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/).

Asama has a history of periodic heightened seismicity; the last reported seismicity increase occurred in September 1996 (BGVN 21:11). A previously unreported seismic increase began on 18 September 2000. During 18-24 September the Japan Meteorological Agency (JMA) recorded 138, 431, 310, 243, 96, 33, and 14 earthquakes per day, respectively.

During 22-23 June 2002 another period of heightened seismicity occurred at Asama that was similar to the September 2000 activity (figure 15). The earthquakes began at 0100 on 22 June and at 0900 JMA issued a Volcanic Advisory stating that 210 volcanic tremor events had occurred during 0100-0800. The report also stated that the temperature of the crater floor had increased since May 2002; on 19 June the floor was at 180°C. Prior to the heightened seismicity, on 2 and 4 June plumes rose 700 and 1,000 m above Asama's summit, respectively.

Figure 15. Plot showing volcanic earthquakes registered at Asama during 22-24 June 2002. The number of earthquakes peaked on 22 June around 0300 and gradually decreased, reaching background levels on 24 June. Courtesy of Asama Volcano Observatory, ERI-University of Tokyo.

The Asama Volcano Observatory (ERI, University of Tokyo) reported that the number of B-type earthquakes peaked around 0300 on 22 June, with more than 30 earthquakes recorded per hour at a station located on the middle of Asama's eastern slope. Several A-type earthquakes, with a maximum magnitude of 2.1, occurred during 0300-0700. The B- and A-type earthquakes occurred 1.5 and 3.5 km beneath the volcano, respectively.

The restricted area surrounding Asama's summit was increased from 2 km to a 4-km radius on 22 June. After the 22nd, seismicity gradually decreased and JMA reported that by the afternoon of 24 June neither volcanic tremor nor notable changes in ground deformation had been recorded.

Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asamayama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has observed activity dating back at least to the 11th century CE. Maekake has had several major Plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

Information Contacts: Tsuneomi Kagiyama, Earthquake Research Institute, University of Tokyo; Yukio Hayakawa, Gunma University, Japan (URL: http://www.hayakawayukio.jp/).

On 12 April 2002, NASA's Jet Propulsion Laboratory reported that new images taken by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (Aster) on NASA's Terra satellite showed signs of activity at Chiliques. This volcano was previously considered to be dormant; however, on 6 January, a nighttime thermal infrared image from Aster showed a hot spot in the summit crater, as well as several others along the upper flanks, indicating new volcanic activity (figure 1). Examination of an earlier nighttime thermal infrared image from 24 May 2000 showed no such hot spots.

Figure 1. Aster images of Chiliques. The larger view is a daytime image acquired on 19 November 2000, created by displaying ASTER bands 1, 2, and 3. The inset is a nighttime thermal infrared image of Chiliques on 6 January 2002. Both images cover an area of 7.5 x 7.5 km and are centered at 23.6°S latitude, 67.6°W longitude. Courtesy Michael Abrams, NASA's Jet Propulsion Laboratory.

General Reference. de Silva, S.L., and Francis, P.W., 1991, Volcanoes of the Central Andes: Berlin: Springer-Verlag, 216 p.

Geologic Background. Volcán Chiliques is a structurally simple stratovolcano located immediately south of Laguna Lejía. The summit contains a 500-m-wide crater. Several youthful lava flows, some of which are considered to be of possible Holocene age (de Silva, 2007 pers. comm.), descend its flanks. The largest of these extends 5 km NW. Older lava flows reach up to 10 km from the summit on the N flank. This volcano had previously been considered to be dormant; however, in 2002 a NASA nighttime thermal infrared satellite image from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) showed low-level hot spots in the summit crater and upper flanks.

Information Contacts: Michael Abrams, Jet Propulsion Laboratory, California Institute of Technology, National Aeronautics and Space Administration, Pasadena, CA 91109 (URL: http://www.jpl.nasa.gov/).

The following report documents several climbs to the summit of Volcán de Colima, carried out in order to accurately measure the size of the growing lava dome, measure fumarole temperatures, and sample gases when possible. Strict safety precautions were followed and climbs were only undertaken during periods of low seismicity. Time is local (calibrated to RESCO seismographic clock). Coordinates and most calculations were obtained by GPS navigator (accuracies of 3-6 m indicated by the instrument) and GARMIN software.

Between 19 August 2001 and 29 June 2002, Nick Varley, Juan Carlos Gavilanes-Ruiz, Mitchell Ventura-Fishgold, Philippa Swannell, and Ruri Ursúa-Calvario performed four ascents to the growing dome, obtaining fresh lava samples, as well as ballistic-projectile samples ejected by the pre-extrusion explosion that occurred on 22 February 2001 (table 12). The lava sample of 18 February 2002 was obtained by Carlos Navarro-Ochoa (a block from a rockfall at the active lava front).

Table 12. The authors took fresh lava samples at Colima at these specified dates and locations. Latitude and longitude are given in degrees, minutes, and decimal minutes. Courtesy Universidad de Colima and Instituto de Geofísica.

During each ascent GPS and geometric measurements were taken in order to calculate the volume of the dome and the current rate of extrusion. Figures 53 and 54 show the preliminary calculations of these variations. The samples collected during the ascents were analyzed by Juan Carlos Mora-Chaparro.

Figure 53. Increase in volume of lava dome and flows measured at Colima during May 2001-April 2002. Courtesy Universidad de Colima and Instituto de Geofísica.

Figure 54. Variation in effusion rates seen at Colima from May 2001 to April 2002. Courtesy Universidad de Colima and Instituto de Geofísica.

Ascent to the crater, 19 August 2001. On this occasion Varley and Gavilanes descended into the crater and circumnavigated the dome discovered on 26 May 2001. The volume of the dome had increased by ~77% since then, and a new lobe had appeared. The GPS tracks recorded around the dome revealed a maximum distance of 103 m in its N-S axis, and a maximum of 122 m in the E-W axis. A zone of incandescent fumaroles (with temperatures up to 877°C) was found on the NE slope of the dome and on the adjacent crater floor (figure 55). This high-temperature zone was located in the same position as the high-temperature group of fumaroles that existed above the previous dome and was monitored between 1995 and 1998. This suggests that the location of the main conduit has not changed since then. During the nearly 4-hour-long stay (0950-1400) on the crater rim and inside the crater, only two small rockfalls were heard.

Figure 55. Incandescent fumarole on the E flank of the growing dome inside the major crater on 19 August 2001. A lava sample was obtained 1 m to the left. Photo taken by J.C. Gavilanes-Ruiz. Courtesy Universidad de Colima and Instituto de Geofísica.

Samples of high-temperature fumarolic gases were taken during this ascent. Unlike previous samples from Colima, they were relatively uncontaminated by atmospheric air. The results of the analyses are shown in table 13. The temperature ranges recorded in the N crater-floor field and in the N and NE crater-rim field are shown in table 14.

Table 13. Volume of gases of high-temperature fumarolic gas collected on 19 August 2001 at Colima. R/Ra represents the isotopic ratio of helium normalized to the atmospheric ratio. (Gas volumes are in mol%). Courtesy Universidad de Colima and Instituto de Geofísica.

Table 14. Temperature ranges of fumarole fields at Colima during 19 August 2001-26 November 2002. Courtesy Universidad de Colima and Instituto de Geofísica.

Ascent to the crater and to the base of the active dome, 26 Nov 2001. During this excursion Varley and Ventura descended into the crater and measured temperatures of the new fumarole field on the S border of the growing dome (figure 56). Meanwhile, Gavilanes and Ursúa measured the fields located on the N and NE borders of the main crater and performed GPS measurements. Gas condensates were sampled from the NE fumarole field. Rock samples were taken from the andesitic spine (figure 57) first observed almost one month previously by personnel of Proteccion Civil of the State of Jalisco. The spine was located in the same area where the maximum temperatures were found on 19 August 2001. The mean frequency of rockfalls from the active dome caused by the lava effusion was once every 5 minutes, with larger events occurring approximately once every 30 minutes. Ranges of fumarole temperatures measured on the S side of the dome and in the NE field are shown in table 14.

Figure 56. Composite photos giving a wide-angle view of the growing dome and collapsing spine from the E border of the main crater on 26 November 2001. The circle (left) locates Nick Varley and Mitch Ventura who were measuring fumarole temperatures in the S sector of the main crater. GPS data indicated that by this day the dome measured 98 m along its N-S axis. Photo taken by J.C. Gavilanes-Ruiz. Courtesy Universidad de Colima and Instituto de Geofísica.

Figure 57. Photo taken on 26 November 2001 showing Ruri Ursúa standing on the E inner border of the main crater of Colima. The highest part of the growing dome can be seen in the background, the andesitic spine in the center of the photo (~ 10 m high in the visible part). Photo taken by J.C. Gavilanes-Ruiz. Courtesy Universidad de Colima and Instituto de Geofísica.

Ascent to the dome, 22 Feb 2002. During this ascent several light ashfall-producing, small explosive events were observed (figure 58). One event expelled several bombs (up to 20 cm in diameter) to a height of ~20 m above the dome. The explosions appeared to originate from the central to W side of the dome. Small rockfalls were occurring approximately once every 15 to 20 minutes on the E side of the dome. Due to the potential of rockfalls, a temperature was only obtained from the fumarole field to the N. There had been an increase in the size of this field, which was located outside of the crater, high on the N flank. The temperature range is shown in table 14.

Figure 58. The ~10 m-high E border of the growing dome at Colima on 22 February 2002. The area covered by the outermost blocks is the remaining ~ 20 m-wide part of the 1987 crater. Photo taken by Nick Varley. Courtesy Universidad de Colima and Instituto de Geofísica.

Ascent to the lava flow's front, 7 June 2002. On this ascent Varley and Gavilanes, climbing the S flank of the volcano, reached a point (19°30.218N, 103°37.392W) located at the same elevation (3,090 m) and approximately 75 m to the E of the front of the active lava flow emplaced on the upper part of the Cordobán Central ravine. The maximum length of the Cordobán Central 2002 lava flow was estimated to be 1,290 m on 7 June 2002. During this 6-hour-long ascent, the average frequency of rockfalls originating from both the lava flow front and the active dome was on the order of one rockfall every 10 minutes. No pyroclastic flows were observed.

Ascent to El Volcancito. On 11 June 2001 Juan Carlos Gavilanes and Alejandro Elizalde ascended to the dome formed in 1869-1872 called El Volcancito in order to repair the meteorological station (19°30.996 N, 103°36.511 W). El Volcancito is located on Colima's E summit (1,010 m horizontal distance, and N62°E of the center of the active dome of Volcán de Colima at 19°30.746 N, 103°37.020 W). Only one rockfall was observed on the E face while the team was 1,750 m from the dome, during the period from 1200-1545. In comparison to the 22 February 2001 observations performed from the same distance, no substantial changes in the size of the dome were apparent from El Volcancito (figure 59).

Figure 59. Alejandro Elizalde repairing the meteorological station located on El Volcancito dome. Volcancito sits ~1 km NE of Volcán de Colima's active summit dome, which can be seen capping the summit in the background. Photo taken by J.C. Gavilanes-Ruiz. Courtesy Universidad de Colima and Instituto de Geofísica.

Ascent to the dome, 29 June 2002. Varley and Gavilanes remained on the N (figure 60), NE, and E borders of the active dome during 1147-1540. On the NE and N borders they measured angular heights and distances between the crater's lip and the upper part of the new dome borders. Only a small volume of lava blocks was observed to have fallen outside of the crater rim on the N border, extending only 4 m. No rockfalls were observed. The team tried to reach the center of the dome, but the complicated array of big scoriaceous and fragile new lava blocks, with abundant 3-to 7-m-deep void spaces between them (figure 61), impeded movement. They measured temperatures at the N fumarole field (table 14) and obtained a condensate gas sample. They also saw and/or heard several short-lived and high-pressure emissions of volcanic gas (table 15).

Figure 60. Photo on 29 June 2002 showing Nick Varley walking adjacent to the crater's N rim. The dark blocks of lava (on the right) represent loose debris that has fallen from the active dome. Photo taken by J.C. Gavilanes-Ruiz. Courtesy Universidad de Colima and Instituto de Geofísica.

Figure 61. Photo on 29 June 2002 showing J.C. Gavilanes-Ruiz (enclosed by the circle) walking on the NE border of the active dome. Photo taken by Nick Varley. Courtesy Universidad de Colima and Instituto de Geofísica.

Table 15. High-pressure emissions of volcanic gas at Colima on 29 June 2002. Courtesy Universidad de Colima and Instituto de Geofísica.

Petrographical and chemical analyses were conducted on recent rock samples from Volcán de Colima at the Instituto de Geofísica, UNAM. The results were compared with similar analyses reported by Mora et al. (2002) from the 1998, 1999, and 2001 samples (table 16).

Table 16. Chemical composition of Colima lava. Numbers in parentheses correspond to the sample numbers in table 12. Fe₂O₃t = Fe total (except 1913). References: ¹Mora et al. (2002), ²Luhr and Carmichael (1982). New data courtesy Universidad de Colima and Instituto de Geofísica.

Chemical analyses indicated that the new rocks registered a slight decrease in SiO₂ and Al₂O₃ contents, and a slight increase in MgO with respect to the 1998 samples. Trace elements registered a decrease of Ba, and increases of Cu, Cr, and Ni (table 16).

Chemical analyses of rocks from 1818 to 2002 eruptions (Luhr, J.F. and Carmichael, I.S.E., 1982; Mora et al. 2002), show maximum variations of ~4 wt.% SiO₂ (57 to 61 wt.%), and ~1.6 wt.% MgO (3.0 to 4.6 wt.%). The most mafic compositions were recorded in the products of the largest explosive eruptions (1818 and 1913). Notable disequilibrium textures observed in phenocrysts, as well as the shift to less evolved compositions in the new dome (2002 samples) with respect to the 1998 eruptive products may indicate an input of magma from a deeper chamber or an injection of new magma into the more shallow magma chamber. Therefore, we think that these detailed petrographic and chemical studies of the more recent eruptive products may provide valuable information for the monitoring of this volcano.

References. Luhr, J.F., and Carmichael, I.S.E., 1982, The Colima Volcanic Complex, Mexico: Part III, Ash and scoria-fall deposits from the upper slopes of Volcán Colima: Contrib. Mineral. Petrol., v. 80, p. 262-275.

Mora, J.C., Macías, J.L., Saucedo, R., Orlando A., Manetti, P., and Vaselli, O., 2002, Petrology of the 1998-2000 products of Volcán de Colima, Mexico: Accepted in the Special Issue of the Journal of Volcanology and Geothermal Research "Volcán de Colima, México, and its Activity in 1997-2000" (in press).

Geologic Background. The Colima complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide scarp, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent recorded eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: N. Varley, J. C. Gavilanes-Ruiz, Facultad de Ciencias and Centro Universitario de Investigaciones en Ciencias del Ambiente, Universidad de Colima; J.C. Mora, J.L. Macias, R. Castro, R. Arias, Instituto de Geofísica, UNAM.

On 27 and 28 May the Alaska Volcano Observatory (AVO) detected anomalous seismicity at Great Sitkin, a volcano located 1,895 km SW of Anchorage, Alaska. On 27 May two periods of seismic tremor lasted for 20 and 55 minutes and on 28 May earthquake swarms began at 0306 and 1228. The earthquake swarms each began with a relatively large event (ML 2.2 and ML 4.3) followed by tens to hundreds of smaller aftershocks, most located 5-6 km SE of the crater at depths of 0-5 km. Both the tremor and earthquake swarms represent significant changes from background seismicity at Great Sitkin. However, aftershocks declined significantly overnight, and seismicity returned to background levels with a lack of recorded tremor since 27 May. Satellite imagery showed no signs of surface volcanic activity, and no reports of anomalous activity were received by AVO.

Geologic Background. The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger volcano capped by a small, 0.8 x 1.2 km ice-filled summit caldera was constructed within a large late-Pleistocene or early Holocene scarp formed by massive edifice failure that truncated an older edifice and produced a submarine debris avalanche. Deposits from this and an even older debris avalanche from a source to the south cover a broad area of the ocean floor north of the volcano. The summit lies along the eastern rim of the younger collapse scarp. Deposits from an earlier caldera-forming eruption of unknown age cover the flanks of the island to a depth up to 6 m. The small younger caldera was partially filled by lava domes emplaced in 1945 and 1974, and five small older flank lava domes, two of which lie on the coastline, were constructed along northwest- and NNW-trending lines. Hot springs, mud pots, and fumaroles occur near the head of Big Fox Creek, south of the volcano. Eruptions have been recorded since the late-19th century.

Information Contacts: Tom Murray and John Eichelberger, Alaska Volcano Observatory (AVO) (URL: http://www.avo.alaska.edu/).

Seismicity at Karymsky was above background during late March through at least mid-July 2002. Local shallow events occurred at the same rate previously reported in BGVN 27:03 (~10 events per hour). The rate increased briefly during mid-May to ~10-15 events per hour. The character of the seismicity indicated that weak gas-and-ash explosions and avalanches possibly occurred. Thermal anomalies and occasional plumes were visible on satellite imagery throughout the report period (table 2).

Table 2. Thermal anomalies and plumes visible on AVHRR satellite imagery at Karymsky during 30 March-9 July 2002. No airborne ash was detected in any image. Courtesy KVERT.

According to a pilot's report, at 1115 on 15 April an explosion ejected ash to a height of 3.0 km above the volcano. MODIS imagery on 17 April revealed at least five traces of ashfall extending to ~25 km in various directions.

During a helicopter flight on 28 April, observers reported an ash explosion to 500 m above the crater. Ash deposits were visible on the W (most intense) and E flanks of the volcano. A new ~100-m-high cone was visible on 28 April inside the active crater.

On 10 May the new cone was visible along with a lava flow 1.3 km down the S-SW slope of the volcano (figure 9). It reached ~300 m wide. The flow was unusual because it had an andesitic composition, rather than the typical basaltic composition that was common in lava flows down the SW flank during 1996-2000. Seismic data on 29 June indicated a possible ash-and-gas explosion to a height of ~4.0 km at 1631. On 9 July at 1032, a helicopter pilot reported a plume to a height of 3.0 km. The Concern Color Code remained at Yellow throughout the report period.

Figure 9. View of Karymsky from a helicopter on 10 May 2002. The billowing plume at the time of this photo concealed the new intracrater cone at the summit; winds carried the plume approximately ENE. The active crater generated a conspicuous lava flow down the S-SW slope that reached ~1.3 km long and ~300 m wide (~ 20% of its length continued beyond the lower right-hand margin of this photo). Caption help courtesy of Victor Ivanov (Institute of Volcanology). Photo by Nikolay I. Seliverstov (Institute of Volcanology); provided courtesy of KVERT.

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

Information Contacts: Olga Chubarova, Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; Tokyo Volcanic Ash Advisory Center (VAAC),Tokyo, Japan (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

Submarine volcanic eruptions occurred at Kick-'em-Jenny during 4-6 December 2001 (BGVN 26:11). Following the 6 December seismicity, no further volcanic or seismic activity were recorded. On 8 December the Alert Level was reduced from Orange to Yellow.

On 12 March 2002, the NOAA Research Vessel Ronald H. Brown conducted extensive mapping of Kick-'em Jenny using the SeaBeam® sonar mapping system (SeaBeam® is a registered trademark of L-3 Communications SeaBeam Instruments). The resulting bathymetric map (figure 3) shows several interesting features.

Figure 3. Bathymetric sonar map of the Kick-'em-Jenny created on 12 March 2002. Courtesy Seismic Research Unit, University of the West Indies.

The volcano's crater is clearly visible (immediately right of center on the image) on top of a symmetrical cone of about 1 km diameter. The crater is nearly perfectly circular with a diameter of ~330 m and a maximum depth of ~80 m. The crater center is located precisely at 12.3004° N, 61.6378° W. The dome, first noticed in 1978 when it almost filled the crater, has now disappeared except for a few remnants on the crater floor. The sonar image shows a breach of the crater to the NE. A prominent escarpment arcs around the E side of the cone and extends at least a few kilometers to the NE and S of the volcano. A series of ridges, principally in the cone's N to W sectors, trend radial or sub-radial to the cone's crater.

The topographic image furnished a bases for some new studies. Temperature-depth profiles were obtained within the crater and on the flanks, water samples were collected at a range of depths, and rock samples were collected from the summit region.

The Seismic Research Unit of the University of the West Indies reported that complete analysis of the results will take some time but preliminary analysis of the bathymetry confirms that the depth to the summit of the volcano has increased since the last detailed survey in 1989. Depth to the highest point on the crater rim is now ~183 m. The difference between this depth and the depths of ~160 m measured from 1978 to 1989 is probably accounted for by the fact that the dome that filled the crater beginning in 1977 has now completely disappeared.

Geologic Background. Kick 'em Jenny, an active submarine volcano 8 km off the N shore of Grenada, rises 1,300 m from the sea floor. Recent bathymetric surveys have shown evidence for a major arcuate collapse structure, which was the source of a submarine debris avalanche that traveled more than 15 km W. Bathymetry also revealed another submarine cone to the SE, Kick 'em Jack, and submarine lava domes to its S. These and subaerial tuff rings and lava flows at Ile de Caille and other nearby islands may represent a single large volcanic complex. Numerous eruptions have occurred since 1939, mostly documented by acoustic signals. Prior to the 1939 eruption, when an eruption cloud rose 275 m above the ocean and was witnessed by a large number of people in northern Grenada, there had been no written mention of the volcano. Eruptions have involved both explosive activity and the quiet extrusion of lava flows and lava domes in the summit crater; deep rumbling noises have sometimes been heard onshore. Recent eruptions have modified the morphology of the summit crater.

Information Contacts: John Shepard, Richie Robertson, Jan Lindsay, and Joan Latchman, Seismic Research Unit, University of the West Indies, St. Augustine, Trinidad, W.I. (URL: http://www.uwiseismic.com/).

During mid-September 2001 through at least mid-June 2002 activity at Kliuchevskoi was characterized by brief periods of increased seismicity and minor surface activity. Earthquakes up to M 3 occurred (table 3) along with weak spasmodic tremor with a maximum amplitude up to 1.5 x 10^-6 m/s (table 4). Gas-and-steam plumes often accompanied the increased seismicity and were visible reaching up to 2.0 km above the crater (table 5).

Table 3. Seismicity at Kliuchevskoi during mid-September 2001 through mid-June 2002. Courtesy KVERT.

Table 4. Tremor recorded at Kliuchevskoi during mid-September through mid-June 2002. Courtesy KVERT.

Table 5. Plumes visible at Kliuchevskoi during 13 September 2001 to 20 June 2002. Plumes were visible from Klyuchi town unless noted otherwise. Heights are above the crater. Courtesy KVERT.

On 13 November a swarm of shallow M 3 earthquakes caused the Kamchatkan Volcanic Eruption Response Team (KVERT) to increase the Alert Level from Green to Yellow. According to a pilot's report, at 1315 on 19 November powerful fumarolic activity was observed. Seismicity decreased during the following days and on 23 November KVERT decreased the Color Code to Green. Seismicity remained at or near background levels with only slight increases in activity until 31 May when a series of earthquakes (up to M 2.3) was recorded in the volcano's edifice. As a result, the Color Code was increased to Yellow.

During 31 May-7 June ~20 earthquakes occurred daily at a depth of ~30 km (table 3). Overflight observations on 9 June indicated fresh ash on the volcano's slopes. The deposits were not accompanied by visually or seismically detected explosions. At the end of the report period, seismicity was slightly above background with a small gas-and-steam plume visible from nearby villages.

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 and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

From 25 February through 16 June 2002 a generally white, variably dense, low-pressure plume rose 150-820 m above the summit of Merapi. Seismicity was dominated by avalanche earthquakes (table 14). During the week of 25-31 March, one shallow volcanic earthquake was reported. The Volcanological Survey of Indonesia (VSI) reported that Merapi emitted varying amounts of SO₂ (table 15).

Table 14. Seismicity (low-frequency, avalanche, and multiphase) and crater characteristics at Merapi during 25 February-16 June 2002. Magnetic field strength was measured at Pusang-Lempong and is reported in nanoteslas (nT). "--" indicates that the information was not reported. Courtesy VSI.

Table 15. COSPEC-measured SO₂ gas emission at Merapi during 3 March-16 June 2002. "--" indicates that the information was not reported. Courtesy VSI.

In total, 69-108 lava avalanches per week were observed during mid-February through late March. The avalanches generally traveled 2.5-2.75 km towards the upstream ends of the Senowo, Sat, and Lamat rivers, and partly to the Bebeng river. During 25 February-3 March, a total of four minor pyroclastic flows traveled to the upstream part of the Bebeng river to a maximum distance of 1.0 km (3 on 25 February and 1 on 3 March). Field observations of the summit on 28 February revealed very thin solfatara sublimation at Gendol and Woro craters. Temperatures at the craters were 354-755°C (table 14). No further pyroclastic flows occurred until 29 and 30 March, when 7 and 2 flows, respectively, traveled 1.8 km down to the upstream ends of the Sat and Senowo rivers. Low-frequency (LF) earthquakes, which had been recorded during the previous few weeks, increased (table 14), and high-intensity rain fell but did not trigger lahars.

Table 15 shows Merapi's SO₂ fluxes. The molar concentrations of volcanic gases from Gendol crater on 28 February were as follows: 0.21% H₂, 0.02% (O₂ + Ar), 0.54% N₂, 3.87% CO₂, 0.01% CO, 1.00% H₂S, 5.49% HCl, 88.86% H₂O. One pyroclastic flow was reported during 25-31 March.

During early April, two minor pyroclastic flows traveled 1.3 km toward the Sat river. Activity at Merapi increased significantly; LF earthquakes reached 135 events within the week. The most intense rain was ~65 mm/hour near the Babadan post observatory on 4 April, but it did not trigger lahars. On 14 April, two minor pyroclastic flows reached 1.8 km maximum distance. Seismicity began to decrease but was still higher than normal. Deformation data from Reflector 4 at the Babadan post observatory indicated 7 mm of deflation, and the lava dome morphology did not change.

No further pyroclastic flows were reported through at least mid-June. Seismicity and general activity at Merapi was reportedly decreasing. Merapi remained at Alert Level 2 throughout the report period.

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 March through at least late June 2002, volcanic activity at Popocatépetl consisted of small-to-moderate, but at times explosive, eruptions of steam, gas, and generally minor amounts of ash, along with episodes of harmonic tremor. Ash clouds rose up to ~2 km above the summit. Because of the remote location and high elevation of the summit, the dome growth within the crater was often hard to constrain, although seismicity and occasional flights over the summit did shed light on the situation. The following report is compiled from updates from the Centro Nacional de Prevencion de Desastres (CENAPRED) and from reports issued by the Washington Volcanic Ash Advisory Center (VAAC).

March began with activity at low and steady levels with up to 18 small steam-and-gas emissions per day and occasional episodes of harmonic tremor. The amount of ash emitted was generally minor. Occasional M 3 volcano-tectonic (VT) events were recorded. Low fumarolic activity began on 4 March and was frequently visible throughout the report period. Overflight observations on 7 March confirmed the presence of a lava dome in the crater (figure 44). A gas-and-steam plume reached ~2 km above the crater on 9 March. According to CENAPRED, the activity implied the possibility of low-level explosive activity in the coming days or weeks.

Figure 44. Aerial view from the NE on 7 March 2002 of the crater of Popocatépetl. The darkest circle in the center of the crater represents the newest lava-dome growth. Courtesy CENAPRED.

Activity increased during 26-27 March when 42 gas and steam emissions reached 200-500 m above the crater, accompanied by small amounts of ash and low-amplitude harmonic tremor. The Washington VAAC issued a volcanic ash warning based upon seismic observations that indicated a possible ash-bearing eruption, but no ash was visible in satellite images. Activity decreased to levels similar to earlier in the month and continued at those levels through early April.

At 0438 on 8 April, observers recorded a moderate eruption with explosive characteristics accompanied by some visible incandescence. An accompanying ash cloud moved E towards the coastline and diffused within 24 hours. After a M 2.3 VT earthquake was recorded at 0545 on 8 April, activity returned to steady levels.

Activity remained low through mid-April, with the exception of a brief period around 11 April when observers detected a slight increase in low-amplitude tremor and fumarolic activity. An increased number of small-to-moderate exhalations per day (up to 52) accompanied by episodes of low-level harmonic and high-frequency tremor, and weak VT earthquakes characterized increased activity that began in late April and lasted through early May. According to CENAPRED, this activity was most likely related to motion of small amounts of magma towards the surface and growth of the lava dome within the crater.

An air photo taken on 29 April (figure 45) by the Department of Federal Roads showed a small dome ~170 m in diameter. On 1 May CENAPRED reported an ash plume moving W at 1.0 km above the summit. No ash was visible on satellite imagery.

Figure 45. Air photo of the Popocatépetl crater taken by the Department of Federal Roads on 29 April 2002. The darkest circle in the left-center of the photo is the newest lava dome, measuring 170 m across. Subsequent flights indicated that explosive activity on 12 May destroyed part of this dome. Courtesy CENAPRED.

Activity increased slightly during mid-May with 33 small-to-moderate exhalations and 1 hour of low-amplitude tremor on 10 May. At 0609 on 12 May, a small explosive eruption occurred, ejecting incandescent fragments on the N flank up to 500 m from the crater. During the next few days, CENAPRED reported increased numbers of exhalations per day (up to 124 on 14 May) of steam, gas, and sometimes small amounts of ash. It was later determined from overflight observations that this explosive activity destroyed part of the growing dome.

This period of increased activity decreased beginning around 17 May. During the rest of May, activity was again characterized by numerous (up to 66) small-to-moderate gas-and-steam exhalations accompanied by small amounts of ash and periods of harmonic tremor. Fumarolic activity continued at the surface. A pilot reported an ash cloud in the region on 21 May.

Activity declined to steady, low levels through June with the average number of exhalations per day dropping to less than 10, occasional isolated harmonic tremor episodes of ~15 minutes duration, and as many as five VT earthquakes per day (M 2.5).

On 17 June at 1136 an ash plume extended up to 2 km above the summit and drifted to the WSW. Shortly thereafter, CENAPRED recorded high-frequency tremor for almost 8 hours and four VT events (M 2.0-2.2). The resulting ash cloud moved across Mexico to the SW. During the following days the volcano quieted but continued to emit gas, steam, and ash in small quantities with episodes of harmonic tremor lasting less than an hour. On 27 and 29 June ash plumes reaching up to 2 km above the summit were accompanied by periods of harmonic tremor lasting up to 2 hours. The Alert Level remained at Yellow throughout the report period.

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

Information Contacts: Centro Nacional de Prevencion de Desastres (CENAPRED), Delfin Madrigal 665, Col. Pedregal de Santo Domingo, Coyoacán, 04360, México D.F. (URL: https://www.gob.mx/cenapred/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

Since mid-July 2001, Semeru was at Alert Level 1 (on a scale of 1-4). On 8 March 2002 two pyroclastic flows traveled 2.5 km downslope to the Besuk Kembar river. The same day, tectonic and volcanic earthquakes increased, prompting the Volcanological Survey of Indonesia (VSI) to raise the Alert Level to 2. Tectonic and volcanic earthquakes continued, along with explosions, avalanches, pyroclastic flows, and tremor (table 7). Plumes, sometimes containing ash, were visible reaching up to 500 m above the summit (table 8).

Table 7. Seismicity registered at Semeru during 3 March-16 June 2002. "--" indicates that information was not reported. Courtesy VSI.

Table 8. Plumes observed at Semeru during 8 March-16 June 2002. Courtesy VSI.

On 31 March two tremor earthquakes occurred with amplitudes of ~3-17 mm. During mid-April, a tremor earthquake occurred with an amplitude of 0.2 mm. Lava avalanches continued to travel up to 750 m down to Besuk Kembar. Seismic signals thought to indicate local floods registered 15-21 April. Incandescence was observed up to 25 m above the crater rim during 1820-2025 on 18 April. During that time, seismicity was dominated by low-frequency earthquakes, with amplitudes of 2-3 mm. During 27 May-2 June ash explosions produced white-gray plumes that reached ~200-400 m above the summit, while lava avalanches traveled ~100 m away. Semeru remained at Alert Level 2 through at least 16 June 2002.

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/).

Stephen O'Meara and four Volcano Watch International (VWI) team members (Robert Benward, Tippy D'Auria, Scott Ireland, and Larry Mitchell) visually monitored Soufrière Hills for 10 days beginning on 19 February 2002. The observations took place on Jack Boy Hill, a spot at ~180 m elevation 6 km N of the volcano. In addition, for 3 hours on the night of 25 February, the group joined Montserrat Volcano Observatory (MVO) scientists Peter Dunkley and Richard Herd on the runway at Bramble Airport. Except for a storm on 20 February, the weather facilitated exceptionally clear views of the dome during both day and night. The team employed a variety of telescopes and other optical equipment and had an interest in astronomy as well as the volcano (O'Meara, 2002).

Benward brought along a homemade night-vision scope (near-infrared image intensifier) that captured images of the dome, even through local atmospheric conditions where visible light was weakened or scattered. The intensifier was coupled to camera lenses. It could be used visually or attached to a video camera (figure 47). The camera's phosphor viewing screen yielded green-colored images of the hot portions of the dome.

Figure 47. The night-vision scope (image intensifier) put together by Robert Benward and used to obtain images of Soufriere Hills' growing dome. In this configuration the intensifier lies between two other components: a telephoto camera lens (left) and a video camera (right). Courtesy of Steve O'Meara, Robert Benward, and Sky and Telescope magazine.

One purpose of the VWI team's visit to Montserrat was to chronicle changes in the volcano's visible behavior with approach to the time of the full Moon and its perigee (when the Moon is closest to the Earth). The idea was that the tidal influence associated with the full Moon and its perigee might lead to enhanced activity. With approach of the full Moon, there did seem to be a rise in visible indicators, particularly plume height, a strong pulse of extruded spines, and less-substantial increases in the numbers of rockfalls and pyroclastic flows.

As background on tidal forces, the paths of both the Moon around the Earth, and Earth around the Sun are elliptical throughout the lunar cycle (29.53 days) and solar cycle (the year), meaning that the separations and resulting gravitational forces vary with time. The Earth-Moon separations change by ~50,000 km; when they are smallest (perigee) and largest (apogee) the respective tidal forces are higher or lower than usual. In addition, the gravitational attractions of Moon and Sun on the Earth may act along a common line or at changing angles relative to each other. Particularly large tides affect the Earth's crust and oceans when the Sun and the Moon are lined up with the Earth; this occurs at the new and full phases of the Moon. These orientations lead to what are called spring tides (a name not associated with the season of Spring, but which implies a "welling up"). The amount of tidal enhancement is roughly the same whether the Sun and Moon are lined up on opposite sides of the Earth (full Moon) or on the same side of the Earth (new Moon). In contrast, when the Moon is at first quarter or last quarter (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon produce tidal bulges called neap tides. These are generally weaker than the above-described spring tides.

A two-month record of seismicity and tides at the volcanically active Axial seamount on the Juan de Fuca ridge during 1994 found both bi-weekly and diurnal patterns in earthquakes and volcanic tremor (Tolstoy and others, 2002). The authors concluded that microearthquakes took place at tidal minima.

Montserrat, Moon, and magma. The full Moon occurred at 0518 on 27 February; perigee, ~11 hours later, at 1630. The team's 10-day stay was too short to see more than a partial lunar cycle, but soon after full Moon and perigee, the numbers for the observed visible indicators appeared to drop considerably.

After an initial study of dome activity on 19 February and a storm on 20 February, the group began taking regular visible observations on 21 February. At that time, activity appeared to be on the increase and a high-level of activity was sustained throughout the observation period. According to MVO: "The level of volcanism at Soufrière Hills during 22 February-1 March was higher than it had been in previous weeks." The growing dome was quite active, displaying near continuous rockfall and small pyroclastic flows, most of which traveled E to the Tar River Valley, though some activity was directed to the S and W. During the 10-day observation interval, the dome also rapidly extruded very large spines.

By midnight on 27 February the team had recorded and tabulated 440 observations of notable rockfalls and pyroclastic flows. On the whole during this interval, the number of these events per hour stood well below 10, typically ranging from ~4 to ~8. One low, late on 23 February, only reached 1 event per hour. The average number of these events per hour reached a low of ~5 during 21-23 February rising to ~8 on 27 February. The highest hourly total recorded during the observing period occurred on 27 February with 13 of these events during 0000-0100 and 10 during 1120-1320. These times fall on either side of the full Moon; the second total lies at the midpoint between the full Moon and lunar perigee.

Visible activity decreased sharply on 28 February. The team, which departed on 3 March, made sporadic observations until 1 March. Their observations on and after 28 February suggested dome activity had remained substantially lower than during 21-27 February.

During their interval of observation the team found a direct correlation between the number of large visible events and the size of the dome's emerging and collapsing spines. The mass of each spine also increased during the observation period; the largest spine was observed on 26 February, the day before the full Moon and perigee.

Each of the spines collapsed in less than a day, only to regrow rapidly. The largest (shown on figures 48-53) reached 90 m tall; it enabled the summit to attain 1,080-m elevation, the highest the summit has been during the entire eruption to date (according to the MVO weekly update). It grew rapidly; specifically, it was not present from 1830 to 2100 on the evening of 25 February, but was fully grown by 0600 the following morning. When seen at 0330 on 26 February the new spine appeared as an incandescent obelisk about one-fifth its maximum size. The majority of this massive spine then grew to its record height in 3 hours.

Figure 48. A S-view taken from Jack Boy Hill of Soufrière Hills dome shown with the yet-highest-reaching spine seen to date, which was photographed shortly after sunrise on 26 February 2002. The spine appears as a triangular peak at the summit; it soon began to collapse. Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 49. A S-looking night shot taken from Jack Boy Hill at 0300 on 26 February that depicts Soufriere Hills in a highly incandescent state, with a large and growing spine extruding out of the top of the dome. Disrupted and displaced dome materials, including falling blocks, incandescent rockfalls, and pyroclastic flows, have left a conspicuous apron of hot material on the dome's left (W) side. Surprisingly little ash and steam appear to be present. Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 50. A daytime shot taken from Jack Boy Hill showing part of a comparatively large pyroclastic flow at Soufriere Hills on 24 February 2002. Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 51. The ragged summit of the dome at Soufriere Hills as it lies beneath a small plume at sunset. Taken from Jack Boy Hill looking S on 25 February 2002. Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 52. A night shot of the dome at Soufriere Hills showing the summit dome that was soon to extrude a large spine (not yet visible). This photo was taken from the airport (several kilometers NE of the dome) in conditions of moonlight on 25 February at about 2100. Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 53. Soufriere Hills' glowing dome showing triangular spine in the moonlight with stars in the night sky. Taken from the airport (several kilometers NE of the dome). Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 54 is one of several plots constructed to illustrate the results. It was made by omitting the smaller events, which the team judged from small to medium using a qualitative visual scale that ran from S1 to S₃ and continued upwards from M1 to M₃ (where event sizes are abbreviated as S for "small" and M for "medium" and termed as S-class or M-class, respectively). Thus, the largest events seen were M₃ (i.e., they saw no events in these time periods that they classified as "large"). On their scale, events of size S₃ and M1 were judged to be of very similar magnitude. Figure 54 shows the increase in larger event size seen during 21-26 February, culminating in the highest numbers late on 26 February to early on 27 February.

Figure 54. A plot of the number of larger observed rockfall and pyroclastic-flow events seen at Soufrière Hills during 21-27 February 2002. The events counted in this plot excluded the smallest two categories (S1 and S2 classes, see text). High tides were shown (thin vertical lines) for those cases where they occurred during an interval in which observations were conducted; otherwise they are absent. The symbols along the top of the plot indicate processes described in the key. The symbol sizes were increased or reduced for events judged to be of larger or smaller size. For example, the largest spine grew on 26 February (large dark triangle). Courtesy of Steve and Donna O'Meara, Volcano Watch International.

Figure 54 shows six high tides that occurred at times when observations were conducted (on 21, 22, 24, 26 and 27 February). Five of the six of these tides coincided with observation intervals with the day's highest number of the largest events (the M-class events).

Plume height. As shown on figure 55, an increase in plume height took place around the time of first quarter Moon followed by a decrease, then a gradual rise in plume height, until it reached a maximum at the time of perigee on 27 February. Although atmospheric conditions could clearly affect the extent and height of a plume, the team found the pattern of the plotted data compelling. The plot may disclose tidal effects.

Figure 55. Plume heights (in degrees above a reference horizon) at Soufriere Hills plotted against time as observed during 19-28 February 2002. Courtesy of Steve and Donna O'Meara, Volcano Watch International.

References. O'Meara, S., 2002, Firelight nights: Stargazing from the Caribbean's Emerald Isle; A group of American amateur astronomers helps residents of Montserrat and its neighboring island explore the universe: Sky & Telescope, August 2002, p. 79-83.

Tolstoy, M., Vernon, F.L., Orcutt, J.A., and Wyatt, F.K., 2002, Breathing of the seafloor, tidal correlations of seismicity at Axial Volcano: Geological Society of America (GSA), Geology, v. 30, no. 6, p. 503-506.

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: Steve and Donna O'Meara, Robert Benward, Tippy D'Auria, Scott Ireland, and Larry Mitchell, Volcano Watch International, PO Box 218, Volcano, Hawaii 96785.

During 11 March-16 June 2002 at Talang, seismicity was dominated by small explosion earthquakes (table 4). A thin white plume reached 50-100 m above the summit and sometimes drifted E. Hotspring temperatures were 42-64°C. As of 13 May, the Volcanological Survey of Indonesia (VSI) reported that no seismic data were available because of a broken seismograph. During April and early May seismicity had been decreasing. Talang remained at Alert Level 2 (on a scale of 1-4) throughout the report period.

Table 4. Earthquakes at Talang during 11 March-12 May 2002. The seismograph was broken as of 13 May, so no seismicity data was available through at least 16 June. Courtesy VSI.

Geologic Background. Talang, which forms a twin volcano with the extinct Pasar Arbaa volcano, lies ESE of the major city of Padang and rises NW of Dibawah Lake. Talang has two crater lakes on its flanks; the largest of these is 1 x 2 km wide Danau Talang. The summit exhibits fumarolic activity, but which lacks a crater. Historical eruptions have mostly involved small-to-moderate explosive activity first documented in the 19th century that originated from a series of small craters in a valley on the upper NE flank.

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

Uplift (up to ~10 cm) occurred during 1996-2000 over a broad region centered 5 km W of South Sister in the Three Sisters region (BGVN 26:05). At the time scientists did not know exactly when the uplift had occurred, whether it would continue, or its specific cause. Although most of these questions remain, some new data are available.

On 18 March 2002 scientists from the USGS Cascades Volcano Observatory and Central Washington University reported that they, in cooperation with staff from the U.S. Forest Service's (USFS) Willamette and Deschutes National Forests, confirmed that slow uplift of the area was continuing at approximately the same rate as previously reported (i.e., a maximum rate of ~2.5 cm/year).

About a month later NASA's Jet Propulsion Laboratory (JPL) released a simulated natural-color image from the Aster high-resolution imaging instrument on the satellite Terra. Aster uses 14 spectral bands, at wavelengths from visible to thermal-infrared, and it has a spatial resolution of 15-90 m. By draping the Aster data over digital topography from the U.S. Geological Survey's National Elevation Dataset, they created a new perspective view of the Three Sisters and adjacent Cascade volcanoes (figure 2). The image was timely because of concerns about continued uplift in the area. BGVN 26:05 included a radar interferogram showing ground uplift pattern during 1996-2000, movement centered ~ 5 km W of South Sister.

Figure 2. The Three Sisters volcanic area appears in this perspective view from the SW quadrant. The view uses a simulated natural-color image from the satellite-borne Aster imaging system, which has been draped over digital topography taken from the U.S. Geological Survey's National Elevation Dataset. N lies to the upper-right; the distance between the summits of North Sister and South Sister is ~ 7 km. The image was released on 12 April 2002. Courtesy NASA's Jet Propulsion Laboratory.

Analyses of spring water samples collected during late summer 2001 were similar to those from earlier surveys but isotopic studies of carbon and helium in the most recent samples, which were not done previously, suggested a magmatic source. Taken together, the ground deformation, seismic, spring water chemistry, and gas emission results suggest that uplift was caused by slow accumulation of magma at a depth of 6-7 km beneath the surface. If magma intrusion were to continue, it could eventually lead to a volcanic eruption; however, an eruption is unlikely without months to years of precursory activity. In addition to continued or accelerating uplift, precursors to an eruption would include earthquakes, typically swarms of small events generated by fracturing of rock as magma moves upward, and large emissions of volcanic gases, such as carbon dioxide, which is released from the magma.

The Pacific Northwest Seismograph Network (PNSN) has reported three earthquakes in the Three Sisters region since January 2001. On 21 August 2001 a M 1.9 earthquake occurred at a depth of 4.9 km; on 5 November 2001 a M 1 earthquake occurred at a depth of 1 km; and on 18 January 2002 an M 2.4 earthquake occurred at a depth of 3.0 km. The recent earthquakes are consistent with background seismicity at Three Sisters. As of mid-July 2002, the number of earthquakes and gas emissions remained at low-t-obackground levels while steady uplift continued.

General Reference. Scott, W.E., 1987, Holocene rhyodacite eruptions on the flanks of South Sister volcano, Oregon: Geol Soc Amer Spec Pap, v. 212, p. 35-53.

Geologic Background. The north-south-trending Three Sisters volcano group dominates the landscape of the Central Oregon Cascades. All Three Sisters stratovolcanoes ceased activity during the late Pleistocene, but basaltic-to-rhyolitic flank vents erupted during the Holocene, producing both blocky lava flows north of North Sister and rhyolitic lava domes and flows south of South Sister volcano. Glaciers have deeply eroded the Pleistocene andesitic-dacitic North Sister stratovolcano, exposing the volcano's central plug. Construction of the main edifice ceased at about 55,000 yrs ago, but north-flank vents produced blocky lava flows in the McKenzie Pass area as recently as about 1600 years ago. Middle Sister volcano is located only 2 km to the SW and was active largely contemporaneously with South Sister until about 14,000 years ago. South Sister is the highest of the Three Sisters. It was constructed beginning about 50,000 years ago and was capped by a symmetrical summit cinder cone formed about 22,000 years ago. The late Pleistocene or early Holocene Cayuse Crater on the SW flank of Broken Top volcano and other flank vents such as Le Conte Crater on the SW flank of South Sister mark mafic vents that have erupted at considerable distances from South Sister itself, and a chain of dike-fed rhyolitic lava domes and flows at Rock Mesa and Devils Chain south of South Sister erupted about 2000 years ago.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey (USGS), Building 10, Suite 100, 1300 SE Cardinal Court, Vancouver, WA 98683 (URL: https://volcanoes.usgs.gov/observatories/cvo/); Volcano Hazards Team, USGS, 345 Middlefield Road, Menlo Park, CA 94025-3591 USA (URL: http://volcanoes.usgs.gov/); Pacific Northwest Seismograph Network (PNSN), University of Washington Geophysics Program, Box 351650, Seattle, WA 98195-1650 USA (URL: http://www.geophys.washington.edu/SEIS/PNSN/); Jet Propulsion Laboratory, California Institute of Technology, National Aeronautics and Space Administration, Pasadena, CA 91109 (URL: http://www.jpl.nasa.gov/).

Our last report described activity at Villarrica during January 2001 (BGVN 27:02) through January 2002, when incandescent lava was observed in the crater and ballistics were ejected ~80-150 m. At that time explosions generally occurred every ~1-10 minutes and degassing sounds were occasionally heard.

During February through at least May 2002, sporadic observations showed a general decrease in activity. Degassing noises were sometimes heard; however, no incandescence or ballistics were reported. A crater visit on 9 April revealed that no incandescence or explosive noises occurred. The surface of the lava lake, last seen on 19 January, remained low (~200 m below the crater rim). On 10 April, explosions occurred every ~10-13 minutes.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic-andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Proyecto de Observacion Villarrica (POVI), Wiesenstrasse 8, 86438 Kissing, Germany (URL: http://www.povi.cl/).