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).

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/).

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/).

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/).

Minor activity continued at Minami-dake until mid-March, although the highest ash plume of the month rose 2,100 m above the crater on the 6th. Twelve explosive eruptions occurred on 18 March. Overall during March there were 88 eruptions, 69 of which were explosive. The monthly total ashfall measured 10 km W of the crater was 22 g/m². Seismicity recorded 2.3 km NW of the crater during March consisted of 970 earthquakes and 773 tremors.

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

According to the Institute of Volcanology (IV), the eruption on 2 January (BGVN 21:01) began around 0800. This activity was preceded by an upsurge in seismicity that started in April 1995. At 1926 on 31 December 1995, a M 5.8 earthquake occurred in the Kronotsky gulf, 50-60 km NE of the volcano. On 1 January at 2057 an earthquake of M 5.2 in the Karymsky region was followed at 2157 by a M 6.9 event centered ~25 km S of the volcano. During the next day there were more than 10 aftershocks of M >= 5.0. On 2 January at 1540, a group of IV volcanologists arrived by helicopter. Eruptive centers were observed near the summit and 5-6 km S in Karymsky Lake (maximum depth 115 m), which fills the Akademii Nauk caldera.

The eruption began with formation of a vent with a diameter of 20-30 m, located 50 m below the summit. Violent emissions of ash-rich gas jets rose to 1 km from another vent on the SW slope. Steam-and-gas jets, occasionally with black-colored matter, were also ejected to several hundred meters from beneath the surface of Karymsky Lake. The presumed eruptive center was 100-200 m from the shore in the NW sector of the lake. Turbulent steam-and-gas plumes rose 5-6 km above the surface from a 200-m-diameter area. Ice covering the lake had completely melted.

On 3 January the near-summit vent increased in size to 50 m in diameter. Gas and steam blasts alternated with ash ejections from the two simultaneously active vents on the volcano. Ash was usually ejected from the upper vent, and a white-colored plume was emitted from the lower vent. Ash ejections lasted 2-3 minutes, and gas blasts lasted 1.5-2 minutes. An ash-and-gas column rose 1-1.2 km and was blown E and SE by the wind. The surface of Karymsky Lake steamed intensely, sending clouds 800-1,000 m above the lake. Areas of green water were visible through breaks in the clouds, and a newly-formed black beach was seen. In the N and NE sector of the lake a narrow spit, beginning from the source of the Karymsky river and extending 250-300 m to the center of the lake, had formed. The water level in the lake had dropped a few meters. The upper reaches of the river had dried up, but on 2 January waves from the submarine eruption (up to 10 m high or more) overflowed the N shore, flooding a wide valley 1.5 km below the source. During a surveillance flight on 4 January, large areas of the valley were covered by black mud. The beach contained three fumarolic vents along the NE-trending fault zone. Within a radius of 500-800 m of the source of the Karymsky River, the surrounding snow-covered hills contained thousands of holes with diameters ranging from 10 cm to 1.5-2 m formed by lithic blocks ejected from the lake. The water level of the lake continued to fall because of intense evaporation.

Light-gray dacitic ash covered an area of about 150-200 km². At a distance of 8 km from the volcano fractions ranging from 0.16 to 0.06 mm dominated. Estimates made by S.A. Fedotov indicated that on 2 and 3 January the ash ejection rate from the summit crater reached 3-4 tons/second.

Routine observations from 2 January through 11 February showed that the climactic phase of the subaqueous eruption continued for no more than 12-15 hours. That eruption consisted of frequent explosions during which a vapor-gas mixture with lithic material was ejected to the surface. In the N sector of the lake at the shore W of the Karymsky River, damaged trees provided evidence of two eruptive sources 500-600 m from each other. This zone contained the main concentration of bomb material ejected from the lake. A portion of the shoreline (150-200 m long and 5-15 m wide) E of the river sank several meters into the lake. The main eruption center was 500 m from the shore, but smaller peripheral centers were also observed. As a result of the eruption, in the NNW sector of the lake, a beach in the form of a wide 0.4 km² cape was produced, as well as a narrow spit extending SE from the old shore. The length of the new shoreline was 2.4 km, and a large shoal was observed around the new peninsula. According to the preliminary estimates, the ejected deposits in the lake are at least 1 km² in area and 5-10 x 10⁶ m³ in volume.

Thermal springs that discharge at the S shore of Karymsky Lake were destroyed by ejecta from this eruption, and several new mud pots were formed; chemical composition of the solutions was unchanged. Near the center of the new beach, composed of sand-gravel and bomb material, a chain of five explosive vents with diameters from 1.5 to 30 m was observed. At the N end was a thermal site with a diameter of ~50 m that exhibited intense vapor emission and was covered by sublimates; visiting scientists detected a hydrogen sulfide odor. A dry funnel with a diameter of ~3 m and high gas emission at a temperature of 97°C was in the center of this site. Other explosion funnels had water at a depth of 1.2-1.5 m with temperatures from 33 to 70°C. The three funnels closest to the lake and on the opposite shore had gas emissions with temperatures of 97-98°C.

On 4 January run-off from the lake ceased owing to damming by ejected material. Analyses of water samples from the lake, river, and various hot springs in the area indicated that there had been chemical contributions to the lake water by an underlying magma body.

Geologic Background. The lake-filled Akademia Nauk caldera is one of three volcanoes constructed within the mid-Pleistocene, 15-km-wide Polovinka caldera. The eroded Beliankin stratovolcano, in the SW part of Polovinka caldera, has been active in postglacial time (Sviatlovsky, 1959). Two nested calderas, 5 x 4 km Odnoboky and 3 x 5 km Akademia Nauk (also known as Karymsky Lake or Academii Nauk), were formed during the late Pleistocene, the latter about 30,000 years ago. Eruptive products varied from initial basaltic-andesite lava flows to late-stage rhyodacitic lava domes. Two maars, Akademia Nauk and Karymsky, subsequently formed at the southern and northern margins of the caldera lake, respectively. The northern maar, Karymsky, erupted about 6,500 radiocarbon years ago and formed a small bay. The first recorded eruption from Akademia Nauk took place on 2 January 1996, when a day-long explosive eruption of unusual basaltic and rhyolitic composition occurred from vents beneath the NNW part of the caldera lake near Karymsky maar.

Information Contacts: G.A. Karpov, Ya.D. Muravyev, R.A. Shuvalov, S.M. Fazlullin, and V.N. Chebrov, Institute of Volcanology, Far East Division, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, 683006, Russia.

Intense seismicity was felt by Akutan residents beginning on the evening of 10 March and through the next day (BGVN 21:02). Students in Akutan (13 km E of the summit; figure 1) carefully counted the frequency and intensity of the earthquakes during the day on 11 March. The resulting information was the first quantitative dataset about the earthquakes and suggested that this was an earthquake swarm rather than a classic mainshock-aftershock sequence. The strongest shocks rattled small objects on tables and caused some cabinet doors to open; ground shaking was continuous. The largest of the earthquakes had a magnitude of about 5.1, and there were several of M 4-5, most of them were probably in the M 2.5 to 4 range. There were no operating seismometers on Akutan Island at the onset of seismic unrest; the nearest seismometer was at Sand Point, ~380 km NE. The intense seismic activity began subsiding at about 1700 on 11 March but remained at a level substantially above normal. Seismicity continued through most of that night with many events strongly felt in Akutan. Seismicity declined on 12 March, and late that day a seismologist from the Alaska Volcano Observatory (AVO) who reached Akutan with a seismometer and a portable recording system determined that the earthquakes were volcano-tectonic.

Figure 1. Terrain-corrected synthetic aperture radar (SAR) image showing Akutan and Makushin volcanoes in the Aleutian Islands, Alaska. Courtesy of the Alaska SAR Facility; data copyright European Space Agency.

At about 1700 on 13 March, felt-earthquakes began occurring at a rate of greater than 1/minute, a higher rate than on 11 March. Damage associated with these earthquakes included objects tumbling off shelves, and ground shaking was again continuous. The strongest of these events were felt as far away as Dutch Harbor/Unalaska 50 km SW of Akutan. The number of earthquakes recorded in Akutan was over 800/day during the intense earthquake swarm on 13-14 March. A slight decrease in the rate of activity occurred at about 0500 on 14 March, but felt earthquakes still occurred every 2-3 minutes. There were a few earthquakes with M >= 5, and more between 4 and 5. This swarm began subsiding about 18 hours after onset. Because of the continued high seismicity AVO initiated use of a Level of Concern Color Code system and designated the current level to be Orange on 14 March, indicating an eruption was possible at any time within the next few days. On the night of 14 March, AVO's seismologist in Akutan reported 4-5 felt events.

On 15 March the rate and intensity of recorded earthquakes, although much lower than earlier in the week, remained well above background. At about 1700, a geologist flying into Akutan glimpsed a part of the N flank and summit area through broken clouds, but observed no evidence of eruptive activity. AVO scientists in Akutan felt only a few earthquakes that night. The number of earthquakes recorded on 16 March was much lower than during the swarms of 11 and 13 March. However, the rate and intensity of earthquake activity remained well above background. The weather continued to be poor, hampering visual observations. The number of daily earthquakes remained about the same through 19 March. Scientists in Akutan reported feeling only a few earthquakes each of those nights.

The Level of Concern Color Code was downgraded to Yellow on 20 March based on decreasing seismicity over the previous six days to 60-80 events/day. The Yellow code indicates that the threat of imminent eruption has declined, and the possibility that the volcano will return to quiet over a period of weeks without eruption has increased. An airline passenger reported seeing the snow-filled summit crater, with very slight normal wisps of steam from the central cinder cone, and no evidence of eruptive activity. The level of seismicity remained above background, and several earthquakes each day were felt in Akutan.

By 22 March a total of five seismic stations in four locations had been installed and all data were being sent to the Fairbanks and Anchorage laboratories in real time. Maximum separation of the stations was ~9 km. Four of the stations were located around Akutan Harbor, and the fifth was on the E slopes of the volcano about midway between the village of Akutan and the summit. The seismic array will remain in its present geometry until additional stations can be placed by helicopter this summer. By 24 March all AVO personnel had left, and around-the-clock monitoring using the new seismic stations was being conducted from AVO.

The number of earthquakes continued during 21-25 March at a rate of ~60-80/day, decreased slightly by 27 March to ~40-60/day, and remained at that level through 29 March. As of 5 April seismicity continued to slowly diminish. Earthquakes were distributed widely beneath the E half of the island with a cluster, shallower than 10 km, located ~8-10 km due E of the summit cinder cone and ~5 km W of the village of Akutan. The rate of seismicity during 6-12 April was about half that of the previous week, with ~10-20 earthquakes/day, most too small to be felt by local residents. Seismicity decreased again by half during 13-19 April, to ~5-10 small earthquakes/day.

Geologic Background. Akutan contains a 2-km-wide caldera with a large cinder cone in the NE part of the caldera that has been the source of frequent explosive eruptions and occasional lava effusion that covers the caldera floor. An older, largely buried caldera was formed during the late Pleistocene or early Holocene. Two volcanic centers are located on the NW flank. Lava Peak is of Pleistocene age, and a cinder cone lower on the flank produced a lava flow in 1852 that extended the shoreline of the island and forms Lava Point. The 60-365 m deep younger caldera was formed during a major explosive eruption about 1,600 years ago and contains at least three lakes. A lava flow in 1978 traveled through a narrow breach in the north caldera rim almost to the coast. Fumaroles occur at the base of the caldera cinder cone, and hot springs are located NE of the caldera at the head of Hot Springs Bay valley and along the shores of Hot Springs Bay.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.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; Alaska SAR Facility, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775 USA.

During February and March, Crater C continued to emit gas, lava, and sporadic Strombolian eruptions. For both months, OVSICORI-UNA reported that the intensity of explosive activity was slightly below that of January, however, columns still rose ~1 km. Prevailing winds blew towards the NW, W, and SW. Arenal continued to cause acidic rains and to eject volcanic bombs, blocks, and ash. The volcano's steep slopes and receding vegetation have led to gullying in unconsolidated material and cold avalanches down local drainages.

ICE reported the distribution of lava flows down Arenal's W slopes (figure 76). A flow active from May 1995 through early January 1996 ceased and a new one trending WNW began in late January, its downslope end reaching 1,200-m elevation in early March. Also, on the N flank in early March, some rolling blocks destroyed forest down to 1,150 m elevation. Deposits reminiscent of those from pyroclastic flows were found in late December at 950 m elevation.

Figure 76. Sketch map of Arenal area showing distribution of lavas as of March 1996. Courtesy of G. J. Soto.

ICE noted that December explosions were ~15-30 minutes apart; after mid-January the explosions were ~27 ± 21 minutes apart (1 sigma standard deviation); steam often vented in the N quadrant of Crater C. Nightime observers saw cyclic changes in the intensity of crater glow repeating every 39 ± 22 seconds; the changes were attributed to convection in the intra-crater lava pool. ICE also repeatedly measured ash deposition rates adjacent to the crater (table 13).

Table 13. Mass of Arenal's ash collected at a site 1.8 km W of the active vent. Courtesy of ICE.

OVSICORI-UNA reported the respective values of tremor duration and local seismicity during February and March: 386 and 261 hours and 758 and 624 events. Events of frequency below 3.5 Hz typically accompanied those eruptions that ejected pyroclastics.

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

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Oficina de Sismolog¡a y Vulcanolog¡a, Departamento de Geolog¡a, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

On 7 March the Institute of Volcanic Geology and Geochemistry (IVGG) reported a noteworthy increase in seismicity beneath Avachinsky and an increase in the height of the steam plume to ~100 m above the volcano. The steam plume suggested a possible increase in heat flux. The IVGG reported that the possibility of an eruption within the next few weeks to months has increased significantly. Elevated seismicity was previously reported in late 1993 and early 1994 (BGVN 19:01).

Geologic Background. Avachinsky, one of Kamchatka's most active volcanoes, rises above Petropavlovsk, Kamchatka's largest city. It began to form during the middle or late Pleistocene, and is flanked to the SE by Kozelsky volcano, which has a large crater breached to the NE. A large collapse scarp open to the SW was created when a major debris avalanche about 30,000-40,000 years ago buried an area of about 500 km2 to the south, underlying the city of Petropavlovsk. Reconstruction of the volcano took place in two stages, the first of which began about 18,000 years before present (BP), and the second 7,000 years BP. Most eruptions have been explosive, with pyroclastic flows and hot lahars being directed primarily to the SW by the collapse scarp, although there have also been relatively short lava flows. The frequent historical eruptions have been similar in style and magnitude to previous Holocene eruptions.

Information Contacts: Tom Miller, Alaska Volcano Observatory (URL: https://www.avo.alaska.edu/); Vladimir Kirianov, Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia.

Adverse weather conditions that prevented observation of the summit in late February (BGVN 21:02) continued throughout March. Ash puffs from Bocca Nuova crater (BN) were seen during some clear periods on 1 and 5 March, and on the morning of 6 March several black ash emissions were observed. Between 1200 and 1300 a sequence of ash puffs was produced from Northeast Crater (NEC). At 1530, another dense black ash puff was emitted from BN. At sunset the snow mantle was discontinuously covered by a thin ash layer. Ash emissions were again observed during some clearings on 7 March.

On 11 March around 2300 a one-hour long increase in tremor amplitude was recorded at the summit stations. During the afternoon of 12 March the weather improved and after sunset pulsating red glows were observed above NEC by the surveillance camera. Glow produced by the Strombolian activity after 1730 was almost continuous until changing to pulses at 1840 and disappearing at 2100. At the climax, red tracks of volcanic bombs were recognizable up to 150 m above the crater rim. The eruptive episode was marked by increased seismic tremor amplitude similar to that of the previous night.

On the morning of 14 March weather conditions became worse and the video link was interrupted. The video link was restored on 21 March and some minor ash emissions were observed. The observations by the video camera remained intermittent due to the poor weather. Around 2000 on 30 March a remarkable increase in low-frequency events and explosion earthquakes was recorded at all stations of the seismic network; poor weather prevented visual confirmation. The phenomena continued until 2100 on 31 March and during the daytime strong pulsing steam emissions, sometimes with ash, were observed at NEC and BN.

Strombolian activity that began the day after the eighth fire fountaining episode (9-10 February) continued in April, building several nested spatter and scoria cones on the NEC floor; these rose as high as the crater rim.

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

Information Contacts: Mauro Coltelli, CNR Istituto Internazionale di Vulcanologia (IIV), Piazza Roma 2, Catania, Italy (URL: http://www.ingv.it/en/).

Aviators from the Japan Marine Safety Agency (JMSA) began observing yellowish-green discoloration of seawater during 25-28 November 1995 (BGVN 20:11/12). Similar discoloration was seen on 12, 22, and 23 January 1996 (BGVN 21:01), and also on 26 January, as reported by the Japan Meteorological Agency.

Information from the Volcano Research Center revealed that JMSA observers once again noted yellowish brown discolored seawater in the area on 4 April. According to the reports, the colored area expanded like a belt up to ~3 km long. Strong emission of colored water was recognized from two points. Although white-colored suspension was observed on the surface, floating pumices were not recognized. Yellowish-green to yellowish-brown water observed on 12 April formed a plume ~4 km long and 200 m wide, including 3-4 spots from which colored-water was gushing out intermittently. No pumices were recognized.

Geologic Background. Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the island of Minami-Ioto. Water discoloration is frequently observed, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto ("New Sulfur Island") in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Hydrographic Department, Maritime Safety Agency, 3-1 Tsukiji, 5-Chome, Chuo-ku, Tokyo 104, Japan.

Volcanic tremor was registered for six minutes starting at 1810 on 5 March by the JMA station 4.1 km WSW of the crater. During this activity, two main vents opened on and near the S side of Showa 4-nen (1929) crater. A line of vents extending ~200 m N-S formed on the S part of the crater floor. Strong eruptive activity was observed until 7 March and then decreased. Volcanic earthquakes had increased somewhat prior to the eruption, but seismicity remained low afterwards through mid-April.

Geologic Background. Much of the truncated Hokkaido-Komagatake andesitic volcano on the Oshima Peninsula of southern Hokkaido is Pleistocene in age. The sharp-topped summit lies at the western side of a large breached crater that formed as a result of edifice collapse in 1640 CE. Hummocky debris avalanche material occurs at the base of the volcano on three sides. Two late-Pleistocene and two Holocene Plinian eruptions occurred prior to the first historical eruption in 1640, which began a period of more frequent explosive activity. The 1640 eruption, one of the largest in Japan during historical time, deposited ash as far away as central Honshu and produced a debris avalanche that reached the sea. The resulting tsunami caused 700 fatalities. Three Plinian eruptions have occurred since 1640; in 1694, 1856, and 1929.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

During March, the dark-blue lake dropped 30 cm with respect to December 1995. Constant bubbling continued along the N, NW, W, and SE shores. The NW-flank site of the December 1994 eruption continued to emit low volumes of gas. During February and March seismic station IRZ2, 5 km SW of the crater, registered 31 and 19 microseisms, respectively. These events were only detected locally. Significant tilt was not detected over the deformation network.

Geologic Background. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the main crater, which contains a small lake. The first well-documented eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

Small-amplitude volcanic tremor was detected on 4 March. Tremor was last registered on three days in January 1996 (BGVN 21:02) and once in October 1995.

Geologic Background. Viewed from the east, Iwatesan volcano has a symmetrical profile that invites comparison with Fuji, but on the west an older cone is visible containing an oval-shaped, 1.8 x 3 km caldera. After the growth of Nishi-Iwate volcano beginning about 700,000 years ago, activity migrated eastward to form Higashi-Iwate volcano. Iwate has collapsed seven times during the past 230,000 years, most recently between 739 and 1615 CE. The dominantly basaltic summit cone of Higashi-Iwate volcano, Yakushidake, is truncated by a 500-m-wide crater. It rises well above and buries the eastern rim of the caldera, which is breached by a narrow gorge on the NW. A central cone containing a 500-m-wide crater partially filled by a lake is located in the center of the oval-shaped caldera. A young lava flow from Yakushidake descended into the caldera, and a fresh-looking lava flow from the 1732 eruption traveled down the NE flank.

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

According to the Institute of Volcanology (IV), the eruption on 2 January began around 0800. This activity was preceded by an upsurge in seismicity that started in April 1995. At 1926 on 31 December 1995, a M 5.8 earthquake occurred in the Kronotsky gulf, 50-60 km NE of the volcano. On 1 January at 2057 an earthquake of M 5.2 in the Karymsky region was followed at 2157 by a M 6.9 event centered ~25 km S of the volcano. During the next day there were more than 10 aftershocks of M >= 5.0. On 2 January at 1540, a group of IV volcanologists arrived by helicopter. Eruptive centers were observed near the summit and 5-6 km S in Karymsky Lake (maximum depth 115 m), which fills the Akademii Nauk caldera.

The eruption began with formation of a vent with a diameter of 20-30 m, located 50 m below the summit. Violent emissions of ash-rich gas jets rose to 1 km from another vent on the SW slope. Steam-and-gas jets, occasionally with black-colored matter, were also ejected to several hundred meters from beneath the surface of Karymsky Lake. The presumed eruptive center was 100-200 m from the shore in the NW sector of the lake. Turbulent steam-and-gas plumes rose 5-6 km above the surface from a 200-m-diameter area. Ice covering the lake had completely melted.

On 3 January the near-summit vent increased in size to 50 m in diameter. Gas and steam blasts alternated with ash ejections from the two simultaneously active vents on the volcano. Ash was usually ejected from the upper vent, and a white-colored plume was emitted from the lower vent. Ash ejections lasted 2-3 minutes, and gas blasts lasted 1.5-2 minutes. An ash-and-gas column rose 1-1.2 km and was blown E and SE by the wind. The surface of Karymsky Lake steamed intensely, sending clouds 800-1,000 m above the lake. Areas of green water were visible through breaks in the clouds, and a newly-formed black beach was seen. In the N and NE sector of the lake a narrow spit, beginning from the source of the Karymsky river and extending 250-300 m to the center of the lake, had formed. The water level in the lake had dropped a few meters. The upper reaches of the river had dried up, but on 2 January waves from the submarine eruption (up to 10 m high or more) overflowed the N shore, flooding a wide valley 1.5 km below the source. During a surveillance flight on 4 January, large areas of the valley were covered by black mud. The beach contained three fumarolic vents along the NE-trending fault zone. Within a radius of 500-800 m of the source of the Karymsky River, the surrounding snow-covered hills contained thousands of holes with diameters ranging from 10 cm to 1.5-2 m formed by lithic blocks ejected from the lake. The water level of the lake continued to fall because of intense evaporation.

Light-gray dacitic ash covered an area of about 150-200 km². At a distance of 8 km from the volcano fractions ranging from 0.16 to 0.06 mm dominated. Estimates made by S.A. Fedotov indicated that on 2 and 3 January the ash ejection rate from the summit crater reached 3-4 tons/second.

Routine observations from 2 January through 11 February showed that the climactic phase of the subaqueous eruption continued for no more than 12-15 hours. That eruption consisted of frequent explosions during which a vapor-gas mixture with lithic material was ejected to the surface. In the N sector of the lake at the shore W of the Karymsky River, damaged trees provided evidence of two eruptive sources 500-600 m from each other. This zone contained the main concentration of bomb material ejected from the lake. A portion of the shoreline (150-200 m long and 5-15 m wide) E of the river sank several meters into the lake. The main eruption center was 500 m from the shore, but smaller peripheral centers were also observed. As a result of the eruption, in the NNW sector of the lake, a beach in the form of a wide 0.4 km² cape was produced, as well as a narrow spit extending SE from the old shore. The length of the new shoreline was 2.4 km, and a large shoal was observed around the new peninsula. According to the preliminary estimates, the ejected deposits in the lake are at least 1 km² in area and 5-10 x 10⁶ m³ in volume.

Thermal springs that discharge at the S shore of Karymsky Lake were destroyed by ejecta from this eruption, and several new mud pots were formed; chemical composition of the solutions was unchanged. Near the center of the new beach, composed of sand-gravel and bomb material, a chain of five explosive vents with diameters from 1.5 to 30 m was observed. At the N end was a thermal site with a diameter of ~50 m that exhibited intense vapor emission and was covered by sublimates; visiting scientists detected a hydrogen sulfide odor. A dry funnel with a diameter of ~3 m and high gas emission at a temperature of 97°C was in the center of this site. Other explosion funnels had water at a depth of 1.2-1.5 m with temperatures from 33 to 70°C. The three funnels closest to the lake and on the opposite shore had gas emissions with temperatures of 97-98°C.

On 4 January run-off from the lake ceased owing to damming by ejected material. Analyses of water samples from the lake, river, and various hot springs in the area indicated that there had been chemical contributions to the lake water by an underlying magma body.

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: G.A. Karpov, Ya.D. Muravyev, R.A. Shuvalov, S.M. Fazlullin, and V.N. Chebrov, Institute of Volcanology, Far East Division, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, 683006, Russia.

Unusually heightened activity along Kīlauea's East Rift zone on 1-4 February was followed by a pause that began on 4 February and ended at midnight on 14 February (BGVN 21:01). Tilt in the N-S direction increased roughly 3-fold; in the E-W direction, roughly 4-fold. Daily counts of shallow, summit SPC earthquake counts rose from around 10/day to 580/day.

During the 14-day pause, lava continued circulating inside the lava pond at Pu`u `O`o cone but no lava was seen flowing in downstream tubes. The lava pond rose to 60-70 m below the rim as the eruption restarted and lava re-entered the tubes around mid-day on 14 February.

Lava subsequently broke out of the tubes to reach the surface at numerous locations (including those at 750-, 720-, 700-, 450-, and 90-m elevations and on the coastal flats). About 31 hours later lava reached the ocean via the old tube system. Despite these numerous sites where lava had been escaping from tubes on 14-15 February, in the days following lava generally ceased reaching the surface and feeding lava flows. Lava did emerge at elevations of 60 to 100 m and on the coastal plain between Kamokuna and Kamoamoa (figure 99). The surface of the lava pond dropped by 19-21 February, possibly reaching 90 m below the rim. Still, on 23 February aa emerged at 270 m elevation. The same day, 23 February, explosive activity at the Kamokuna bench built a new littoral cone.

Figure 99. New land and active entries in the Kamokuna area, March 1996. Index map shows the swath of 1992-96 lava flows. Courtesy of HVO.

As late as 9 March, flows confined to the area below Pulama Pali and the coast covered much of the S half of the Kamoamoa flow field but failed to reach either adjacent grasslands or the sea. During the early hours of 29 February the entire lower coastal bench, a roughly 30 x 100 m area, fell into the ocean. The event was recorded seismically at an instrument 10 km distant. Since then, freshly erupted lava began constructing a new coastal bench.

During the second half of February through early March, low-amplitude tremor in the East Rift continued; during 13-26 February and 12-25 March tremor amplitudes were ~3x background but showed fluctuations. During the February interval microearthquake counts were low beneath the summit and rift zones. Shallow, long-period microearthquake counts were high on 4-5 March and briefly again on 8 March. On 2, 5, 7, and 8 March there were four events > M 3.0 in the 7-33 km range. Deep tremor from the usual SW source was recorded in three episodes during 14-15 March: a total of 90 minutes on 14 March and 108 minutes on 15 March. Counts of shallow, long-period earthquakes increased during 19-23 March reaching a maximum daily total of 1,750.

On 24 March, 2 hours of elevated tremor (4-5x background) took place without accompanying shallow short-period earthquakes. That same day, the summit inflated rapidly for an hour and then deflated for several hours. The rate of inflation was similar to that of 1 February but the summit acquired only 3 µrads of tilt compared with the 15 µrads seen on 1 February. As the summit deflated on the afternoon of 24 March, the eruption site on the East Rift zone probably received a small magma surge resulting in moderate-sized breakouts in the early afternoon. The breakouts, which originated from the lava tube at the 820-, 750-, and near the 600-m elevations, produced small pahoehoe flows that were mostly stagnant by the next morning. On the night of the 24th, bright glow from Pu`u `O`o indicated turbulence in the lava pond. Except for these flows on 24 March, surface lavas mainly appeared below the base of Pulama Pali.

At the coast, spectacular explosions, some as high as 70 m, began on 19 March. Though diminishing thereafter, they persisted until at least 6 days. Observers saw lava bubble-bursts, lava fountains, and steam jets. These explosions built up five new littoral cones ~130 m W of the earlier Kamokuna entries inside the National Park (figure 99).

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii National Park, HI 96718, USA.

According to the Sakura-jima Volcanological Observatory of Kyoto University, the number of earthquakes has increased around Shin-dake since January. The total number of earthquakes recorded was 32 in January, 40 in February, and 77 in March.

A group of young stratovolcanoes forms the E end of Kuchinoerabu-jima Island, midway between Suwanose-jima and Kyushu. Several villages on the 4 x 12 km island are located within a few kilometers of the active crater of Shin-dake and have suffered damage from historical eruptions. Shin-dake is the summit cone, and has been the site of all 13 eruptions known since 1840. The last eruption was a weak 30-minute explosion on 28 September 1980 that sent an ash plume 2-3 km high.

Geologic Background. A group of young stratovolcanoes forms the eastern end of the irregularly shaped island of Kuchinoerabujima in the northern Ryukyu Islands, 15 km W of Yakushima. The Furudake, Shindake, and Noikeyama cones were erupted from south to north, respectively, forming a composite cone with multiple craters. All historical eruptions have occurred from Shindake, although a lava flow from the S flank of Furudake that reached the coast has a very fresh morphology. Frequent explosive eruptions have taken place from Shindake since 1840; the largest of these was in December 1933. Several villages on the 4 x 12 km island are located within a few kilometers of the active crater and have suffered damage from eruptions.

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

Seismicity increased during 24-27 March, and volcanic tremors were detected late in the month. The total number of earthquakes in March was 507. The height of the ash-free plume remained at 100-300 m for most of the month, with increases to 600 m on 12 and 27 March.

Geologic Background. Kujusan is a complex of stratovolcanoes and lava domes lying NE of Aso caldera in north-central Kyushu. The group consists of 16 andesitic lava domes, five andesitic stratovolcanoes, and one basaltic cone. Activity dates back about 150,000 years. Six major andesitic-to-dacitic tephra deposits, many associated with the growth of lava domes, have been recorded during the Holocene. Eruptive activity has migrated systematically eastward during the past 5000 years. The latest magmatic activity occurred about 1600 years ago, when Kurodake lava dome at the E end of the complex was formed. The first reports of historical eruptions were in the 17th and 18th centuries, when phreatic or hydrothermal activity occurred. There are also many hot springs and hydrothermal fields. A fumarole on Hosho lava dome was the site of a sulfur mine for at least 500 years. Two geothermal power plants are in operation at Kuju.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

Moderate explosive activity continued at Crater 2 during March, however, there was possibly a slight decline compared to last September. Intermittent Vulcanian explosions released ash clouds that rose several hundred meters above the crater. These explosions resulted in minor ashfalls to the volcano's SE. A weak but steady crater-glow was observed on a few nights. In accord with these observations, 6-30 daily explosion earthquakes registered at a station 4 km away. There was no visible activity from Crater 3.

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

Information Contacts: Ben Talai, RVO.

When visible, South and Main Craters only gave off weak to moderate white vapor. There were no audible sounds from either crater and no sighting of glow at night. Seismic monitoring at Manam was absent during March. Measurements from the water tube tiltmeters at Tabele Observatory (4 km SW of the summit) indicated no deflation to slight deflation.

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: Ben Talai, RVO.

Volcanic earthquakes and tremors were detected near the end of March by instruments maintained by Hokkaido University.

This small island 55 km W of Hokkaido in the Japan Sea consists of two coalescing volcanoes. An eruption in August 1741 produced heavy ashfall on the Hokkaido mainland. A violent explosion and landsliding from the Nishi-yama cone accompanied a large tectonic earthquake and a major tsunami that killed 1,475 people, most on the W coast of the Oshima Peninsula. Subsequent eruptions through early 1742 produced a new central cone and lava flows. Minor explosive activity was documented in 1759, 1786, and 1790.

Geologic Background. Oshima-Oshima, a 10 km2 island about 105 km W offshore from the SW tip of Hokkaido, is the emergent summit of two coalescing basaltic-to-andesitic stratovolcanoes. Higashiyama, at the east end of the island, is cut by a 2-km-wide caldera covered on its west side by Nishiyama volcano. The western cone failed during an eruption in 1741 CE, producing a mostly submarine debris avalanche that traveled 16 km and leaving a scarp open to the north. A tsunami associated with the collapse swept the coasts of Hokkaido, western Honshu, and Korea, and caused nearly 1,500 fatalities. The 1741 eruption concluded with the construction of a basaltic pyroclastic cone at the head of the amphitheater. No eruptions have occurred since the late-18th century.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

Intense and prolonged rainfall, associated with the passage of two typhoons on 1 October and 3 November 1995, triggered lahars and floods along the Pasig-Potrero River.

On 1 October 1995, Typhoon Mameng delivered 337 mm of rain on the Sacobia pyroclastic fan triggering five distinct and fairly continuous lahar episodes over a 14-hour period. The largest episode had an estimated peak discharge of 400 m³/sec at Mancatian, Porac. Within each episode were discrete peak readings that could be generated by several causes: variations in rainfall intensity and duration, bank caving, repeated damming and breaching along channel constrictions, as well as any combination of these. Flows were described as steaming at observation point Delta 5 (15.5 km from Pinatubo), but progressive dilution through the incorporation of older materials cooled the flows to ambient temperature by the time they reached Bacolor.

The Typhoon Mameng deposit can be distinguished from earlier lahar deposits by the abundance of pebble to boulder-size clasts rip-ups of older, pre-1991, eruption materials scoured from the channel. The Mameng deposit was provisionally classified in two distinct debris flow units: a gray pumiceous, pebbly sand unit (A), and a brown, lithic-rich, coarse gravel unit (B). Unit A occurs as small overbank deposits at Mancatian, and as laterally extensive coalescing lobes from San Antonio, Bacolor down to the Gapan-San Fernando-Ologampo (GSO) road. Its occurrence as an overbank facies at Mancatian suggests that this unit correlates to the above mentioned peak flow episode. Unit B corresponds to subsequent flows and waning episodes: it occurred as in-channel, gravel terrace deposits from Delta 5 observation point downstream to the GSO road, and from thereon as an overbank facies where it overlaid unit A.

A total area of 25 km² was buried beneath 0.5 to 6 m thick of sediment. An estimated sediment volume of 50 x 10⁶ was deposited during these events, with roughly 40% consisting of either old, pre-1991 eruption deposits or post-1991 eruption lahar materials.

On 3 November 1995, 150 mm of rain fell on Mount Pinatubo with the passage of Typhoon Rosing. Lahars observed at Delta 5 watch point were relatively cold hyperconcentrated flows, based on the absence of steaming. Estimated peak discharge was about 120 m³/sec. Based on flow sensor data from the Pasig-Potrero river, the peak flow was channel-confined down to the GSO road and lasted ~2.6 hours. It eroded ~30 m of the left bank along the Porac-Angels Road. Sediments were mostly clayey remobilized Mameng deposits.

Former flows had already filled the stretch of the channel at a point 2 km upstream of the GSO road down to the S portion of Bacolor. When the peak flow reached the channel-filling stage, it caused flows to bifurcate and incise a new channel (figure 34) ~50 m W of the typhoon Mameng channel. Average in-channel deposition was 2 m thick; average overflow deposition, ~0.3-m thick. Overflow units were observed along the banks of the previous channel along the GSO road and leveled to a recently emplaced steel bridge. Flows reached farther downstream causing flooding and siltation near Mesalipit and Tinajeros. The other channel backflowed following considerable aggradation along the GSO road. The new channel delivered muddy flows that induced flooding and siltation in the W portion of San Fernando, particularly in the Barangays of St. Nino and St. Lucia.

Figure 34. Map of the 1991-95 lahars at Pinatubo. Courtesy of PHIVOLCS.

Lahars have occurred during every rainy season since the eruption of 15 June 1991. Pinatubo's last reported lahars were triggered by the heavy rainfalls of July 1995, when 30 x 10⁶ m³ of debris, deposited over a 12 km² area, forced mass evacuation of Porac and Bacalor (BGVN 20:07).

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

Information Contacts: Raymundo S. Punongbayan, director, Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, (DOST), 5th & 6th Floors Hizon Building, 29 Quezon Avenue, Quezon City, Philippines.

During a February visit, the temperature of the turquoise-green crater lake was 26°C and its surface had risen 2 m with respect to its January level. Except that this lake-level rise had covered some active fumaroles, their behavior was similar to previous months. Fumaroles on the SE, S, and SW sides of the crater had temperatures of 93-95°C. One fumarole along the lake's W shore had migrated upward along a crack.

When visited during March, the lake appeared sky blue in color, its surface had dropped by 0.5 m compared to the previous month, and the water temperature was 30°C. Fumaroles, their gas emission rates, and temperatures were similar to previous months. A distance survey across the crater found that a 21 ppm/year expansion had occurred since mid-1995. At a spot adjacent to the lake, the survey found an 18 mm contraction since October 1995.

The pyroclastic cone, the major source of gas emission, discharged plumes 200-400 m high. Where accessible the temperatures of the emitted gases were around 94°C; gas emissions sounded like releases from a pressure valve, particularly those venting along the inaccessible N wall.

February and March seismicity consisted of a total 1,100 and 983 events, respectively, the majority being low frequency. This was a roughly 10-fold decrease since a peak in October 1995. Tremor duration was <10 hours, down from over 250 hours in November and December 1995.

Geologic Background. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA); Gerardo J. Soto, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE).

A new eruption began early on 5 March with continuous tremor followed by small ash emissions (BGVN 21:02). Low-level ash emissions continued through 11 March with some larger events on 10 and 11 March. Those episodes generated plumes that extended SW over the Pacific Ocean.

After 11 March and through the 19th, the overall level of activity appeared to have reached a steady state. Fumarolic activity alternated with 4-6 short-duration ash emissions each day from the same vents as the 1994-95 episode. These emissions formed short-lived ash columns that were carried away by the wind. Light ashfalls were reported from several towns around the volcano, particularly to the E and S. Seismicity, as low-level tremor accompanied by minor A- and B-type volcanic earthquakes, also showed almost stationary patterns and energy release rates. No deformation was detected by the 3-tiltmeter network on the N flank.

Satellite imagery during this interval revealed intermittent plumes extending E at altitudes around 6-7 km. Late on 13 March the plume was visible as far as 340 km ESE of the summit at 5-7 km altitude; ashfall was reported in Puebla, 70 km E. Large plumes of very thin dispersed ash blowing E over the Gulf of Mexico were observed through 15 March, with denser plumes closer to the volcano. During 15-19 March, when observed on satellite imagery, plumes averaged ~20 km wide and 60 km long before they dissipated; altitudes were in the 5-7 km range.

Ash emission increased between 2000 on 19 March and 0300 on 20 March, when characteristic signals of eight emission events or 'puffs' were detected by the seismic monitoring network. Afterwards, the emission-event rate returned to the previous range of 4-6 events/day. This, combined with stronger winds towards the E, produced light ashfalls on towns in that direction. The 'puff' events were detected on top of a moderate level of volcanic seismicity, consisting of A- and B-type events and low-level tremor, as well as strong signals from Pacific-coast tectonic earthquakes unrelated to the volcanic activity.

On 21 March the ash emission rate remained stable. The next day, the puffs' frequency increased to ~9/day, but their size decreased. Average height of the ash plumes was ~500 m above the summit, and duration <5 minutes. This activity continued without significant changes until 25 March, when the rate of ash emissions reached 8 puffs between 1030 and 1230 before returning to a rate of 8-10/day. This condition prevailed until 28 March, when another increase in the level of activity was detected similar to that on 25 March. The ash puffs were easily recognized in the seismograms as 30-40 seconds of tremor followed by an impulsive signal, similar to seismic events in the 1994-95 episode. Although the release of seismic energy increased after 25 March, the levels never reached high values, and remained well below the energy level of 5 March. Seismicity decreased again in late March.

Plumes after 20 March continued to be visible on satellite imagery, and were interpreted based on wind data to generally have been below 7 km altitude, with some slightly higher. However, poor weather and low levels of activity limited the number of plumes identified. Aviation notices from Mexico City and observers at the Puebla airport through 4 April continued to report ash at low levels, usually within ~20-30 km of the summit, blowing in easterly directions.

On 29 March during a COSPEC flight, Lucio Cardenas, Juan Jose Ramirez, and Hugo Delgado observed a new lava dome with an area of 400 m² on the E side of the crater floor along the rim of the inner crater (a lava dome destroyed during the 1920-27 eruption). This new lava dome was observed coming from a source outside that inner crater but flowing into the it. Another helicopter flight later that day confirmed that block-lava was flowing from a vent located between the vents opened on December 1994 and the 1919 craterlet near the center of the crater. This lava slowly flowed towards the craterlet. When the dome was checked again on 1 April lava had filled most of the inner crater (nearly 60 m deep) and increased its area to nearly 600 m². Assuming that the lava started to flow towards the craterlet on 25 March, and that it had been almost filled by 1 April, a rough estimate of the lava extrusion rate is 5,000-6,000 m³/day.

The formation of this craterlet was described in detail by Dr. Atl, the painter-volcanologist who later studied the Parícutin eruption in detail. According to him, the bottom of the volcano crater was almost flat before 1919. That year, extruded lava formed a small dome ~35 m high and 60-70 m diameter in the base. That dome collapsed in 1923 forming the craterlet. The volume of the internal cone of the craterlet is estimated to be 40,000 m³.

A series of SO₂ flux measurements was begun after January 1994 (BGVN 19:11 and 19:12). During 1995 measurements rose to nearly 8,000 metric tons/day (t/d) in March, but gradually decreased to 2,000 t/d in June. A persistent decrease in gas emissions starting in July reduced the SO₂ flux to nearly 100 t/d by December 1995. During the 5 March 1996 event, renewed ash emissions coincided with SO₂ fluxes of up to 15,000 t/d; by late March it was decreasing, but emission levels remained high (>5,000 t/d). Currently, the COSPEC measurements are carried out by the Instituto de Geofisica (National University of Mexico), sponsored by the Secretaria de Gobernacion (Ministry of the Interior) through CENAPRED (Disaster Prevention National Center) using an instrument borrowed from the University of Colima and a plane owned by the Mexican Navy.

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

Information Contacts: Servando De la Cruz-Reyna (CENAPRED and Instituto de Geofisica, UNAM); Roberto Quaas Weppen, Enrique Guevara Ortiz, Bertha López Najera, and Alicia Martinez Bringas, Centro Nacional de Prevencion de Desastres (CENAPRED), México D.F., México; Hugo Delgado Granados, Instituto de Geofisica, UNAM, Circuito Cientifico C.U., 04510 Mexico D.F., México; Jim Lynch, NOAA Synoptic Analysis Branch.

During March the intra-caldera cone Tavurvur produced ash explosions at 2-5 minute intervals; these rose to ~400-1,500 m altitude and then generally drifted SE. As a result, over the last 4 months ~10 cm of ash accumulated on the abandoned village of Talwat (2 km SE of Tavurvur). Vulcan only produced weak fumarolic emissions.

Seismicity fluctuated slightly during March, remaining at a level slightly lower than the peak reached in mid-February. Low-frequency earthquakes, events associated with Tavurvur ash emissions, took place 100-250 times/day (a total of 4,708 times during March). There were also five brief intervals where non-harmonic tremor took place. Only six high-frequency earthquakes occurred; some were kilometers outside the caldera to the NE in the area most seismically active since the 1994 eruption.

No significant ground deformation affected the caldera during the month. Overall, during the recent eruptive phase, the only observed ground deformation has been a slight (20 µrad) deflation at the tiltmeters nearest to Tavurvur.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: Ben Talai, Rabaul Volcano Observatory, P.O. Box 386, Rabaul, Papua New Guinea.

Several small-to-moderate eruptions took place in early November 1995 (BGVN 20:10 and 20:11/12). Mild seismicity continued after the eruption; during February seismic station RIN3, located 5 km SW of the active crater, registered seven microseisms (six low-frequency, one high-frequency). These microseisms were only detected locally.

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

Information Contacts: Erick Fernández, Elicer Duarte, Vilma Barboza, Rodolfo Van der Laat, and Enrique Hernandez, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Gerardo J. Soto, Oficina de Sismolog¡a y Vulcanolog¡a, Departamento de Geolog¡a, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

From 24 November to 12 December 1995, the first detailed study of Sangay volcano (figures 1 and 2) was carried out by an Instituto Geofísico/ORSTOM team (Escuela Politécnica Nacional, Quito), with helicopter support from the Ecuadorian Army and the assistance of five local guides from Alao. During this time, activity was characterized by continuous fumarolic steaming, frequent phreatic explosions, occasional crater glow, and dome rockfalls. Previous reports from August 1976, August 1983, and June-August 1988 (SEAN 01:10, 08:07, and 13:08) identified four summit vents aligned WSW-ENE, which are here numbered from 1 to 4 going from W to E.

Figure 1. Present cone of Sangay in December 1995 viewed from the base camp 4.3 km SW. The recent pyroclastic-flow deposit on which the campsite is located is in the foreground, among the badlands corresponding to an older edifice. At the summit can be seen the W lava dome (Vent 1) and its now inactive lava tongues. Photo by M. Monzier, courtesy of ORSTOM.

Figure 2. View of the Sangay summit in December 1995 looking NE from the base camp showing the lava dome and associated lava flows from Vent 1. Behind this dome, a steam plume rises from the main crater (Vent 3). Photo by M. Monzier, courtesy of ORSTOM.

In 1976, Vent 1 consisted of a fracture from which lava was slowly issuing, but by August 1983 it had built a lava dome. This small dome was apparently more active in August 1988, and sent a lava flow 400 m down the W flank, where it split into two lobes. In late 1995 this dome was possibly still growing, and was the source of some fumarolic activity and many rockfalls, making the W and SW slopes of the cone dangerous to cross. Apparently there have been no new lava flows from this vent since August 1988. Vent 2, a small 15-m-diameter crater immediately ENE of Vent 1 has frequently been the site of explosive activity (1976 and 1983), but apparently was less active in 1988 and was quiet during the 1995 visit. The ENE crater (Vent 4) remained inactive but with occasional fumarolic activity.

Vent 3, at 80-100 m across, is the largest and deepest crater. In 1976 and 1983 only fumarolic activity was observed from this crater, but lava was reported in 1988. During the 1995 visit it was the site of frequent phreatic explosions, some separated by hours, others coming as often as every 26 minutes. Several explosions were followed by a rhythmic, pulsating roar that lasted for up to 50 oscillations. White vapor plumes, ejected with the audible explosions, rose several hundred meters above the summit. Light blue gas plumes and occasional red glow at night immediately above this crater implied the presence of lava. Frequent rockfalls from the upper S flank of the cone suggested that some lava may be escaping, breaking off, and rolling down the S slopes.

During the visit a portable MEQ-800 Sprengnether seismograph with a vertical, 1-Hz L4C geophone was operated at the La Playa base camp, 4.3 km SW of the main crater at 3,600 m elevation. A preliminary study of the smoked-paper seismograms showed three types of seismic signals, frequently associated with observed explosions in the crater (figures 3 and 4): tremor, long-period, and hybrid events. Tremor events had a monochromatic signature with a period of 1 second and lasted < 60 seconds. The long-period events had emergent arrivals and a constant period of ~0.7 seconds; they were often associated with observed explosions. Hybrid events began with a long-period event (0.7 seconds) and were followed by a signal similar to that of the tremor (1 second). Some hybrid events were associated with audible and observed explosions followed by a roar like pulsating, rhythmic exhalations. No local high-frequency events were detected.

Figure 3. Types of volcanic earthquakes at Sangay recorded by the seismic station 4.3 km SW in December 1995. Courtesy of ORSTOM.

Figure 4. Volcanic seismicity recorded at Sangay, 26 November-10 December 1995. Courtesy of ORSTOM.

Recent lavas and pyroclastic-flow, debris-flow, and lahar deposits are ubiquitous around the cone and testify to Sangay's nearly continuous activity. The site of the La Playa camp (figure 5) is on an andesitic pyroclastic-flow deposit containing bombs up to 4 m in diameter which was emplaced between 1956 and 1965. An accident with two fatalities happened in August 1976 (SEAN 01:10). A previously unreported accident occurred in December 1993 when the main crater exploded just as two mountaineers looked over its rim. Both were blinded by the heat and fragment impacts and remained lost in the jungle on the cone's lower slopes until rescued three days later.

Figure 5. Preliminary geological/structural map of Sangay volcano based on fieldwork, aerial photographs, and 1:50,000 topographic maps from the Instituto Geografico Militar, Quito. Key: M = metamorphic formations; I, II, III = successive volcanic edifices; C1 and C2 = avalanche calderas; AD = avalanche deposits. Campsites are shown as black dots (La Playa = basecamp, Z = Zumbacocha and D = Duende are secondary camps).

In addition to the present cone (Sangay III), two previous edifices were identified and sampled, both of which had been destroyed by collapse. The remnant calderas are found on the E side of the present cone and are breached E toward the Amazon plain. Their probable avalanche deposits lie at the E foot of the cone. A preliminary geologic map of Sangay (figure 5) shows the three successive edifices and the two associated calderas. Edifice I is mainly built of lava, whereas edifices II and III contain both lava and pyroclastic deposits. The products of edifices I and II appear to be more varied in composition (greater differentiation) than those of Sangay III, where mafic andesites seem to predominate.

This isolated stratovolcano E of the Andean crest is one of Ecuador's most active volcanoes having been in frequent eruption for the past several centuries. The steep-sided glacier-covered volcano towers above the tropical jungle on the E side; on the other sides heavy rains have caused plains of ash to be sculpted into steep-walled canyons up to 600 m deep. The first historical eruption was reported in 1628, and more or less continuous eruptions took place from 1728 until 1916, and again from 1934 to the present.

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

Information Contacts: M. Monzier and C. Robin, ORSTOM, A.P. 17-11-6596, Quito, Ecuador; M. Hall, P. Mothes, and P. Samaniego, Instituto Geofísico, Escuela Politécnica Nacional, A.P. 17-01-2759, Quito, Ecuador.

Logistical support from the Méxican Navy enabled researchers to measure seven fumarole and hot spring temperatures on 2 February 1995 in the summit region of Socorro Island's Mount Everman. Previous measurements, taken on 5-12 February 1993, were made at the same sites. These sites, labeled A-G, are shown on sketch maps and tables in Siebe and others (1995) and BGVN 18:01. In this most recent series of measurements, all temperatures were 90°C, except site D, which was 74°C. The 1993 measurements were made in conjunction with a local submarine eruption that also produced T-wave signals. These new measurements showed a several-degree increase over many of those in 1993.

The 1993 eruption was seen at the ocean surface over the island's submarine W flanks; during this visit further signs of eruption were absent from the ocean's surface and from distant hydrophones. Unfortunately, local hydrophones on the S end of the island were not operational. Several hundred meters N of the summit, on North Dome, the visitors saw recently killed vegetation and dead trees on the margins of some hydrothermally active pits. They also noted soft warm ground, dead bracken, and newly established pits, suggesting reactivation. In other cases green trees grew in the pit walls. While the majority of the fumaroles appeared similar to those in 1993, the observers noted three 'mud volcanoes'; two were active and the third issued deep rumblings. A stream in the vicinity of the summit and North Dome had a temperature of 60°C and numerous 80°C springs were seen both along its bed and nearby.

Reference. Siebe, C., Komorowski, J-C., Navarro, C., McHone, J., Delgado, H., and Cortes, A., 1995, Submarine eruption near Socorro Island, Mexico: Geochemistry and scanning electron microscope studies of floating scoria and reticulite: Journal of Volcanology and Geothermal Research, v. 68, p. 239-71.

Geologic Background. Socorro, the SE-most of the Revillagigedo Islands south of Baja California, is the summit of a massive, predominately submarine basaltic shield volcano capped by a largely buried, 4.5 x 3.8-km-wide summit caldera. A large tephra cone and lava dome complex, Cerro Evermann, forms the summit, and along with other cones and vents, fills much of the Pleistocene caldera. Rhyolitic lava domes have been constructed along flank rifts oriented to the N, W, and SE, and silicic lava flows from summit and flank vents have reached the coast and created an extremely irregular shoreline. Late-stage basaltic eruptions produced cones and flows near the coast. Only minor explosive activity, some of which is of uncertain validity, has occurred from flank vents in historical time dating back to the 19th century. In 1951 a brief phreatic eruption ejected blocks, and the gas column reached 1200 m altitude. A submarine eruption occurred during 1993-94 from a vent 3 km W of the island during which large scoriaceous blocks up to 5 m in size floated to the surface without associated explosive activity.

Information Contacts: Andrew M. Burton, OCEAN, Organizatión para la Conservación Estudio y Análisis de la Naturaleza, A.C., 22 de Diciembre No. 1, Col. Manuel Avila Camancho, Naucalpan, Edo. de México.

During March ash plumes continued to blow over the Capital and environs, and the rate of dome extrusion escalated. Later, on 3 April, explosions at the dome and pyroclastic flows down the Tar River prompted an evacuation of the southern part of the island.

Seismicity during March from both rockfalls and deeper sources continued in a manner consistent with dome growth. Tremor was repeatedly recorded at Gages station. Although there were exceptions, deformation mainly continued as a shortening of line lengths equivalent to ~1 mm/day (similar to trends seen since mid-November). The chief exception was on the W flank (Amersham to Chances Steps line), which on March 11 showed a surprising 3 cm lengthening since last measured on February 19. This is a reversal of the shortening that occurred from October to late December on this line.

Numerous rockfalls and avalanches from the dome in early March chiefly appeared on the dome's SW and NW sides. Next, they were repeatedly seen on the NW but some also started in the dome's central area (week 2). Rockfalls then shifted from the dome's NW margin to the E margin (week 3). Later rockfalls descended the NW, W, and E margins (week 4).

Rapidly growing spines continued to be common during much of March. They were noted on the dome's SW (weeks 1 and 2) and NW (week 4). On the NW, one spine achieved the greatest absolute height of any yet seen. It extruded rapidly, rising 10 m over an interval of about one day on 26-27 March. Over a 24-hour interval beginning at 1600 on 21 March, another spine's vertical growth measured ~7 m.

The dome's topography was mapped during week 2 from Farrell's lookout (on the WNW). The resulting map allowed workers to estimate the dome's mid-March volume as ~6.7 x 10⁶ m³, a value comparable to previous, cruder estimates made in the field. It appeared that the dome's growth rate increased 7- to 10-fold in the last few months. Specifically, the late-November and December rate was ~0.2-0.3 m³/second whereas the March rate was closer to 2 m³/sec. On 3 and 12 March the growing dome's summit elevations were 845 and 875 m, a 30 m rise in ~9 days. Later, on 20 March, a visit to Gages Wall revealed that, even though this sector had few rockfalls around the time of the visit, the dome's talus apron had grown to within ~15 m of the wall's top.

During week 2, fine ash carried from some larger rock falls was deposited on the upper W flanks. On 17 March, viewers on Farrell's lookout were enveloped in a warm ash cloud following a rockfall that occurred without a noticeable explosive component. That same day an explosion may have helped drive an ash column to 2,300 m.

Other relatively large ash clouds appeared repeatedly during late March and early April. On 27 March there were ash clouds generated at 0642, 0700, 0848, and 1725. The 0642 event produced an ash column that reached a height of 2,000-2,300 m and blew W blanketing areas in vicinity of the Capital. The 0642 event accompanied a seismic signal comprised of seven pulses in a 14-minute interval; the 0700 event generated a smaller ash column accompanied by three seismic pulses. Except for these intervals of unusual seismicity and frequent signals from large rockfalls, seismicity during the 24-hour interval prior to the 27 March events had been generally quiet. Helicopter observations shortly after the 0700 event disclosed that ash had been channeled to the E down a drainage called the Hot River Ghaut. Hot ash had traveled for ~1 km from the dome, igniting dead trees along its path. Observers witnessed the 0848 event, but it was much smaller and areally restricted.

Several other plumes on 31 March led to a nearly one-hour interval late that day when unusually intense seismicity registered at all the stations. The seismicity was correlated with ash plumes that blew W. On 1 April a helicopter flight confirmed the largest block-and-ash flows yet seen. Although runout distances were similar to those seen on 27 March (on the order of 1 km from the base of the Castle Peak dome), those on 1 April entrained bigger blocks and had a more widely dispersed dilute component that burned a broader swath of trees and foliage around the Tar River Soufriere (~1 km NE of Castle Peak's summit).

Until a small explosive event at 0652 on 3 April, the majority of the airborne ash was thought to have come from rockfalls and avalanches off the dome. This explosion, and several other significant ones the same day, discharged from a fissure on the dome's E flank, a spot that also appeared as the source of recent rockfalls. At various times on 3 April, continuous ash emissions came from the crater area. The activity continued to build during the day, with many small explosive seismic signals and continuous tremor recorded at the closest seismic station on Chances Peak.

At 1518, a pyroclastic flow occurred in the Tar River area. It traveled ~1.9 km down this drainage and burned vegetation and set fire to sulfur at the Tar River Soufriere. It also extended 1.9 km down the Hot River Valley (to where the road crosses the river), stopping ~400 m upslope of the Tar River Estate house. Although no inhabited areas were affected by the pyroclastic flow, the settlement of Long Ground lies ~2 km NE of Castle Peak's summit. The flow generated an ash plume that rose to ~6,700 m. Much of the ash blew N in light and variable winds. Other pyroclastic flows occurred at 1808 and 1818. These events, some of which were captured on NASA GOES satellite images, prompted scientists to note the possibility of further explosive eruptions during the next few days and to urge residents to move to the island's N end. The 3 April evacuation continued through at least 30 April.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat (URL: http://www.mvo.ms/); NOAA/NESDIS Synoptic Analysis Branch (SAB), Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Weak ash eruptions were observed on 5 and 6 March; occasional ashfalls were reported on the island. Nine explosions were observed in 1995 and there were small eruptions during 10-13 January (BGVN 21:01). Activity has been high since 1950.

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

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

As in previous months, during March Ulawun emitted weak to moderate volumes of white vapor. The seismograph did not operate.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: Ben Talai, RVO.

Volcanic tremor on 24 March was associated with minor tiltmeter changes. A pyroclastic flow on 10 February (BGVN 21:02) was the first in a year. Dome growth followed by collapses that generated pyroclastic flows was a common occurrence during the 1990-95 eruption.

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

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.