Manam is a 10-km-wide island that consists of two active summit craters: the Main summit crater and the South summit crater and is located 13 km off the northern coast of mainland Papua New Guinea. Frequent mild-to-moderate eruptions have been recorded since 1616. The current eruption period began during June 2014 and has more recently been characterized by intermittent ash plumes and thermal activity (BGVN 47:11). This report updates activity that occurred from November 2022 through May 2023 based on information from the Darwin Volcanic Ash Advisory Center (VAAC) and various satellite data.

Ash plumes were reported during November and December 2022 by the Darwin VAAC. On 7 November an ash plume rose to 2.1 km altitude and drifted NE based on satellite images and weather models. On 14 November an ash plume rose to 2.1 km altitude and drifted W based on RVO webcam images. On 20 November ash plumes rose to 1.8 km altitude and drifted NW. On 26 December an ash plume rose to 3 km altitude and drifted S and SSE.

Intermittent sulfur dioxide plumes were detected using the TROPOMI instrument on the Sentinel-5P satellite, some of which exceeded at least two Dobson Units (DU) and drifted in different directions (figure 93). Occasional low-to-moderate power thermal anomalies were recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system; less than five anomalies were recorded each month during November 2022 through May 2023 (figure 94). Two thermal hotspots were detected by the MODVOLC thermal alerts system on 10 December 2022. On clear weather days, thermal activity was also captured in infrared satellite imagery in both the Main and South summit craters, accompanied by gas-and-steam emissions (figure 95).

Figure 93. Distinct sulfur dioxide plumes were captured, rising from Manam based on data from the TROPOMI instrument on the Sentinel-5P satellite on 16 November 2022 (top left), 6 December 2022 (top right), 14 January 2023 (bottom left), and 23 March 2023 (bottom right). Plumes generally drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 94. Occasional low-to-moderate power thermal anomalies were detected at Manam during November 2022 through May 2023, as shown in this MIROVA graph (Log Radiative Power). Only three anomalies were detected during late November, one in early December, two during January 2023, one in late March, four during April, and one during late May. Courtesy of MIROVA.

Figure 95. Infrared (bands B12, B11, B4) satellite images show a consistent thermal anomaly (bright yellow-orange) in both the Main (the northern crater) and South summit craters on 10 November 2022 (top left), 15 December 2022 (top right), 3 February 2023 (bottom left), and 24 April 2023 (bottom right). Gas-and-steam emissions occasionally accompanied the thermal activity. Courtesy of Copernicus Browser.

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

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); 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/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers activity during November 2022 through April 2023 based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

Activity was relatively low during November and December 2022. Daily white gas-and-steam plumes rose 25-100 m above the summit and drifted in different directions. Gray ash plumes rose 200 m above the summit and drifted NE at 1047 and at 2343 on 11 November. On 14 November at 0933 ash plumes rose 300 m above the summit and drifted E. An ash plume was reported at 0935 on 15 December that rose 100 m above the summit and drifted NE. An eruptive event at 1031 later that day generated an ash plume that rose 700 m above the summit and drifted NE. A gray ash plume at 1910 rose 100 m above the summit and drifted E. Incandescent material was ejected above the vent based on an image taken at 1936.

During January 2023 daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions. Gray-to-brown ash plumes were reported at 1638 on 3 January, at 1410 and 1509 on 4 January, and at 0013 on 5 January that rose 100-750 m above the summit and drifted NE and E; the gray-to-black ash plume at 1509 on 4 January rose as high as 3 km above the summit and drifted E. Gray ash plumes were recorded at 1754, 2241, and 2325 on 11 January and at 0046 on 12 January and rose 200-300 m above the summit and drifted NE. Toward the end of January, PVMBG reported that activity had intensified; Strombolian activity was visible in webcam images taken at 0041, 0043, and 0450 on 23 January. Multiple gray ash plumes throughout the day rose 200-500 m above the summit and drifted E and SE (figure 135). Webcam images showed progressively intensifying Strombolian activity at 1919, 1958, and 2113 on 24 January; a gray ash plume at 1957 rose 300 m above the summit and drifted E (figure 135). Eruptive events at 0231 and 2256 on 25 January and at 0003 on 26 January ejected incandescent material from the vent, based on webcam images. Gray ash plumes observed during 26-27 January rose 300-500 m above the summit and drifted NE, E, and SE.

Figure 135. Webcam images of a strong, gray ash plume (left) and Strombolian activity (right) captured at Krakatau at 0802 on 23 January 2023 (left) and at 2116 on 24 January 2023 (right). Courtesy of PVMBG and MAGMA Indonesia.

Low levels of activity were reported during February and March. Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in different directions. The Darwin VAAC reported that continuous ash emissions rose to 1.5-1.8 km altitude and drifted W and NW during 1240-1300 on 10 March, based on satellite images, weather models, and PVMBG webcams. White-and-gray ash plumes rose 500 m and 300 m above the summit and drifted SW at 1446 and 1846 on 18 March, respectively. An eruptive event was recorded at 2143, though it was not visible due to darkness. Multiple ash plumes were reported during 27-29 March that rose as high as 2.5 km above the summit and drifted NE, W, and SW (figure 136). Webcam images captured incandescent ejecta above the vent at 0415 and around the summit area at 2003 on 28 March and at 0047 above the vent on 29 March.

Figure 136. Webcam image of a strong ash plume rising above Krakatau at 1522 on 28 March 2023. Courtesy of PVMBG and MAGMA Indonesia.

Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions during April and May. White-and-gray and black plumes rose 50-300 m above the summit on 2 and 9 April. On 11 May at 1241 a gray ash plume rose 1-3 km above the summit and drifted SW. On 12 May at 0920 a gray ash plume rose 2.5 km above the summit and drifted SW and at 2320 an ash plume rose 1.5 km above the summit and drifted SW. An accompanying webcam image showed incandescent ejecta. On 13 May at 0710 a gray ash plume rose 2 km above the summit and drifted SW (figure 137).

Figure 137. Webcam image of an ash plume rising 2 km above the summit of Krakatau at 0715 on 13 May 2023. Courtesy of PVMBG and MAGMA Indonesia.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during November 2022 through April 2023 (figure 138). Some of this thermal activity was also visible in infrared satellite imagery at the crater, accompanied by gas-and-steam and ash plumes that drifted in different directions (figure 139).

Figure 138. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during November 2022 through April 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 139. A thermal anomaly (bright yellow-orange) was visible at Krakatau in infrared (bands B12, B11, B4) satellite images on clear weather days during November 2022 through May 2023. Occasional gas-and-steam and ash plumes accompanied the thermal activity, which drifted in different directions. Images were captured on 25 November 2022 (top left), 15 December 2022 (top right), 27 January 2023 (bottom left), and 12 May 2023 (bottom right). Courtesy of Copernicus Browser.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 350 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of frequent Strombolian explosions and lava flows (BGVN 48:02). This report updates activity during January through April 2023 primarily characterized by Strombolian explosions and lava flows based on reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 253, table 16). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 254). According to data collected by the MODVOLC thermal algorithm, a total of 9 thermal alerts were detected: one on 2 January 2023, one on 1 February, five on 24 March, and two on 26 March. The stronger pulses of thermal activity likely reflected lava flow events. Infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 8 March (figure 255).

Figure 253. Explosive activity persisted at Stromboli during January through April 2023, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during January through April (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 17/2023, Stromboli, Bollettino Settimanale, 18/04/2023 - 24/04/2023).

Table 16. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during January-April 2023; information from webcam observations. Courtesy of INGV weekly reports.

Figure 254. Intermittent thermal activity at Stromboli was detected during January through April 2023 and varied in strength, as shown in this MIROVA graph (Log Radiative Power). A pulse of activity was captured during late March. Courtesy of MIROVA.

Figure 255. Infrared (bands B12, B11, B4) satellite images showing persistent thermal anomalies at both summit crater on 1 February 2023 (top left), 23 March 2023 (top right), 8 March 2023 (bottom left), and 27 April 2023. A particularly strong thermal anomaly was visible on 8 March. Courtesy of Copernicus Browser.

Activity during January-February 2023. Strombolian explosions were reported in the N crater area, as well as lava effusion. Explosive activity in the N crater area ejected coarse material (bombs and lapilli). Intense spattering was observed in both the N1 and N2 craters. In the CS crater area, explosions generally ejected fine material (ash), sometimes to heights greater than 250 m. The intensity of the explosions was characterized as low-to-medium in the N crater and medium-to-high in the CS crater. After intense spattering activity from the N crater area, a lava overflow began at 2136 on 2 January that flowed part way down the Sciara del Fuoco, possibly moving down the drainage that formed in October, out of view from webcams. The flow remained active for a couple of hours before stopping and beginning to cool. A second lava flow was reported at 0224 on 4 January that similarly remained active for a few hours before stopping and cooling. Intense spattering was observed on 11 and 13 January from the N1 crater. After intense spattering activity at the N2 crater at 1052 on 17 January another lava flow started to flow into the upper part of the Sciara del Fuoco (figure 256), dividing into two: one that traveled in the direction of the drainage formed in October, and the other one moving parallel to the point of emission. By the afternoon, the rate of the flow began to decrease, and at 1900 it started to cool. A lava flow was reported at 1519 on 24 January following intense spattering in the N2 area, which began to flow into the upper part of the Sciara del Fuoco. By the morning of 25 January, the lava flow had begun to cool. During 27 January the frequency of eruption in the CS crater area increased to 6-7 events/hour compared to the typical 1-7 events/hour; the following two days showed a decrease in frequency to less than 1 event/hour. Starting at 1007 on 30 January a high-energy explosive sequence was produced by vents in the CS crater area. The sequence began with an initial energetic pulse that lasted 45 seconds, ejecting predominantly coarse products 300 m above the crater that fell in an ESE direction. Subsequent and less intense explosions ejected material 100 m above the crater. The total duration of this event lasted approximately two minutes. During 31 January through 6, 13, and 24 February spattering activity was particularly intense for short periods in the N2 crater.

Figure 256. Webcam images of the lava flow development at Stromboli during 17 January 2023 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 04/2023, Stromboli, Bollettino Settimanale, 16/01/2023 - 22/01/2023).

An explosive sequence was reported on 16 February that was characterized by a major explosion in the CS crater area (figure 257). The sequence began at 1817 near the S2 crater that ejected material radially. A few seconds later, lava fountains were observed in the central part of the crater. Three explosions of medium intensity (material was ejected less than 150 m high) were recorded at the S2 crater. The first part of this sequence lasted approximately one minute, according to INGV, and material rose 300 m above the crater and then was deposited along the Sciara del Fuoco. The second phase began at 1818 at the S1 crater; it lasted seven seconds and material was ejected 150 m above the crater. Another event 20 seconds later lasted 12 seconds, also ejecting material 150 m above the crater. The sequence ended with at least three explosions of mostly fine material from the S1 crater. The total duration of this sequence was about two minutes.

Figure 257. Webcam images of the explosive sequence at Stromboli on 16 February 2023 taken by the SCT and SCV infrared and visible cameras. The lava appears light yellow-green in the infrared images. Courtesy of INGV (Report 08/2023, Stromboli, Bollettino Settimanale, 13/02/2023 - 19/02/2023).

Short, intense spattering activity was noted above the N1 crater on 27 and 28 February. A lava overflow was first reported at 0657 from the N2 crater on 27 February that flowed into the October 2022 drainage. By 1900 the flow had stopped. A second lava overflow also in the N crater area occurred at 2149, which overlapped the first flow and then stopped by 0150 on 28 February. Material detached from both the lava overflows rolled down the Sciara del Fuoco, some of which was visible in webcam images.

Activity during March-April 2023. Strombolian activity continued with spattering activity and lava overflows in the N crater area during March. Explosive activity at the N crater area varied from low (less than 80 m high) to medium (less than 150 m high) and ejected coarse material, such as bombs and lapilli. Spattering was observed above the N1 crater, while explosive activity at the CS crater area varied from medium to high (greater than 150 m high) and ejected coarse material. Intense spattering activity was observed for short periods on 6 March above the N1 crater. At approximately 0610 a lava overflow was reported around the N2 crater on 8 March, which then flowed into the October 2022 drainage. By 1700 the flow started to cool. A second overflow began at 1712 on 9 March and overlapped the previous flow. It had stopped by 2100. Material from both flows was deposited along the Sciara del Fuoco, though much of the activity was not visible in webcam images. On 11 March a lava overflow was observed at 0215 that overlapped the two previous flows in the October 2022 drainage. By late afternoon on 12 March, it had stopped.

During a field excursion on 16 March, scientists noted that a vent in the central crater area was degassing. Another vent showed occasional Strombolian activity that emitted ash and lapilli. During 1200-1430 low-to-medium intense activity was reported; the N1 crater emitted ash emissions and the N2 crater emitted both ash and coarse material. Some explosions also occurred in the CS crater area that ejected coarse material. The C crater in the CS crater area occasionally showed gas jetting and low intensity explosions on 17 and 22 March; no activity was observed at the S1 crater. Intense, longer periods of spattering were reported in the N1 crater on 19, 24, and 25 March. Around 2242 on 23 March a lava overflow began from the N1 crater that, after about an hour, began moving down the October 2022 drainage and flow along the Sciara del Fuoco (figure 258). Between 0200 and 0400 on 26 March the flow rate increased, which generated avalanches of material from collapses at the advancing flow front. By early afternoon, the flow began to cool. On 25 March at 1548 an explosive sequence began from one of the vents at S2 in the CS crater area (figure 258). Fine ash mixed with coarse material was ejected 300 m above the crater rim and drifted SSE. Some modest explosions around Vent C were detected at 1549 on 25 March, which included an explosion at 1551 that ejected coarse material. The entire explosive sequence lasted approximately three minutes.

Figure 258. Webcam images of the lava overflow in the N1 crater area of Stromboli on 23 March 2023 taken by the SCT infrared camera. The lava appears light yellow-green in the infrared images. The start of the explosive sequence was also captured on 25 March 2023 accompanied by an eruption plume (e) captured by the SCT and SPT infrared webcams. Courtesy of INGV (Report 13/2023, Stromboli, Bollettino Settimanale, 20/03/2023 - 26/03/2023).

During April explosions persisted in both the N and CS crater areas. Fine material was ejected less than 80 m above the N crater rim until 6 April, followed by ejection of coarser material. Fine material was also ejected less than 80 m above the CS crater rim. The C and S2 crater did not show significant eruptive activity. On 7 April an explosive sequence was detected in the CS crater area at 1203 (figure 259). The first explosion lasted approximately 18 seconds and ejected material 400 m above the crater rim, depositing pyroclastic material in the upper part of the Sciara del Fuoco. At 1204 a second, less intense explosion lasted approximately four seconds and deposited pyroclastic products outside the crater area and near Pizzo Sopra La Fossa. A third explosion at 1205 was mainly composed of ash that rose about 150 m above the crater and lasted roughly 20 seconds. A fourth explosion occurred at 1205 about 28 seconds after the third explosion and ejected a mixture of coarse and fine material about 200 m above the crater; the explosion lasted roughly seven seconds. Overall, the entire explosive sequence lasted about two minutes and 20 seconds. After the explosive sequence on 7 April, explosions in both the N and CS crater areas ejected material as high as 150 m above the crater.

Figure 259. Webcam images of the explosive sequence at Stromboli during 1203-1205 (local time) on 7 April 2023 taken by the SCT infrared camera. Strong eruption plumes are visible, accompanied by deposits on the nearby flanks. Courtesy of INGV (Report 15/2023, Stromboli, Bollettino Settimanale, 03/04/2023 - 09/04/2023).

On 21 April research scientists from INGV made field observations in the summit area of Stromboli, and some lapilli samples were collected. In the N crater area near the N1 crater, a small cone was observed with at least two active vents, one of which was characterized by Strombolian explosions. The other vent produced explosions that ejected ash and chunks of cooled lava. At the N2 crater at least one vent was active and frequently emitted ash. In the CS crater area, a small cone contained 2-3 degassing vents and a smaller, possible fissure area also showed signs of degassing close to the Pizzo Sopra La Fossa. In the S part of the crater, three vents were active: a small hornito was characterized by modest and rare explosions, a vent that intermittently produced weak Strombolian explosions, and a vent at the end of the terrace that produced frequent ash emissions. Near the S1 crater there was a hornito that generally emitted weak gas-and-steam emissions, sometimes associated with “gas rings”. On 22 April another field inspection was carried out that reported two large sliding surfaces on the Sciara del Fuoco that showed where blocks frequently descended toward the sea. A thermal anomaly was detected at 0150 on 29 April.

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

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Nishinoshima is a small island located about 1,000 km S of Tokyo in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. Eruptions date back to 1973; the most recent eruption period began in October 2022 and was characterized by ash plumes and fumarolic activity (BGVN 47:12). This report describes ash plumes and fumarolic activity during November 2022 through April 2023 based on monthly reports from the Japan Meteorological Agency (JMA) monthly reports and satellite data.

The most recent eruptive activity prior to the reporting internal occurred on 12 October 2022, when an ash plume rose 3.5 km above the crater rim. An aerial observation conducted by the Japan Coast Guard (JCG) on 25 November reported that white fumaroles rose approximately 200 m above the central crater of a pyroclastic cone (figure 119), and multiple plumes were observed on the ESE flank of the cone. Discolored water ranging from reddish-brown to brown and yellowish-green were visible around the perimeter of the island (figure 119). No significant activity was reported in December.

Figure 119. Aerial photo of gas-and-steam plumes rising 200 m above Nishinoshima on 25 November 2022. Reddish brown to brown and yellowish-green discolored water was visible around the perimeter of the island. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, November 2022).

During an overflight conducted by JCG on 25 January 2023 intermittent activity and small, blackish-gray plumes rose 900 m above the central part of the crater were observed (figure 120). The fumarolic zone of the E flank and base of the cone had expanded and emissions had intensified. Dark brown discolored water was visible around the perimeter of the island.

Figure 120. Aerial photo of a black-gray ash plume rising approximately 900 m above the crater rim of Nishinoshima on 25 January 2023. White fumaroles were visible on the E slope of the pyroclastic cone. Dense brown to brown discolored water was observed surrounding the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, January, 2023).

No significant activity was reported during February through March. Ash plumes at 1050 and 1420 on 11 April rose 1.9 km above the crater rim and drifted NW and N. These were the first ash plumes observed since 12 October 2022. On 14 April JCG carried out an overflight and reported that no further eruptive activity was visible, although white gas-and-steam plumes were visible from the central crater and rose 900 m high (figure 121). Brownish and yellow-green discolored water surrounded the island.

Figure 121. Aerial photo of white gas-and-steam plumes rising 900 m above Nishinoshima on 14 April 2023. Brown and yellow-green discolored water is visible around the perimeter of the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, April, 2023).

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during November 2022 through April 2023 (figure 123). A cluster of six to eight anomalies were detected during November while a smaller number were detected during the following months: two to three during December, one during mid-January 2023, one during February, five during March, and two during April. Thermal activity was also reflected in infrared satellite data at the summit crater, accompanied by occasional gas-and-steam plumes (figure 124).

Figure 123. Intermittent low-to-moderate thermal anomalies were detected at Nishinoshima during November 2022 through April 2023, according to this MIROVA graph (Log Radiative Power). A cluster of anomalies occurred throughout November, while fewer anomalies were detected during the following months. Courtesy of MIROVA.

Figure 124. Infrared (bands B12, B11, B4) satellite images show a small thermal anomaly at the summit crater of Nishinoshima on 9 January 2023 (left) and 8 February 2023 (right). Gas-and-steam plumes accompanied this activity and extended S and SE, respectively. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Karangetang (also known as Api Siau), at the northern end of the island of Siau, Indonesia, contains five summit craters along a N-S line. More than 40 eruptions have been recorded since 1675; recent eruptions have included frequent explosive activity, sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters and collapses of lava flow fronts have produced pyroclastic flows. The two active summit craters are Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). The most recent eruption began in late November 2018 and has more recently consisted of weak thermal activity and gas-and-steam emissions (BGVN 48:01). This report updates activity characterized by lava flows, incandescent avalanches, and ash plumes during January through June 2023 using reports from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin VAAC (Volcano Ash Advisory Center), and satellite data.

Activity during January was relatively low and mainly consisted of white gas-and-steam emissions that rose 25-150 m above Main Crater (S crater) and drifted in different directions. Incandescence was visible from the lava dome in Kawah Dua (the N crater). Weather conditions often prevented clear views of the summit. On 18 January the number of seismic signals that indicated avalanches of material began to increase. In addition, there were a total of 71 earthquakes detected during the month.

Activity continued to increase during the first week of February. Material from Main Crater traveled as far as 800 m down the Batuawang (S) and Batang (W) drainages and as far as 1 km W down the Beha (W) drainage on 4 February. On 6 February 43 earthquake events were recorded, and on 7 February, 62 events were recorded. White gas-and-steam emissions rose 25-250 m above both summit craters throughout the month. PVMBG reported an eruption began during the evening of 8 February around 1700. Photos showed incandescent material at Main Crater. Incandescent material had also descended the flank in at least two unconfirmed directions as far as 2 km from Main Crater, accompanied by ash plumes (figure 60). As a result, PVMBG increased the Volcano Alert Level (VAL) to 3 (the second highest level on a 1-4 scale).

Figure 60. Photos of the eruption at Karangetang on 8 February 2023 that consisted of incandescent material descending the flanks (top left), ash plumes (top right and bottom left), and summit crater incandescence (bottom right). Courtesy of IDN Times.

Occasional nighttime webcam images showed three main incandescent lava flows of differing lengths traveling down the S, SW, and W flanks (figure 61). Incandescent rocks were visible on the upper flanks, possibly from ejected or collapsed material from the crater, and incandescence was the most intense at the summit. Based on analyses of satellite imagery and weather models, the Darwin VAAC reported that daily ash plumes during 16-20 February rose to 2.1-3 km altitude and drifted NNE, E, and SE. BNPB reported on 16 February that as many as 77 people were evacuated and relocated to the East Siau Museum. A webcam image taken at 2156 on 17 February possibly showed incandescent material descending the SE flank. Ash plumes rose to 2.1 km altitude and drifted SE during 22-23 February, according to the Darwin VAAC.

Figure 61. Webcam image of summit incandescence and lava flows descending the S, SW, and W flanks of Karangetang on 13 February 2023. Courtesy of MAGMA Indonesia.

Incandescent avalanches of material and summit incandescence at Main Crater continued during March. White gas-and-steam emissions during March generally rose 25-150 m above the summit crater; on 31 March gas-and-steam emissions rose 200-400 m high. An ash plume rose to 2.4 km altitude and drifted S at 1710 on 9 March and a large thermal anomaly was visible in images taken at 0550 and 0930 on 10 March. Incandescent material was visible at the summit and on the flanks based on webcam images taken at 0007 and 2345 on 16 March, at 1828 on 17 March, at 1940 on 18 March, at 2311 on 19 March, and at 2351 on 20 March. Incandescence was most intense on 18 and 20 March and webcam images showed possible Strombolian explosions (figure 62). An ash plume rose to 2.4 km altitude and drifted SW on 18 March, accompanied by a thermal anomaly.

Figure 62. Webcam image of intense summit incandescence and incandescent avalanches descending the flanks of Karangetang on 18 March 2023. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Summit crater incandescence at Main Crater and on the flanks persisted during April. Incandescent material at the S crater and on the flanks was reported at 0016 on 1 April. The lava flows had stopped by 1 April according to PVMBG, although incandescence was still visible up to 10 m high. Seismic signals indicating effusion decreased and by 6 April they were no longer detected. Incandescence was visible from both summit craters. On 26 April the VAL was lowered to 2 (the second lowest level on a 1-4 scale). White gas-and-steam emissions rose 25-200 m above the summit crater.

During May white gas-and-steam emissions generally rose 50-250 m above the summit, though it was often cloudy, which prevented clear views; on 21 May gas-and-steam emissions rose 50-400 m high. Nighttime N summit crater incandescence rose 10-25 m above the lava dome, and less intense incandescence was noted above Main Crater, which reached about 10 m above the dome. Sounds of falling rocks at Main Crater were heard on 15 May and the seismic network recorded 32 rockfall events in the crater on 17 May. Avalanches traveled as far as 1.5 km down the SW and S flanks, accompanied by rumbling sounds on 18 May. Incandescent material descending the flanks was captured in a webcam image at 2025 on 19 May (figure 63) and on 29 May; summit crater incandescence was observed in webcam images at 2332 on 26 May and at 2304 on 29 May. On 19 May the VAL was again raised to 3.

Figure 63. Webcam image showing incandescent material descending the flanks of Karangetang on 19 May 2023. Courtesy of MAGMA Indonesia.

Occasional Main Crater incandescence was reported during June, as well as incandescent material on the flanks. White gas-and-steam emissions rose 10-200 m above the summit crater. Ash plumes rose to 2.1 km altitude and drifted SE and E during 2-4 June, according to the Darwin VAAC. Material on the flanks of Main Crater were observed at 2225 on 7 June, at 2051 on 9 June, at 0007 on 17 June, and at 0440 on 18 June. Webcam images taken on 21, 25, and 27 June showed incandescence at Main Crater and from material on the flanks.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed strong thermal activity during mid-February through March and mid-May through June, which represented incandescent avalanches and lava flows (figure 64). During April through mid-May the power of the anomalies decreased but frequent anomalies were still detected. Brief gaps in activity occurred during late March through early April and during mid-June. Infrared satellite images showed strong lava flows mainly affecting the SW and S flanks, accompanied by gas-and-steam emissions (figure 65). According to data recorded by the MODVOLC thermal algorithm, there were a total of 79 thermal hotspots detected: 28 during February, 24 during March, one during April, five during May, and 21 during June.

Figure 64. Strong thermal activity was detected during mid-February 2023 through March and mid-May through June at Karangetang during January through June 2023, as recorded by this MIROVA graph (Log Radiative Power). During April through mid-May the power of the anomalies decreased, but the frequency at which they occurred was still relatively high. A brief gap in activity was shown during mid-June. Courtesy of MIROVA.

Figure 65. Incandescent avalanches of material and summit crater incandescence was visible in infrared satellite images (bands 12, 11, 8A) at both the N and S summit crater of Karangetang on 17 February 2023 (top left), 13 April 2023 (top right), 28 May 2023 (bottom left), and 7 June 2023 (bottom right), as shown in these infrared (bands 12, 11, 8A) satellite images. The incandescent avalanches mainly affected the SW and S flanks. Sometimes gas-and-steam plumes accompanied the thermal activity. Courtesy of Copernicus Browser.

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

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); IDN Times, Jl. Jend. Gatot Subroto Kav. 27 3rd Floor Kuningan, Jakarta, Indonesia 12950, Status of Karangetang Volcano in Sitaro Islands Increases (URL: https://sulsel.idntimes.com/news/indonesia/savi/status-gunung-api-karangetang-di-kepulauan-sitaro-meningkat?page=all).

Ahyi seamount is a large, conical submarine volcano that rises to within 75 m of the ocean surface about 18 km SE of the island of Farallon de Pajaros in the Northern Marianas. The remote location of the seamount has made eruptions difficult to document, but seismic stations installed in the region confirmed an eruption in the vicinity in 2001. No new activity was detected until April-May 2014 when an eruption was detected by NOAA (National Oceanic and Atmospheric Administration) divers, hydroacoustic sensors, and seismic stations (BGVN 42:04). New activity was first detected on 15 November by hydroacoustic sensors that were consistent with submarine volcanic activity. This report covers activity during November 2022 through June 2023 based on daily and weekly reports from the US Geological Survey.

Starting in mid-October, hydroacoustic sensors at Wake Island (2.2 km E) recorded signals consistent with submarine volcanic activity, according to a report from the USGS issued on 15 November 2022. A combined analysis of the hydroacoustic signals and seismic stations located at Guam and Chichijima Island, Japan, suggested that the source of this activity was at or near the Ahyi seamount. After a re-analysis of a satellite image of the area that was captured on 6 November, USGS confirmed that there was no evidence of discoloration at the ocean surface. Few hydroacoustic and seismic signals continued through November, including on 18 November, which USGS suggested signified a decline or pause in unrest. A VONA (Volcano Observatory Notice for Aviation) reported that a discolored water plume was persistently visible in satellite data starting on 18 November (figure 6). Though clouds often obscured clear views of the volcano, another discolored water plume was captured in a satellite image on 26 November. The Aviation Color Code (ACC) was raised to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level (VAL) was raised to Advisory (the second lowest level on a four-level scale) on 29 November.

Figure 6. A clear, true color satellite image showed a yellow-green discolored water plume extending NW from the Ahyi seamount (white arrow) on 21 November 2022. Courtesy of Copernicus Browser.

During December, occasional detections were recorded on the Wake Island hydrophone sensors and discolored water over the seamount remained visible. During 2-7, 10-12, and 16-31 December possible explosion signals were detected. A small area of discolored water was observed in high-resolution Sentinel-2 satellite images during 1-6 December (figure 7). High-resolution satellite images recorded discolored water plumes on 13 December that originated from the summit region; no observations indicated that activity breached the ocean surface. A possible underwater plume was visible in satellite images on 18 December, and during 19-20 December a definite but diffuse underwater plume located SSE from the main vent was reported. An underwater plume was visible in a satellite image taken on 26 December (figure 7).

Figure 7. Clear, true color satellite images showed yellow-green discolored water plumes extending NE and W from Ahyi (white arrows) on 1 (left) and 26 (right) December 2022, respectively. Courtesy of Copernicus Browser.

Hydrophone sensors continued to detect signals consistent with possible explosions during 1-8 January 2023. USGS reported that the number of detections decreased during 4-5 January. The hydrophone sensors experienced a data outage that started at 0118 on 8 January and continued through 10 January, though according to USGS, possible explosions were recorded prior to the data outage and likely continued during the outage. A discolored water plume originating from the summit region was detected in a partly cloudy satellite image on 8 January. On 11-12 and 15-17 January possible explosion signals were recorded again. One small signal was detected during 22-23 January and several signals were recorded on 25 and 31 January. During 27-31 January a plume of discolored water was observed above the seamount in satellite imagery (figure 8).

Figure 8. True color satellite images showed intermittent yellow-green discolored water plumes of various sizes extending N on 5 January 2023 (top left), SE on 30 January 2023 (top right), W on 4 February 2023 (bottom left), and SW on 1 March 2023 (bottom right) from Ahyi (white arrows). Courtesy of Copernicus Browser.

Low levels of activity continued during February and March, based on data from pressure sensors on Wake Island. During 1 and 4-6 February activity was reported, and a submarine plume was observed on 4 February (figure 8). Possible explosion signals were detected during 7-8, 10, 13-14, and 24 February. During 1-2 and 3-5 March a plume of discolored water was observed in satellite imagery (figure 8). Almost continuous hydroacoustic signals were detected in remote pressure sensor data on Wake Island 2,270 km E from the volcano during 7-13 March. During 12-13 March water discoloration around the seamount was observed in satellite imagery, despite cloudy weather. By 14 March discolored water extended about 35 km, but no direction was noted. USGS reported that the continuous hydroacoustic signals detected during 13-14 March stopped abruptly on 14 March and no new detections were observed. Three 30 second hydroacoustic detections were reported during 17-19 March, but no activity was visible due to cloudy weather. A data outage was reported during 21-22 March, making pressure sensor data unavailable; a discolored water plume was, however, visible in satellite data. A possible underwater explosion signal was detected by pressure sensors at Wake Island on 26, 29, and 31 March, though the cause and origin of these events were unclear.

Similar low activity continued during April, May, and June. Several signals were detected during 1-3 April in pressure sensors at Wake Island. USGS suggested that these may be related to underwater explosions or earthquakes at the volcano, but no underwater plumes were visible in clear satellite images. The pressure sensors had data outages during 12-13 April and no data were recorded; no underwater plumes were visible in satellite images, although cloudy weather obscured most clear views. Eruptive activity was reported starting at 2210 on 21 May. On 22 May a discolored water plume that extended 4 km was visible in satellite images, though no direction was recorded. During 23-24 May some signals were detected by the underwater pressure sensors. Possible hydroacoustic signals were detected during 2-3 and 6-8 June. Multiple hydroacoustic signals were detected during 9-11 and 16-17 June, although no activity was visible in satellite images. One hydroacoustic signal was detected during 23-24 June, but there was some uncertainty about its association with volcanic activity. A single possible hydroacoustic signal was detected during 30 June to 1 July.

Geologic Background. Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the ocean surface ~18 km SE of the island of Farallon de Pajaros in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Information Contacts: US Geological Survey, Volcano Hazards Program (USGS-VHP), 12201 Sunrise Valley Drive, Reston, VA, USA, https://volcanoes.usgs.gov/index.html; Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kadovar is a 2-km-wide island that is the emergent summit of a Bismarck Sea stratovolcano. It lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the volcano, filling an arcuate landslide scarp open to the S. Submarine debris-avalanche deposits occur to the S of the island. The current eruption began in January 2018 and has comprised lava effusion from vents at the summit and at the E coast; more recent activity has consisted of ash plumes, weak thermal activity, and gas-and-steam plumes (BGVN 48:02). This report covers activity during February through May 2023 using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

Activity during the reporting period was relatively low and mainly consisted of white gas-and-steam plumes that were visible in natural color satellite images on clear weather days (figure 67). According to a Darwin VAAC report, at 2040 on 6 May an ash plume rose to 4.6 km altitude and drifted W; by 2300 the plume had dissipated. MODIS satellite instruments using the MODVOLC thermal algorithm detected a single thermal hotspot on the SE side of the island on 7 May. Weak thermal activity was also detected in a satellite image on the E side of the island on 14 May, accompanied by a white gas-and-steam plume that drifted SE (figure 68).

Figure 67. True color satellite images showing a white gas-and-steam plume rising from Kadovar on 28 February 2023 (left) and 30 March 2023 (right) and drifting SE and S, respectively. Courtesy of Copernicus Browser.

Figure 68. Infrared (bands B12, B11, B4) image showing weak thermal activity on the E side of the island, accompanied by a gas-and-steam plume that drifted SE from Kadovar on 14 May 2023. Courtesy of Copernicus Browser.

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

Information Contacts: 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/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

San Miguel in El Salvador is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century and consists of the summit known locally as Chaparrastique. Flank eruptions have produced lava flows that extended to the N, NE, and SE during the 17-19th centuries. The most recent activity has consisted of minor ash eruptions from the summit crater. The current eruption period began in November 2022 and has been characterized by frequent phreatic explosions, gas-and-ash emissions, and sulfur dioxide plumes (BGVN 47:12). This report describes small gas-and-ash explosions during December 2022 through May 2023 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

Activity has been relatively low since the last recorded explosions on 29 November 2022. Seismicity recorded by the San Miguel Volcano Station (VSM) located on the N flank at 1.7 km elevation had decreased by 7 December. Sulfur dioxide gas measurements taken with DOAS (Differential Optical Absorption Spectroscopy) mobile equipment were below typical previously recorded values: 300 tons per day (t/d). During December, small explosions were recorded by the seismic network and manifested as gas-and-steam emissions.

Gas-and-ash explosions in the crater occurred during January 2023, which were recorded by the seismic network. Sulfur dioxide values remained low, between 300-400 t/d through 10 March. At 0817 on 14 January a gas-and-ash emission was visible in webcam images, rising just above the crater rim. Some mornings during February, small gas-and-steam plumes were visible in the crater. On 7 March at 2252 MARN noted an increase in degassing from the central crater; gas emissions were constantly observed through the early morning hours on 8 March. During the early morning of 8 March through the afternoon on 9 March, 12 emissions were registered, some accompanied by ash. The last gas-and-ash emission was recorded at 1210 on 9 March; very fine ashfall was reported in El Tránsito (10 km S), La Morita (6 km W), and La Piedrita (3 km W). The smell of sulfur was reported in Piedra Azul (5 km SW). On 16 March MARN reported that gas-and-steam emissions decreased.

Low degassing and very low seismicity were reported during April; no explosions have been detected between 9 March and 27 May. The sulfur dioxide emissions remained between 350-400 t/d; during 13-20 April sulfur dioxide values fluctuated between 30-300 t/d. Activity remained low through most of May; on 23 May seismicity increased. An explosion was detected at 1647 on 27 May generated a gas-and-ash plume that rose 700 m high (figure 32); a decrease in seismicity and gas emissions followed. The DOAS station installed on the W flank recorded sulfur dioxide values that reached 400 t/d on 27 May; subsequent measurements showed a decrease to 268 t/d on 28 May and 100 t/d on 29 May.

Figure 32. Webcam image of a gas-and-ash plume rising 700 m above San Miguel at 1652 on 27 May 2023. Courtesy of MARN.

Geologic Background. The symmetrical cone of San Miguel, one of the most active volcanoes in El Salvador, rises from near sea level to form one of the country's most prominent landmarks. A broad, deep, crater complex that has been frequently modified by eruptions recorded since the early 16th century caps the truncated unvegetated summit, also known locally as Chaparrastique. Flanks eruptions of the basaltic-andesitic volcano have produced many lava flows, including several during the 17th-19th centuries that extended to the N, NE, and SE. The SE-flank flows are the largest and form broad, sparsely vegetated lava fields crossed by highways and a railroad skirting the base of the volcano. Flank vent locations have migrated higher on the edifice during historical time, and the most recent activity has consisted of minor ash eruptions from the summit crater.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia).

Ebeko, located on the N end of Paramushir Island in the Kuril Islands, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruption period began in June 2022 and has recently consisted of frequent explosions, ash plumes, and thermal activity (BGVN 47:10). This report covers similar activity during October 2022 through May 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Activity during October consisted of explosive activity, ash plumes, and occasional thermal anomalies. Visual data by volcanologists from Severo-Kurilsk showed explosions producing ash clouds up to 2.1-3 km altitude which drifted E, N, NE, and SE during 1-8, 10, 16, and 18 October. KVERT issued several Volcano Observatory Notices for Aviation (VONA) on 7, 13-15, and 27 October 2022, stating that explosions generated ash plumes that rose to 2.3-4 km altitude and drifted 5 km E, NE, and SE. Ashfall was reported in Severo-Kurilsk (Paramushir Island, about 7 km E) on 7 and 13 October. Satellite data showed a thermal anomaly over the volcano on 15-16 October. Visual data showed ash plumes rising to 2.5-3.6 km altitude on 22, 25-29, and 31 October and moving NE due to constant explosions.

Similar activity continued during November, with explosions, ash plumes, and ashfall occurring. KVERT issued VONAs on 1-2, 4, 6-7, 9, 13, and 16 November that reported explosions and resulting ash plumes that rose to 1.7-3.6 km altitude and drifted 3-5 km SE, ESE, E, and NE. On 1 November ash plumes extended as far as 110 km SE. On 5, 8, 12, and 24-25 November explosions and ash plumes rose to 2-3.1 km altitude and drifted N and E. Ashfall was observed in Severo-Kurilsk on 7 and 16 November. A thermal anomaly was visible during 1-4, 16, and 20 November. Explosions during 26 November rose as high as 2.7 km altitude and drifted NE (figure 45).

Figure 45. Photo of an ash plume rising to 2.7 km altitude above Ebeko on 26 November 2022. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

Explosions and ash plumes continued to occur in December. During 1-2 and 4 December volcanologists from Severo-Kurilsk observed explosions that sent ash to 1.9-2.5 km altitude and drifted NE and SE (figure 46). VONAs were issued on 5, 9, and 16 December reporting that explosions generated ash plumes rising to 1.9 km, 2.6 km, and 2.4 km altitude and drifted 5 km SE, E, and NE, respectively. A thermal anomaly was visible in satellite imagery on 16 December. On 18 and 27-28 December explosions produced ash plumes that rose to 2.5 km altitude and drifted NE and SE. On 31 December an ash plume rose to 2 km altitude and drifted NE.

Figure 46. Photo of an explosive event at Ebeko at 1109 on 2 December 2022. Photo has been color corrected. Photo by S. Lakomov, IVS FEB RAS.

Explosions continued during January 2023, based on visual observations by volcanologists from Severo-Kurilsk. During 1-7 January explosions generated ash plumes that rose to 4 km altitude and drifted NE, E, W, and SE. According to VONAs issued by KVERT on 2, 4, 10, and 23 January, explosions produced ash plumes that rose to 2-4 km altitude and drifted 5 km N, NE, E, and ENE; the ash plume that rose to 4 km altitude occurred on 10 January (figure 47). Satellite data showed a thermal anomaly during 3-4, 10, 13, 16, 21, 22, and 31 January. KVERT reported that an ash cloud on 4 January moved 12 km NE. On 6 and 9-11 January explosions sent ash plumes to 4.5 km altitude and drifted W and ESE. On 13 January an ash plume rose to 3 km altitude and drifted SE. During 20-24 January ash plumes from explosions rose to 3.7 km altitude and drifted SE, N, and NE. On 21 January the ash plume drifted as far as 40 km NE. During 28-29 and 31 January and 1 February ash plumes rose to 4 km altitude and drifted NE.

Figure 47. Photo of a strong ash plume rising to 4 km altitude from an explosive event on 10 January 2023 (local time). Photo by L. Kotenko, IVS FEB RAS.

During February, explosions, ash plumes, and ashfall were reported. During 1, 4-5 and 7-8 February explosions generated ash plumes that rose to 4.5 km altitude and drifted E and NE; ashfall was observed on 5 and 8 February. On 6 February an explosion produced an ash plume that rose to 3 km altitude and drifted 7 km E, causing ashfall in Severo-Kurilsk. A thermal anomaly was visible in satellite data on 8, 9, 13, and 21 February. Explosions on 9 and 12-13 February produced ash plumes that rose to 4 km altitude and drifted E and NE; the ash cloud on 12 February extended as far as 45 km E. On 22 February explosions sent ash to 3 km altitude that drifted E. During 24 and 26-27 February ash plumes rose to 4 km altitude and drifted E. On 28 February an explosion sent ash to 2.5-3 km altitude and drifted 5 km E; ashfall was observed in Severo-Kurilsk.

Activity continued during March; visual observations showed that explosions generated ash plumes that rose to 3.6 km altitude on 3, 5-7, and 9-12 March and drifted E, NE, and NW. Thermal anomalies were visible on 10, 13, and 29-30 March in satellite imagery. On 18, 21-23, 26, and 29-30 March explosions produced ash plumes that rose to 2.8 km altitude and drifted NE and E; the ash plumes during 22-23 March extended up to 76 km E. A VONA issued on 21 March reported an explosion that produced an ash plume that rose to 2.8 km altitude and drifted 5 km E. Another VONA issued on 23 March reported that satellite data showed an ash plume rising to 3 km altitude and drifted 14 km E.

Explosions during April continued to generate ash plumes. On 1 and 4 April an ash plume rose to 2.8-3.5 km altitude and drifted SE and NE. A thermal anomaly was visible in satellite imagery during 1-6 April. Satellite data showed ash plumes and clouds rising to 2-3 km altitude and drifting up to 12 km SW and E on 3 and 6 April (figure 48). KVERT issued VONAs on 3, 5, 14, 16 April describing explosions that produced ash plumes rising to 3 km, 3.5 km, 3.5 km, and 3 km altitude and drifting 5 km S, 5 km NE and SE, 72 km NNE, and 5 km NE, respectively. According to satellite data, the resulting ash cloud from the explosion on 14 April was 25 x 7 km in size and drifted 72-104 km NNE during 14-15 April. According to visual data by volcanologists from Severo-Kurilsk explosions sent ash up to 3.5 km altitude that drifted NE and E during 15-16, 22, 25-26, and 29 April.

Figure 48. Photo of an ash cloud rising to 3.5 km altitude at Ebeko on 6 April 2023. The cloud extended up to 12 km SW and E. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

The explosive eruption continued during May. Explosions during 3-4, 6-7, and 9-10 May generated ash plumes that rose to 4 km altitude and drifted SW and E. Satellite data showed a thermal anomaly on 3, 9, 13-14, and 24 May. During 12-16, 23-25, and 27-28 May ash plumes rose to 3.5 km altitude and drifted in different directions due to explosions. Two VONA notices were issued on 16 and 25 May, describing explosions that generated ash plumes rising to 3 km and 3.5 km altitude, respectively and extending 5 km E. The ash cloud on 25 May drifted 75 km SE.

Thermal activity in the summit crater, occasionally accompanied by ash plumes and ash deposits on the SE and E flanks due to frequent explosions, were visible in infrared and true color satellite images (figure 49).

Figure 49. Infrared (bands B12, B11, B4) and true color satellite images of Ebeko showing occasional small thermal anomalies at the summit crater on 4 October 2022 (top left), 30 April 2023 (bottom left), and 27 May 2023 (bottom right). On 1 November (top right) ash deposits (light-to-dark gray) were visible on the SE flank. An ash plume drifted NE on 30 April, and ash deposits were also visible to the E on both 30 April and 27 May. Courtesy of Copernicus Browser.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Home Reef is a submarine volcano located in the central Tonga islands between Lateiki (Metis Shoal) and Late Island. The first recorded eruption occurred in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, a large volume of floating pumice, and an ephemeral island 500 x 1,500 m wide, with cliffs 30-50 m high that enclosed a water-filled crater. Another island-forming eruption in 2006 produced widespread pumice rafts that drifted as far as Australia; by 2008 the island had eroded below sea level. The previous eruption occurred during October 2022 and was characterized by a new island-forming eruption, lava effusion, ash plumes, discolored water, and gas-and-steam plumes (BGVN 47:11). This report covers discolored water plumes during November 2022 through April 2023 using satellite data.

Discolored plumes continued during the reporting period and were observed in true color satellite images on clear weather days. Satellite images show light green-yellow discolored water extending W on 8 and 28 November 2022 (figure 31), and SW on 18 November. Light green-yellow plumes extended W on 3 December, S on 13 December, SW on 18 December, and W and S on 23 December (figure 31). On 12 January 2023 discolored green-yellow plumes extended to the NE, E, SE, and N. The plume moved SE on 17 January and NW on 22 January. Faint discolored water in February was visible moving NE on 1 February. A discolored plume extended NW on 8 and 28 March and NW on 13 March (figure 31). During April, clear weather showed green-blue discolored plumes moving S on 2 April, W on 7 April, and NE and S on 12 April. A strong green-yellow discolored plume extended E and NE on 22 April for several kilometers (figure 31).

Figure 31. Visual (true color) satellite images showing continued green-yellow discolored plumes at Home Reef (black circle) that extended W on 28 November 2022 (top left), W and S on 23 December 2022 (top right), NW on 13 March 2023 (bottom left), and E and NE on 22 April 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. Home Reef, a submarine volcano midway between Metis Shoal and Late Island in the central Tonga islands, was first reported active in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, large amounts of floating pumice, and an ephemeral 500 x 1,500 m island, with cliffs 30-50 m high that enclosed a water-filled crater. In 2006 an island-forming eruption produced widespread dacitic pumice rafts that drifted as far as Australia. Another island was built during a September-October 2022 eruption.

Information Contacts: Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Semisopochnoi is located in the western Aleutians, is 20-km-wide at sea level, and contains an 8-km-wide caldera. The three-peaked Mount Young (formerly Cerberus) was constructed within the caldera during the Holocene. Each of these peaks contains a summit crater; the lava flows on the N flank appear younger than those on the S side. The current eruption period began in early February 2021 and has more recently consisted of intermittent explosions and ash emissions (BGVN 47:12). This report updates activity during December 2022 through May 2023 using daily, weekly, and special reports from the Alaska Volcano Observatory (AVO). AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

Activity during most of December 2022 was relatively quiet; according to AVO no eruptive or explosive activity was observed since 7 November 2022. Intermittent tremor and occasional small earthquakes were observed in geophysical data. Continuous gas-and-steam emissions were observed from the N crater of Mount Young in webcam images on clear weather days (figure 25). On 24 December, there was a slight increase in earthquake activity and several small possible explosion signals were detected in infrasound data. Eruptive activity resumed on 27 December at the N crater of Mount Young; AVO issued a Volcano Activity Notice (VAN) that reported minor ash deposits on the flanks of Mount Young that extended as far as 1 km from the vent, according to webcam images taken during 27-28 December (figure 26). No ash plumes were observed in webcam or satellite imagery, but a persistent gas-and-steam plume that might have contained some ash rose to 1.5 km altitude. As a result, AVO raised the Aviation Color Code (ACC) to Orange (the second highest level on a four-color scale) and the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale). Possible explosions were detected during 21 December 2022 through 1 January 2023 and seismic tremor was recorded during 30-31 December.

Figure 25. Webcam image of a gas-and-steam plume rising above Semisopochnoi from Mount Young on 21 December 2022. Courtesy of AVO.

Figure 26. Webcam image showing fresh ash deposits (black color) at the summit and on the flanks of Mount Young at Semisopochnoi, extending up to 1 km from the N crater. Image was taken on 27 December 2022. Image has been color corrected. Courtesy of AVO.

During January 2023 eruptive activity continued at the active N crater of Mount Young. Minor ash deposits were observed on the flanks, extending about 2 km SSW, based on webcam images from 1 and 3 January. A possible explosion occurred during 1-2 January based on elevated seismicity recorded on local seismometers and an infrasound signal recorded minutes later by an array at Adak. Though no ash plumes were observed in webcam or satellite imagery, a persistent gas-and-steam plume rose to 1.5 km altitude that might have carried minor traces of ash. Ash deposits were accompanied by periods of elevated seismicity and infrasound signals from the local geophysical network, which AVO reported were likely due to weak explosive activity. Low-level explosive activity was also detected during 2-3 January, with minor gas-and-steam emissions and a new ash deposit that was visible in webcam images. Low-level explosive activity was detected in geophysical data during 4-5 January, with elevated seismicity and infrasound signals observed on local stations. Volcanic tremor was detected during 7-9 January and very weak explosive activity was detected in seismic and infrasound data on 9 January. Weak seismic and infrasound signals were recorded on 17 January, which indicated minor explosive activity, but no ash emissions were observed in clear webcam images; a gas-and-steam plume continued to rise to 1.5 km altitude. During 29-30 January, ash deposits near the summit were observed on fresh snow, according to webcam images.

The active N cone at Mount Young continued to produce a gas-and-steam plume during February, but no ash emissions or explosive events were detected. Seismicity remained elevated with faint tremor during early February. Gas-and-steam emissions from the N crater were observed in clear webcam images on 11-13 and 16 February; no explosive activity was detected in seismic, infrasound, or satellite data. Seismicity has also decreased, with no significant seismic tremor observed since 25 January. Therefore, the ACC was lowered to Yellow (the second lowest level on a four-color scale) and the VAL was lowered to Advisory (the second lowest level on a four-color scale) on 22 February.

Gas-and-steam emissions persisted during March from the N cone of Mount Young, based on clear webcam images. A few brief episodes of weak tremor were detected in seismic data, although seismicity decreased over the month. A gas-and-steam plume detected in satellite data extended 150 km on 18 March. Low-level ash emissions from the N cone at Mount Young were observed in several webcam images during 18-19 March, in addition to small explosions and volcanic tremor. The ACC was raised to Orange and the VAL increased to Watch on 19 March. A small explosion was detected in seismic and infrasound data on 21 March.

Low-level unrest continued during April, although cloudy weather often obscured views of the summit; periods of seismic tremor and local earthquakes were recorded. During 3-4 April a gas-and-steam plume was visible traveling more than 200 km overnight; no ash was evident in the plume, according to AVO. A gas-and-steam plume was observed during 4-6 April that extended 400 km but did not seem to contain ash. Small explosions were detected in seismic and infrasound data on 5 April. Occasional clear webcam images showed continuing gas-and-steam emissions rose from Mount Young, but no ash deposits were observed on the snow. On 19 April small explosions and tremor were detected in seismic and infrasound data. A period of seismic tremor was detected during 22-25 April, with possible weak explosions on 25 April. Ash deposits were visible near the crater rim, but it was unclear if these deposits were recent or due to older deposits.

Occasional small earthquakes were recorded during May, but there were no signs of explosive activity seen in geophysical data. Gas-and-steam emissions continued from the N crater of Mount Young, based on webcam images, and seismicity remained slightly elevated. A new, light ash deposit was visible during the morning of 5 May on fresh snow on the NW flank of Mount Young. During 10 May periods of volcanic tremor were observed. The ACC was lowered to Yellow and the VAL to Advisory on 17 May due to no additional evidence of activity.

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 Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical 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 eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

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/).

Ambae, also known as Aoba, is a large basaltic shield volcano in Vanuatu. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas. Periodic phreatic and pyroclastic explosions have been reported since the 16th century. A large eruption more than 400 years ago resulted in a volcanic cone within the summit crater that is now filled by Lake Voui; the similarly sized Lake Manaro fills the western third of the caldera. The previous eruption ended in August 2022 that was characterized by gas-and-steam and ash emissions and explosions of wet tephra (BGVN 47:10). This report covers a new eruption during February through May 2023 that consisted of a new lava flow, ash plumes, and sulfur dioxide emissions, using information from the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and satellite data.

During the reporting period, the Alert Level remained at a 2 (on a scale of 0-5), which has been in place since December 2021. Activity during October 2022 through March 2023 remained relatively low and mostly consisted of gas-and-steam emissions in Lake Voui. VMGD reported that at 1300 on 15 November a satellite image captured a strong amount of sulfur dioxide rising above the volcano (figure 99), and that seismicity slightly increased. The southern and northern part of the island reported a strong sulfur dioxide smell and heard explosions. On 20 February 2023 a gas-and-ash plume rose 1.3 km above the summit and drifted SSW, according to a webcam image (figure 100). Gas-and-steam and possibly ash emissions continued on 23 February and volcanic earthquakes were recorded by the seismic network.

Figure 99. Satellite image of the strong sulfur dioxide plume above Ambae taken on 15 November 2022. The Dobson Units (DU) exceeded 12. Courtesy of VMGD.

Figure 100. Webcam image of a gas-and-ash plume rising above Ambae at 1745 on 20 February 2023. The plume drifted SSW. Courtesy of VMGD.

During April, volcanic earthquakes and gas-and-steam and ash emissions were reported from the cone in Lake Voui. VMGD reported that activity increased during 5-7 April; high gas-and-steam and ash plumes were visible, accompanied by nighttime incandescence. According to a Wellington VAAC report, a low-level ash plume rose as high as 2.5 km above the summit and drifted W and SW on 5 April, based on satellite imagery. Reports in Saratamata stated that a dark ash plume drifted to the WSW, but no loud explosion was heard. Webcam images from 2100 showed incandescence above the crater and reflected in the clouds. According to an aerial survey, field observations, and satellite data, water was no longer present in the lake. A lava flow was reported effusing from the vent and traveling N into the dry Lake Voui, which lasted three days. The next morning at 0745 on 6 April a gas-and-steam and ash plume rose 5.4 km above the summit and drifted ESE, based on information from VMGD (figure 101). The Wellington VAAC also reported that light ashfall was observed on the island. Intermittent gas-and-steam and ash emissions were visible on 7 April, some of which rose to an estimated 3 km above the summit and drifted E. Webcam images during 0107-0730 on 7 April showed continuing ash emissions. A gas-and-steam and ash plume rose 695 m above the summit crater at 0730 on 19 April and drifted ESE, based on a webcam image (figure 102).

Figure 101. Webcam image showing a gas-and-ash plume rising 5.4 km above the summit of Ambae at 0745 on 6 April 2023. Courtesy of VMGD.

Figure 102. Webcam image showing a gas-and-ash plume rising 695 m above the summit of Ambae at 0730 on 19 April 2023. Courtesy of VMGD.

According to visual and infrared satellite data, water was visible in Lake Voui as late as 24 March 2023 (figure 103). The vent in the caldera showed a gas-and-steam plume drifted SE. On 3 April thermal activity was first detected, accompanied by a gas-and-ash plume that drifted W (figure 103). The lava flow moved N within the dry lake and was shown cooling by 8 April. By 23 April much of the water in the lake had returned. Occasional sulfur dioxide plumes were detected by the TROPOMI instrument on the Sentinel-5P satellite that exceeded 2 Dobson Units (DU) and drifted in different directions (figure 104).

Figure 103. Satellite images showing both visual (true color) and infrared (bands B12, B11, B4) views on 24 March 2023 (top left), 3 April 2023 (top left), 8 April 2023 (bottom left), and 23 April 2023 (bottom right). In the image on 24 March, water filled Lake Voui around the small northern lake. A gas-and-steam plume drifted SE. Thermal activity (bright yellow-orange) was first detected in infrared data on 3 April 2023, accompanied by a gas-and-ash plume that drifted W. The lava flow slowly filled the northern part of the then-dry lake and remained hot on 8 April. By 23 April, the water in Lake Voui had returned. Courtesy of Copernicus Browser.

Figure 104. Images showing sulfur dioxide plumes rising from Ambae on 26 December 2022 (top left), 25 February 2023 (top right), 23 March 2023 (bottom left), and 5 April 2023 (bottom right), as detected by the TROPOMI instrument on the Sentinel-5P satellite. These plumes exceeded at least 2 Dobson Units (DU) and drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone with numerous scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Axial Seamount appears to have undergone an eruption starting 23-24 April 2015 Pacific Time (local time= UTC-08 hours, or Day Light Saving time, 8 March to 1 November = UTC-07 hours on the US west coast). At this time, evidence of the eruption stems from increases in seismicity, deformation, and seawater temperature around Axial, recorded in the weeks after 23 April UTC. The length of the eruption is currently unknown; however, it likely lasted days to weeks (William Chadwick, personal communication). Two expeditions to Axial are planned for this coming July and August on R/V Thompson, during which further investigation regarding the 2015 eruption will be completed (personal communication).

Axial Seamount sits along the Juan de Fuca ridge ~480 km off the coasts of Oregon and Washington (figure 1 in BGVN 23:01). Eruptions were detected seismically and geodetically in 1998 and 2011 and confirmed shortly after each eruption during submersible dives. Since about 410 AD, the volcano produced over 50 lava flows (Clague and others, 2013). On this basis, Axial is described as the NE Pacific's most active submarine volcano.

In this Bulletin report, we present information on the April 2015 eruption taken from several sources referred to in the text and provided in the Reference list near the end of this report. The sources include (a) an Oregon State University (OSU) online press release (OSU, 2015); (b) blog posts on the website of NOAA's Pacific Marine Environmental Laboratory's (PMEL) Earth-Ocean Interactions Program (EOI) (Chadwick and Nooner, 2015); (c) a post on the Networked Observations and Visualization of the Axial Environment (NOVAE) website (Delaney, 2015); and (d) seismic data posted on William Wilcock's website (Wilcock, 2015).

The last two Bulletin reports (BGVN 36:07 and 37:10) detailed Axial's previous eruption, which occurred during 6-12 April 2011 and erupted 99 x 10⁶ m³ of lava.

Instrumentation. Since September 2014, Axial Seamount has been a part of the Ocean Observatories Initiative's (OOI) Cabled Array, which is operated by the University of Washington. As explained in their online literature (OOI, posting date uncertain1), OOI is funded by the National Science Foundation and is "an integrated infrastructure project composed of science-driven platforms and sensor systems to measure physical, chemical, geological and biological properties and processes from the seafloor to the air-sea interface." The Cabled Array, which straddles the Juan de Fuca and North American plates, is one of OOI's seven arrays worldwide (OOI, posting date uncertain1). Within the Cabled Array, there are seven primary nodes (PN) that are connected to a shore station in Pacific City, Oregon, through ~ 900 km of modified telecommunications cable (OOI, posting data uncertain2). This cable provides the PNs with high power and bandwidth that allows for near real-time interactive and adaptive investigation (OOI, posting date uncertain1). Each PN then delivers power and communication to a range of scientific instructions connected to it, such as those connected to the Axial Seamount PN3B (figure 10).

Figure 10. Bathymetry map highlighting the various instruments connected to the Axial Seamount Primary Node (PN) 3B. PN3B is one of the seven PNs that comprise OOI's Cabled Array. The white cable (RSN Primary) that goes to the right of the image, connects PN3B and two other PNs (not shown) to the shore station in Pacific City, Oregon. For scale, there is a 250 m square on the map, located next to MJ03F. Image courtesy of Networked Observations and Visualization of the Axial Environment (NOVAE).

April 2015 eruption. Based on personal communication with Chadwick, around 0530 UTC on 24 April 2015 (2230 Pacific Time on 23 April), increased seismicity was recorded by the instruments in Axial's caldera.

This increased seismicity was the first indication of activity. On 24 April UTC, the daily earthquake count reached nearly 8,000 events/day (figure 11). Before the eruption, the daily earthquake count was generally less than 1,000 events/day (Delaney, 2015). From Wilcock (2015), Bulletin editors obtained a histogram of the daily earthquake count from 16 November 2014 to 14 June 2015; on the histogram, the daily earthquake count from 24 April UTC is represented by the largest bar (figure 11). Wilcock (2015) also provides animations of the preliminary locations of the earthquakes' epicenters at Axial from 23-24 April. The majority of the epicenters were largely clustered on the E side of the caldera, not far from the location of the 2011 eruption (Wilcock, 2015; Delaney, 2015).

Figure 11. Preliminary earthquake histogram showing the daily earthquake counts at Axial Seamount from 16 November 2014 to 14 June 2015. On 24 April UTC, nearly 8,000 earthquakes were recorded (largest bar on the histogram). Dates on the histogram are in UTC. Taken from Wilcock (2015).

Around 0630 UTC on 24 April 2015 (2330 Pacific Time on 23 April), deformation at Axial Seamount increased considerably (Delaney, 2015). According to Delaney (2015), the instrument that measures bottom pressure and tilt on the seafloor at the Central Caldera site (figure 10) began to indicate deflation (subsidence). This deflationary event was reported to have lasted approximately 12 hours with the seafloor dropping as much as 2.4 m (Delaney, 2015; OSU, 2015). Bulletin editors learned from personal communication with Chadwick that the duration of subsidence at Axial may actually be on the order of ~10 days with the greatest amount of subsidence occurring within the first 24 hours of activity.

Although there was initial uncertainty regarding whether the observed deflation and earthquakes were associated with an eruption (or merely an intrusion), Chadwick and Nooner (2015) cited increases in seawater temperature data that occurred in the days and weeks following 24 April UTC. The observed increases suggested that lava erupted somewhere nearby on the seafloor (Chadwick and Nooner, 2015). The temperature data was collected by the three bottom pressure/tilt instruments, located in Central and Eastern Caldera and the International District Vent District (figure 10). The graph in figure 12 displays the increases documented in the temperature data (PMEL/EOI, 2015).

Figure 12. Graph presenting seawater temperature data versus date (UTC, 1 April 2015 to 10 June 2015) collected by the bottom pressure and tilt instruments that are part of the OOI Cabled Array. These instruments detected increases in temperature that started on 24 April (indicated on graph) and peaked on 13 May 2015. By 10 June, the temperatures appear to have decreased to levels similar to before the eruption. The increases in temperature appear to be on the order of ~0.6-0.8 °C. Courtesy of PMEL/EOI, 2015

For reference, the 2.4 m deflation that occurred is similar to the amount detected in 2011 (BGVN 36:07). Delaney (2015) quoted Chadwick as saying that the amount of deflationary subsidence suggested that a similar amount of magma erupted in 2015 and during Axial's 2011 eruption.

References. Chadwick W and Nooner S, 2015, Successful forecast of the 24 April 2015 eruption at Axial Seamount, NOAA's Pacific Marine Environmental Laboratory's (PMEL) Earth-Ocean Interactions Program (EOI), URL: http://www.pmel.noaa.gov/eoi/axial_blog.html, accessed on 15 June 2015

Clague D A, Breyer B M, Paduan J B, Martin J F, Chadwick W W, Caress D W, Portner R A, Guilderson T P, McGann M L, Thomas H, Butterfield D A, Embley R W, 2013, Geologic history of the summit of Axial Seamount, Juan de Fuca Ridge. Geochem Geophys Geosystems, 14: 4403-4443

Delaney, J, 2015, Axial Activity Intensifies, URL: http://novae.ocean.washington.edu/file/Axial_Activity_Intensifies, accessed on 16 June 2015

NOVAE (Networked Observations and Visualization of the Axial Environment), 2015 http://novae.ocean.washington.edu/story/Latest_News, accessed on 15 June 2015

OSU (Oregon State University), 2015, Researchers think Axial Seamount off Northwest coast is erupting – right on schedule, URL: http://oregonstate.edu/ua/ncs/archives/2015/apr/researchers-think-axial-seamount-northwest-coast-erupting-–-right-schedule, accessed on 15 June 2015

OOI (Ocean Observatories Initiative), posting date uncertain1, OOI Frequently Asked Questions, URL: http://oceanobservatories.org/about/frequently-asked-questions/#01, accessed on 22 June 2015

Ocean Observatories Initiative (OOI), Cabled Array (URL: http://oceanobservatories.org/, accessed on 17 June 2015).

PMEL/EOI (NOAA's Pacific Marine Environmental Laboratory's Earth-Ocean Interactions Program), 2015, Data from the 24 April 2015 eruption at Axial Seamount, URL: http://www.pmel.noaa.gov/eoi/rsn/24April2015_event.html, accessed on 15-22 June 2015

Wilcock, W, 2015, Welcome to William Wilcock's desktop computer, URL: http://alben.ocean.washington.edu/, accessed on 16 June 2015.

Geologic Background. Axial Seamount rises 700 m above the mean level of the central Juan de Fuca Ridge crest about 480 km W of Cannon Beach, Oregon, to within about 1,400 m of the ocean surface. It is the most magmatically and seismically active site on the Juan de Fuca Ridge between the Blanco Fracture Zone and the Cobb offset. The summit is marked by an unusual rectangular-shaped caldera (3 x 8 km) that lies between two rift zones and is estimated to have formed about 31,000 years ago. The caldera is breached to the SE and is defined on three sides by boundary faults of up to 150 m relief. Hydrothermal vents with biological communities are located near the caldera fault and along the rift zones. Hydrothermal venting was discovered north of the caldera in 1983. Detailed mapping and sampling efforts have identified more than 50 lava flows emplaced since about 410 CE (Clague et al., 2013). Eruptions producing fissure-fed lava flows that buried previously installed seafloor instrumentation were detected seismically and geodetically in 1998 and 2011, and confirmed shortly after each eruption during submersible dives.

Information Contacts: William Chadwick, NOAA and Oregon State University, Hatfield Marine Science Center, 2115 S.E. Oregon State University Drive, Newport, OR 97365, USA; Scott Nooner, Department of Geography and Geology, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5944, USA (URL: http://people.uncw.edu/nooners/Nooner/); Oregon State University, News and Research Communications, Corvalis, OR (URL: http://oregonstate.edu/ua/ncs/); William Wilcock, School of Oceanography, University of Washington, Marine Sciences Building, 1501 NE Boat Street, Seattle, WA 98195, USA (URL: http://faculty.washington.edu/wilcock/).

This Bulletin report on Ontakesan (Kiso-Ontakesan, Ontake) covers activity from November 2003 to November 2014. During this reporting interval, two eruptions occurred, both broadly described as phreatic, yet containing a minor component of identified juvenile magmatic material. The first eruption was on an unknown date in late March 2007, and the second eruption was on 27 September 2014. The 2014 eruption took place with a sudden onset and with few if any precursory warnings. The volcano is a famous tourist area to see color changes in autumn foliage, it also contains considerable alpine touristic infrastructure, including lodges. The 2014 eruption took place during the autumn color season on a Saturday. Hundreds of people were on the mountain at the time. The 2014 eruption included ashfall, pyroclastic flows, and related density currents. The eruption killed ~57 people and an additional 6 were still missing as of 27 October 2014 (Kyodo, 2014). The 2014 impulsive eruption was documented by an outstanding number of close-up photographs and videos taken by eyewitnesses.

Between the 2007 and 2014 eruptions, activity receded to background levels. Our last Bulletin report (BGVN 28:11) noted occasional white plumes during 2000 to 2003.

The data for this report was collected chiefly from online reports by the Japanese Meteorological Agency (JMA), the Geological Survey of Japan (GSJ), and the National Institute of Advanced Industrial Science and Technology (AIST).

Historically, activity at Ontakesan has consisted mainly of phreatic explosions every several hundred years. However, recent research cited in JMA (date unknown) indicates that over the past 10,000 years, four magmatic eruptions have also occurred. Although there are no reported records of historical eruptions before 1979, fumarolic activity was noted as ongoing for several hundred years near to Ontakesan's summit (Jigokudani and Hachotarumi). The period of activity after 1979 represents what appears to be Ontakesan's most active during the past 250 years (Oikawa, 2008).

2006–2007. The March 2007 eruption was preceded by minor inflation and an increase in seismic activity (JMA, date unknown). During mid- to late December 2006, instruments detected inflation of the volcanic edifice and an increase in shallow seismicity directly below the summit. During January 2007, instruments recorded 90 earthquakes on the 16th and 164 earthquakes on the 17th. On 25 January 2007, tremor occurred with the largest recorded amplitude in at least a year. Tremor had, according to a JMA plot, remained near zero during all of 2006. The January 2007 tremor was described as very low frequency, containing a 15- to 20-second-long component. Furthermore, GPS observations detected a small amount of crustal deformation preceding the seismic activity, indicating a slight inflation at Ontakesan.

On 16 March 2007, fumarolic activity increased; fumes at the summit were occasionally detected by a surveillance camera (at "Mitake Kurozawa," but the exact location was not found in English on maps in Ontakesan reports). Seismic and other data considered by JMA showed that an earthquake had occurred during late March, originating directly below Ontakesan. The specific dates of the seismicity and earthquake were not specified in the available reporting. Based on the data collected, JMA inferred that the earthquake had resulted from a magmatic intrusion that had advanced toward the surface reaching ~4 km below the volcano's summit.

A field study two months later, on 29 May 2007, noted fresh volcanic ash from Ontakesan's 79-7 crater. The ash reached ~200 m NE of the crater. This finding of fresh ash was believed to indicate that the 2007 eruption was not merely phreatic but involved some escape of juvenile components to the surface. The exact date of the eruption was undeterminable.

JMA cited a model by Nakamichi and others (2009) regarding the intrusion of a magmatic body and the subsequent 2007 eruption, to describe phreatic eruptions at Ontakesan (figure 12). The depths shown reflect but one set of depth values for the top of the intrusion ((a) in this case 3 km depth below the summit) and the zone of groundwater ((b) centered at ~2.4 km depth below the summit). Thus, figure 12 illustrates a basic model of the various processes involved in a phreatic eruption, which is defined by Harris (2000) as a ". . . steam eruption that produces no fresh magma. A common precursor of eruptive activity, it is caused when groundwater, heated by a magmatic source, flashes into steam."

Figure 12. Diagrams (a and b) that JMA cited to help explain magma intrusion, steam generation, and related signals associated with a phreatic eruption. Courtesy of JMA, citing Nakamichi and others (2009).

Figure 12a considers an earlier pre-eruptive stage, where magma advanced upward to 3 km below the volcano's summit. Volcano-tectonic (VT) earthquakes resulted from breaking rock. Note that in the March 2007 eruption, the magma was thought to have advanced to 4 km depth below the summit and the tremor noted then was very-long period (VLP, because of the above-mentioned long-period (15-20 second) component).

Figure 12b considers a later stage of the intrusion event, where the magma ceased to advance towards the surface. The magma heated the groundwater, which expanded into steam. The pressure from the heated water and steam broke rock as it advanced towards the surface. Acoustical signals include VLP and long period (LP) earthquakes. In this model, the steam escaped through the ancestral vents, thus producing a phreatic eruption.

Eruption in September 2014. Available JMA and GSJ reports say very little about the period leading up to the September 2014 eruption. According to a news source (Asahi Shimbun, 2014b), during the early part of September, daily tremor peaked at 85 on 11 September and were followed by 3 to 27 tremor events per day starting on 12 September.

The JMA Executive Committee (2014) reported that on the morning of 27 September, a few hours before the eruption at 1152 Local Time (LT=UTC+9), there was no major unrest. They did record mild seismic events, tilt, and increased steaming; the noted data appearing from 1130 to 1210. The report noted that seismic signal recorded 11 minutes prior to the eruption was nearly flat, with only one small event, in contrast to the robust signal associated with the subsequent eruption. Tilt began 7 minutes prior to and peaked during the explosion.

GSJ (2014) stated: "A volcanic eruption occurred on September 27, 2014 at Mount Ontake on the border between Nagano and Gifu Prefectures. According to the Japan Meteorological Agency, the eruption began at about 11:52 JST [Japan Standard Time] (=UTC+9h) on September 27. It is estimated that the plume from this eruption reached a maximum height of 7000 m, and a pyroclastic flow cascaded down the mountain in a southwesterly direction for a distance of more than 3 km."

Additionally, about forty minutes later, the Tokyo Volcanic Ash Advisory Center (VAAC) noted that the plume resulting from the eruption ascended to 11 km above sea level (a.s.l.) extending to the E. The JMA raised the Alert Level to 3, and the level remained elevated throughout the reporting interval. Figure 13 provides an oblique view of the volcano with the approximate area of the eruption and the summit mountain lodge labelled.

Figure 13. A DigitalGlobe Google image of Ontakesan with a label indicating the approximate area where the sudden eruption vented (on the S flank). For scale, the prominent crater atop this part of the cone is ~200 m in diameter. Several bodies were found within the mountain lodge and other infrastructure on and near the summit rim. Courtesy of BBC (2014b).

The 27 September eruption was captured by a camera system (Yamamoto, 2014; AIST, 2014b; GSJ, 2014) (figure 14). The term pyroclastic flow generally applies to laterally moving mixtures of hot gas and particles such as tephra and lithics; a broader term that includes ash cloud surges, etc. is pyroclastic density current (see Roche, 2015, and the references therein). In the case of the 2014 eruption, density currents occurred near the summit and on the western flank; some of the complexity may have been due to secondary explosions well downslope of the vent area (AIST, 2014b; Boyle and others, 2014). The density currents descended at variously reported maximum speeds of 30 to 72 km per hour (AIST, 2014b; Yamamoto, 2014). According to Boyle and others (2014), the pyroclastic density currents traveled more than 3 km down Ontakesan's S flank.

Figure 14. Four photos of the Ontakesan 2014 eruption captured by the Chubu Regional Development Bureau's camera at Takigoshi on 27 September. [1] (1154 LT) The pyroclastic flow descended. [2] (1156 LT) A secondary plume rose. [3] (1201 LT) The plume released a downburst of ash. [4] (1205 LT) The plume developed laterally, spreading along the ground surface. Taken from Yamamoto (2014).

On 28 September, the day following the eruption, the GSJ conducted several aerial observations, utilizing media helicopters (GSJ, 2014). At 0800 LT, the height of the plume was ~500 m, heading S to SW. The scientists noted the eruption formed a new line of craters (figure 15) running NW to SE. These new craters resided 250–300 m to the SE of those formed during a previous eruption in 1979. The line of craters is roughly parallel to the 1979 line, but covers a wider area.

Figure 15 Aerial view of Ontakesan from 28 September at 1636 LT. The Kengamine summit area, as depicted from the NW, released white plumes heading S to SE. The smaller plume in the foreground originated from a fissure crater on the W end of the new line of craters. The distant plume originated from the line of craters formed at Jigokudani, located SW of the summit . Additional photos of the eruption, similar to this one, are provided by the authors. Source: GSJ (2014).

GSJ also conducted expeditions to the Kaida Plateau, located ~6 km E to NE of the Kengamine summit, and collected ash samples on 28 September. A photo depicted in GSJ (2014) shows a light coating (on the order of several millimeters) of gray ash covering the leaves and horizontal surfaces of plants in the region. The geologists described the ash as "medium-to-fine-grain sand-sized particles" with a maximum diameter of 0.5 mm. Most of the ash was altered rock fragments and less than 10% was unaltered red-orange and crystalline fragments (as seen in figure 16). GSJ (2014) reported that, as a result of this ash analysis, the recent eruption was considered as phreatic (rather than predominantly magmatic).

Figure 16. Samples collected at Ontakesan. (Top) A majority of this ash sample was altered. (Bottom) A sample with some fragments of unaltered ash. The AIST report also includes a detailed description of the methods used to gather/ analyze ash samples. Courtesy of AIST (2014a).

GSJ (2014) also noted that scientists had charted the main axis of ash fall from the 2014 eruption, based on the findings from their expeditions. The ash distribution extended towards Ontakesan's E to NE (red arrow, figure 17).

Figure 17. Geological map of Ontakesan containing annotations relevant to the 2014 eruption. The main axis of ash distribution is noted by the red arrow. The line of craters formed by the 2014 eruption are marked approximately by a series of small red dots and a yellow line near the base of the red arrow. The 1979 line of craters is depicted as a row of green dots. The blue line denotes the known margin of the Younger Ontake Volcano formed ~100,000 years ago. Source: GSJ (2014).

News sources contributed the following. The ash resulting from the eruption was ~50 cm thick near the crater and up to 20 cm thick in lower areas (Adonai, 2014). Asahi Shimbun (2014b) also quoted a scholar, Takayuki Kaneko, as allegedly stating that the ash was "moist" and stuck together like "sesame seeds."

According to JMA, from 1 to 7 October, Ontakesan continued to emit ash, but the resultant plume height could not be determined due to poor visibility. On 7 October, the plume was observed to rise 300 m above the crater rim, drifting E. Tremor continued to be detected; the number of earthquakes detected from 27 September to 6 October are compiled in figure 18 (JMA daily reports).

Figure 18. The number of earthquakes detected per day during 27 September to 6 October 2014. Data courtesy of JMA (daily reports); figure by Bulletin editors.

According to JMA, Ontakesan emitted ash plumes during 8–9 October, white plumes on 10 October, and plumes with only small amounts of ash during 10–14 October. During 8–14 October, tremors were below the detection limits. White plumes rose 100–200 m above the crater rim, drifting NE and SE, during 16–18 October. On 19 October, plumes rose to 600 m above crater.

Impact of eruption on people. As previously mentioned, according to news sources, the 27 September eruption killed 57 hikers on Ontakesan's slopes, and as of 27 October another 6 were missing (Kyodo, 2014). Furthermore, more than 70 were injured (RT, 2014). According to the Associated Press in Tokyo (2014), the explosion was the deadliest volcanic eruption in Japan in the post-WWII period.

According to the BBC (2014a), nearly 300 people were hiking on Ontakesan on the day of the eruption. The news article characterized the accounts of the eruption as consisting of falling ash and boulders, at times with sufficient density to cause several minutes of total darkness. Kuroda Terutoshi (Kuroda Terutoshi, 2014), posted a video of the eruption to YouTube that he took while hiking. The expanding ash plume engulfed a cabin on the slopes above him, and he was soon surrounded by the plume, which included dark ballistics and ashfall. The ash plume grew rapidly and continued downhill, as depicted in two other videos (Asahi Shimbun, 2014a and BBC News, 2014).

Boston Globe (2014) stated that rescue and recovery missions began on 28 September deploying more than 500 Japanese military and police. Metal and landmine detectors played a role in locating victims buried under ash (Asahi Shimbun, 2014c). National Geographic (2014) and Ogrodnik (2014) provide several photos of the rescue missions, derived from various news sources. According to Malm (2014), several of the casualties found during rescue operations were "still holding their smartphones." Lies and Meyers (2014) noted the halting of some initial search and recovery efforts on 30 September owing to increased tremor the night before raising concerns then about the return of volcanic activity.

On 6 October, Typhoon Phanfone (No. 18) came near to the Ngano Prefecture where Ontakesan is located (Asahi Shimbun, 2014b, c). Accompanied by heavy rains, a mixture of volcanic ash and rain formed into mud, making it hard for large helicopters to land near the summit. Rescue missions were halted on 15 October due to the wintery conditions despite people still missing. The article said that the search for bodies was expected to resume in the springtime.

References. T (?Pfeiffer, T), 2014, Ontake-san volcano (Japan): death toll from yesterday's eruption rises to more than 30, 28 September 2014, Volcano Discovery (URL: http://www.volcanodiscovery.com/on-take/news/48168/Ontake-san-volcano-Japan-death-toll-from-yesterday-s-eruption-rises-to-more-than-30.html) [accessed in June 2015]

AIST, 2014a, Analysis of Volcanic Ash Falling from the September 2014 Eruption of the Ontake Volcano, 28 September 2014, National Institute of Advanced Industrial Science and Technology (URL: https://www.gsj.jp/hazards/volcano/kazan-bukai/yochiren/ontake_ash_140928E.pdf)

AIST, 2014b, The Pyroclastic Flow Generated by the September 27, 2014 Eruption of the Ontake Volcano, 29 September 2014, National Institute of Advanced Industrial Science and Technology (URL: https://www.gsj.jp/hazards/volcano/kazan-bukai/yochiren/ontake_flow_140928E.pdf)

Asahi Shimbun, 2014a, Ontakesan eruption prediction was difficult, the Japan Meteorological Agency did not find any omens, 27 September 2014 (URL: http://www.asahi.com/articles/ASG9W5KFJG9WULBJ00D.html) [accessed in June 2015]

Asahi Shimbun, 2014b, Volcanologists: with few telltale signs, Mt. Ontakesan's eruption hard to predict, 28 September 2014, (URL: http://ajw.asahi.com/article/behind_news/social_affairs/AJ201409280022) [accessed in June 2015]

Asahi Shimbun, 2014c, Update: More victims of Mt. Ontakesan eruption found as typhoon nears, 4 October 2014 (URL: http://ajw.asahi.com/article/behind_news/social_affairs/AJ201410040056)

Associated Press in Tokyo, 2014, Hikers killed by Japanese volcano left poignant pictures of final moments, 5 October 2014, The Guardian (URL: http://www.theguardian.com/world/2014/oct/05/-sp-hikers-killed-japanese-volcano-pictures-final-moments) [accessed in June 2015]

Australian Associated Press with Australia Geographic staff, 2014, Japan Volcano Ontake an Extremely Rare Eruption, 29 September 2014, Australia Geographic (URL: www.australiangeographic.com.au/news/2014/09/japan-volcano-ontake-an-extremely-rare-eruption) [accessed in June 2015]

BBC News, 2014, Video: Japan volcano shoots rock & ash on Mount Ontake - BBC News, 29 September 2014, YouTube (URL: https://www.youtube.com/watch?v=aQtkoLxqUNQ) [accessed in June 2015]

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BBC, 2014b, Japan Mount Ontake Volcano: Death Toll Reaches 47, 1 October 2014, British Broadcasting Corporation (URL: www.bbc.com/news/world-asia-29440982) [accessed in June 2015]

Boston Globe, 2014, Military Helicopters Rescue People Stranded After Mt Ontake Eruption, 28 September 2014, Boston.com (URL: http://www.boston.com/news/world/asia/2014/09/28/military-helicopters-rescue-people-stranded-after-ontake-eruption/iW6wHPhW5K0rAsYM0EUe7H/video.html) [accessed in June 2015]

Boyle, D., Charlton, C., 2014, 36 Now Feared Dead in Japanese Volcano Disaster, 28 September 2014, Daily Mail (URL: http://www.dailymail.co.uk/news/article-2772458/More-30-hikers-dead-near-Japanese-volcano-erupted-without-warning-spewinf-eight-inch-blanket-ash.html) [accessed in June 2015]

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JMA, date unknown, 53. Ontakesan, Japan Meteorological Agency (JMA) (URL: http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/souran_eng/volcanoes/053_ontakesan.pdf) [accessed in June 2015]

JMA Executive Committee, 2014, Prediction of Volcanic Eruptions Coordinating Committee Expanded Executive Committee, 21 pages, 28 September 2014, Japan Meteorological Agency (JMA) (URL: www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/kaisetsu/CCPVE/shiryo/kakudai140928/kakudai140928_no01.pdf)

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Kyodo, 2014, Ontake victims mourned a month after eruption as tourism industry scrambles to recover, 27 October 2014, Japan Times (URL: http://www.japantimes.co.jp/news/2014/10/27/national/ontake-victims-mourned-month-eruption-tourism-industry-scrambles-recover/) [accessed in June 2015]

Lies, E. and Meyers, C., 2014, Recovery of Japan Volcano Victims Suspended Amid Signs of Rising Activity, 30 September 2014, Reuters (URL: www.reuters.com/article/2014/09/30/us-japan-volcano-idUSKCN0HP07G20140930) [accessed in June 2015]

Nakamichi, H., Kumagai, H., Nakano, M., Okubo, M., Kimata, F., Ito, Y., Obara, K., 2014, Source mechanism of a very-long-period event at Mt Ontake, central Japan: Response of a hydrothermal system to magma intrusion beneath the summit, 10 November 2009, Journal of Volcanology and Geothermal Research (URL: http://www.sciencedirect.com/science/article/pii/S0377027309003588)

Malm, S., 2014, One last picture... that cost their lives: More than half the victims killed by erupting volcano in Japan were found clutching smartphones, 12 November 2014, Daily Mail (URL: http://www.dailymail.co.uk/news/article-2831305/One-selfie-cost-lives-half-victims-killed-erupting-volcano-Japan-clutching-smartphones-photos-lava-coming-them.html) [accessed in June 2015]

National Geographic, 2014, Pictures: Japanese Volcano that Killed Hikers, 29 September 2014 (URL: http://news.nationalgeographic.com/news/2014/09/pictures/140929-japan-volcano-ontake-pictures/) [accessed in June 2015]

Ogrodnik, I., 2014, IN PHOTOS: Powerful images as Japanese Volcano Mount Ontake Erupts, 1 October 2014, Global News (URL: http://globalnews.ca/news/1587737/in-photos-powerful-images-as-japans-mount-ontake-erupts/) [accessed in June 2015]

Oikawa, T., 2008, Re-examination of historical records and fumarolic activity of eruption records of the Ontake volcano, Japanese with English abstract, Geological Survey of Japan (GSJ) (URL: https://www.gsj.jp/data/bulletin/59_05_01.pdf)

Roche, O., 2015, Nature and Velocity of Pyroclastic Density Currents Inferred from Models of Entertainment of Substrate Lithic Clasts, Earth and Planetary Science Letters, 418, 115-125 (URL: http://adsabs.harvard.edu/abs/2015E&PSL.418..115R)

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Yamamoto, T., 2014, The Pyroclastic Density Currents Generated by the September 27, 2014 Phreatic Eruption of Ontake Volcano, Bulletin of the Geological Survey of Japan, number 65 (URL: https://www.gsj.jp/data/bulletin/65_09_03.pdf)

Geologic Background. The massive Ontakesan stratovolcano, the second highest volcano in Japan, lies at the southern end of the Northern Japan Alps. Ascending this volcano is one of the major objects of religious pilgrimage in central Japan. It is constructed within a largely buried 4 x 5 km caldera and occupies the southern end of the Norikura volcanic zone, which extends northward to Yakedake volcano. The older volcanic complex consisted of at least four major stratovolcanoes constructed from about 680,000 to about 420,000 years ago, after which Ontakesan was inactive for more than 300,000 years. The broad, elongated summit of the younger edifice is cut by a series of small explosion craters along a NNE-trending line. Several phreatic eruptions post-date the roughly 7300-year-old Akahoya tephra from Kikai caldera. The first historical eruption took place in 1979 from fissures near the summit. A non-eruptive landslide in 1984 produced a debris avalanche and lahar that swept down valleys south and east of the volcano. Very minor phreatic activity caused a dusting of ash near the summit in 1991 and 2007. A significant phreatic explosion in September 2014, when a large number of hikers were at or near the summit, resulted in many fatalities.

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

Introduction. This Bulletin report covers activity at Tungurahua from November 2010 through December 2011, during which Ecuador's Instituto Geofísico-Escuela Politécnica Nacional (IG) reported three Eruptive Episodes (EE). The last Bulletin (BGVN 38:03) reported explosions and earthquakes at Tungurahua through late July 2010, followed by a lull in activity until October 2010.

This reporting period was characterized by ashfall; lahars; emissions of ash, water vapor, and gases such as SO₂; strombolian activity; explosions; various types of earthquakes; and pyroclastic flows (PFs) among other events. Temporally, the majority of these events occurred during EE7 (November-December 2010), EE8 (April-May 2011), and EE9 (November-December 2011). Spatially, the processes such as lahars and PFs followed drainages, the geography of which is partly aided by maps and information in the next section. Intervening periods of low activity occurred during January-March 2011 and June-October 2011. The data presented here was gathered from various types of reports, special bulletins, and announcements published by IG.

Setting. Figure 54 displays a map of Tungurahua that includes ravines on its flanks and nearby inhabited areas. For ravines not seen in figure 54, such as those on the E flank, see figure 22 in BGVN 29:01. Table 8 in BGVN 29:01 gives the distances and direction of towns around Tungurahua, some of which are mentioned in this report. Mapa de Peligros–Tungurahua (Map of Hazards–Tungurahua) can also be viewed on IG's website, for a more general view of Tungurahua and environs.

Figure 54. Map showing ravines and several inhabited areas around Tungurahua. Part of the network of stations that monitors seismic-acoustic activity is also shown. Deposits (noted in the key) were laid down in the August 2006 eruption. The Chambo and Patate Rivers join to form the Pastaza River, which flows E. Taken from Kelfoun and others (2009).

Eruptive episodes (EEs). In 2006, IG partnered with Japan's International Cooperation Agency to improve the monitoring of Tungurahua's seismic-acoustic activity, through the deployment of a network of broadband seismic sensors and infrared sensors. By 2008, the network had five stations (BMAS, BPAT, BRUN, BBIL and BULB), each located 5-7 km from Tungurahua's crater (figure 55). The sensors record seismic and acoustic signals propagated by volcanic explosions, emission tremors, booms, and chugging-type events. Information from the sensors and other observations, are used to define each EE. Table 18 shows the nine EEs registered from 2006-2011 based on information from IG's 2011 annual report on Tungurahua's explosive activity.

Figure 55. Map of the network of stations that monitor seismic-acoustic activity at Tungurahua. Each station is equipped with broadband seismic sensors and infrared sensors. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

Table 18. The nine Explosive Episodes (EEs) recorded by IG at Tungurahua since 2006. The table also shows the number of explosions recorded during each EE. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

Activity through mid-November 2010. After a four-month lull in activity, Tungurahua revived in November 2010, displaying somewhat elevated seismicity that included a series of volcano-tectonic (VT) earthquakes and a few low-energy explosions. IG's weekly report no.46, detailing activity from 15-21 November, stated that activity remained low with 88 long period (LP) earthquakes; 4 VT earthquakes; weak fumarolic activity; and weak emissions of water vapor and volcanic gases. Rains of varying intensities produced lahars from 15-19 November. On 16 November, the largest lahars of the week descended all of Tungurahua's ravines, with the most significant lahars descending the Mapayacu, Bilbao, and Vazcún ravines (figure 54).

EE7: 22 November-25 December 2010. A sudden increase in activity on 22 November, characterized by explosions in the afternoon and at night, marked the beginning of Tungurahua's EE7, according to IG's 2010 annual report on explosive activity. At 1408, a low-energy explosion was heard around Tungurahua and at the Observatorio del Volcán Tungurahua (OVT), located 14 km to the NW in Guadalupe. Movements of blocks along the upper flanks and an emission of low-to-moderate ash content that moved S accompanied the explosion. At 2235, a sudden large explosion generated an ash plume that rose to more than 7 km in altitude. This explosion triggered the ballistic expulsion of incandescent blocks, which descended ~1.5 km below the crater. Reports of ashfall and falls of gravel were received from communities to the W of Tungurahua, such as Choglontus.

Later, smaller explosions that produced emission columns 2-3 km above the crater were reported. Rains during the late afternoon generated a lahar that descended the Mapayacu ravine and temporarily dammed the Puela River. The Washington Volcanic Ash Advisory Center (VAAC) reported a 7.6 km altitude ash plume, during the night of 22 November, and ash emissions that rose 6.4 km in altitude on the morning of 23 November. EE7 continued until 25 December 2010. Figure 56 shows the number of explosions per day registered during this episode.

Figure 56. Histogram showing the number of explosions per day recorded seismically at Tungurahua from 22 November to 25 December 2010 during EE7. 110 explosions were recorded, with the highest number of explosions occurring on 9 December. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

According to IG's special bulletin no. 20, starting on 24 November 2010, the continuous ash emissions indicated the process of an open system, characterized by the constant generation of emission columns with variable amounts of ash due to an open and unsealed magma ascent path. Ashfall was reported in Ulba, Baños, Juive, Runtún, Bilbao, and Choglontus (settlements located on the NE-SW flanks).

Tungurahua's elevated activity continued through 3 December 2010 with (a) ash emissions, some rising ~3-4 km above the crater; (b) ashfall affecting communities such as Bilbao, Choglontus, El Manzano, and Cahuají; (c) strombolian activity with fountains up to ~1 km above the crater; (d) booming noises of varying magnitude; and (e) the expulsion of incandescent blocks, which rolled as much as ~1 km below the crater. On 29 November, an increase in SO₂ was registered by the satellite-based Ozone Monitoring Instrument (OMI). IG also noted an increased SO₂ flux through their fixed gas-monitoring stations.

On 4 December 2010, there was a very rapid and sudden increase in Tungurahua's seismic activity after no explosion signals were registered on the preceding days (figure 56). From 0830, the rapid increase in activity was manifested by (a) a speedy rise in the registered internal vibration; (b) an increase in the intensity and duration of booms and explosions; (c) an increase in the volume of emissions with higher ash levels; and (d) blocks moving more than 1.5 km below the crater. Residents in cities on the skirts of the volcano reported feeling vibrations. Due to this sudden increase in activity, the Alert Level was raised to Red by the National Secretariat of Risk Management, and populations around Tungurahua were evacuated.

Around 0939 on 4 December, observers saw pyroclastic flows (PFs) on Tungurahua's W and N flanks. At 0946, several PFs descended the Vazcún ravine. By 1130, PFs were still descending ravines on the W side, such as Mandur, Choglontus, and La Rea. The last PF of the day, at 1404, descended through the Juive sector. All the PFs on 4 December, descended ~2 km from the crater; the largest descended the Vazcún, Juive, and Mandur ravines.

During the afternoon of 4 December, IG noted a reduction in the intensity of activity, although a plume with moderate-to-high ash content rose ~3 km above the crater. Throughout the day, incandescent blocks rolled ~2 km down the flanks. During the evening, the seismic and surface activity continued to decrease. The Alert Level was lowered from Red to Orange.

An IG announcement, released on 4 December 2010, noted that this sudden increase in activity was unexpected in an open system. Special bulletin no. 23, also from 4 December, stated that Tungurahua's increased activity and the generation of PFs were associated with a rapid increase in the volume of magma entering the lower ducts of the volcanic vent and upon finding an open system, the magma was able to quickly rise to the crater and overflow as PFs.

After 4 December 2010, Tungurahua's activity returned to moderate levels. On 5 and 6 December, ashfall occurred in communities to the NW, W and SW. On 7 December, special bulletin no. 24 described activity marked by a constant emission of gases and ash, rising 2-3 km above the crater, occasionally accompanied by incandescent blocks, ejected hundreds of meters above the crater, before falling onto Tungurahua's flanks and rolling 1-2 km. According to that bulletin, secondary transport of ash also affected communities to the W of Tungurahua; crops in that area were reportedly covered by ash, no more than 1 mm thick.

On 9 December 2010, the maximum number of explosions for this EE was recorded (figure 56), and a PF descended also the Cusúa ravine. Tungurahua's explosive activity decreased thereafter. Between 9 and 23 December, activity included: incandescent blocks ejected above the crater rolled down Tungurahua's flanks; plumes containing variable amounts of ash often rose as high as 2-3 km above the crater; ashfall was reported in nearby communities; explosions caused vibrations of windows, "cannon shots", and plumes; and lahars. According to special bulletin no. 26, from 13 December, there was an increase in the number of LP earthquakes and from15 December, several VT earthquakes were recorded.

Between 24 and 25 December 2010, IG registered a series of small-to-moderate-sized explosions. Figure 56 shows 10 explosions were recorded on 25 December, after which explosive activity was reported to have stopped. Special bulletin no. 1, from 5 January 2011, stated Tungurahua's monitoring system had detected: smaller plumes with lower ash content; no explosions since 25 December 2010; fewer LP earthquakes and emission tremors (26 December 2010, 36 LPs and 4 emission tremors were recorded, while on 2 January 2011, 18 LPs and 1 emission tremor); decreased SO₂ emissions (26 December, 2,200 tons/day of SO₂ were recorded, but days before 5 January, 200 tons/day were recorded); and less pressure in the upper part of the cone. IG also reported 10 small VT earthquakes, interpreted to represent the entrance of new magma into the volcanic system and precursors to an increase in explosive activity.

Lull during January-March 2011. Weekly reports in January described moderate activity until 5 January, after which activity was moderate to low through 18 January. Special bulletin no.2 stated that between 5 and 17 January emissions mainly consisted of water vapor. January ended with activity being low and it remained so for the next two months (figure 57).

Figure 57. Histogram showing the number of explosions recorded per day at Tungurahua from January to December 2011. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

EE8: 22 April-26 May 2011. Tungurahua's monitoring stations began to register an episode of volcanic tremor at 1714 on 20 April 2011. A plume with low-to-moderate ash content rose 3 km above the crater and drifted SW. This was the first major sign of surface activity since the beginning of 2011. An IG announcement from 20 April stated that small seismic fracture events were observed. Deformation observed in the NW quadrant since February 2011 had recently increased. During the night of 20 April, IG personnel witnessed strombolian activity, accompanied by constant booms of moderate intensity. Incandescent blocks also rolled 1 km down the flanks.

At 1512 on 22 April 2011, a moderate-sized explosion signaled the beginning of Tungurahua's EE8, according to the 2011 annual report on explosive activity. EE8 continued until 26 May and was characterized by a total of 64 explosions (figure 58). According to IG's weekly report no. 16, detailing activity from 18-24 April, low-intensity rains produced muddy water in some of Tungurahua's ravines. During this interval, plumes with moderate-to-high ash levels rose 5 km above the crater. On some nights, strombolian activity was observed, and ashfall of varying intensities affected numerous settlements including Choglontus, Bilbao (figure 59), El Manzano, Puela, Cusúa, Guadalupe, Baños, Ulba, and Cevallos. SO₂ was measured at 571 tons/day on 19 April and increased each day until it reached a maximum of 6,015 tons/day on 24 April.

Figure 58. Histogram showing the number of explosions recorded per day at Tungurahua from 22 April to 26 May 2011. In total, 64 explosions were recorded, with the maximum number of explosions occurring on 21 May. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

Figure 59. (A) Ash covering the surface of a solar panel in Bilbao (8 km W of Tungurahua) on 24 April 2011. (B) Ash covering vegetation in Bilbao on 24 April 2011. In Bilbao, ash accumulated over 12 hours due to constant emissions and reached a thickness of ~3 mm. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

According to special bulletin no. 5 from 26 April 2011, during 20-26 April ash emissions were accompanied by booms of varying intensity, lava fountains, and incandescent blocks rolling down the upper flanks. Data collected by inclinometers showed continued deformation in the NW quadrant.

On 26 April 2011, volcanic activity increased (figure 58), starting around 1245, with ash emissions rising 8-12 km above the crater. Areas to the NW and W experienced ashfall. OVT received reports that activity caused vibrations of the ground in areas close to the volcano. In Baños (8 km N), vibrations of doors and windows were reported. The National Secretariat of Risk Management declared an Orange Alert Level for areas around the volcano and evacuated families in Cusúa, Bilbao, and Chacauco. A widening of the volcanic cone was also detected on 26 April.

Another increase in activity occurred on 29 April 2011 and lasted for 48 hours. At ~0100, an increase in seismic amplitude and the presence of harmonic tremor were registered. Emissions with moderate-to-high ash content were often observed rising 2-3 km above the crater, moving NE. Baños, Runtún, Ulba, Juive and Cusúa received notable ashfall, while in Guadalupe, ash was lightly deposited. Emissions also rose 7 km above the crater and drifted mainly SE toward uninhabited areas. Strombolian eruptions also ejected blocks from the crater.

According to special bulletin no. 7, published on 3 May, the intensity of Tungurahua's emissions started to slowly decrease on 1 May, until returning to levels similar to those observed at the beginning of this EE. Special bulletin no. 7 also stated that Tungurahua's seismic and deformation monitoring systems continued to show evidence of internal pressurization. That bulletin also mentioned that IG and the French Institute for Research and Development (IRD) had estimated that ~1.6x106 m³ to 3x106 m³ of ash was deposited in areas around Tungurahua during an unstated amount of time; the maximum thickness reached 15.5 mm (figure 60).

Figure 60. Map of accumulated ash thickness in areas around Tungurahua as measured on 2 May 2011. The interval over which the ash accumulated was unstated. Thicknesses were measured in mm. The English translations for the Spanish words in the key are: Espesor, thickness; Cabecera Cantonal, cantonal capital; and Cantones, districts. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

Special bulletin no. 8 from 6 May 2011 stated that since 3 May, surface activity had continued to decrease with respect to the height, frequency, and ash content of the plumes and in the number of explosions. Nevertheless, IG warned that the decrease in emissions did not signal the end of this EE. Evidence of increasing internal pressure, presumably caused by the ascent of a new volume of magma towards the upper volcanic ducts, continued to be registered through deformation recorded by GPS and inclinometers, rock fracture earthquakes, and constant emission of volcanic gases. SO₂ fluxes surpassed 2,000 tons/day.

After 6 May, Tungurahua experienced a slight lull in its eruptive activity for ~ 10 days; however, roaring was sometimes still heard, ashfall was reported, and on 10 May an explosion produced an ash plume that rose 5 km above the crater. More consistent and elevated activity resumed at 2222 on 16 May when gas-and-ash plumes rose between 1.5 and 2 km above the crater and drifted NE. At dawn on 17 May, significant ashfall was reported in Río Negro, ~25 km E of Baños. Special bulletin no. 9 (issued 17 May) said that a slight decrease in deformation was recorded by inclinometers as well as a reduction in the gas fluxes (less than 300 tons/day were recorded through 16 May). On the afternoon of 17 May, new fumaroles were also identified ~ 1,000 m below the summit on the W flank.

Through 19 May 2011, Tungurahua continued emitting vapor and gas containing variable amounts of ash. Ashfall affected communities in the NE-E-SE sectors such as Río Negro, Baños, Runtún, Cusúa, and Bilbao. The greatest measured thickness was 3 mm in Trigal, located to the SE. At midday on 19 May, observers witnessed explosions of significant sizes producing ash plumes that rose 3 km above the crater and drifted W and SW.

During 18-25 May 2011, seismic activity was characterized by 37 explosions, the majority of which occurred on 21 May (figure 58). Some of the explosions generated "cannon shots" that vibrated structures in nearby areas. Activity was characterized by high seismicity, including LP events and tremors. On 25 May, surface activity had decreased, but seismicity was moderate-to-high, deformation continued, and SO₂ was being still emitted.

Constant rains during 27-28 May generated lahars and the swelling of rivers. On 27 May, a large lahar descended the Pingullo ravine, halting traffic along the road between Baños and Penipe. Muddy water moved down the Juive, Vazcún, Pondoa, Bilbao, Mapayacu, Ulba, and Achupashal ravines. On 28 May, flows of muddy water traveled down the Bilbao, Pondoa, Juive, and La Pirámide ravines; swelling of the Chambo and Puela rivers was reported; and the Ulba River doubled its volume and carried large blocks of rock. At the end of May 2011, activity returned to moderate levels.

Lull from June to October 2011. A second lull in explosive activity occurred (figure 57). On 29 July, 20 landslides took place along the Baños access road, a vital escape route.

On 7 October 2011, the Washington VAAC reported an ash plume that rose ~1 km above Tungurahua's crater and drifted W. On 24 October, another ash plume rose slightly over 2 km above the crater and drifted W. However, IG stated they had not observed any ash emissions since June. On 9 November, the Washington VAAC reported an ash plume, while IG reported only gas-and-steam emissions. On 15 October, IG also reported the reactivation of a fumarole, first observed in May 2011, on Tungurahua's W flank. The fumarole's activity lessened during the following week.

EE9: 27 November-22 December 2011. Three explosions (figure 61) on 27 November marked the beginning of Tungurahua's EE9, which continued to 22 December.

Figure 61. Histogram showing the number of explosions recorded per day at Tungurahua from 27 November to 22 December 2011. During this EE, 55 explosions were recorded and the maximum number of explosions was registered on 4 and 5 December. Source: Instituto Geofísico-Escuela Politécnica Nacional (IG).

On 27 November 2011, seismic activity began steadily increasing at 1540. At 1700, explosions of varying sizes were detected. Booms and "cannon shots" produced by the explosions were heard in areas close to Tungurahua. As activity progressed, explosions generated PFs that descended ravines on the NW and W flanks. The explosions also ejected incandescent blocks that rolled to ~1 km below the crater. Four VT earthquakes were also registered beginning at 1650. IG received reports of ashfall in El Manzano and Bilbao; deposits of coarse-grained ash in Pillate; ash with shards in Cotaló; and only shards in Cusúa. Later, PFs descended the S and SW flanks. Distances traveled by the PFs on the NW, W, S and SW flanks were unstated.

During the last days of November 2011, activity remained high. On 28 November, an explosion ejected incandescent material and generated a PF in the Achupashal ravine. There were also almost constant booms of moderate-to-high intensity, lava fountains, and ejected incandescent blocks that traveled ~1 km down the W and NW flanks. A constant ash plume rose 3 km over the summit. Small PFs descended the S flank, and ash fell in El Manzano, Choglontus, and Runtún. At night, incandescent blocks were observed being ejected above the crater before they descended 400-500 m down the flanks.

On 29 November, two PFs were observed: the first traveled ~500 m down the NW flank; the second traveled ~1 km down the Pingullo ravine. Almost-constant booms of moderate-to-high intensity and muddy water flowing down several ravines on the W side were noted. Ash-and-gas plumes rose up to 4 km above the crater, and tremor was constant. On 30 November, plumes rose to average heights of 2 km and minor ashfall in Baños and Río Verde was reported.

Activity remained high during the beginning of December. On the night of 1 December, a decrease in tremor amplitude coincided with an increase in the number of explosions. Strong "cannon shots" were generated by some of the explosions and incandescent blocks ejected onto the flanks rolling 1 km downslope. Heavy rains during the night led to the descent of lahars in the Mapayacu and La Pampa ravines and the swelling of the Vazcún River. Early on 2 December, an increase in tremor amplitude corresponded with moderate to strong booms. An ash plume later rose 1.5 km above the crater, drifting E.

During the night of 2 December and dawn of 3 December, IG noted two periods with increased tremor amplitude, during which a great volume of incandescent blocks was expelled and high intensity booms resonated through nearby communities. On 3 December, several PFs traveled ~1.5 km in the upper parts of La Rea and Ingapirca ravines and ravines on the NW flank. A plume with high ash content rose 2-3 km above the crater. Ash fell in settlements, including Bilbao, Cusúa, El Manzano, and Penipe.

During 4-5 December, the maximum number of explosions for EE9 was recorded (figure 61). On both days, seismicity consisted of constant tremor intermixed with explosions. On 4 December, strong booms and "cannon shots", both associated with explosive activity, were reported by communities close to Tungurahua. A plume with moderate-to-high ash content rose 4 km high, a small PF was noted, and ash fell in Bilbao, Cusúa, and El Manzano. On 5 December, an ash plume rose 1 km above the crater and drifted N and NE, leading to ashfall in Baños and Runtún. Booms of low to moderate intensity were also reported. On 7 December, explosions generated "cannon shots" of moderate intensity; blocks descended the flanks generating noises; a plume with moderate ash content rose 1.5 km above the crater and drifted W; and Cusúa and Chacauco received ashfall. On 8 December, similar conditions prevailed, but some plumes also rose as high as 4 km above the crater.

Activity declined on 9 December according to special bulletin no. 18. Emissions contained only water vapor. SO₂ fluxes had fallen to 500 tons/day, which was significantly less than the maximum 8,000 tons/day measured after 28 November. Special bulletin no. 18 noted that despite these declines, seismic events such as LP and VT earthquakes were still recorded (between 60 and 170 LP earthquakes were registered daily). That special bulletin also remarked that the network for deformation monitoring did not register any evidence of the ascent of a new volume of magma towards the surface. This decline in activity lasted until 21 December.

From about 0830 on 22 December, several seismic events indicated the movement of fluids within Tungurahua's interior. Three explosions produced ash plumes that rose less than 500 m above the crater, the second explosion of which led to a PF that descended 300-400 m down the NW flank. That afternoon, a high-energy explosion generated two PFs that descended the Achupashal and La Hacienda ravines. An ash plume rose 4 km above the crater and drifted NW. The PFs descended to a maximum of 2 km below the crater, without reaching inhabited or cultivated areas. Ashfall was reported in N and W communities. Ash-and-gas plumes continued, and at night strombolian activity ejected blocks ~500 m above the crater, which landed and rolled 500 m below the crater.

For the rest of December 2011, activity was considered moderate. After 22 December, ash was often deposited in areas to the SW of Tungurahua such as El Manzano and Choglontus, where up to 2-4 mm of ash was deposited. On 23 December constant gas-and-ash emissions rose ~200 m above the crater and were carried SW. Ashfall was reported in El Manzano, Choglontus and Chacauco. On 24 December, no surface observations were made, but there were reports of ashfall in El Manzano, Choglontus and Cahuají. On 25 December, an emission with moderate-to-high ash content was observed rising 500 m above the crater and drifted W. Ashfall was reported in El Manzano. From 27 to 29 December, the area around Tungurahua was cloudy and no reports of booms or ashfall were received. On the last day of 2011, small emissions of water vapor and minor rains were reported.

References. Kelfoun, K., Samaniego, P., Palacios, P., Barba, D., 2009. Testing the suitability of frictional behaviour for pyroclastic flow simulation by comparison with a well-constrained eruption at Tungurahua volcano (Ecuador). Bulletin of Volcanology, v. 71(9), p. 1057-1075 (URL: http://dx.doi.org/10.1007/s00445-009-0286-6)

Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Information Contacts: Instituto Geofísico (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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/).