and 18 August were 259°C and 251°C respectively. The four fumaroles located near seismic station TELN were measured three times in 2010. With a digital thermocouple, INETER determined that maximum temperatures from the flank fumaroles gradually increased from April to August: 72.3°C, 81°C, and 105°C. Minimum measured temperatures were irregular and ranged from 66.4 to 76°C.

Few earthquakes were sufficiently large to registered and be located during 2010 (table 2), but INETER reported that those recorded were smaller, and there was frequent tremor. The seismic network for this volcano was installed in 1994. The two local seismic stations, TEL3 and TELN, operated with 3-component sensors but during this reporting interval TELN was offline until July 2010. Microseismicity was reported during four months in 2010 (March, September, November, and December). The highest rate occurred in March, exceeding 150 events per day. In September, more than 120 events were recorded per day, and in November and December, the rate was 80 microseisms per day.

Table 2. Located earthquakes at Telica recorded in the interval from January 2010 through October 2011. Only months with earthquakes reported are shown. High values in May-June were during an eruption. Values are based on monthly reports from INETER.

INETER reported that field investigators encountered significant gas plumes from Telica's summit in April and July. The W edge of the crater and a small vent on the E interior wall were constant sources. There were notable rockfalls from the crater rim; an observation from 28 July 2010 mentioned the NE and SE walls in particular experience rockfalls of sufficient magnitude to increase the summit crater's size.

January 2011. Investigators from INETER visited Telica this month for instrument maintenance and monitoring activities. Rockfalls from the S crater wall were noted on 11 January by staff. According to Halldor Geirsson, Mel Rodgers (Univ. of South Florida) was in the vicinity during 25-31 January and noted strong degassing and occasional rockfalls. On 14 January thermal data was collected from the central crater and fumaroles on the outer flank. The maximum temperature recorded from the crater was 295?C. The four fumaroles located on the W flank (figure 20a) had recorded temperatures ranging from 68?C to 72?C.

Seismicity during January 2011 was generally high, with ~ 907 earthquakes recorded. Most were long-period (LP) with dominant frequencies of 1-3 Hz. From 19 to 23 January events were absent. Seismic tremor was recorded throughout January at 30-40 RSAM units with scattered intervals of greater than 100 RSAM units.

February 2011. Collaborative fieldwork was conducted on 26 February between INETER and scientists from the Institute of Renewable Energy (Spain). This team measured temperatures and took thermal images of the fumaroles located both within the crater and on the W flank. Maximum temperatures within the crater ranged from 62-75?C.

Elevated seismicity continued through February with 676 recorded events. These events were similar to those recorded in previous months; LP events had dominant frequencies of 1-3 Hz. Figure 21 presents an example of frequency analysis for one earthquake at Telica. Volcanic-tectonic (VT) events rarely occurred. Tremor was recorded at 30-40 RSAM units.

Figure 21. The seismic trace of a characteristic LP event (a) from Telica processed with spectrum-analysis software to obtain the dominant frequency ranges. This event was recorded on 1 February 2011 and lasted for more than 20 seconds (b, a zoom on the trace). The dominant frequencies (c, frequency vs. amplitude) ranged from 1 to 2 Hz with some signals to ~10 Hz. Courtesy of INETER.

Ashfall in March 2011. On 6 March residents living near Telica felt an earthquake during the night and during the following day they observed small plumes rising from the volcano. INETER scientists visited on 8 March for routine data collection and to investigate reports of fresh ashfall. Light ash was still visible on leaves and rooftops and appeared directed towards the N and SE. Residents also reported strong sulfurous odors during the explosive events. Field investigators found evidence of juvenile material along the SE rim of the crater.

INETER collected thermal data on 8 March from fumaroles located within the crater and near the seismic station (TELN) on the E flank. The maximum temperature measured within the crater was 137?C. Two fumaroles were identified within the crater on the W side; these sites had recorded temperatures in the range of 47-71?C. Thermal measurements near TELN ranged from 51-60?C. This site did not emit steam or other gases.

The INETER field crew noticed that degassing appeared to be more intense during the 8 March field visit compared to their previous 26 February visit. The view to the crater was often obscured from the point of view of seismic station TELN (figure 20b).

In their March report, INETER discussed the relevance of the new temperatures measured during the 8 March field campaign. The issue was the apparent decrease compared to temperatures recorded from Telica in January. INETER staff acknowledged the limits of monthly temperature readings but looked forward to longer-term correlations with seismic data.

Seismicity in March remained high with ~ 572 events recorded. The majority of the earthquakes were LP events with dominant frequencies between 1-3 Hz. Few VT events were recorded. Seismic tremor was between 30-40 RSAM units.

April 2011. New seismic data from an early warning system (Sistema de Alerta Temprana, SAT) was presented for the first time in INETER's April report. The new 6-station network contained five stations with 3-component sensors and one station with a single-component sensor. The network included two short-period stations, TEL3 and TEL4 (note that TEL4 was TELN), that continued to send data.

From late March through the second week of April, small explosions, LP earthquakes, and seismic swarms were detected. There was a three-day lull in early April, but VT earthquakes began occurring with increasing magnitudes and small explosions from the summit occurred at least once per week throughout the month. By 30 April, explosions were registered having 30-minute durations and were followed by periods of degassing and low-altitude ash plumes. For an interval in late April, seismicity was very high, with more than 600 events recorded each day with a range of M 0.1-3.3. A maximum of 380 RSAM units was recorded and seismic tremor ranged between 30-40 RSAM. The VT earthquakes were strong enough to be noticed by residents in local communities.

During field investigations on 11 April, INETER measured temperatures within the crater, recording a of maximum 254?C. One fumarole within the W wall of the crater had decreased temperature by 10?C while the other had increased by 1?C compared to values from March. The four fumaroles near TEL4 had recorded temperatures in the range of 53-69?C.

Eruptions in May 2011. The escalation of seismic activity and recurrence of ash plumes seen since March prompted INETER to issue an alert to civil defense on 13 May warning that eruptive activity was possible. During the first week of May seismicity increased to 500 microseismic events per day (in general, microseismic events in April occurred at a rate of ~ 220 per day) and VT events suddenly became rare (figure 22).

Figure 22. Telica's seismicity recorded during May 2011: epicenters shown on map and cross section (roughly aligned with map) were located with the new, early warning system (the 6 stations shown as blue triangles). Note that the focal depths are clustered under the volcano at depths mainly below 5 km. Courtesy of INETER.

Large explosions from Telica were registered at midnight on 13 May. After that, INETER reported that residents in three different communities observed pink-colored ash had fallen, and residents had also felt earthquakes. Frequent explosions of abundant gas and falls of coarse-to-fine ash occurred 14-15 May. Seismicity during 14 May was dominated by M 1.0-3.3 events with depths between 1 and 5 km (figure 23). Ash fell over the community of La Quemada, located 4 km N of the volcano. Residents heard loud noises from the volcano.

Figure 23. This minor explosion at Telica was observed on 16 May 2011. Courtesy of Halldor Geirsson (Pennsylvania State University).

On 16 May observers first saw gray ash clouds rising from Telica's summit. Later, this activity visibly escalated and ashfall was observed in continuous plumes (figure 23). The highest plumes reached 1.2 km altitude and ash fell to the SE over communities. By mid-May the number of seismic events had increased to 800 microseismic events per day, most of which were explosions with a few VT events (figure 24). INETER noted that seismic stations recorded up to 140 RSAM units on 16 May.

Figure 24. The number of seismic events registered per day during March-May 2011. This record was dominated by microseismicity and explosions from Telica. Courtesy of INETER.

Fieldworkers from INETER and others, deployed a large tarp, collected ~ 80 kg of tephra during 16-18 May. Preliminary assessments determined the nature of material that landed on the tarp included dominant lithics, fragmented rock and crystalline material of 0.5-1.0 mm diameter, and round fragments of pink-colored tephra (as opposed to the gray, sand-size grains from a small event on 12 May). These observations were also reported in field notes by Halldor Geirsson and in an abstract by Witter and others (2011). During a lull in activity on 16 May 2011, the team visited Telica's summit. Upon approaching the N rim, they heard no sounds coming from the crater, and they measured temperatures on the crater floor of ~ 395?C. They observed fresh, inward-directed rockfalls from the crater's rim and found the N wall unstable and dangerous, seemingly on the verge of falling. The team also observed a dark area on the SE wall and floor of the crater, which suggested that recent explosions had concentrated tephra on these surfaces.

Washington VAAC 15 and 17 May. The Washington Volcanic Ash Advisory Center (VAAC) reported that the GOES-13 satellite detected at least two plumes on 15 and 17 May 2011. This satellite imagery confirmed gas-rich plumes on both days at ~ 1.8 km altitude, but ash content could not be determined from available images. INETER reported that these events were accompanied by elevated seismicity on 15 May and associated ashfall occurred 4 km N of the summit. After three emissions of gas on 17 May, ash and tephra fell SE of the summit. The last time plumes from Telica appeared in VAAC reports was in early months of 2007 when plumes reached an altitude of ~ 1.5 km for three days in January and February drifting SW.

Explosion with sustained ash plumes. On 18 May INETER posted two online reports ("Volcanic Communications" 5 and 6) indicating the peak of activity starting on that day. After two hours of sustained explosive activity producing ash plumes ~ 600 m high, the largest explosion suddenly occurred at 1350 on the 18th. A column of ash rose to an altitude of 2.6 km and was maintained for six minutes (figure 25). More than 15 explosions were seismically recorded (with a maximum of 350 RSAM units). The episodic explosions produced ash, steam, and, at times, lightning within the plume. Temperatures within the crater were gauged at a maximum 432?C, and flank fumaroles were measured to be 60-126?C.

Figure 25. A sustained ash plume issued from Telica during the peak of activity in 21 May 2011. Courtesy of Halldor Geirsson.

In a 19 May statement to news agencies, the municipal committee for disaster prevention and mitigation (COMUPRED) reported the evacuation of 390 families from nine villages near the volcano. The villages included Agua Fría (150 m from the edifice) and Los Patos, the most distant of the villages at 8 km from the edifice.

On 19 May, public meetings were held that included INETER, civil defense, and national disaster response (SINAPRED) representatives. A widely discussed issue was how the heavy ashfall and volcanic gases were affecting water quality. Officials favored monitoring local wells within 5 km of Telica's edifice.By 20 May, Telica's explosions became infrequent: three were registered that day. Only ash and resulting plumes rose to 500-800m altitude. Microseismicity remained high (850 events) and 60 earthquakes (M 0.8-2.2) were located at depths of 0.7-2.2 km (table 2).

On 20 May, explosions were also infrequent but four large events from the crater emitted plumes with heights of 500-700 m. In addition, at 1500 on the 20th, one large, continuous explosion occurred that lasted 36 minutes. Observers described a plume containing gas, steam, tephra, and small rocks. The largest ballistics did not reach farther than the crater rim and lightning was observed in the plume. INETER noted that wind conditions allowed the plume to reach 2 km altitude. Nine hundred microseismic events were recorded on 21 May and 57 earthquakes (M 0.5-2.0) were located with average depth of 1 km.

Well monitoring on 21 May revealed slight changes in local water quality. Sites located NE and within 5 km of the summit showed elevated quantities of sulfates, chlorides, alkalinity and PH.

New SO₂ monitoring efforts. On 22 May, Universidad Tecnológica de Chalmers (Switzerland) and SNET (El Salvador) installed two portable Mini-DOAS stations (Differential Optical Absorption Spectrometer) at Los Angeles and Mendoza to measure SO₂ levels.. These stations were installed downwind from Telica, SW of the edifice (figure 26). While the fixed stations collected data, traverses were made across the plume with mobile Mini-DOAS.

Figure 26. Location map of SO2 monitoring stations, Los Angeles and Mendoza, located SW of Telica Volcano (green stars). The 22 May 2011, Mini-DOAS traverse is highlighted in red. Courtesy of INETER.

The first Mini-DOAS results from fixed stations were obtained on 24 May. Data from the Los Angeles and Mendoza monitoring stations showed that flow of SO₂ oscillated between 50 and 150 tons/day. Data collected from traverses below the plume on 23-27 May were processed by the Universidad Tecnológica de Chalmers (Switzerland). The traverse results compared well with the fixed stations: reported values ranged from 40 to 130 tons/day. Peak SO₂ values from the fixed stations appeared as follows: Mendoza station, 420 tons/day on (28 May); Los Angeles station, 194 tons/day (30 May).

May 2011 eruption declines. INETER reported that ash explosions became infrequent during 23-30 May. That said, a cluster of eight explosions occurred sequentially on 24 May and created plumes reaching 600 m above the crater. Microseismicity remained high throughout the rest of the month. During 23-24 May, the largest number of earthquakes were located. Approximately 100 earthquakes (M 0.7-2.5) occurred with hypocenters at depths of 1-15 km.

According to the information supplied by the Directiorate of Meteorology of INETER, on 24 May ash expelled from Telica drifted SE at 8-15 km/hour at altitudes of 1.5 km. In their 24 May report, INETER warned that eruptive conditions could continue during the remainder of the month. They recommended the authorities of the Institute Nicargüense of Aeronaútica Civil (INAC) to caution air traffic about persistent and dispersed volcanic ash.

SO₂ monitoring in June. On 3 June INETER conducted field investigations and measured SO₂ with Mini-DOAS and Mobile DOAS. There were eight successful on-land traverses below the plume, each covering 18 km. Mobile DOAS data indicated a decrease in SO₂: the maximum value recorded was 39 tons/day. SO₂ flux from the two fixed stations, Mendoza and Los Angeles, also showed reduced levels during the early part of the month but an increase appeared from both sites in 13-15 June. INETER suggested that the low SO₂ flux in early June may have been influenced by local wind patterns. Observers in the area noticed that the summit plume was very dispersed during this time. Wind velocities reported by NOAA were as low as 1.2 m/s on 3 June.

During a field visit by INETER on 14 June, the investigators managed to count 17 explosions that expelled ash and gas. The explosions occurred within short intervals of time, from two to three minutes and the longest interim was 10 minutes. The field team visited fumaroles S of the TEL4 seismic station and recorded temperatures from three fumaroles with values ranging from 64-76?C.

Field data collected on 30 June included a maximum temperature of 590?C from the crater (figure 27). The team observed incandescence within the crater and from a new vent near the NE wall. There were jetting and collapse sounds emitting from the crater.

Figure 27. Telica's crater temperatures measured January-October 2011. Note multiple measurements taken during the peak of activity in May and two measurements in September. The value measured in February was obtained with an infrared camera (Institute of Renewable Energy). Courtesy of INETER.

During June the number of earthquakes diminished but seismicity remained high. Approximately 500 earthquakes were registered per day. Approximately 100 earthquakes (up to M 2.8) were located (half the number located in May) at a maximum depth of 28 km (table 2). The majority of the events were volcanic-tectonic (VT) and doublet earthquakes (paired events). The dominant frequencies of the earthquakes shifted in June to 4.0-8.0 Hz.

Routine monitoring in July. During fieldwork on 12 July, INETER measured SO₂ flux with Mobile DOAS. Five traverses, each one 8.5 km in distance, were recorded. The average value of SO₂ was higher than the previous month, 484 tons/day. In their monthly report, INETER discussed the strong impact of inferred wind speed on their new gas measurements. During the month wind patterns were variable with speeds average ~ 5.8 m/s. They commented that the plume was noticeably less dispersed when they conducted the gas measurements.

On 22 July routine fieldwork was conducted at Telica. Residents of La Joya had heard loud jetting noises and at night saw incandescence at the summit. During the day, the team also heard jetting but did not see any explosive activity or feel earthquakes. Crater temperatures averaged 265?C (five measurements), very low compared to the previous month (figure 27). Temperatures taken from fumaroles S of the seismic station ranged from 68-72?C (three fumaroles).

Incandescence during August to October 2011. Halldor Geirsson noted that incandescence was seen in August 2011 (by Mel Rodgers and INETER staff). Further anomalous activity was not reported that month. A night visit took place on 9 September. The INETER team observed incandescence from the crater and measured temperatures with a thermal camera recording a maximum 458?C. No jetting sounds were heard.

On 13 September INETER measured SO₂ with Mobile DOAS. There were seven traverses along an 8.5-km stretch of road to cross below Telica's gas plume. SO₂ flux was significantly lower than the previous month with an average of 81 tons/day.

On 13 October, SO₂ traverses were attempted, but no gas was detected. Wind patterns had been disrupted by a low-pressure system that caused major flooding along Nicaragua's W coast.

During INETER's 27 October field visit, the team observed incandescence during the day and fragments of molten spatter were released during moderate gas explosions (figure 28). They also observed minor gas emissions and loud jetting persisted from the crater. INETER took five measurements of crater temperature, which averaged 280?C. Temperatures from three fumaroles S of seismic station TEL4 were 66-72?C. Vegetation was noticeably affected by volcanic gases; numerous dead plants were photographed during the 27 October visit.

Figure 28. Two photos both showing the same scene, centered on active vents within Telica's summit crater on 27 October 2011. Original photo (left); black and white copy (right). Gas explosions and spatter escaped from these vents during the field visit. Courtesy of INETER.

References. Bellos, A., Mulholland, K., O'Brien, K.L., Qazi, S.A., Gayer, M., Checchi, F., 2010, The burden of acute respiratory infections in crisis-affected populations: a systematic review: Conflict and Health, v. 4, no. 3.

Freundt, A., Kutterolf, S., Schmincke, H.-U., Hansteen, T., Wehrmann, H., Peréz, W., Strauch, W., Navarro, M., 2006, Volcanic hazards in Nicaragua: Past, present, and future, in Rose, W.I., Bluth, G.J.S., Carr, M.J., Ewert, J.W., Patino, L.C., and Vallance, J.W., eds., Volcanic hazards in Central America: Geological Society of America Special Paper 412, p. 141-165.

Malilay, J., Real, M.G., Vanegas, A.R., Noji, E., and Sinks, T., 1996, Public Health Surveillance after a Volcanic Eruption: Lessons from Cerro Negro, Nicaragua, 1992: Bulletin of Pan American Health Organization, v. 30, no. 3.

Witter, M.R., Geirsson, H., La Femina, P.C., Roman, D.C., Rodgers, M., Muñoz, A., Morales, A., Tenorio, V., Chavarria, D., Feineman, M.D., Furman, T., and Longley, A., 2011, May 2011 eruption of Telica Volcano, Nicaragua: Multidisciplinary observations, Abstract V53E-2670, Fall Meeting, AGU, San Francisco, California.

Geologic Background. Telica, one of Nicaragua's most active volcanoes, has erupted frequently since the beginning of the Spanish era. This volcano group consists of several interlocking cones and vents with a general NW alignment. Sixteenth-century eruptions were reported at symmetrical Santa Clara volcano at the SW end of the group. However, its eroded and breached crater has been covered by forests throughout historical time, and these eruptions may have originated from Telica, whose upper slopes in contrast are unvegetated. The steep-sided cone of Telica is truncated by a 700-m-wide double crater; the southern crater, the source of recent eruptions, is 120 m deep. El Liston, immediately E, has several nested craters. The fumaroles and boiling mudpots of Hervideros de San Jacinto, SE of Telica, form a prominent geothermal area frequented by tourists, and geothermal exploration has occurred nearby.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Halldor Geirsson, The Pennsylvania State University, Department of Geosciences, 536 Deike Building, University Park, PA 16802, USA; La Prensa: (URL: http://www.laprensa.com.ni/2011/05/19/departamentos/60948); Mel Rodgers, University of South Florida, Department of Geology, 4202 East Fowler Ave., SCA528, Tampa, FL 33620, USA.

Following two M 3 earthquakes in the region on 13 December 2011, fishermen in Salif City, Yemen, reported an eruption in the Zubair island group that began as late as 18 December 2011. Moderate Resolution Imaging Spectroradiometer (MODIS) images of the area also first revealed a plume on 18 December, and this and later MODIS images fixed the vent's location at a spot in the N portion of Yemen's Jebel Zubair (Zubair Group, figure 1). A new island emerged in this vicinity and was large enough to resolve in satellite imagery by 23 December 2011. The latitude and longitude given in the title for Jebel Zubair (15.05°N, 42.18°E) indicate the largest island of the Zubair Group (figure 1); the new island emerged at approximately 15.158°N, 42.101°E.

Figure 1. Map and index map of the 10 islands of the Zubair Group (Yemen) with our indication of the site of the new eruption and its associated emergent island. The islands emerge in the southern Red Sea, dotting an elongate region of about 8 x 27 km. Islands represented by gray shading; other features identified by legend at left. Plotted earthquake epicenters are given in table 1. The cross section at bottom is along line A-B. Modified from Gass and others (1973); index map modified from MapsOf.net.

Initial reports. According to an article published in the Yemen Observer on 19 December, the fishermen who first reported the eruption stated that it was near Saba island (figure 1). They stated that they could see the eruption from 3 hours travel time away. The fishermen reported that the volcano had been "popping up red lava that reached 20-30 meters high." The same day, an EOS-AURA Ozone Monitoring Istrument (OMI) image showed an SO₂ cloud in the area (figure 2). According to Volcano Discovery, a reader from Yemen confirmed the reported eruption, and added that an earthquake was felt on 19 December. Two other seismic events in the Zubair Group were recorded by the Seismological and Volcanological Observatory Center (SVOC) on 13 December (table 1, figure 1).

Figure 2. An SO2 cloud over the area of the Zubair island group captured by the AURA satellite's OMI imager between 1023 and 1205 on 19 December 2011. Scale at right is in Dobson Units (DU). The mass of SO2 depicted on this image was 0.403 kilotons (kt); the area of the cloud was ~45,352 km2; the maximum SO2 values on the image occured at 15.28°N and 14.28°E and reached 1.4 DU. Courtesy of Simon Carn and NASA Global Sulfur Dioxide Monitoring Aura/OMI.

Table 1. Seismic events recorded in the Zubair Group in December 2011. Courtesy of the Seismological and Volcanological Observatory Center (SVOC).

On 20 December, the Toulouse Volcanic Ash Advisory Center (VAAC) reported a white plume that may have contained some unidentifiable ash (as reported that day from an aircraft in the area). Their report included a remark that the eruption seemed to be a continuing submarine eruption that began on 18 December. They stated that the plume was not identifiable in their satellite data. On 22 December, the Emirates News Agency published an article reporting that the head of SVOC stated that, based on preliminary data, there was no danger to marine navigation.

Small plumes were visible on MODIS imagery beginning on 18 December (figure 3). While cloud cover and dust plumes rendered the images speculative on a few days, others provided a clear view of the plumes, and highlighted their origin (figure 3f). The plumes did not appear to originate from one of the Zubair islands, but instead from just N of Rugged and ~ 1.5 km SW of Haycock islands (figure 4; also see "Eruption site" on figure 1). The lack of plumes prior to 18 December 2011, and the persistence of plumes after, indicates that the eruption began breaking the surface of the Red Sea sometime during 17-18 December.

Figure 3. Satellite images of the Zubair Group captured during 17-22 December 2011. Images b-f show small plumes (circled) emanating from ~1.5 km SW of Haycock and just N of Rugged. A dust plume somewhat obscures the visibility in (b) and clouds are present in (c) and (d). Pixel resolution in each image is 250 m. All images acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA's Aqua satellite except (d), which was acquired by the MODIS instrument aboard NASA's Terra satellite. Images courtesy of NASA's Land Atmosphere Near Real-time Capability for EOS (LANCE).

Figure 4. NASA Earth Observatory images captured by the Advanced Land Imager (ALI) aboard NASA's Earth Observing-1 (EO-1) satellite on (a) 24 October 2007 and (b) 23 December 2011. The 23 December 2011 image shows that an apparent new island is the eruption site, less than 1 km to the N of Rugged Island. An eruptive plume is seen rising and drifting to the N. Courtesy of Jesse Allen and Michon Scott, NASA Earth Observatory.

New island. Finally, following approximately a week of widespread speculation on the exact location of the plume's source, NASA Earth Observatory published a high resolution satellite image of a new eruption (acquired 23 December 2011), clearly showing the off-island source of the eruptive plumes (figure 4b). From comparison with an image acquired on 24 October 2007, the 23 December 2011 image clearly shows that the eruption site was less than 1 km due N of Rugged Island, and was an apparent new island (figure 4). Their report stated that "The image . . . shows an apparent island where there had previously been an unbroken water surface." As of 28 December 2011, all information seems to point to the formation of a new, as yet unnamed, island in the Red Sea.

Reference. Gass, I.G., Mallick, D.I.J., and Cox, K.G., 1973, Volcanic islands of the Red Sea, Journal of the Geological Society of London, v. 129, p. 275-310 (DOI: 10.1144/gs

Geologic Background. The 5-km-long Jebel Zubair Island is the largest of a group of small islands and submerged shoals that rise from a shallow platform in the Red Sea rift. The platform and eruptive vents forming the islands and shoals are oriented NNW-SSE, parallel to the rift. An early explosive phase was followed by a brief period of marine erosion, then by renewed explosive activity accompanied by the extrusion of basaltic pahoehoe lava flows. This latest phase of activity occurred on the morphologically youngest islands of Zubair, Centre Peak, Saba, and Haycock. Historical explosive activity was reported from Saddle Island in the 19th century. Spatter cones and pyroclastic cones were erupted along fissures that form the low spine of Zubair Island. Eruptions that began in late 2011 built two new islands, increasing the total number in the group to 12.

Information Contacts: MapsOf.net (URL: http://mapsof.net/); The Yemen Observer, P.O. Box 19183, Sana'a, Rep. of Yemen (URL: http://www.yobserver.com/); Seismological and Volcanological Observatory Center (SVOC), P.O. Box 87175, Dhamar, Yemen (URL: http://www.nsoc.org.ye/); Simon Carn, NASA Global Sulfur Dioxide Monitoring, Aura/OMI (URL: https://so2.gsfc.nasa.gov/); Toulouse Volcanic Ash Advisory Center (VAAC), Météo France, 42 Avenue Gaspard Coriolis, 31057 Toulouse Cedex 1, France (URL: http://www.meteo.fr/vaac/eindex.html); Emirates News Agency, 3790 Abu Dhabi, United Arab Emirates (URL: http://wam.org.ae/); NASA's Land Atmosphere Near Real-time Capability for EOS (LANCE) (URL: http://lance.nasa.gov/); NASA Earth Observatory (Jesse Allen and Michon Scott), NASA Goddard Space Flight Center (URL: http://earthobservatory.nasa.gov/).

In discussing the week ending on 12 September, "Earthweek" (Newman, 1997) incorrectly claimed that a volcano named "Mount Pinukis" had erupted. Widely read in the US, the dramatic Earthweek report described terrified farmers and a black mushroom cloud that resembled a nuclear explosion. The mountain's location was given as "200 km E of Zamboanga City," a spot well into the sea. The purported eruption had received mention in a Manila Bulletin newspaper report nine days earlier, on 4 September. Their comparatively understated report said that a local police director had disclosed that residents had seen a dormant volcano showing signs of activity.

In response to these news reports Emmanuel Ramos of the Philippine Institute of Volcanology and Seismology (PHIVOLCS) sent a reply on 17 September. PHIVOLCS staff had initially heard that there were some 12 alleged families who fled the mountain and sought shelter in the lowlands. A PHIVOLCS investigation team later found that the reported "families" were actually individuals seeking respite from some politically motivated harassment. The story seems to have stemmed from a local gold rush and an influential politician who wanted to use volcanism as a ploy to exclude residents. PHIVOLCS concluded that no volcanic activity had occurred. They also added that this finding disappointed local politicians but was much welcomed by the residents.

PHIVOLCS spelled the mountain's name as "Pinokis" and from their report it seems that it might be an inactive volcano. There is no known Holocene volcano with a similar name (Simkin and Siebert, 1994). No similar names (Pinokis, Pinukis, Pinakis, etc.) were found listed in the National Imagery and Mapping Agency GEOnet Names Server (http://geonames.nga.mil/gns/html/index.html), a searchable database of 3.3 million non-US geographic-feature names.

The Manila Bulletin report suggested that Pinokis resides on the Zamboanga Peninsula. The Peninsula lies on Mindanao Island's extreme W side where it bounds the Moro Gulf, an arm of the Celebes Sea. The mountainous Peninsula trends NNE-SSW and contains peaks with summit elevations near 1,300 m. Zamboanga City sits at the extreme end of the Peninsula and operates both a major seaport and an international airport.

[Later investigation found that Mt. Pinokis is located in the Lison Valley on the Zamboanga Peninsula, about 170 km NE of Zamboanga City and 30 km NW of Pagadian City. It is adjacent to the two peaks of the Susong Dalaga (Maiden's Breast) and near Mt. Sugarloaf.]

References. Newman, S., 1997, Earthweek, a diary of the planet (week ending 12 September): syndicated newspaper column (URL: http://www.earthweek.com/).

Manila Bulletin, 4 Sept. 1997, Dante's Peak (URL: http://www.mb.com.ph/).

Simkin, T., and Siebert, L., 1994, Volcanoes of the world, 2nd edition: Geoscience Press in association with the Smithsonian Institution Global Volcanism Program, Tucson AZ, 368 p.

Information Contacts: Emmanuel G. Ramos, Deputy Director, Philippine Institute of Volcanology and Seismology, Department of Science and Technology, PHIVOLCS Building, C. P. Garcia Ave., University of the Philippines, Diliman campus, Quezon City, Philippines.

Xinhua News Agency filed a news report on 27 February under the headline "Volcano erupts in Somalia" but the veracity of the story now appears doubtful. The report disclosed the volcano's location as on the W side of the Gedo region, an area along the Ethiopian border just NE of Kenya. The report had relied on the commissioner of the town of Bohol Garas (a settlement described as 40 km NE of the main Al-Itihad headquarters of Luq town) and some or all of the information was relayed by journalists through VHF radio. The report claimed the disaster "wounded six herdsmen" and "claimed the lives of 290 goats grazing near the mountain when the incident took place." Further descriptions included such statements as "the volcano which erupted two days ago [25 February] has melted down the rocks and sand and spread . . . ."

Giday WoldeGabriel returned from three weeks of geological fieldwork in SW Ethiopia, near the Kenyan border, on 25 August. During his time there he inquired of many people, including geologists, if they had heard of a Somalian eruption in the Gedo area; no one had heard of the event. WoldeGabriel stated that he felt the news report could have described an old mine or bomb exploding. Heavy fighting took place in the Gedo region during the Ethio-Somalian war of 1977. Somalia lacks an embassy in Washington DC; when asked during late August, Ayalaw Yiman, an Ethiopian embassy staff member in Washington DC also lacked any knowledge of a Somalian eruption.

A Somalian eruption would be significant since the closest known Holocene volcanoes occur in the central Ethiopian segment of the East African rift system S of Addis Ababa, ~500 km NW of the Gedo area. These Ethiopian rift volcanoes include volcanic fields, shield volcanoes, cinder cones, and stratovolcanoes.

Information Contacts: Xinhua News Agency, 5 Sharp Street West, Wanchai, Hong Kong; Giday WoldeGabriel, EES-1/MS D462, Geology-Geochemistry Group, Los Alamos National Laboratory, Los Alamos, NM 87545; Ayalaw Yiman, Ethiopian Embassy, 2134 Kalorama Rd. NW, Washington DC 20008.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

In December 2002 information appeared in Mongolian and Russian newspapers and on national TV that a volcano in Central Mongolia, the Har-Togoo volcano, was producing white vapors and constant acoustic noise. Because of the potential hazard posed to two nearby settlements, mainly with regard to potential blocking of rivers, the Director of the Research Center of Astronomy and Geophysics of the Mongolian Academy of Sciences, Dr. Bekhtur, organized a scientific expedition to the volcano on 19-20 March 2003. The scientific team also included M. Ulziibat, seismologist from the same Research Center, M. Ganzorig, the Director of the Institute of Informatics, and A. Ivanov from the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences.

Geological setting. The Miocene Har-Togoo shield volcano is situated on top of a vast volcanic plateau (figure 1). The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Pliocene and Quaternary volcanic rocks are also abundant in the vicinity of the Holocene volcanoes (Devyatkin and Smelov, 1979; Logatchev and others, 1982). Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Figure 1. Photograph of the Har-Togoo volcano viewed from west, March 2003. Courtesy of Alexei Ivanov.

Observations during March 2003. The name of the volcano in the Mongolian language means "black-pot" and through questioning of the local inhabitants, it was learned that there is a local myth that a dragon lived in the volcano. The local inhabitants also mentioned that marmots, previously abundant in the area, began to migrate westwards five years ago; they are now practically absent from the area.

Acoustic noise and venting of colorless warm gas from a small hole near the summit were noticed in October 2002 by local residents. In December 2002, while snow lay on the ground, the hole was clearly visible to local visitors, and a second hole could be seen a few meters away; it is unclear whether or not white vapors were noticed on this occasion. During the inspection in March 2003 a third hole was seen. The second hole is located within a 3 x 3 m outcrop of cinder and pumice (figure 2) whereas the first and the third holes are located within massive basalts. When close to the holes, constant noise resembled a rapid river heard from afar. The second hole was covered with plastic sheeting fixed at the margins, but the plastic was blown off within 2-3 seconds. Gas from the second hole was sampled in a mechanically pumped glass sampler. Analysis by gas chromatography, performed a week later at the Institute of the Earth's Crust, showed that nitrogen and atmospheric air were the major constituents.

Figure 2. Photograph of the second hole sampled at Har-Togoo, with hammer for scale, March 2003. Courtesy of Alexei Ivanov.

The temperature of the gas at the first, second, and third holes was +1.1, +1.4, and +2.7°C, respectively, while air temperature was -4.6 to -4.7°C (measured on 19 March 2003). Repeated measurements of the temperatures on the next day gave values of +1.1, +0.8, and -6.0°C at the first, second, and third holes, respectively. Air temperature was -9.4°C. To avoid bias due to direct heating from sunlight the measurements were performed under shadow. All measurements were done with Chechtemp2 digital thermometer with precision of ± 0.1°C and accuracy ± 0.3°C.

Inside the mouth of the first hole was 4-10-cm-thick ice with suspended gas bubbles (figure 5). The ice and snow were sampled in plastic bottles, melted, and tested for pH and Eh with digital meters. The pH-meter was calibrated by Horiba Ltd (Kyoto, Japan) standard solutions 4 and 7. Water from melted ice appeared to be slightly acidic (pH 6.52) in comparison to water of melted snow (pH 7.04). Both pH values were within neutral solution values. No prominent difference in Eh (108 and 117 for ice and snow, respectively) was revealed.

Two digital short-period three-component stations were installed on top of Har-Togoo, one 50 m from the degassing holes and one in a remote area on basement rocks, for monitoring during 19-20 March 2003. Every hour 1-3 microseismic events with magnitude <2 were recorded. All seismic events were virtually identical and resembled A-type volcano-tectonic earthquakes (figure 6). Arrival difference between S and P waves were around 0.06-0.3 seconds for the Har-Togoo station and 0.1-1.5 seconds for the remote station. Assuming that the Har-Togoo station was located in the epicentral zone, the events were located at ~1-3 km depth. Seismic episodes similar to volcanic tremors were also recorded (figure 3).

Figure 3. Examples of an A-type volcano-tectonic earthquake and volcanic tremor episodes recorded at the Har-Togoo station on 19 March 2003. Courtesy of Alexei Ivanov.

Conclusions. The abnormal thermal and seismic activities could be the result of either hydrothermal or volcanic processes. This activity could have started in the fall of 2002 when they were directly observed for the first time, or possibly up to five years earlier when marmots started migrating from the area. Further studies are planned to investigate the cause of the fumarolic and seismic activities.

At the end of a second visit in early July, gas venting had stopped, but seismicity was continuing. In August there will be a workshop on Russian-Mongolian cooperation between Institutions of the Russian and Mongolian Academies of Sciences (held in Ulan-Bator, Mongolia), where the work being done on this volcano will be presented.

References. Devyatkin, E.V. and Smelov, S.B., 1979, Position of basalts in sequence of Cenozoic sediments of Mongolia: Izvestiya USSR Academy of Sciences, geological series, no. 1, p. 16-29. (In Russian).

Logatchev, N.A., Devyatkin, E.V., Malaeva, E.M., and others, 1982, Cenozoic deposits of Taryat basin and Chulutu river valley (Central Hangai): Izvestiya USSR Academy of Sciences, geological series, no. 8, p. 76-86. (In Russian).

Geologic Background. The Miocene Har-Togoo shield volcano, also known as Togoo Tologoy, is situated on top of a vast volcanic plateau. The 5,000-year-old Khorog (Horog) cone in the Taryatu-Chulutu volcanic field is located 135 km SW and the Quaternary Urun-Dush cone in the Khanuy Gol (Hanuy Gol) volcanic field is 95 km ENE. Analysis of seismic activity recorded by a network of seismic stations across Mongolia shows that earthquakes of magnitude 2-3.5 are scattered around the Har-Togoo volcano at a distance of 10-15 km.

Information Contacts: Alexei V. Ivanov, Institute of the Earth Crust SB, Russian Academy of Sciences, Irkutsk, Russia; Bekhtur andM. Ulziibat, Research Center of Astronomy and Geophysics, Mongolian Academy of Sciences, Ulan-Bator, Mongolia; M. Ganzorig, Institute of Informatics MAS, Ulan-Bator, Mongolia.

An eruption at Mount Elgon was mistakenly inferred when fumes escaped from this otherwise quiet volcano. The fumes were eventually traced to dung burning in a lava-tube cave. The cave is home to, or visited by, wildlife ranging from bats to elephants. Mt. Elgon (Ol Doinyo Ilgoon) is a stratovolcano on the SW margin of a 13 x 16 km caldera that straddles the Uganda-Kenya border 140 km NE of the N shore of Lake Victoria. No eruptions are known in the historical record or in the Holocene.

On 7 September 2004 the web site of the Kenyan newspaper The Daily Nation reported that villagers sighted and smelled noxious fumes from a cave on the flank of Mt. Elgon during August 2005. The villagers' concerns were taken quite seriously by both nations, to the extent that evacuation of nearby villages was considered.

The Daily Nation article added that shortly after the villagers' reports, Moses Masibo, Kenya's Western Province geology officer visited the cave, confirmed the villagers observations, and added that the temperature in the cave was 170°C. He recommended that nearby villagers move to safer locations. Masibo and Silas Simiyu of KenGens geothermal department collected ashes from the cave for testing.

Gerald Ernst reported on 19 September 2004 that he spoke with two local geologists involved with the Elgon crisis from the Geology Department of the University of Nairobi (Jiromo campus): Professor Nyambok and Zacharia Kuria (the former is a senior scientist who was unable to go in the field; the latter is a junior scientist who visited the site). According to Ernst their interpretation is that somebody set fire to bat guano in one of the caves. The fire was intense and probably explains the vigorous fuming, high temperatures, and suffocated animals. The event was also accompanied by emissions of gases with an ammonia odor. Ernst noted that this was not surprising considering the high nitrogen content of guano—ammonia is highly toxic and can also explain the animal deaths. The intense fumes initially caused substantial panic in the area.

It was Ernst's understanding that the authorities ordered evacuations while awaiting a report from local scientists, but that people returned before the report reached the authorities. The fire presumably prompted the response of local authorities who then urged the University geologists to analyze the situation. By the time geologists arrived, the fuming had ceased, or nearly so. The residue left by the fire and other observations led them to conclude that nothing remotely related to a volcanic eruption had occurred.

However, the incident emphasized the problem due to lack of a seismic station to monitor tectonic activity related to a local triple junction associated with the rift valley or volcanic seismicity. In response, one seismic station was moved from S Kenya to the area of Mt. Elgon so that local seismicity can be monitored in the future.

Information Contacts: Gerald Ernst, Univ. of Ghent, Krijgslaan 281/S8, B-9000, Belgium; Chris Newhall, USGS, Univ. of Washington, Dept. of Earth & Space Sciences, Box 351310, Seattle, WA 98195-1310, USA; The Daily Nation (URL: http://www.nationmedia.com/dailynation/); Uganda Tourist Board (URL: http://www.visituganda.com/).