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/).

The following summarizes activity at Sakura-jima during January-June 1998 and January-May 1999. Information concerning events in 1998 were provided through communications from Yosihiro Sawada forwarded by Dan Shackelford. More recent information is available at the Japanese Meteorological Agency website.

The amount of tephra deposited around the volcano peaked during 1991-92 (up to 5.8 x 10⁶ tons/month) but has since decreased; present deposits are <3 x 10⁵ tons/month.

Activity during January-June 1998. Ten explosions and eruptions were recorded in January 1998. One explosion, on 7 January, produced a plume that rose 1,200 m above the summit. An explosive outburst on 24 January produced a column of incandescent ejecta accompanied by volcanic lightning. The total ashfall measured at Kagoshima Local Meteorological Observatory (KLMO) during January amounted to 10 g/m².

Silent ash emissions (eruptions without explosions) occurred on 8, 16, 25, and 27 February 1998. A plume on 16 February rose 1,400 m above the crater. In March 1998, 22 eruptions (without explosions) were recorded. Eruption plumes on 2 and 3 March rose 1,200 m. KLMO recorded 37 g/m² of ash for March. In April 1998, there were 19 eruptions, including eight explosions. The highest plume during April was observed 1,500 m above the crater on 30 April. April ashfall totaled only 1 g/m².

Forty-one eruptions were observed in May 1998, including 27 explosive eruptions. The highest plume observed rose 2,500 m above crater on 24 May. A volcanic earthquake swarm lasted seven hours on 19-20 May. The total May ashfall was 105 g/m². All five eruptions recorded during June were explosive. On 6 June an eruption plume rose 1,100 m above the crater. The total June ashfall deposit was 5 g/m² thick.

Activity during January-May 1999. January 1999 was characterized by a low level of volcanic earthquakes, although 13 eruptions occurred. During February-April 1999 observers recorded 44 eruptions, including 15 explosions. Eruptive activity was relatively high during 11-16 March when there were 10 eruptions, including eight explosions. An eruption column rose 1,800 m on 20 April.

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: Yosihiro Sawada, Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Dan Shackelford, 3124 E. Yorba Linda Blvd., Apt. H-33, Fullerton, CA 92831-2324 USA.

Deception has been monitored for part of every austral summer since 1987; its flooded caldera forms a 5 x 9 km bay that is breached to the SW, giving Deception Island a ring shape. In December 1998, as part of the Spanish Antarctic Research Project, a new seismic network was deployed to monitor activity on the island. The instrumentation included two seismic antennas and two continuous-recording stations.

The latest seismic survey was conducted under the auspices of the Spanish Antarctic Project; it revealed a significant increase in the daily number of seismic events (i.e., volcano-tectonic (VT) earthquakes, volcanic tremor, and long-period (LP) events when compared with activity recorded during previous summer field surveys. The noteworthy increase started in early January 1999 (BGVN 24:01) and continued in February (figure 14).

Figure 14. Change in the number of seismic events during 31 December 1998-19 February 1999 at Deception Island. Earthquakes shown encompass long period (LP) and volcano-tectonic (VT). Courtesy of the Spanish Antarctic Research Project.

The active area is in a region between Fumarolic Bay and Murature Point and the seismic activity appears to be located near the boundary of the geological structure activated by the 1970 eruption.

The focal depth of the seismic events was shallow (less than 1 km). The seismic energy release was higher than observed in the recent past, and a few shocks were felt. The maximum magnitude of the VT earthquakes is estimated to be ~3.5. Some episodes of volcanic tremor were felt by the scientific staff while working close to the site at Murature Point. An anomalous increase of dead krill was noticed in the bay a few days before the seismic crisis.

Members of the project interpreted these observations in the following manner: A fresh intrusion of magma occurred at a depth of ~500 m. The VT earthquakes are the brittle response of the shallow structure surrounding this injection, and the LP events and volcanic tremor result from the interaction of the shallow aquifer of the island with this injection of hot material. This implies a significant evolution of activity, marked by a change from stable stationary geothermal processes to a more dynamic intrusive process.

Geologic Background. Ring-shaped Deception Island, at the SW end of the South Shetland Islands, NE of Graham Land Peninsula, was constructed along the axis of the Bransfield Rift spreading center. A narrow passageway named Neptunes Bellows provides an entrance to a natural harbor within the 8.5 x 10 km caldera that was utilized as an Antarctic whaling station. Numerous vents along ring fractures circling the low 14-km-wide island have been reported active for more than 200 years. Maars line the shores of 190-m-deep Port Foster caldera bay. Among the largest of these maars is 1-km-wide Whalers Bay, at the entrance to the harbor. Eruptions during the past 8,700 years have been dated from ash layers in lake sediments on the Antarctic Peninsula and neighboring islands.

Information Contacts: Jesus Ibañez, Gerardo Alguacil, José Morales, José Peña, Javier Almendros and Enrique Carmona, Instituto Andaluz de Geofísica, Univ. de Granada, Spain; Alicia Garcia and Ramón Ortiz, Dpto. de Volcanologia, MNCN-CSIC, José Gutierrez Abascal 2, 28006 Madrid, Spain; Edoardo del Pezzo and Marcello Martini, Osservatorio Vesuviano, Napoles, Italy; Bernard Chouet, USGS, Menlo Park, CA 94025 USA; Gilberto Saccorotti, Univ. Salerno, 84100 Baronissi (Salerno), Italy.

The following report summarizes activity observed at Etna from March through May 1999. The information for this report was compiled by Boris Behncke at the Dipartimento di Scienze Geologiche (formerly Istituto di Geologia e Geofisica), University of Catania (DSGUC), and posted on his internet web site. The compilation was based on personal visits to the summit, observations from Catania, and other sources cited in the text.

Almost all activity has been limited to the eruptive fissure that became active on 4 February at the southeastern base of Southeast Cone (SEC). During early March, lava continued to flow into the Valle del Bove, forming a lava field (figure 77) composed of numerous lobes. The activity was visible from Catania and other locations on 5 March, with the flow field incandescent over its length from the W rim of Valle del Bove down to ~2,000 m elevation.

Figure 77. Sketch map of Etna showing the Valle del Bove and the area of lava flow-field that has developed since 4 February 1999, as of 4 June 1999. The lavas erupted during 1989-93 are shown in various shades of gray, previous flows (since 1971) are shown in lighter shades. 1995 to early 1999 lavas are not shown. Key: NE=Northeast Crater; V=Voragine; BN=Bocca Nuova; SE=Southeast Cone; TDF=Torre del Filosofo; MO=Montagnola; RS=Rifugio Sapienza; MC=Monti Centenari. Courtesy of Boris Behncke.

Observations on 3 March. On 3 March, the area of activity was visited by Giovanni Sturiale and Boris Behncke (DSGUC), and Christophe Baudin, a visitor from Belgium. The most recent lava to flow through the notch in the N rim of SEC, on 4 February, was covered with a thin layer of reddish ash and showed no heat emission. The inclined floor of the notch was ~3-5 m wide and covered with rubble; the width at its rim was 15-20 m. Only a few glimpses were caught of the interior of the crater, but at several tens of meters depth there was an inner terrace surrounding a narrow central pit. The width of the crater at its rim was at most 50 m. Gas and vapor escaped from several fumarolic areas on the SW and E crater rims. On the SE side of the crater there was a fuming pit ~15-20 m wide. Below this pit, on the outer flank of the cone, a vigorously steaming fissure segment extended ~100 m downslope; below this was the oval-shaped main vent of the 4 February episode.

Eruptive activity from the 4 February fissure consisted of very weak, and intermittent, ejections of pyroclastics, and quiet outflow of degassed lava. A cluster of about 10 hornitos at the upper end of the fissure were covered with sulfur. Relatively regular observations during 27 February-3 March (information from Giuseppe Scarpinati, Carmelo Monaco, and Christophe Baudin) indicated irregular activity at the hornitos. There was vigorous spattering at one of the main hornitos on 27 February, a new hornito began to grow on 28 February about 70-80 m downslope from the main group, and on 2 March a short-lived lava extrusion occurred from the base of one of the uppermost hornitos. On these occasions the hornitos were the site of high-pressure gas emissions that produced loud hissing.

During the 3 March visit, all hornitos were unusually quiet. High-pressure gas emission occurred from a few locations 50-80 m downslope. No flowing lava was visible, but a row of skylights lay 100-150 m downslope from the hornitos. About 100 m farther downslope the lava appeared at the surface in a well-defined flow channel. Several other lava flows were slowly moving across the lava field. At the rim of the Valle del Bove one main flow spilled into this vast depression, forming a pronounced ridge where it disappeared in another lava tube. Lava resurfaced a few hundred meters further downslope through numerous ephemeral vents, forming narrow flows that extended to the floor of the Valle. The farthest active lava was at ~2,000 m elevation, above the Monti Centenari, a cluster of 1852-53 cinder cones of which only the summits now protrude (figure 77).

The mean output was several cubic meters per second; the volume of lava produced in one month of activity was between 5 and 10 x 10⁶ m³. Although this is a very rough estimate, with an error of ~50%, it indicates more production than other long-lived effusive eruptions in the summit area, which had effusion rates of < 1 m³/s.

Observations on 11 and 13 March. A group of DSGUC geologists (Behncke, Mariangela Porravecchio, Giuseppe Paradiso, and Antonella Lentini) visited the eruptive fissure at the base of the SEC on 11 March. Above the rim of Valle del Bove, all lava was flowing in tubes. There are apparently two main lava tubes ~30 m apart at the rim of Valle del Bove. The more southerly tube ended just above the crest of the Valle where the lava appeared at the surface, and two ephemeral vents emitted lava further downslope. The northerly lava tube extended much further into the Valle, and surface flows appeared halfway down its W slope, at ~2,500 m elevation. The active flow-fronts appeared to be somewhere above 2,000 m elevation. No activity had occurred at the hornitos in the uppermost part of the fissure, but degassing occurred from several incandescent fumaroles. Geologists from Palermo University measured temperatures of ~1,030°C in one of these fumaroles.

There appeared to be little activity elsewhere, although something that appeared to be a phreatic steam explosion came from Bocca Nuova's southeastern vent area. It formed a convoluted cloud but contained little or no ash, and it produced no sound.

On the late afternoon of 13 March the eruptive fissure was again visited by DSGUC geologists (Behncke, Monaco, Betty Giampiccolo, and Marcello Bianca). There were only minor changes compared to two days earlier. The southern lava flow sent numerous branches spilling over the rim of Valle del Bove. Little had changed at the northern lava flow. The output was still as high as 5 m³/s, and the effusive episode that began on 4 February was estimated to have produced >15 x 10⁶ m³ of lava, an amount typical of a "slow" flank eruption (Etnean flank eruptions are generally classified into those with relatively low effusion rates, such as the 1983 and 1991-93 eruptions, and high-effusion rate eruptions like those of 1981 or 1989).

During late March lava continued to flow into Valle del Bove. While a major flow issued from an ephemeral vent about halfway down the W slope of Valle del Bove, several smaller lobes were visible on the crest of the Valle on the evening of 24 March.

Observations on 7 April. Effusive activity continued in early April, though at a slightly diminishing rate. Lava flows continued to spill into the Valle del Bove, but their fronts stopped before reaching the base of the steep slope. A visit was made on 7 April by Behncke and geologists of Catania and Switzerland together with Marco Fulle (Trieste Astronomical Observatory) and Roberto Carniel (Stromboli On-line). Numerous small surface flows were active in two areas above the rim of Valle del Bove, one about halfway between the rim and the hornitos at the upper end of the 4 February fissure, and the other just above the rim. In the lower area, three or four small flows slowly advanced a short distance from their vents, which lay around spectacular tumulus, or pressure ridges, formed as magma pushed from below, raising blocks and slabs of older lava up to 5 m. The constant effusion since 4 February had formed an impressive, delta-like ridge on the Valle del Bove rim.

The upper area of activity, at ~2,850 m elevation, had five or six vents around a smaller tumulus. Two vents produced voluminous flows that descended tens of meters and had spectacular cascades. A small vent produced a flow that moved in a 20-cm-wide, S-shaped channel; this vent froze over in less than two hours, and a new ephemeral vent became active 20 m downslope. Strong gas emission occurred from two places in the upper part of the 4 February fissure. The cluster of hornitos at the end of the fissure was quiet while profuse steaming occurred from the upper part of the fracture that split the southeastern side of SEC on 4 February. On the W wall of the Valle del Bove, lava issued from a number of ephemeral vents about halfway down the slope, feeding flows that advanced a few hundred meters.

While lava effusion continued unabated, the rate of lava production appeared lower than during the first six weeks of the eruption, possibly in the range of 1-3 m³/s. The volume of lava produced thus far exceeded 20 x 10⁶ m³.

Observations on 14 April. Decreased lava effusion from the 4 February fissure was observed by Behncke and a German television team on 14 April. Near the W rim of Valle del Bove, lava came to the surface in a few places and produced very small flows. Halfway between the Valle del Bove rim and the hornitos, two main vents fed chanellized flows. Activity was more vigorous than the lower area, but had decreased markedly since one week before.

A notable feature is the formation of pressure ridges, tumuli, and small-volume extrusions from cracks in older lavas. Most of this was caused by very slow intrusion of lava from tubes towards the surface once the tube was blocked or slowed, forcing the lava upwards. Lava oozed through the cracks and formed new flows, but in many cases the lava formed bulbous protuberances which often resembled the lobes of pillow lavas forming underwater.

Observations on 30 April. Behncke and others descended into the Valle del Bove on 30 April. One main effusive vent, at ~2,700 m elevation (~100 m below the rim of Valle del Bove), was feeding several flows which in part disappeared into lava tubes and resurfaced tens of meters downslope. The mean effusion rate appeared to be around 1 m³/s or slightly less. The maximum flow length was ~300 m. The farthest flow fronts were stagnant above the floor of the Valle del Bove. There was no explosive activity around the effusive vents. Only one feeder tube appeared to be active, located in the central part of the field on the W slope of Valle del Bove.

Observations on 12 May. During early May, little significant change affected the activity. Lava flowing from the area of the 4 February fissure through a lava tube appeared at the surface at ~2,600 m elevation on the W wall of Valle del Bove. Active lava fronts did not extend below 2,000 m.

On 12 May, Behncke and Scarpinati visited the summit area, including Bocca Nuova and SEC, and entered the Valle del Bove. The summit craters were quiet, apart from near-continuous passive emissions of light brown ash from the NW vent of Bocca Nuova. This activity, which was most likely caused by internal collapse, was entirely noiseless and ash plumes barely rose above the crater rim. A deposit several centimeters thick covered the S, SE, and E sides of the main summit cone.

After descending from the main summit cone to the saddle which separates it from the SE Cone, Behncke climbed to the summit of the SEC. The crater was practically gas-free, so it could be seen that its floor had collapsed and the conduit was no longer open. There was, however, some gas and vapor emission from the upper part of the fracture which had split the SE flank of the cone on 4 February.

There was no visible activity anywhere above the Valle del Bove rim, the only surface flows appearing ~200-250 m below the original eruptive fissure, in Valle del Bove. Lava was issuing from two ephemeral vents on the N and E sides of a large tumulus. It was ~10-15 m across and consisted of uplifted, tilted, craggy blocks of older lava and minor volumes of more recent smooth-surfaced pahoehoe. The N vent fed a small well-channelized flow while from the vent on the E side of the tumulus lava was squeezed out like toothpaste.

Behncke and Scarpinati heard continuous cracking and knocking sounds from below, and small rockfalls from the sides of the tumulus were frequently observed; rocks at the surface of the tumulus were slowly fracturing. It was evident that magma was forcefully pushing from below, and lava was rising slowly within cracks between the blocks at the surface. After about 15 minutes of observation, Behncke and Scarpinati left the unstable tumulus area and continued their observation from 15 m upslope. For another 20-30 minutes, the tumulus gradually extended in all directions. Fracturing on the slope above the tumulus indicated a larger volume of magma arriving at the end of the feeder tube and nearing the surface.

After about 35-45 minutes of observation, the large blocks of older lava began to fall apart while ever larger rockfalls and collapses occurred on the flanks of the tumulus. After this, the entire tumulus area became highly mobile, and its E and SE sides, perched above the steep Valle del Bove slope, began to slide downhill, producing spectacular cascades of incandescent blocks and exposing the fluid, incandescent interior. The most dramatic phase, which lasted no longer than 5 minutes, saw the virtual unfolding of the whole structure as older blocks slipped into the Valle del Bove. Where the observers had walked only 30 minutes before, an incandescent chasm 15 m wide and 5-6 m deep opened, and lava slowly flowed from draining lava tubes. Fresh lava welled up at the W end of the collapse depression, rapidly filling it and spilling over its SE rim, causing further rockfalls. Some rocks at least 20 m³ in volume fell, with fresh incandescent lava attached to them. The overflowing lava appeared to be more voluminous than that which had previously issued from the two vents.

The previously active flows, cut off from their supply, soon stagnated, and small lava tubes with still-incandescent walls became visible. Fresh lava spilled down in a southeasterly direction, forming two branches which traveled 100-150 m in about 30 minutes.

Observations on 15-19 May. Surface activity resumed at the 4 February fissure on 15 May, after about a month of lava flowing through tubes to the Valle del Bove. A luminous spot in the hornito area was sighted through binoculars on the evening of 15 May by Giuseppe Scarpinati from his home in Acireale. Earlier that day Harry Pinkerton (Environmental Sciences Division, University of Lancaster) and others working in the area had noted no active lava. The next day (16 May), lava was slowly extruded from three small vents. The rate of emission increased during the following two days, and a larger vent fed sluggish flows that advanced across the N part of the flow-field.

On 19 May, Behncke, Antonella Lentini, Mariangela Porravecchio (DGSUC), Valentina Giambarresi (Catania University), and others visited Etna's summit area and the effusive vents. A part of this group climbed to the SEC summit. Since the previous visit by Behncke on 12 May, all emissions of vapor from the obstructed crater floor had ceased. The crater floor was ~70 m below the W rim, and the crater walls were vertical in most places. Fumarolic activity was occurring from numerous locations around the crater rim.

While the group stayed on the SEC summit, ash emissions from Bocca Nuova and Northeast Crater produced dilute plumes ~100-150 m above the respective crater rims. Rumbling sounds came from the direction of the Voragine or (less likely) Northeast Crater. No visible emissions were associated with the noise, and no more sounds were heard when everybody had returned to the hornito area.

The new vent 80-100 m below the hornitos lay in a drained channel ~3 m wide in its central portion. The shift of the currently active vent downslope was evident in the form of a ridge built by a series of crusted-over vents. The emission of lava was frequently accompanied by strong degassing, indicating that the lava here was more gas-rich than that issuing from the ephemeral vent on the Valle del Bove slope. Some 20 m downslope from this active vent, one of the initial three vents (of 16 May) was still extruding small amounts of bulbous lava, forming a tumulus.

The visit to the ephemeral vent on the Valle del Bove slope was highly instructive regarding the properties of effusive vents and lava channels. Lava was still flowing from two vents on the floor of the depression formed during the 12 May collapse of the large tumulus. Flow fronts did not extend to the Valle del Bove floor. Four or five well-channelized flows ~1 m wide were moving down the slope; towards their fronts two of these flows were seen to thicken and broaden to 2-3 m height and 5 m width.

The floor of the depression left after the tumulus collapse was partially covered with new lava, mostly aa with one small lobe of very smooth pahoehoe, and the main effusive vent had shifted ~2 m downslope. A second effusive vent 4-5 m downflow produced a small volume of lava.

Investigation of what remained of the collapsed tumulus revealed that the two effusive vents observed on 12 May had not been completely destroyed. Vent 1 lay on the N side of the tumulus, and while still active it had fed lava into a well-defined channel. Vent 2, on the E side of the tumulus, had squeezed out lava much like toothpaste, which had spilled down the steep E face of the tumulus. Both vents were cut off from lava supply as the tumulus collapsed, and when the area of vent 1 was observed shortly thereafter, lava was still draining from the flow channel. Observation on 19 May revealed that the channel had completely drained, its depth being ~1.5 m (compared to a width of 0.8-0.9 m). This is a much higher depth-width ratio than in most other lava channels on Etna and many other volcanoes, but normally lava channels are not drained as completely as in this case since the supply of lava decreases gradually, allowing some of the lava to remain and "freeze" on the channel floor.

Vent 2, one week after the cessation of its activity, was a roughly circular hole open towards the downslope side, and evolving into a thoroughly drained flow channel that had been hidden under the lava when observed before the tumulus collapse. Like at vent 1, this flow channel was deeper than wide. The vent 2 channel is on much steeper terrain, but this apparently had no effect on the depth and width of the flow channel. The vent itself, ~1.5 m wide at its rim, widened at depth to a subcircular cavity from whose floor some bulbous lava had oozed, probably, at some stage of the tumulus collapse. This indicates that there had been a lava pocket ~1 m below the vent. Vent 2 was surrounded by peculiar lava features. There was something like a "basal" lava type, of chocolate brown color, and with a very smooth ripply surface, onto which patches of black, scoriaceous lava were attached. Many of these features are almost certainly related to the frequent and rapid shifting of the locus of extrusion, and the shearing of actively extruding lava along solidified lava on the vent or flow channel walls.

Effusive activity from the 4 February fissure continued at a slowly decreasing rate through the end of May. This decrease was accompanied by an apparent increase of activity in the summit craters, possibly caused by the rise of the magma in the central conduit system below these craters, as less magma escaped through the 4 February fissure. A new surge of activity at this fissure occurred in mid-June; more detail will be provided in a future Bulletin.

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

Information Contacts: Boris Behncke, Dipartimento di Scienze Geologiche, Palazzo delle Scienze, Università di Catania, Corso Italia 55, 95129 Catania, Italy.

During March and April 1999, seismic activity continued at low levels, similar to those reported in previous months (BGVN 24:02). The biggest source of seismic energy release, 2.1 x 10¹⁵ ergs, was associated with volcano-tectonic (VT) events attributed to fracturing. In total, 75 VT earthquakes were registered, located at depths between 0.4 and 20 km below the summit. The largest event, on 3 April, had a magnitude of 2.7 and was located SW of the volcano. During this two-month period the sum of the seismic energy released by 46 long-period events and 27 tremor episodes amounted to 9.8 x 1013 ergs. Radon-222 emissions at four stations located around the volcano measured between 23 and 3,608 pCi/l, values similar to those seen in previous months.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: Observatorio Vulcanológico y Sismológico de Pasto (OVSP), Carrera 31, 18-07 Parque Infantil, PO Box 1795, Pasto, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

"White ash emissions" were noted between 9 March and 24 May, rising as high as 700 m above the crater rim but more often 100-200 m. On 11 March it was noted that larger eruptions, accompanied by booming noises and thick ash emission, had decreased to every 10-15 minutes from every 5 minutes during the previous observations on 2 February. Signals from seismic events were dominated by those from the eruptive events, which occurred at a rate of over 100 per day in March and reduced to half that number in April and May.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

A large tectonic earthquake near Iwate on 3 September 1998 (BGVN 23:09) was followed by elevated rates of seismicity and ground deformation that declined in subsequent months (figure 3). In late 1998 volcanic tremor was recorded up to ten times per month and earthquakes around the Moho discontinuity occurred >10 times per month.

Figure 3. Daily numbers of earthquakes at Iwate (recorded at the Matsukawa station) during 1 January 1998-14 May 1999. Courtesy JMA.

At 1909 on 22 May 1999, a M 3.6 earthquake occurred around the W edge of Nishi-Iwate. The earthquake was followed 8 minutes later by another of M 2.2. These volcanic earthquakes were the largest of a swarm centered at 3-6 km depth. No episodes of tremor were observed and no surface manifestations were detected by video monitoring or during a helicopter flight.

The National Coordination Committee for Prediction of Volcanic Eruption said that the largest earthquakes in the swarm occurred in the W region of the volcano, where shallow (5-10 km in depth) low-frequency volcanic earthquakes had been increasing (figure 4). They also observed that deep (~30 km in depth) low-frequency earthquakes had been occurring regularly with some minor increases and decreases in frequency. GPS observation showed relatively low but steady ground-deformation in the western region of the volcano. Gas measurement showed an increase in below-ground temperature and also indicated the gas chemistry had become more magmatic since September 1998. Their conclusion was that Iwate was showing a slight increase in activity.

Figure 4. Hypocenters of earthquakes around Iwate during 1 April-13 May 1999. Courtesy JMA.

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: Hiroyuki Hamaguchi, Faculty of Science, Tohoku University, Sendai 980, Japan; Kazuo Sekine, Sendai District Meteorological Observatory, Japan Meteorological Agency, 1-3-15 Gorin, Miyagino-ku, Sendai 983, Japan; Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

Between 9 March and 24 May, a "thick white ash plume" was emitted from the main crater and rose to 300-500 m, while a "thin white ash plume" rose to ~150 m from Crater II. Occasionally incandescence was seen in the column rising from the main crater to heights of 25 m. Some A-type earthquakes occurred throughout the reporting period, but seismicity was dominated by tectonic events that frequently exceeded 200/week.

Karangetang (Api Siau) lies at the northern end of the island of Siau, N of Sulawesi, and contains five summit craters strung along a N-S line. One of Indonesia's most active volcanoes, Karangetang has had more than 40 recorded eruptions since 1675. Twentieth century eruptions have included frequent explosions, sometimes accompanied by pyroclastic flows and lahars.

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: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

The following summarizes activity at Satsuma-Iwo-jima (also called Tokara-Iwo-jima), an island on the NW rim of Kikai Caldera. Information concerning events in 1997-98 was provided through communications from Yosihiro Sawada, forwarded by Dan Shackelford. More recent information is available at the Japanese Meteorological Agency (JMA) website.

JMA initiated seismic observation at Kikai in September 1997; from the beginning, several volcanic earthquakes were recorded each day. The number of earthquakes increased suddenly in April 1998 to 60-80/day with some days having more than 100 events. Earthquakes were at this high level during a field inspection on 4-5 May 1998. High numbers of earthquakes continued well into June, then gradually waned, before returning to levels seen in March (~10 events/day). Events decreased to <20/day by late June 1998, but increased again to 20-40/day during September, and to more than 60/day in late 1998.

During the inspection in May 1998, JMA staff found a newly deposited ash layer 5 cm thick around the crater, suggesting that an eruption had occurred in late-April or early-May. The Geological Survey of Japan (GSJ) analyzed the ash and concluded that it was composed of silicic and altered lava fragments of Iwo-dake lava (rhyolite). Residents of this volcanic island witnessed ash falls in August and October 1998. In early November GSJ scientists saw intermittent ash emissions from the crater and found ash deposits in the middle of the SE flank.

Volcanic earthquakes occurred 50-100 times/day during January and February 1999, and 90-130 times/day after February. Hypocenters of these earthquakes were located just below Iwo-dake. Island residents observed ash falling on 24 January [and 14 February 1999].

Geophysical activity is monitored by the Sakura-jima Volcano Observatory, Kyoto University, and JMA; geochemical data are maintained by GSJ.

Geologic Background. Multiple eruption centers have exhibited recent activity at Kikai, a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world's largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post-caldera eruptions formed Iodake (or Iwo-dake) lava dome and Inamuradake scoria cone, as well as submarine lava domes. Recorded eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Satsuma-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Satsuma-Iojima.

Information Contacts: Yosihiro Sawada, Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Dan Shackelford, 3124 E. Yorba Linda Blvd., Apt. H-33, Fullerton, CA 92831-2324 USA.

The eruption of Pu`u `O`o continued to generate a variety of effects on the pali (cliff or fault scarp) and coastal plain during April and May as lava traveled from the vent through a lava-tube system to the ocean. At the coastal lava-entry area the lava bench repeatedly changed as new deposits built up and collapsed. Frequent explosions threw lava fragments into the ocean, onto the bench, and to the top of the adjacent sea cliff.

In the past, the supply of magma to the vent has been cut off for short periods; the 23rd such pause in the eruption began at about 1300 on 4 May and ended around 2200 on 5 May, only about 33 hours. That was long enough for both the steam plume at the ocean-entry area to stop (figure 136) and for a sluggish 6-week-old pahoehoe flow on the coastal plain to cease advancing. The eruption resumed slowly, and as lava moved through the tube system, only a few small sluggish flows broke out onto the pali and coastal plain (figure 137). Lava finally reached the ocean through the preexisting tube system on 7 May.

Figure 136. View of Kīlauea on the island of Hawaii's SE flank looking W on the morning of 6 May 1999, about 12 hours after the eruptive pause ended. The entire area from the ocean and lava bench (bottom) to Pu`u `O`o (top) can be seen. Courtesy of HVO.

Figure 137. A short-lived pahoehoe flow at Kīlauea that began after pause #23 (4-5 May 1999) had ended. Courtesy of HVO; photograph by S.R. Brantley.

Figure 138. A pahoehoe flow from Kīlauea on the coastal plain emerging after pause #23 and inflating with new lava. Note the crack at the top of the flow; this crack formed when the top crust fractured as the molten interior of the flow swelled or inflated with new lava. Courtesy of HVO.

Figure 139. Another view of the same Kīlauea pahoehoe flow shown on figure 138 taken as the lava spread laterally. Typical of pahoehoe flows, this moved forward as lava spread across the ground in budding toes and small sheets. The lava flow inflated as molten material continued to move through its main body. Courtesy of HVO.

Figure 140. Location of the source of a breakout from the main lava tube on the coastal plain as seen on 31 March. The surface of the new pahoehoe flow is about 2 m below the top of the rise at its source. Courtesy of HVO; photograph by J. Kauahikaua.

Lava broke out from the tube system on 26 March about 2 km from the ocean. The breakout fed a wide, slow-moving pahoehoe flow for the next five weeks. The flow had advanced to within ~700 m of the ocean when the supply of lava was shut off by the pause on 4 May.

No significant changes have occurred recently at the Pu`u `O`o cone. Observations into the deep crater of Pu`u `O`o are usually not possible because of the thick plume of steam and SO₂ gas. The vent continued to release an average of 2,000 tons of SO₂ gas each day during April and May. Lava was sometimes visible in the northernmost pit on the crater floor; and when viewed on 6 May, a small spatter cone was visible in this pit.

Explosions at lava bench on 13 April. A series of strong explosions from the active lava bench on 13 April was likely related to the progressive collapse of the leading edge of the delta. As the lava tubes were sheared off by the collapses, seawater entered the tube system and a much larger than usual volume of lava was suddenly exposed to seawater. Both processes led to strong steam-driven explosions that hurled lava bombs and hot rocks into the air as high as 80 m and inland nearly 100 m from the bench's edge (figure 141). These ballistics did not land behind the warning signs posted by the National Park Service about 90 m from the sea cliff above the bench.

Figure 141. Lava bombs flying inland from the site of steam explosions at the ocean entry point on 13 April. Courtesy of HVO; photograph by J. Wightman.

USGS observers said that when they arrived at the ocean entry area at 1400 on 13 April two plumes rose from the edge of the active bench. The widest part of the bench was ~40-45 m, where a few hours earlier the bench was as wide as 80 m. Starting just before 1600 the W plume area began venting steam from a hole (probably a skylight) just inland from the outer edge of the bench; the venting sounded like a jet engine. A few moments later explosions from this new vent hurled spatter into the air and formed bubble fountains as large as 10-15 m in diameter.

This activity turned highly explosive within a few minutes, hurling spatter and rocks into the air (average height of spatter was between 50 and 60 m). Ejected materials fell to the ground atop the sea cliff more than 75 m from the source. Explosive activity continued steadily for about 90 minutes and returned intermittently during the next few hours. One explosive episode caused lightning in the plume. When the witnesses returned to their car, parked 5 km away at the end of the Chain of Craters road, they found a thin layer of tiny brown flakes of glass from the explosions on the windshield.

Background. Kīlauea is one of five coalescing volcanoes that comprise the island of Hawaii. Historically its eruptions originate primarily from the summit caldera or along one of the lengthy E and SW rift zones that extend from the caldera to the sea. The latest Kīlauea eruption began in January 1983 along the east rift zone. The eruption's early phases, or episodes, occurred along a portion of the rift zone that extends from Napau Crater on the uprift (toward the summit) end to ~8 km E on the downrift end (toward the sea). Activity eventually centered on the area and crater that were later named Pu`u `O`o. Between July 1986 and January 1992, the Kupaianaha lava lake was active ~3 km NE (downrift) of Pu`u `O`o. It was during this period that the town of Kalapana and most of the 181 homes lost were destroyed. In December 1991, one month before the shutdown of Kupaianaha, eruptive activity returned to Pu`u `O`o. More than 1 km³ of lava was erupted from January 1983 through January 1997.

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 Volcanoes National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

Seismicity at Kliuchevskoi was above background levels during most of May. Earthquakes were concentrated near the summit crater and at depths of 25-30 km. On 29-30 April, a plume rose 200-400 m above the crater. An ash explosion began at 1330 on 1 May and on the evening of 2 May a fumarolic plume rose 2,700 m above the crater. During 3-5 May plumes rose 200-1,500 m above the crater before extending a few kilometers NE.

Short-lived explosive eruptions began at 1143 on 7 May, as seen from the nearby town of Klyuchi [(30 km NNE)]. Activity began with a powerful gas-and-steam blowout that became dark gray as ash mixed with steam rose above the summit. Ash explosions continued to occur every three minutes until the series ended abruptly at 1217. The height of the ash column reached 3,000 m and the plume extended 8 km NW. Authorities increased the color-coded warning level to yellow. Less vigorous gas-and-steam explosions, with plume heights of 400-700 m, occurred during the day at intervals of 7-10 minutes. At 1453 an ash-poor explosion column rose 2,500 m above the crater. Explosions were observed every 3-5 minutes with plumes 200-1,000 m above the crater during much of 8-9 May. A plume released on 8 May extended 30 km to the S and at 1230 on 9 May a plume rose 2,000 m above the crater. Moderate seismic and fumarolic activity returned and continued until the end of May.

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

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Following several months of intense activity that began on 5 February (BGVN 24:04), Anak Krakatau became relatively quiet in late April. From the end of April until the end of May, only several explosions were heard. On 26 April a weak explosion sent a white-gray ash plume 200-500 m high. Between 4 and 17 May there were two blasts per week, each accompanied by a glow and white-gray ash reaching between 100 and 400 m high. In the week from 18 to 24 May, in addition to two explosions, a shock on the morning of 20 May registered at 2 on the MMI scale.

Anak Krakatau was very active from 1992 to 1997, depositing 6.8 x 10⁶ m³ of lava flows. The island was enlarged by 378,527 m² and the height of the cone increased from 159 to ~300 m.

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: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

The current phase of activity at Lewotobi Lakilaki began at 0547 on 21 March, when observers noted a vigorous steam plume rising 250 m above the summit. This was noteworthy because when the volcano is in a non-active phase its steam plume rises no higher than ~25 m. On 30 March a rumbling noise was heard and the volcano's status was raised to Level II, or "Alert." The following day observers twice noted ash plumes ~250 m high accompanied by a rumbling noise. On 1 April observers saw such plumes at three different times.

From 27 April to 3 May the ash eruption continued. The observed ash was whitish-gray, of weak to moderate pressure, and extending 300 m above the summit. Eruption events were sometimes accompanied by detonations. On 29 April, glowing material was ejected 50-75 m above the crater and it fell over an area with 50 m radius. Thin ash fell over the Boru area the following day at 1600. The seismic record totals for the week showed four volcanic type-B, nine tectonic, four eruption, and nine emission events.

Ash eruptions reached heights of 500 m during 4-10 May and were sometimes accompanied by strong detonations. On 7 and 9 May glowing materials were again ejected to 50-75 m heights, falling within a 50 m radius. On 7 May ash fell around the areas of Boru, Riang Boru, Hokeng, and Wolorona. Deposits were ~1 mm thick. Seismic events for the week decreased, with three eruptive and five emission events. From 11 through 17 May ash eruptions continued to reach heights of 500 m and were sometimes accompanied by strong detonations. During the week of 18-24 May emission heights decreased to 300 m and blast sounds became less frequent. Thin ash fell during 19 May on the Boru and Riagulu regions.

Geologic Background. The Lewotobi edifice in eastern Flores Island is composed of the two adjacent Lewotobi Laki-laki and Lewotobi Perempuan stratovolcanoes (the "husband and wife"). Their summits are less than 2 km apart along a NW-SE line. The conical Laki-laki to the NW has been frequently active during the 19th and 20th centuries, while the taller and broader Perempuan has had observed eruptions in 1921 and 1935. Small lava domes have grown during the 20th century in both of the summit craters, which are open to the north. A prominent cone, Iliwokar, occurs on the E flank of Perampuan.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

During 9 March-24 May visual observations suggested stable conditions, with a "white ash plume" rising 25-75 m above the crater rim. But the seismic record showed extreme variation. Between 9 March and 23 March, volcanic A-type events increased from 7 to 53 and volcanic B-type events rose from 15 to 64. Tectonic events decreased from 34 to 17 in that same period. During the week of 23-29 March event numbers dropped to 23 for A-type and 43 for B-type. Tectonic events rose to 35. Weekly event incidence declined in May, hovering under 10 for A-type, under 20 for B-type, and under 25 for tectonic.

Geologic Background. The Lokong-Empung volcanic complex, rising above the plain of Tondano in North Sulawesi, includes four peaks and an active crater. Lokon, the highest peak, has a flat craterless top. The morphologically younger Empung cone 2 km NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century. A ridge extending 3 km WNW from Lokon includes the Tatawiran and Tetempangan peaks. All eruptions since 1829 have originated from Tompaluan, a 150 x 250 m crater in the saddle between Lokon and Empung. These eruptions have primarily produced small-to-moderate ash plumes that sometimes damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Activity of Marapi volcano was subdued during the period from late April to late May. From 27 April-3 May, only the emissions events increased (from 21 to 30) from the previous week; volcanic type-A, type-B, and tectonic events, along with eruptions, all decreased. During 4-17 May observed activity was limited to thin white-gray ash emissions that rose 100-400 m above the summit.

Geologic Background. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in historical time.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Merapi remained active throughout the reporting period of 9 March through 24 May. Although no deaths were reported, the volcano continually threatened surrounding populated areas with lahars, lava avalanches, and pyroclastic flows. Throughout the period, Merapi exhibited weakly pressured, thick, white, sulfur-tinted ash plumes extending 100-700 m above the summit.

During the week of 9-15 March a vigorous "white ash plume" was observed, weakly pressured, with 550 m maximum height above the summit. Observers additionally identified lava glowing along the SW flank, in the direction of the Blongkeng, Lamat, and Sat drainages (the maximum run-out distance was 0.8 km). On 11 March a small pyroclastic flow was noted traveling SW with 0.8 km of run-out distance. During this week multiphase events dominated seismic activity, presumably resulting from the emission of lava and glowing debris. A small lahar traveled down the Sat drainage on 13 March.

During 16-22 March, ash emissions and lava avalanches continued. Avalanches again traveled SW, with maximum run-out distances of 0.4 km. Glow at the lava dome was weak. Surface shocks (assumed to be from lava avalanches) dominated seismicity. From 23 to 29 March the lava avalanches continued in the direction of the SW-flank rivers with 1.8 km maximum run-out distances. These drainages glowed at night. On 24 March a small pyroclastic flow started at the edge of the lava dome, moved in the direction of the Sat River, and attained a 0.6 km run-out distance; no glow was observed at the dome. Seismicity was dominated by surface events mostly interpreted as lava avalanches.

During the week of 27 April to 3 May, sulfur-tinted ash plumes reached their highest point during the recording period, 700 m above the summit. Lava avalanches continued in the direction of Blongkeng, Sat, and Lamat with 0.9 km run-out distances; again night glow was observed in these drainages. In contrast, the body of the lava dome lacked glowing areas. A small pyroclastic flow from the edge of the 1998 lava moved downslope SW for a distance of 1.3 km. Surface events continued to dominate seismicity. A small lahar buried two trucks and a digging machine at Putih on 1 May; no injuries were reported.

Lava avalanches during 4-17 May continued towards the SW-flank drainages with 1-km maximum distances each week. There was a weak glow on the lava dome during the first week and none the second. There was one small pyroclastic flow each week from the edge of the 1998 lava, which again moved SW out to 1 km distance. From 18 to 24 May lava avalanches had 1.8 km run-out distances. Glow within cracks on the lava dome was weak. Seismicity during May continued to be dominated by multiphase shocks and surface events identified as lava avalanches.

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

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

During 9 March-24 May activity initially increased but later diminished. Volcanic activity increased during 9-15 March and people in the local settlement heard booming noises about 20 times/day. From 16 to 22 March, volcanic activity continued at the same scale, but the booming noises weakened. The seismic record also illustrated decreased intensity. Activity continued through the week of 23-29 March without diminishing, but the booming noises ceased. Volcanic and tectonic events increased, with volcanic type-B earthquakes rising from 6 to 15 and tectonic events increasing from 1 to 18. From 27 April to 3 May volcanic earthquakes increased and an eruption emitted white-gray ash to 200 m.

Activity began tapering during 4-10 May. A white plume was observed at heights of 10-20 m. Volcanic shocks decreased. Activity continued to decline during 11-17 May, with a plume ranging from 10 to 200 m heights. Volcanic activity was not recorded during 18-24 May, but observers reported 14 "thin white ash plumes" rising 10-50 m.

The Peuet Sague stratovolcano contains four summit peaks. The crater believed to be active resides SE of one of the peaks of the lava dome (Mount Tutung). This narrow crater has a diameter of about 70 m and a depth of 80 m. The last major eruption occurred in 1918-21 when ash was emitted, a lava dome was formed, and pyroclastic flows spilled into surrounding uninhabited forests. A 1975 team that reached the peak found no eruptive activity, but documented a lake (500 x 800 m) at the foot of Mount Tutung. Within Tutung's crater they found a small (40 x 75 m) blue lake surrounded by four solfataras. Scientists inspecting the summit area in 1984 found burned trees surrounding the main crater, likely due to a 1979 eruption. Local eyewitnesses and pilots reported ash columns above the summit in 1979, 1986, and 1991.

Geologic Background. Peuet Sague is a large volcanic complex in NW Sumatra. The volcano, whose name means "square," contains four summit peaks, with the youngest lava dome being located to the N or NW. This extremely isolated volcano lies several days journey on foot from the nearest village and is infrequently visited. The first recorded historical eruption took place from 1918-21, when explosive activity and pyroclastic flows accompanied summit lava-dome growth. The active crater is located NE of the Gunung Tutung lava dome and has typically produced small-to-moderate explosive eruptions.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

During May, Popocatépetl generally displayed low activity. Seismicity included small, isolated exhalations of gas, steam, and ash. The alert status remained at "Yellow" with a 7-km radius of restricted access.

A gas-and-steam exhalation at 1315 on 5 May lasted 12 minutes and produced a plume to ~1 km above the crater. Gas-and-steam exhalations at 1435 and 1623 on 6 May lasted 30 and 14 minutes, respectively, and both produced plumes rising ~1 km above the crater. Separate seismic events at 2053 and 2133 on 10 May were possibly rockfalls on the S flank. Visibility was obstructed by clouds. The next morning a steam column from the crater was seen rising about 500 m above the summit. Beginning at 2200 on 13 May, a sequence of seismic events with variable amplitudes was accompanied by high-frequency tremor. Stable seismic conditions returned after about an hour.

Activity increased on 15 May and included small rivulets of meltwater. At 0246 on 16 May a moderately large explosion occurred. Later (at 0706) a moderate exhalation, lasting three minutes, produced an ash column rising 2,500 m above the summit before dispersing to the SW. No ashfall was reported. Several volcano-tectonic events were recorded on 17 May; their signals possibly related to small rivulets of meltwater descending the N flank.

During the night of 23 May a swarm of three tectonic earthquakes was centered 2.5 km E of the crater. The first, at 2251, had M 1.7 and was located at a depth of 4.5 km; the next occurred at 2338, had M 1.9, and was located at a depth of 5.2 km; and the last was at 2339, had M 2.0, and was located at a depth of 5.8 km.

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

Information Contacts: Servando De la Cruz-Reyna^1,2, Roberto Quaas^1,2; Carlos Valdés G.², and Alicia Martinez Bringas¹. 1-Centro Nacional de Prevencion de Desastres (CENAPRED), Delfin Madrigal 665, Col. Pedregal de Santo Domingo, Coyoacán, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); 2-Instituto de Geofisica, UNAM, Coyoacán 04510, México D.F., México.

As of 21 February the seismograph was under repair so the volcano was monitored visually. During 9 March-24 May a "white ash plume" rose 10-150 m above the summit. During 27 April - 3 May the plume remained thin, but after 4 May it vacillated between thick and thin.

Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large volcanic cones, 1,949-m-high Doro Api and 1,795-m-high Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent historical eruptions have been recorded since 1512, most of them during in the 20th century.

Geologic Background. Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large trachybasaltic-to-tranchyandesitic volcanic cones, Doro Api and Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent eruptions have been recorded since 1512, most of them during in the 20th century.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Due to increased seismicity, officials raised the status to "Caution" during the week of 23-29 March. Earlier, during the week of 9-15 March a white-gray ash plume attained heights of 400-500 m and the number of events involving debris flows rose from 6 to 42 (with a maximum run-out of 2 km). In addition, weekly volcanic B-type events increased from 1 to 35, tectonic events went from 9 to 27, and explosions increased from 280 to 385. From 23 to 29 March there were 224 earthquakes related to emissions. In addition there was one volcanic A-type, one volcanic B-type, and 14 events associated with moving debris.

On 19 April the Darwin Volcanic Ash Advisory Center (VAAC) issued an advisory to aviators, citing reported eruption ash clouds at 9 km. There was no evidence of the ash cloud from satellite imagery.

Seismic events decreased during 27 April-3 May, but a white-gray ash plume continued to reach up to 600 m. There was a marked increase in seismic events from 4-17 May. From 18-24 May the ash plume varied from white-gray to white-brown and extended to between 400 and 600 m above the summit. Eruptions dominated the seismic record, with a decrease in volcanic events. During the week pilots reported occasional ash clouds drifting NW. On 24 May the plume height reached ~6 km, drifting NW.

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

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

Mount Slamet has been predominantly quiet since its last eruption in 1989. During the week of 27 April-3 May, however, the volcano's status was raised to "Alert." That week and the next, "white ash emissions" reached 400 m and hot-spring temepratures ranged from 40 to 81°C. Tremors constituted the dominate seismic events. Slamet ejected black ash from its crater over 1-2 May, prompting concern. Government officials warned residents to stay away from the area following several outbursts within a 10-day period.

During 4-17 May seismicity was dominated by tremor with 4- to 30-mm amplitudes, and "thin-to-thick white ash plumes" reached 400 m height. During 18-24 May ash plumes only rose 25-100 m; tremor amplitudes declined to 0.5-20 mm. There was an increase in volcanic events, with B-type events increasing from 9 to 68 and A-type increasing from 10 to 26.

Geologic Background. Slamet, Java's second highest volcano at 3428 m and one of its most active, has a cluster of about three dozen cinder cones on its lower SE-NE flanks and a single cinder cone on the western flank. It is composed of two overlapping edifices, an older basaltic-andesite to andesitic volcano on the west and a younger basaltic to basaltic-andesite one on the east. Gunung Malang II cinder cone on the upper E flank on the younger edifice fed a lava flow that extends 6 km E. Four craters occur at the summit of Gunung Slamet, with activity migrating to the SW over time. Historical eruptions, recorded since the 18th century, have originated from a 150-m-deep, 450-m-wide, steep-walled crater at the western part of the summit and have consisted of explosive eruptions generally lasting a few days to a few weeks.

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

January, February, and March were characterized by sporadic low-intensity events and little seismic activity. In early- to mid-January (7, 13, 14, 15, and 16) there was a series of small explosive eruptions, some generating substantial ash clouds. These events were followed by episodes of ash venting and correlated with seismic tremors. There were occasional small-volume pyroclastic flows (mostly generated by dome collapse, but some perhaps due to fountain collapse during earlier eruptions). Several pyroclastic-flow signals (for example, 7 January) had low-frequency precursors and observers heard associated booming noises in the S of the island. Subsequent vigorous ash venting suggested that the collapses came from violent degassing of the dome. SO₂ levels generally decreased during January, although they remained elevated. Ash samples were coarse with lithic and crystal fragments up to 6 mm in size in the Richmond Hills and St. Georges area; those from the dome-collapse pyroclastic flows were very fine-grained.

At the end of January the observatory conducted an extensive photographic and theodolite survey at 12 sites; they also used a GPS-equipped helicopter. The information was used to produce a detailed dome map. The researchers gauged the dome's volume at 76.8 x 10⁶ m³ and measured its highest point (at the top of the White River Valley) at 977 m. Also noted was a deep split in the dome from the 3 July 1998 collapse and subsequent events. The N part of the split comprised two-thirds of the total dome volume (including the three main buttresses above Gages, the N flank, and the Tar River). The scar cuts 100 m into the older English's Crater and has removed a minimum of 5.4 x 10⁶ m³ of old rock.

February activity consisted of a short period of ash venting and pyroclastic flows, and two small mudflows. There was reduced deformation, and the SO₂ flux continued to decline.

Activity in March was dominated by 23 small explosive and ash-venting episodes on 1, 7, 12, 26, and 30 March. The largest produced a 7-km-tall ash cloud, ashfall as far as Salem and Runaway Ghaut, lightning, and pyroclastic flows reaching the Tar River delta. Deformation rates decreased, gas emissions were moderate, and SO₂ fluxes dropped. Seismicity was dominated by signals from ash venting. Events had impulsive origins with gradual declines in amplitude toward the signal's end.

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), Mongo Hill, Montserrat, West Indies (URL: http://www.mvo.ms/).