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/).

During April, Miniami-dake crater produced 14 eruptions, including five that were explosive. Seismic station B, 2.3 km NW of Miniami-dake crater, recorded 364 earthquakes and 120 tremors. On 28 April an ash plume rose 3,500 m above the summit crater. This was the highest ash plume observed during the month. A monthly ashfall total of 8 g/m² of ashfall was measured at the Kagoshima Local Meteorological Observatory (KMO), 10 km W from the crater.

During May, Minami-dake crater produced one explosive eruption. Station B recorded 64 earthquakes and three tremors. The highest ash plume of May rose 3,500 m above the summit crater. The ashfall total at KMO was 6 g/m².

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

Eruptions began on 2 January from the summit of Karymsky and from the lake (Karymsky Lake) within the Akademia Nauk caldera (figure 1), previously considered to be extinct (BGVN 21:01-21:03). Eruptive activity at [Karymsky] continued through the end of April.

Figure 1. Schematic map showing some features of the SW part of the Karymsky Volcanic Center. Karymsky Lake lies within the Akademia Nauk Caldera. Courtesy of the Institute of Volcanology.

Precursory seismicity. Large tectonic earthquakes in the Kronotsky Gulf have historically been among the precursors to eruptions from Karymsky and Maly Semiachik volcanoes. At 1926 on 31 December 1995, a M 5.6 earthquake occurred in the Kronotsky Gulf (50-60 km NE) at a depth of ~60 km. Earthquake swarms are common beneath the large (50 x 35 km) Karymsky Volcanic Center, but an unusually large swarm started on the evening of 1 January with hypocenters to depths of 80 km (figure 2). These followed a M 5.2 foreshock, and at 2157 a shallow M 6.9 earthquake took place centered ~25 km S of Karymsky; this was the largest earthquake recorded beneath the Kamchatkan volcanoes during the past 50 years. Scientists from the Institute of Volcanology and the Kamchatkan Experimental-Methodical Seismological Department of Geophysical Survey, Russian Academy of Sciences, flew to the epicentral zone of the continuing earthquake swarm and observed the onset of the eruption.

Figure 2. Map and cross-sections of epicenters from the earthquake swarm at Karymsky Volcanic Center that began on 1 January 1996. Cross-section A-B (below map) trends approximately NW-SE, and cross-section C-D (left of map) trends approximately NE-SW. Courtesy of the Institute of Volcanology.

Early eruptions at Karymsky volcano. On the afternoon of 2 January the eruption began on Karymsky's upper SW flank 50 m below the old summit crater and from the Akademia Nauk caldera lake, ~6 km S (figure 3). Ash and gas clouds from the summit vent fed a plume rising to 1 km above the crater; the ash-flow rate was estimated to be several cubic meters per second. The eruption cloud extended E towards the ocean and ashfall was visible 40-50 km away.

Figure 3. Simultaneous eruptions of Karymsky (right) and Akademia Nauk (left) volcanoes, 2 January 1996. Distance between the summit vent of Karymsky and subaqueous vents in the Akademia Nauk caldera lake is 6 km. The Karymsky cone is 700 m high. Courtesy of the Institute of Volcanology.

On the evening on 3 January another crater formed on Karymsky; it looked like a 30-m-diameter amphitheater open to the SW. Sub-vertical Vulcanian explosions occurred from this crater to an altitude of 1 km. Over the next few days, explosions sent gas-and-ash emissions 300-1,100 m high almost every minute.

During the first three days of the eruption, ~500-800 x 10³ tons of solid materials, including ash, lapilli, cinder, and bombs, were ejected at Karymsky. During the next 2-3.5 months ~3-4 x 10³ tons of andesite-dacite tephra (SiO2 61%) and a small amount of bombs were ejected. An area with a radius of 15-20 km was covered by an ash layer several millimeters thick. The layer's thickness increased along the ashfall axis, reaching 20-30 mm at 4-5 km from the source.

Early eruptions at Akademia Nauk caldera lake. Violent subaqueous explosions on 2 January took place several times every hour in the N part of the 5-km-wide Akademia Nauk caldera lake (figure 4). Explosion clouds rose to 8 km altitude, but most of the tephra fell back into the lake. Ash from Karymsky Lake covered Akademia Nauk volcano and its surroundings. The head of the Karymsky River had its valley and adjacent flood-lands inundated by high water and mud flows.

Figure 4. One of the powerful subaqueous explosions from the N part of Karymsky Lake (Akademia Nauk Caldera), 2 January 1996. The base of the growing cloud is ~1 km wide. Courtesy of the Institute of Volcanology.

Although the Akademia Nauk caldera lake had been ice-covered during the winter, after the January explosions water temperature reached 25°C, pH decreased from 7.5 to 3.1-3.2, and mineralization increased from 0.1 g/l to 0.9 g/l. Thermal water compositionally similar to those of the Karymsky springs started to discharge at a new shoal in the N part of the lake. According to preliminary estimates, ~0.015 km³ of material was supplied to the lake during the eruption.

After the lake water had cleared, a subaqueous deposit around the main explosion vent (with a diameter of 1 km) was observed. The N part of the deposit, ~1 km², was exposed at the surface, forming an arched spit with the adjoining peninsula (figure 5). According to preliminary estimates, ~5-10 x 10⁶ m³ of tephra including sand and rounded fragments of various sizes, and many bombs, formed the deposit there. Their composition ranged from basaltic andesite to andesite-dacite. The volume of deposits on the bottom of the lake is much greater.

Figure 5. View of Karymsky Lake showing the new 1-km-wide peninsula formed by subaqueous explosion deposits on 2 January 1996. The main vents are to the left of the beach arc. Courtesy of the Institute of Volcanology.

Activity through April. During the ensuing days in January, the eruption style at Karymsky dropped to 5-6 explosions reaching 500-900 m high every hour. More vigorous single explosions were exceptional. On 13-14 January, a block-lava flow from the flank crater traveled 400 m, was 50-70 m wide, and averaged 6-10 m thick. In late January the interval between explosions started to increase from 30 minutes to 2-3 hours.

In February only several explosions were observed each day (figure 6). In late February the number of explosions increased to 5-6/hour, but their intensity decreased. In March the number of explosions decreased but their intensity increased. In April the number of explosions increased. For example, on 23 April they took place every 5 minutes. Two additional lava flows were emitted from the flank crater in April.

A dense geodetic network developed since 1972 at the Karymsky Volcanic Center has been measured repeatedly. During the past 20 years, a horizontal extension of Akademia Nauk caldera was observed that may have indicated filling of a magma chamber under the volcano. Measurements made in February and March revealed an extension of 232 cm along the 3.5-km base and subsidence of 70 cm near the area of subaqueous explosions in the caldera lake.

Figure 6. Typical Vulcanian and Strombolian activity at Karymsky, January-April 1996. Courtesy of the Institute of Volcanology.

Karymsky Volcanic Center. Karymsky and Akademia Nauk are part of the 50 x 35 km Karymsky Volcanic Center (sometimes referred to as the Zhupanovsky volcano-tectonic depression). Located in the Eastern Kamchatka volcanic belt, 30 km from the Kronotsky Gulf and Pacific Ocean, this center contains 21 volcanic edifices, six calderas, and two historically active stratovolcanoes, Karymsky and Maly Semiachik.

The 5-km-diameter Karymsky Caldera formed 7,800 years ago and the Karymsky cone has been growing in the center of the caldera for 5,300 years, ejecting andesitic and dacitic materials. Historical reports on Karymsky's eruptions have been available since 1771. During that period of time, more than 20 prolonged eruptions were separated by quiet periods as long as 10 years. The most recent previous eruption continued from 1970 to 1982.

Akademia Nauk caldera, which was named by the famous Russian volcanologist Vladimir Vlodavetz in 1939, is located immediately to the S in the SW part of the Karymsky Volcanic Center. Its activity began about 50,000 years ago. The N part of the caldera is occupied by Karymsky Lake (4 km wide, 12.5 km² in area, and 80 m deep). The Akademia Nauk chloride-sodium springs, with 1.3 g/l mineralization and temperatures >250°C in the interior part of the hydrothermal system, discharge along the lake's S shore.

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: S.A. Fedotov, V.A. Budhikov, G.A. Karpov, M.A. Maguskin, Ya.D. Muravyev, V.A. Saltykov, and R.A. Shuvalov, Institute of Volcanology, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, 683006, Russia.

Fluctuations in the intensity and frequency of explosive activity were reported by OVSICORI-UNA. Activity during April increased above that of the previous several months but diminished during May. The April increase was accompanied by a corresponding rise in the amount of pyroclastic material produced; columns ascended over 1 km above Crater C in April and somewhat lower in May; these were commonly blown towards the NW, W, and SW. Ashfall measured at the ICE station 1.8 km W of the vent was higher during March-May than earlier in the year (table 14).

Table 14. Ash collected 1.8 km W of Arenal's active vent. Courtesy of Gerardo Soto, ICE.

During April and May, bombs and blocks fell to 1,200 m elevation. New pyroclastic-flow deposits were noted in April. Early April pyroclastic deposits descended the SW flank (to 1,000 m elevation) and those of late April descended the NW flank (to 1,250 m elevation). Light ash fell towards the N and NE in May.

Lava flows emitted in the previous month divided into two arms that both trended about NW. A new, NE-trending flow began during April and by the end of the month its front reached 1,200 m elevation. Sporadic avalanches fell off this front and sometimes reached into forested land. During May, continued descent of the flows to as low as 750 m elevation led to avalanches off their fronts producing small fires in the woods. Accumulating tephra and lava have caused Crater C's floor to rise an average of 5.4 m/year since 1987.

OVSICORI-UNA reported a progressive seismic buildup during April; over the course of the month the number of local earthquakes increased 4- to 6-fold peaking on the 27th. Station VACR (2.7 km NE of the Crater C) registered rather typical numbers of earthquakes for both April and May: 798 and 828 events, respectively. Many of these earthquakes were associated with Strombolian eruptions that took place on 20-28 April.

The number of hours of tremor during April, 434, was the highest measured in more than two years. While there occurred a progressive buildup in the number of earthquakes during April (ending on the 27th), tremor during the same interval fluctuated strongly, with daily totals between about 6 and 23 hours. May tremor totalled 325 hours. Results for monthly earthquakes and tremor obtained by ICE are smaller but also show relative increases (table 15).

Table 15. Average seismicity at Arenal, as recorded in Fortuna station, 3.5 km E of active vent. Courtesy of ICE.

Deformation studies carried out during April and May indicated no significant changes in that time interval. By the end of April 1996 the distance network had indicated a contraction of 22.4 ppm/year during the last two years.

OVSICORI-UNA and a team of seven visiting scientists reported that on 1-9 March Arenal's summit was almost continuously visible due to abnormally clear weather. Two gas plumes were observed, the largest being associated with the continuing Strombolian activity. This plume had extremely variable output and was often ash laden. The smaller plume, which was emitted at a more-or-less constant rate (even during the Strombolian explosions), carried no ash. The separate plumes were thought to signify the existence of two or more summit vents.

The Strombolian activity remained vigorous and variable, with large bombs being regularly thrown over the crater rim, making access to points on the edifice above 1000 m extremely hazardous. The ash column sometimes collapsed, resulting in pyroclastic surges, some of which were witnessed. Ash fallout from the plume was observed to vary from a wet, fine powder to dry particles up to 0.5 mm in diameter. Ash occasionally fell on the lower western flanks of the volcano.

The two lava flows referred to above were active when observed by visiting scientists. One flow was more vigorous; it issued from a steeply leveed channel aligned westwards from the summit for 200 m before diverging northwestwards.

A survey of lava flows erupted during 1995 showed that the westward flow had halted at 750 m and was composed of Arenal's typical basaltic andesite. The visiting scientists saw one anomalously hot area at 850 m elevation on the N levee that was distinguished by escaping steam. The levee on the flow's opposite side had completely collapsed. The flow was beginning to be vegetated by moss and ferns. The westward flow, which halted at 850 m in November 1995, contained vesiculated lava as well as the usual basaltic andesite mixed with blocks of ash. Flow thickness at the front of the surveyed flow that lies to the NW was around 100 m.

SO₂ fluxes were also measured by COSPEC as a follow-up to measurements made at the same time last year. Six days of flux data during 29 February-8 March were collected, the result of more than 40 measurements. Daily averages were 110, 194, 111, 130, 259, and 171 metric tons/day (t/d); the mean for the period was 163 ± 53 t/d (1 sigma). The flux appeared to be small and variable, though less so than at the same time last year (BGVN 20:04). The highest SO₂ flux was associated with mild explosive eruptions. Also evident in the fluxes in some instances were both a strong post-eruption decrease and a possible gradual pre-eruption increase.

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; Hazel Rymer and Mark Davies, Dept. of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; John Stix, Dora Knez, Glyn Williams-Jones, and Alexandre Beaulieu, Dept. de Geologie, Universite de Montreal, Montreal, Quebec, H3C 3J7, Canada; Nicki Stevens, Dept. of Geography, University of Reading, Reading RG2 2AB, United Kingdom; Gerardo J. Soto, Oficina de Sismología y Vulcanología, Departamento de Geología, Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica.

Red glow has been observed over part of the S wall of Naka-dake Crater 1 since 27 April. The floor of this crater was still covered with water in May. Aso, a 24-km wide caldera, produced pyroclastic-flow deposits during the Pleistocene that cover much of Kyushu. Naka-dake, one of the 15 intra-caldera cones of Aso's caldera, has erupted more than 165 times since 553 AD.

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

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

Lidar data from Virginia, USA, again revealed the presence of a volcanic aerosol layer centered at about 22 km altitude in April and May 1996 (table 6), somewhat higher than the 18-19 km measured during August-December 1995 (Bulletin v. 20, no. 10, and table 6). Over Germany, the aerosol layer was concentrated around 15-20 km altitude during January-April 1996, consistent with measurements made during late 1995 (Bulletin v. 21, no. 2). Backscattering ratios continued to show a decreasing trend at Hampton, and remained stable at Garmisch-Partenkirchen. Data from Cuba during January-April 1996 were highly variable, but still comparable to late-1995 data (Bulletin v. 21, no. 2). The base of the aerosol layer was consistently around 15-17.5 km (dropping to 12.7-13.3 km in April), but the layer peak ranged from 18.1 up to 27.1 km. Backscattering ratios were also variable, with seven measurements showing the expected slow decrease to the 1.11-1.17 range, but with the other six being anomalously high in the 1.35-1.51 range.

Table 6. Lidar data from Virginia, Cuba, and Germany showing altitudes of aerosol layers; some layers have multiple peaks. Backscattering ratios from Virginia are for the ruby wavelength of 0.69 µm; those from Germany and Cuba are for the Nd-YAG wavelength of 0.53 µm, with equivalent ruby values in parentheses for data from Germany. The integrated value shows total backscatter, expressed in steradians^-1, integrated over 300-m intervals from 16-33 km for Cuba and from the tropopause to 30 km at Hampton and Garmisch-Partenkirchen. Courtesy of Mary Osborn, Horst Jäger, and Rene Estevan.

Geologic Background. The enormous aerosol cloud from the March-April 1982 eruption of Mexico''s El Chichón persisted for years in the stratosphere, and led to the Atmospheric Effects section becoming a regular feature of the Bulletin thorugh 1989. Lidar data and other atmospheric observations were again published intermittently between 1995 and 2001; those reports are included here.

Information Contacts: Mary Osborn, NASA Langley Research Center (LaRC), Hampton VA 23665, USA; Horst Jäger, Fraunhofer -- Institut für Atmosphärische Umweltforschung, Kreuzeckbahnstrasse 19, D-8100 Garmisch-Partenkirchen, Germany; Rene Estevan and Juan Carlos Antuña, Centro Meteorologico de Camagüey, Apartado 134, Camagüey 70100, Cuba [J.C.A is presently at Univ. Maryland, Dept. of Meteorology, College Park, MD 20742 USA];

Small-amplitude volcanic tremors were detected on 26 April and 26 May. The last eruption occurred in December 1977. Earthquakes began in September 1977, followed by mud and sand spattering and ejection of small blocks in October, and active fuming in November. The small eruption on 7 December 1977 sent ash 500-1,000 m above the crater and produced minor ashfall. Similar ash ejections occurred through January 1978 (SEAN 03:01 and 03:02).

Geologic Background. The Azumayama volcanic group consists of a cluster of stratovolcanoes, shield volcanoes, lava domes, and pyroclastic cones. The andesitic and basaltic complex was constructed in two E-W rows above a relatively high basement of Tertiary sedimentary rocks and granodiorites west of Fukushima city. Volcanic activity has migrated to the east, with the Higashi-Azuma volcano group being the youngest. The symmetrical Azuma-Kofuji crater and a nearby fumarolic area on the flank of Issaikyo volcano are popular tourist destinations. The Azumayama complex contains several crater lakes, including Goshikinuma and Okenuma. Historical eruptions, mostly small phreatic explosions, have been restricted to Issaikyo volcano at the northern end of the Higashiyama group.

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

During the first half of May, aviators of the Maritime Safety Agency and the Maritime Self-Defense Force reported discoloration of seawater at Fukutoku-Okanoba. Similar discoloration has been observed since November 1995 (BGVN 20:11/12, 21:01, 21:03, and 21:04). An overflight on 23 May indicated no discolored seawater.

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

Activity has declined since the eruptive events of March when two vents opened on and near the S side of Showa 4-nen (1929) crater, and a line of vents extending ~200 m N-S formed on the S part of the crater floor. The height of the gas plume remained at 100-200 m. A volcanic earthquake occurred on 15 May. No volcanic tremor was observed.

Komaga-take sits 30 km N of Hakodate City (population 320,000). The andesitic stratovolcano has a 2-km-wide horseshoe-shaped caldera open to the E. The volcano has generated large pyroclastic eruptions, including major historical eruptions in 1640, 1856, and 1929. In the 1640 eruption, debris from a partial summit collapse entered the sea resulting in a tsunami that killed 700 people. Although the 1929 eruption was one of the largest 20th century eruptions in Japan, it may not have had clear geophysical precursors.

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

During May the lake's water was yellow in color and its surface dropped by 40 cm with respect to March 1996. Irazú's seismic station (IRZ2), located 5 km SW of the active crater, registered 55 and 26 events during April and May respectively; all were only detected locally. For the interval April through May dry-tilt measurements failed to show significant changes.

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 12 May. Tremor was last reported on 4 March (BGVN 21:03), and previously in January and 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.

Eruptions began on 2 January from the summit of Karymsky and from the lake (Karymsky Lake) within the Akademia Nauk caldera (figure 2), previously considered to be extinct (BGVN 21:01-21:03). Eruptive activity at [Karymsky] continued through the end of April.

Figure 2. Schematic map showing some features of the SW part of the Karymsky Volcanic Center. Karymsky Lake lies within the Akademia Nauk Caldera. Courtesy of the Institute of Volcanology.

Precursory seismicity. Large tectonic earthquakes in the Kronotsky Gulf have historically been among the precursors to eruptions from Karymsky and Maly Semiachik volcanoes. At 1926 on 31 December 1995, a M 5.6 earthquake occurred in the Kronotsky Gulf (50-60 km NE) at a depth of ~60 km. Earthquake swarms are common beneath the large (50 x 35 km) Karymsky Volcanic Center, but an unusually large swarm started on the evening of 1 January with hypocenters to depths of 80 km (figure 3). These followed a M 5.2 foreshock, and at 2157 a shallow M 6.9 earthquake took place centered ~25 km S of Karymsky; this was the largest earthquake recorded beneath the Kamchatkan volcanoes during the past 50 years. Scientists from the Institute of Volcanology and the Kamchatkan Experimental-Methodical Seismological Department of Geophysical Survey, Russian Academy of Sciences, flew to the epicentral zone of the continuing earthquake swarm and observed the onset of the eruption.

Figure 3. Map and cross-sections of epicenters from the earthquake swarm at Karymsky Volcanic Center that began on 1 January 1996. Cross-section A-B (below map) trends approximately NW-SE, and cross-section C-D (left of map) trends approximately NE-SW. Courtesy of the Institute of Volcanology.

Early eruptions at Karymsky volcano. On the afternoon of 2 January the eruption began on Karymsky's upper SW flank 50 m below the old summit crater and from the Akademia Nauk caldera lake, ~6 km S (figure 4). Ash and gas clouds from the summit vent fed a plume (figure 5) rising to 1 km above the crater; the ash-flow rate was estimated to be several cubic meters per second. The eruption cloud extended E towards the ocean and ashfall was visible 40-50 km away.

Figure 4. Simultaneous eruptions of Karymsky (right) and Akademia Nauk (left) volcanoes, 2 January 1996. Distance between the summit vent of Karymsky and subaqueous vents in the Akademia Nauk caldera lake is 6 km. The Karymsky cone is 700 m high. Courtesy of the Institute of Volcanology.

Figure 5. Continuous gas-and-ash emission from the new vent on the upper flank of Karymsky, 2 January 1996. Courtesy of the Institute of Volcanology.

On the evening on 3 January another crater formed on Karymsky; it looked like a 30-m-diameter amphitheater open to the SW. Sub-vertical Vulcanian explosions occurred from this crater to an altitude of 1 km. Over the next few days, explosions sent gas-and-ash emissions 300-1,100 m high almost every minute.

During the first three days of the eruption, ~500-800 x 10³ tons of solid materials, including ash, lapilli, cinder, and bombs, were ejected at Karymsky. During the next 2-3.5 months ~3-4 x 10³ tons of andesite-dacite tephra (SiO2 61%) and a small amount of bombs were ejected. An area with a radius of 15-20 km was covered by an ash layer several millimeters thick. The layer's thickness increased along the ashfall axis, reaching 20-30 mm at 4-5 km from the source.

Early eruptions at Akademia Nauk caldera lake. Violent subaqueous explosions on 2 January took place several times every hour in the N part of the 5-km-wide Akademia Nauk caldera lake (figure 6). Explosion clouds rose to 8 km altitude, but most of the tephra fell back into the lake. Ash from Karymsky Lake covered Akademia Nauk volcano and its surroundings. The head of the Karymsky River had its valley and adjacent flood-lands inundated by high water and mud flows.

Figure 6. One of the powerful subaqueous explosions from the N part of Karymsky Lake (Akademia Nauk Caldera), 2 January 1996. The base of the growing cloud is ~1 km wide. Courtesy of the Institute of Volcanology.

Although the Akademia Nauk caldera lake had been ice-covered during the winter, after the January explosions water temperature reached 25°C, pH decreased from 7.5 to 3.1-3.2, and mineralization increased from 0.1 g/l to 0.9 g/l. Thermal water compositionally similar to those of the Karymsky springs started to discharge at a new shoal in the N part of the lake. According to preliminary estimates, ~0.015 km³ of material was supplied to the lake during the eruption.

After the lake water had cleared, a subaqueous deposit around the main explosion vent (with a diameter of 1 km) was observed. The N part of the deposit, ~1 km², was exposed at the surface, forming an arched spit with the adjoining peninsula (figure 7). According to preliminary estimates, ~5-10 x 10⁶ m³ of tephra including sand and rounded fragments of various sizes, and many bombs, formed the deposit there. Their composition ranged from basaltic andesite to andesite-dacite. The volume of deposits on the bottom of the lake is much greater.

Figure 7. View of Karymsky Lake showing the new 1-km-wide peninsula formed by subaqueous explosion deposits on 2 January 1996. The main vents are to the left of the beach arc. Courtesy of the Institute of Volcanology.

Activity through April. During the ensuing days in January, the eruption style at Karymsky dropped to 5-6 explosions reaching 500-900 m high every hour. More vigorous single explosions were exceptional. On 13-14 January, a block-lava flow from the flank crater traveled 400 m, was 50-70 m wide, and averaged 6-10 m thick. In late January the interval between explosions started to increase from 30 minutes to 2-3 hours.

In February only several explosions were observed each day (figure 8). In late February the number of explosions increased to 5-6/hour, but their intensity decreased. In March the number of explosions decreased but their intensity increased. In April the number of explosions increased. For example, on 23 April they took place every 5 minutes. Two additional lava flows were emitted from the flank crater in April.

A dense geodetic network developed since 1972 at the Karymsky Volcanic Center has been measured repeatedly. During the past 20 years, a horizontal extension of Akademia Nauk caldera was observed that may have indicated filling of a magma chamber under the volcano. Measurements made in February and March revealed an extension of 232 cm along the 3.5-km base and subsidence of 70 cm near the area of subaqueous explosions in the caldera lake.

Figure 8. Typical Vulcanian and Strombolian activity at Karymsky, January-April 1996. Courtesy of the Institute of Volcanology.

Karymsky Volcanic Center. Karymsky and Akademia Nauk are part of the 50 x 35 km Karymsky Volcanic Center (sometimes referred to as the Zhupanovsky volcano-tectonic depression). Located in the Eastern Kamchatka volcanic belt, 30 km from the Kronotsky Gulf and Pacific Ocean, this center contains 21 volcanic edifices, six calderas, and two historically active stratovolcanoes, Karymsky and Maly Semiachik.

The 5-km-diameter Karymsky Caldera formed 7,800 years ago and the Karymsky cone has been growing in the center of the caldera for 5,300 years, ejecting andesitic and dacitic materials. Historical reports on Karymsky's eruptions have been available since 1771. During that period of time, more than 20 prolonged eruptions were separated by quiet periods as long as 10 years. The most recent previous eruption continued from 1970 to 1982.

Akademia Nauk caldera, which was named by the famous Russian volcanologist Vladimir Vlodavetz in 1939, is located immediately to the S in the SW part of the Karymsky Volcanic Center. Its activity began about 50,000 years ago. The N part of the caldera is occupied by Karymsky Lake (4 km wide, 12.5 km² in area, and 80 m deep). The Akademia Nauk chloride-sodium springs, with 1.3 g/l mineralization and temperatures >250°C in the interior part of the hydrothermal system, discharge along the lake's S shore.

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: S.A. Fedotov, V.A. Budhikov, G.A. Karpov, M.A. Maguskin, Ya.D. Muravyev, V.A. Saltykov, and R.A. Shuvalov, Institute of Volcanology, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, 683006, Russia.

Surface flows were limited to the area below 180 m elevation in late March and early April (figure 100). Through the end of March, the Kamokuna ocean entry exhibited frequent explosive activity. On 6 April the volume of lava entering the ocean diminished as breakouts from the tube increased. By the 8th, the entry was producing a moderate-sized plume, and many small pahoehoe flows were active on the coastal plain. Most of the activity during 9-22 April took place below 165 m elevation, near the base of Pulama Pali. The surface flows on the coastal flats below Paliuli entered the ocean, forming three new entries in addition to the long-lived Kamokuna entry. On 9 April, surface flows entered the sea at the E end of the 1994 Lae'apuki bench (figure 100). Another flow entered the sea in the Kamoamoa area on 15 April, about halfway between the E Lae'apuki and Kamokuna entries. On the 22nd, a small lobe of the flow feeding the E Lae'apuki entry branched off to the W and produced a small new entry. The Pu`u `O`o pond was ~80 m deep as of 18 April and had divided into two active areas separated by a 30-m-wide segment of stationary crust.

Figure 100. Map of recent lava flows from Kīlauea's east rift zone, April 1996. Contours are in meters and the contour interval is approximately 150 m. Courtesy of the USGS Hawaiian Volcano Observatory.

The three active ocean entries were mostly nonexplosive from 23 April to 6 May. On the night of 28 April a large collapse of the Kamokuna bench removed a piece roughly 100 m wide by 400 m long. Surface flow activity was concentrated on the coastal plain. A moderate size "rockfall" registered on 2 May at local seismic stations, suggesting a possible collapse near Pu`u `O`o.

Surface flows during 7-20 May were diminished compared to those of previous weeks and limited to small, short-lived pahoehoe breakouts on the coastal plain inland of the Lae'apuki ocean entry. Lava continued to enter the ocean at Lae'apuki, Kamoamoa, and Kamokuna, with only 10-20% of the total volume entering at Kamoamoa. A major bench collapse at Kamoamoa on 16 May removed the entire bench, along with a significant piece of older inland terrain, for a total area of 375 x 60 m. Coastal explosions were recorded on 9 and 16 May, possibly related to bench activity. The lava pond inside Pu`u `O`o was visible on 16 May and appeared unchanged at a level of 80-90 m below the rim.

On the afternoon of 11 May, two short bursts of rapid summit inflation during a three-hour period were accompanied by shallow seismic tremor up to 6x background level. They were followed by four hours of deflation. This event did not noticeably affect the location or volume of lava flows on the east rift zone.

Through 29 May the eruption continued with three active ocean entries and small pahoehoe breakouts on the coastal plain from the lava tube supplying the Kamoamoa entry. A large pahoehoe sheet flow was observed at 180 m elevation on 29 May. On 29-30 May the eruption gradually shut down over 18 hours. By the morning of 30 May, the ocean entries had died and the 13th pause of Episode 53 had begun. During the pause, the level of the lava pond in Pu`u `O`o cone fluctuated by as much as 30 m, rising to a high point of 58 m below the rim on 3 June. Lava also appeared on the floor of the Great Pit in the outer wall of the cone. This pause in the eruption lasted until 4 June.

Seismicity. Eruption tremor continued with amplitudes averaging ~2-3x background level from 26 March through 3 June. There were three episodes of weak, deep tremor from a SW source on 31 March, 2 April, and 5 April. A fourth tremor of moderate size from the same source occurred on 7 April. Daily counts of shallow, long-period summit events were moderate to low with a maximum of 119 on 27 March. Microearthquake counts then remained generally low beneath the summit and rift zones through 20 May. Shallow, long-period microearthquake counts increased during 21-22 May and again from 30 May to 3 June. A flurry of shallow earthquakes at the uppermost end of the Upper east rift zone began on 30 May. High counts persisted and peaked on 3 June, with >200 events for the day. The number of short-period events was low beneath the summit from 21 May to 3 June.

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 reports of Sakura-jima Volcanological Observatory, Kyoto University, 86 earthquakes occurred around Shin-dake in May. Seismicity has been increasing since January 1996.

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.

The increased seismicity that began in late March and early April (BGVN 21:02 and 21:03) continued during May. The total number of earthquakes in May was 423, of which 283 occurred on the 14th. No volcanic tremor was observed. The plume height remained at 100-400 m for most of the month, but rose to 600 m on 14 May. There were no ashfalls.

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

Crater 2 activity continued in May as in past months (BGVN 21:04) with intermittent Vulcanian explosions producing thin-to-thick white-to-gray/brown ash-and-vapor clouds. These clouds rose several hundred meters above the rim before being blown to the N, NW, and SE and producing fine ashfalls. Occasional explosions were heard. Glows of variable intensity were seen on most nights during the first three weeks. Weak projections of incandescent lava fragments were observed on 12 and 14 May. A daily range of 10-50 explosion earthquakes was recorded at the seismic station until it became non-operational on 24 May. Crater 3 remained quiet apart from a single emission of very thin white/gray vapor on 7 May.

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

Information Contacts: D. Lolok and C. McKee, RVO.

Low-level activity persisted during May as in previous months (BGVN 21:04). Both summit craters emitted white vapor in variable quantity. Blue vapor from South Crater was seen on 28 and 29 May, and weak roaring noises were heard on the evening of 6 May. Between 1 and 5 May the daily occurrence of low-frequency earthquakes ranged from 440 to 690 events/day. This value increased up to 800-1,690 events/day during 6-30 May. On the 31st the seismicity dropped to the early May level.

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

Information Contacts: D. Lolok and C. McKee, RVO.

When observed by visiting scientists on 11-13 March, the lake in the active N crater was at its highest level since 1986, with a depth estimated at 50 m. The lake's color was pale green, its measured temperature, 32°C, and pH, 1.5.

The scientists noted three areas of fumarolic activity in the active crater with the strongest concentrated on the 1953-55 cone immediately S of the lake. Most of this activity was located on the NW side of the cone near lake level; in this area, high-pressure degassing exited from an E-W oriented fracture ~10 m above the lake's surface. These fumaroles have appeared since the beginning of 1996. Low-pressure fumaroles were also observed on the eastern top of the dome, with gas exiting through small cracks and crevices. Maximum temperatures were 93°C, suggesting that these were boiling-point fumaroles.

A second set of at least five individual fumaroles above the lake's W edge within the inner crater began appearing at the end of 1995, with the most recent one, which displayed the highest gas pressure, forming in March 1996. A third set of fumaroles had been observed since April 1995 in the crater's S area where the trail begins ascending to the Mirador; these had low pressure. Temperatures did not exceed 93°C, again indicating boiling-point fumaroles.

Microgravity measurements made in the crater area showed a continuation in the trend of increased gravity on the N crater floor and a new pattern of decreased gravity (~100 µgal in two years) on the S crater floor.

When visited by OVSCICORI-UNA scientists during April and May the surface of the light-gray crater lake had risen 0.4 and 96 cm, respectively, compared to March. The lake's temperature recently increased: in April it was 36°C and in May, 42°C (compared to 26°C in February and 30°C in March). As is typical, fumaroles clustered near the pyroclastic cone. Their temperatures measured 94°C during April and May; however, the most vigorously degassing zones were inaccessible. Some of these degassing zones continued to make loud noises and their condensed gases formed plumes that rose to 500 m above the crater floor. On the SE, S, and SW walls, maximum fumarole temperatures ranged between 91 and 94°C.

In addition to suspended sulfur and constant bubbling seen in the lake, small landslide deposits were noted leading into the lake from the crater walls. Park guards reported that when the wind blew to the S, visitors suffered from coughs and irritated eyes and skin. New fumaroles appeared along the E crater wall, coincident with high-frequency earthquakes and increased steam output at the pyroclastic cone.

Except for signals associated with a small phreatic eruption, seismic station POA2 registered relative quiet during April: 651 total earthquakes, 24 mid-frequency earthquakes, 17 high-frequency earthquakes, and four hours of tremor. During May POA2 registered 1,243 earthquakes, 29 mid-frequency earthquakes, 21 high-frequency earthquakes, and six hours of tremor. Some of the latter signals during May were correlated with increased fumarolic activity and the appearance of new fumaroles in the active crater.

On the morning of 8 April a low-frequency signal lasting for 223 seconds coincided with an eruption. Fieldwork on 12 April disclosed that the eruption had thrown blocks S to SW of the dome. The blocks had dimensions of up to 35 x 45 cm; in an area N of the lake, the diameter of some blocks reached 80 cm. The N, W, and SW walls of the lake were coated with light gray material ejected from the lake floor. Much of the same material fell back into the lake. Insubstantial deformation was seen during April and May.

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), Apartado 86-3000, Heredia, Costa Rica; Hazel Rymer and Mark Davies, Dept. of Earth Sciences, The Open University, Milton Keynes MK7 6AA, United Kingdom; John Stix, Dora Knez, Glyn Williams-Jones, and Alexandre Beaulieu, Dept. de Geologie, Universite de Montreal, Montreal, Quebec, H3C 3J7, Canada; Nicki Stevens, Dept. of Geography, University of Reading, Reading RG2 2AB, United Kingdom.

On 11 May a Strombolian eruption took place at Tavurvur. Until early in May weak to moderate explosions occurred every few minutes and generated pale to dark-gray ash-and-vapor clouds that rose ~ 400-1,000 m before drifting 15-20 km downwind (mostly SE, S, and SW). Large incandescent ejecta were observed at night and roaring noises were heard from as much as 15 km away (BGVN 21:04).

Visible eruptive activity began to change mid-afternoon on 9 May. Vapor emissions reached ~1,500 m and seismicity increased to a peak around 2200, when a series of strong explosions started. By about 0800 on 10 May, the emissions were sub-continuous and explosions sent ash clouds ~2-2.5 km above the vent. The activity declined through the afternoon. Later that day the emission column was ~1.2-1.5 km high, with occasional explosion clouds rising 1.9 km. Shortly before midnight, explosions were occurring at intervals of ~5 minutes.

A moderate increase in activity began at midnight on 11 May. By 0245 it changed to Strombolian mode as explosions were occurring every 30 seconds, with increasing frequency and strength. Large bolts of lightning flashed through the growing eruption column. Slabs of lava ~10-15 m in diameter were ejected hundreds of meters above the vent, and meter-sized blocks were landing on the shore ~1 km from the vent. By 0300 the explosions and the lightning were almost continuous. The eruption column was a constant stream of incandescent lava fragments rising at least 400 m. There was a spontaneous evacuation of some people from nearby Matupit Island. Strong air-shock waves from the explosions were felt within a few kilometers from the summit. Irregular and continuous tremors were recorded, but observers noted that the shaking was due to the blasts and not to earthquakes.

Seismicity peaked at 0315. Within minutes the activity declined, the streaming of ejecta stopped, and the time between explosions increased to 30 seconds. By 0400 the activity had returned to the level observed on 10 May. At 0438 the first of a series of strong explosions, at irregular intervals of 10-40 seconds, sent incandescent ballistic blocks 1.5 km from Tavurvur. The last explosion, at 0728, generated and ash cloud that rose ~2.3 km.

During the following day a few large explosions occurred, but their frequency and strength were declining. The emissions were commonly white and blue vapors with occasional ash. By the end of 12 May the explosions stopped and seismicity consisted of irregular tremor. This type of activity persisted for 2-3 days, until 15 May when explosive activity resumed.

Several phases of intensified activity took place during the following weeks, but these were considerably less intense than that of 10-11 May. Seismicity remained weaker than during the previous five months (figure 26).

Figure 26. Seismicity at Rabaul for the period December 1995-May 1996 with detail over the days of peak activity in May. Courtesy of RVO.

A new electronic tiltmeter was installed on 30 April at Matupit Island, ~2.5 km WSW of Tavurvur. It initially measured moderate WNW downward tilt. This continued until 3 May when the pattern reversed and ESE downward tilt began. On 8 May, after accumulating ~10 µrad of rotation, the tilting pattern again changed and the instrument recorded WNW downward tilt. The WNW downward tilt that started on 8 May was probably related to the 9 May activity. The WNW downward tilt continued until 20 May, with rotation reaching up to 16 µrad. From 20 to 30 May the downward tilt returned to ESE and gradually decreased to zero.

The available COSPEC measurements showed a decline in SO₂ emission rate from the range of ~500-900 metric tons/day (t/d) at the beginning of May to background values of a few hundred tons per day during 8-15 May. At the end of the month the emission rate increased to ~800 t/d. Although the 8-15 May data failed to portray any flux increases associated with the 10-11 May eruption, later, on 18 and 26 May, peaks in SO₂ emissions correlated with some less dramatic periods of enhanced eruptive activity.

A total of 3,993 explosion earthquakes was recorded during May. Episodes of volcanic tremor numbered 106; more than 90% of these tremors took place during the 10-11 May eruption. Four high-frequency earthquakes were recorded during the month. Two of these were within the zone of defined by 1994 caldera seismicity.

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: D. Lolok and C. McKee, Rabaul Volcano Observatory (RVO), P.O. Box 385, Rabaul, Papua New Guinea.

During May seismic station RIN3 registered a total of seven events: two of high frequency and five of low frequency.

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.

Between approximately 1430 on 15 June and 0100 on 16 June, volcanic tremor reached the highest levels recorded during the previous six months. There were no reports of volcanic activity accompanying this tremor episode; however, poor weather conditions prevented observations after the start of the tremor. At about 0600 on 17 June the level of volcanic tremor started to increase again. The first of several eruption plumes was seen around 0650; larger pulses were observed at 0710 and 0825. The plumes rose several kilometers, carrying voluminous amounts of coarse ash. Large blocks rising to heights of 400-500 m fell as far as 600-700 m from the vent. The second pulse was accompanied by a small lahar down the [E]-flank Whangaehu River valley (see map in BGVN 21:04). Ashfall was recorded as far N as Turangi, 32 km away, due to the prevailing southerly wind. The Alert Level was raised to 3, indicating a significant local eruption in progress (see BGVN 20:09).

Volcanic tremor continued to increase until about 1100 when it plateaued at levels similar to those during the 11-12 October 1995 eruptions. By about 1330 the level of tremor was starting to decline, and the style of activity changed to discrete explosive events. Around 1500 the volcano started to erupt every 10-15 minutes, sending ash-laden plumes to several kilometers height (figure 23). During an overflight around the same time, observers confirmed a small lahar down the Whangaehu catchment but no evidence for pyroclastic flows out of the summit crater basin. Light ashfalls occurred over much of the zone extending N from the volcano to the Bay of Plenty between the coastal towns of Tauranga and Whakatane. A significant Strombolian eruption during 2100-2200 on 17 June was characterized by loud detonations and sprays of glowing rocks ejected above the crater, and was accompanied by strong seismicity. Through to about 0300 several discrete eruption earthquakes were recorded, but the size and rate decreased through the morning of 18 June.

Figure 23. Satellite image of the Ruapehu eruption plume, 1512 on 17 June 1996. The ash cloud is rising to about 20 km altitude in clear weather over North Island, New Zealand. The image was created from NOAA-14 data by combining the visible, near infrared, and one thermal infrared wavelength band. Courtesy of Manaaki Whenua Landcare Research.

Observations made on overflights the morning of 18 June confirmed that the new lake was destroyed and the crater floor was dry. The active vent was in the S part of the crater floor, on which thick deposits of bombs and lapilli had accumulated. The bombs and blocks ejected during the night traveled farther than those erupted on 17 June, to ~1.5 km from the vent. Dome Shelter remained intact, as did the seismic signal from the shelter. On 18 June the active vent was producing weakly ash-charged plumes 1,000-2,000 m above the summit, which were blown downwind, forming a low-level haze at 1,500-3,000 m altitude.

Low-frequency volcanic tremor remained elevated, suggesting that molten material continued to move into the base of the volcano. This eruption was continuing at press time in late June, and had caused significant closures of airspace around the Auckland airport and all of North Island. Additional details will be reported next month.

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: B.J. Scott, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand; Manaaki Whenua Landcare Research Ltd., P.O. Box 38491, Wellington, New Zealand (URL: https://www.landcareresearch.co.nz/).

Almost two years of low-level seismicity ended in mid-March 1994 with the occurrence of a high-frequency earthquake swarm followed by long-period events and an explosion on 23 April (BGVN 19:05). Activity returned to low levels through the rest of 1994.

A mid-sized landslide in January 1995 descended the upper reach of the Lagunillas River but caused no significant damage. It was primarily caused by ground and ice-cap instability, not volcanism. Seismicity in July and August 1995 was stronger than in April 1994. Swarms of long-period events reached a maximum count of 1,050 events on 26 July with more than 6.3 x 10⁸ ergs of energy released. Some of the events were related to explosions heard by scientists doing fieldwork some kilometers away from the Arenas Crater, but ash emission was not confirmed. No significant volcano-tectonic activity was registered. Swarms of long-period events during early September 1995 were similar to those of July-August, but were fewer in number and had less energy. This volcanic related seismicity was located mostly toward the Arenas Crater and the SW part of the volcano at shallow depths.

Seismicity during January-April 1996 remained low, except for the first 10 days of January when there was an increase of long-period screw-type events, with a high of seven on the 5th. Most of these events were located at shallow depths near Arenas Crater and over its W side. Screw-type events have become significant since May 1995. Some volcano-tectonic earthquake swarms also occurred during these four months. Two significant swarms were located toward the S part of the volcano, near the RECI seismic station (figure 47). In both swarms, maximum magnitudes were close to 3. Tremor signals were intermittent; some saturated the stations closest to Arenas Crater, but none were correlated to ash emissions. The electronic tiltmeter 800 m from Arenas Crater (FARA) did not show significant variations. During these four months there were a total of 657 volcano-tectonic earthquakes and 1,308 long-period events recorded by the observatory network. This suggests that processes related to fluids within the volcanic conduits were dominant over fracture-related processes.

Figure 47. Location of telemetered stations and significant seismic events recorded at Ruiz during January-April 1996. Courtesy of INGEOMINAS.

Nevado del Ruiz, located 33 km SE of Manizales, is a broad stratovolcano of andesitic and dacitic lavas and andesitic pyroclastic deposits that cover more than 200 km². Steep headwalls of massive landslides cut its flanks, and melting of its summit ice cap during historical eruptions resulted in devastating lahars. The last eruption began with moderate phreatic ejections on 11 September 1985. On 13 November 1985 an explosive eruption produced pyroclastic flows and surges that melted part of the summit ice cap. Major mudflows subsequently devastated Armero and other towns on the flanks of the volcano, causing over 23,000 fatalities. Intermittent minor ash emissions with occasional stronger phreato-magmatic eruptions continued until July 1991.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: John Jairo Sánchez A., Fernando Gil Cruz, Alvaro Pablo Acevedo, John Makario Londoño, and Jairo Patiño Cifuentes, INGEOMINAS Observatorio Vulcanológico y Sismológico de Manizales (OVSM), A.A. 1296, Manizales, Caldas, Colombia.

During May the dome's growth continued, accompanied by small intermittent pyroclastic flows and minor ashfalls that were mostly thought to be generated by rockfalls. Although activity during the first week of May appeared similar to the final week of April, visibility became poor after 5 May. When visible, the dome's new growth was manifested in rapid increases of summit elevation (on 19 April, 865 m; on 30 April, 896 m; on 2 May, 898 m; on 3 May, 909 m). This was followed by an apparent 2-m decrease (i.e. on 4 May, 907 m). Many rockfalls took place on the dome's NE and E flanks. Throughout early May small ash clouds repeatedly blew W depositing very small amounts of ash in the Upper Gages and Amersham areas.

Activity was characterized as slightly less elevated during the second week of May. However, visual observations on 11 May indicated that a small pyroclastic flow had traveled 300 m E of the base of old Castle Peak dome (into the Upper Tar River Valley passing just S of the path of the 3 April pyroclastic flows). Although this flow had set fire to some trees, no significant changes were observed, and small ash clouds again blew W depositing minor ash in the Upper Gages, Amersham, and Fort Barrington areas.

On 12 May the dome area discharged abnormally large ash clouds associated with at least three pyroclastic flows E of the crater down the Tar River. Relatively large ashfalls also took place in the WNW-NW sector at least as far as the coastal area (Fox's Bay). In some places the ashfall reached a maximum thickness of 3 mm. These ashfalls were reported in parts of southern and central Montserrat (including the settlements of Farrell's, Rileys, Windy Hill, Gages, Lees, St. George's Hill, Fox's Bay, Richmond Hill, Garibaldi Hill, Ile Bay, Old Towne, and Salem). Areas affected also included some settlements in the designated safe zone in the N part of the 13-km-long island (including Cork Hill, Weekes', Olveston, and Barzey's) and small amounts of ash fell in the volcano's E sector (Tar River, Long Ground, and Whites).

The 12 May episode began at about 0630 when near-continuous rockfalls took place on the dome's E flank lasting until about 0720. From 0720 to 0945 the rockfalls became intermittent and small but they still produced ash clouds. A further increase in activity produced pyroclastic flows that were seen in the Tar River Valley at around 0945, 0952, 1105 and 1153. The ones at 0945 and 1105 advanced more than 30 m over the sea; the one at 1153 stopped just short of the sea. Activity declined after about 1220 but small-to-moderate rockfalls continued intermittently.

The 12 May pyroclastic flows did not damage any structures but trees were set ablaze in the Tar River Valley area. Excellent views were obtained of the pyroclastic flows.

On 13 May, light ashfalls blew across the volcano's W and SW sectors. On 15 May small ash clouds again blew W; views then suggested that most of the rockfalls producing the ash came from the NE flank of the dome. In addition, on 15 May moderate amounts of steam escaped from the base of the dome's N side; at other times during the second week of May steam mainly escaped from the SW moat.

Rockfalls were especially abundant on 16 and 22 May. In addition, one on 19 May generated an ash plume that reportedly reached an altitude of about 1.2 km. Another on 20 May was associated with a small pyroclastic flow that traveled ~2 km NE of Chances Peak down the Upper Tar River Valley (as far as Hermitage).

Visibility was generally poor for most of the third week of May allowing only brief views into the crater to establish the dome's main areas of growth on the N and NE flanks. When visibility improved on 20 May, nine days after the previous observation on 11 May, the dome contained several smaller spines and a large broad spine at the top. The large spine rose ~20 m and leaned slightly NE. Observers saw no morphological clues for the source of the 12 May pyroclastic flows, possibly because any topographic signs may have been erased by mass wasting during the intervening week. During brief observations from a helicopter, rockfalls mainly cascaded down the dome's N and NE flanks; fewer came down the vigorously steaming SE flank. Very poor visibility returned on 21 and 22 May.

During the week ending on 29 May, visibility gradually improved allowing remote measurement of 200-250°C dome surface temperatures. Observers on 24 May saw at least three spines on top of the dome (none more than 15 m high) and vigorous steaming from both the NW moat and several areas of the dome. A mudflow that descended the Upper Tar River Valley had apparently formed due to heavy rainfall on the previous night (23-24 May). Also noted was a clear scar on the dome's lower NE flank. About a meter deep and perhaps 5- to 10-m wide, the scar provided a path for ongoing rockfalls.

Observations on 26 May indicated dome growth focused on the dome's E, NE, S, and W parts. Also during the week ending on 29 May, the absence of strong wind allowed the development of near vertical ash plumes, some of which ascended up to 2-km altitude. On 29 May observers saw several small pyroclastic flows that started near the upper dome and flowed E down the Tar River Valley, stopping no farther than the Tar River Soufriere.

Seismicity during May is summarized in table 3. Intense hybrid seismicity took place on 2-3 May; otherwise seismic activity for late April through May was dominated by near-continuous broadband tremor, in some cases lasting up to several days. Tremor duration remained qualitative because it was saved on analog recorders; the gains and filters on these recorders were periodically changed in order to look at other types of seismicity, leaving no consistent record for quantitative analysis. In addition to tremor, rockfall signals were also common.

Table 3. Seismic data from Soufriere Hills, May 1996. Courtesy of MVO.

Some of the deformation measurements made during May were taken on the E and S triangles on 26 May. The line lengths on the southern triangle had shortened by 8 to 9 mm since 21 April, while the eastern triangle had shortened by ~1 cm since 20 May. These data obtained by the EDM technique were consistent with recent GPS measurements conducted by the Alan Smith and colleagues from the University of Puerto Rico.

The bulk of the SO₂ flux measurements were made with a car-mounted COSPEC driven under the plume (between Cork Hill and St. Patrick's) at ~20 km/hr (table 4). Wind speeds were measured with a hand-held annemometer before and after each day's runs at Windy Hill (3.4 km N of Chances Peak), the windiest spot accessible by road. Typical SO₂ fluxes were in the range of 25-205 metric tons/day (t/d). An exception was the 13 May measurement of 357 t/d.

Table 4. Correlation spectrometer (COSPEC) SO₂ flux measurements at Soufriere Hills, 28 April-22 May 1996. Courtesy of MVO.

Resettlement. Since 3 April shelters have housed 1,381 residents. About another 3,000 people rented or shared accommodations in the homes of friends and relatives. The W. H. Bramble airport remained open. Pre-fabricated buildings were erected and church and school buildings were converted to temporary shelters; in addition, the government prepared an ancillary hospital and a power station in the safe area; it made road repairs, upgraded fuel storage, relocated livestock on farms, and established programs for sport, culture, counselling, and guidance.

As of 24 April no plan for mass off-island evacuation for the island's 10,000 inhabitants had been implemented; instead the British and CARICOM governments favored voluntary evacuation. Some residents could remain on Montserrat at the N end of the island, in the area considered comparatively safe by Wadge and Isaacs (1988) and by scientists at MVO. Participants who go to the U.K. could be eligible for employment, income support, housing, and the enrollment of children in British schools for two years.

Reference. Wadge, G., and Isaacs, M.C., 1988, Mapping the volcanic hazards from Soufriere Hills Volcano, Montserrat, West Indies using an image processor: Journal of the Geological Society of London, v. 145, no. 4, p. 541-551.

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/); Alan L. Smith, Univ. Puerto Rico, Dept. of Geology, Mayaguez, PR 00680 USA.

Seismicity began slowly increasing in mid-March before a sudden jump in tremor intensity on 15-16 April (BGVN 21:04). Observations made by Marco Fulle confirmed that the elevated seismicity corresponded to increased eruptive activity. During the night of 15-16 April about 100 explosions occurred. Continuous fountains from the N part of vent 1/2 (see sketch in BGVN 21:04) rose 50 m and lasted 1-2 hours. The S part of vent 1/2 produced large explosions to heights of 150-200 m that deposited bombs on the terrace beyond vent 3/2. Activity from vent 3/1 consisted of continuous pulsing of incandescent gas and explosions every 2-3 hours. Vent 3/2 produced simultaneous explosions every 10-30 minutes from two vents. Similar activity and ~50 explosions were seen the night of 20-21 April. Additional observations included glowing ex-hornitos in vent 1/3 with regular steam pulses. Vent 3/2 explosions covered the terrace S of Crater 3 with bombs.

Observations of summit activity made during 21-28 April by Alean, Carniel, and Iacop revealed similar activity consisting of continuous spattering and intermittent explosions from Crater 1 (BGVN 21:04). Seismicity remained at high levels through mid-May (BGVN 21:04).

IIV report of 1 and 6 June explosions. At 2147 on 1 June, local seismic stations maintained by the Istituto Internazionale di Vulcanologia (IIV) recorded a powerful event lasting ~3 minutes. Eyewitnesses at Stromboli village reported a single strong blast followed by the fallout of red bombs on the upper N slope. Incandescent bombs fell on vegetation, causing a fire that was extinguished by Civil Protection aircraft in the late morning of 2 June. More than twenty tourists were visiting the summit at the time of the explosion. Some of them reported light burns caused by hot lapilli fallout and minor injuries made while escaping on the steep slope.

A field survey early on 2 June revealed that the explosion occurred at Crater 1. The chain of hornitos inside Crater 1 was blown out, leaving a large deep depression in the N side of the crater floor. The ejected material completely covered the summit, falling more than 500 m to the S and E, and reaching ~1,000 m on the N sector, where it fell on the vegetation. The deposit was made of black scoriaceous bombs, covered by Pele's hair, reddish blocks, and a small amount of fine material. On the Pizzo area, where people usually stay to observe the activity (250 m SE from the vent), the falling bombs ranged between 10 and 50 cm in size, and they covered the area with a density of 3-4/m².

Strombolian activity after this event shortly returned to a medium intensity and a normal frequency (3-4 events/hour). In the days after there were several hours without any activity alternating to mild Strombolian activity and after 5 June spattering activity lasting several minutes was occasionally observed.

At 0452 on 6 June another strong seismic event from Crater 1 was smaller than the 1 June event and lasted ~1 minute. The eruption was recorded by the surveillance video camera on the Pizzo Sopra La Fossa, 120 m above the vent and 250 m away; the camera had been restored two days earlier. A few people observed the explosion and reported an ash column to a few hundred meters high and bomb fallout on the Sciara del Fuoco. The video showed a very fast gray-brown jet that ascended at ~30 m/second at the upper limit of the camera view; most of the bomb and block fallout was behind the camera. The ash emission lasted ~2 minutes, but at the end only overpressured steam was emitted.

After the explosion, Strombolian activity continued at Crater 1. During fieldwork that afternoon, activity was characterized by low-intensity explosions with emission of bombs and brown ash, interrupted by sporadic strong explosions that produced a larger amount of bombs followed by an almost continuous spattering for 5-15 minutes. All pyroclastic materials fell close to the craters but during the larger explosion some bombs were thrown a few hundred meters from the vents. The Strombolian activity continued through at least 10 June, showing periods of mild explosions interrupted by strong explosions and short periods of continuous spattering.

Observations on 8-9 and 11-12 June. Marco Fulle made observations from Pizzo sopra la Fossa for six hours on the night of 8-9 June. Vent 1/2 exhibited continuous fountaining 50 m high with larger pulses every 5-10 minutes and ejection of meter-sized lava clots. The vent also produced 35 explosions 100-200 m high, with bombs over the Sciara del Fuoco and the terrace up to Crater 2, and meter-sized lava clots inside Crater 1. Vent 3/1 was inactive, but vent 3/2 produced 20 explosions 50 m high with a lot of ash and bombs ejected inside the crater.

Observations from Pizzo sopra la Fossa were again made for six hours on the night of 11-12 June. Vent 1/2 again produced continuous fountaining and 46 explosions. Vent 3/1 remained inactive. Vent 3/2 generated 37 explosions 100-250 m high with minor ash. Fountaining occurred during the explosions and near-vertical jets of bombs fell S of the crater rim and over vent 3/1.

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: Mauro Coltelli, CNR Istituto Internazionale di Vulcanologia (IIV), Piazza Roma 2, Catania, Italy (URL: http://www.ingv.it/en/); Marco Fulle, Osservatorio Astronomico, Via Tiepolo 11, I-34131 Trieste, Italy.

High seismicity during 18-22 May included 50 events on the 19th. Neither volcanic tremor nor any geophysical changes were observed. A seismicity increase also occurred in December 1995 (BGVN 20:11/12).

Geologic Background. Tokachidake volcano consists of a group of dominantly andesitic stratovolcanoes and lava domes arranged on a NE-SW line above a plateau of welded Pleistocene tuffs in central Hokkaido. Numerous explosion craters and cinder cones are located on the upper flanks of the small stratovolcanoes, with the youngest Holocene centers located at the NW end of the chain. Frequent historical eruptions, consisting mostly of mild-to-moderate phreatic explosions, have been recorded since the mid-19th century. Two larger eruptions occurred in 1926 and 1962. Partial cone collapse of the western flank during the 1926 eruption produced a disastrous debris avalanche and mudflow.

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

The number of earthquakes gradually increased to 18 during the first half of May.

The latest eruptive activity consisted of major explosions in August 1977 that were followed by rapid cryptodome growth. More explosions took place in November 1977, became more vigorous and frequent the following summer, and ended in October 1978. Dome growth and seismicity continued for several years and ceased abruptly in 1982 (SEAN 08:12).

Geologic Background. Usuzan, one of Hokkaido's most well-known volcanoes, is a small stratovolcano located astride the southern topographic rim of the 110,000-year-old Toya caldera. The center of the 10-km-wide, lake-filled caldera contains Nakajima, a group of forested Pleistocene andesitic lava domes. The summit of the basaltic-to-andesitic edifice of Usu is cut by a somma formed about 20-30,000 years ago when collapse of the volcano produced a debris avalanche that reached the sea. Dacitic domes erupted along two NW-SE-trending lines fill and flank the summit caldera. Three of these domes, O-Usu, Ko-Usu and Showashinzan, along with seven crypto-domes, were erupted during historical time. The 1663 eruption of Usu was one of the largest in Hokkaido during historical time. The war-time growth of Showashinzan from 1943-45 was painstakingly documented by the local postmaster, who created the first detailed record of growth of a lava dome.

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

The low-level activity of previous months persisted through April and May. White vapor continued to be released in small to moderate volumes, but the rate decreased in May. Seismic activity remained at low levels. The seismograph became non-operational on 23 May.

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: D. Lolok and C. McKee, RVO.

On 1 May a pyroclastic flow was triggered by the partial collapse of an unstable lava dome. Dome collapse causing pyroclastic flows was a common occurrence during the 1990-1995 eruption. Pyroclastic flows began again in February, and tremor was recorded in March. The large Unzen volcanic complex covers much of the Shimabara Peninsula E of Nagasaki. Mayu-yama lava dome was the source of a devastating 1792 avalanche and tsunami.

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.