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/).

Activity at Minami-dake Crater in September consisted of 13 eruptions, including seven explosive ones. The highest ash plume of the month rose 1,500 m on 15 September. Ashfall measured at the Kagoshima Local Meteorological Observatory, 10 km W, was 26 g/m². At a seismic station 2.3 km NE of Minami-dake crater (Station B), 449 earthquakes and 431 tremors were recorded.

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

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

During September, Me-Akan was the site of elevated seismicity and limited tremor. The monthly total number of earthquakes was 252 at Station A, 2.3 km NW from Ponmachineshiri Crater. The highest daily number of earthquakes, on 10 September, was 19. On 13 September a volcanic tremor occurred for one minute duration.

Geologic Background. Akan is a 13 x 24 km caldera located immediately SW of Kussharo caldera. The elongated, irregular outline of the caldera rim reflects its incremental formation during major explosive eruptions from the early to mid-Pleistocene. Growth of four post-caldera stratovolcanoes, three at the SW end of the caldera and the other at the NE side, has restricted the size of the caldera lake. Conical Oakandake was frequently active during the Holocene. The 1-km-wide Nakamachineshiri crater of Meakandake was formed during a major pumice-and-scoria eruption about 13,500 years ago. Within the Akan volcanic complex, only the Meakandake group, east of Lake Akan, has been historically active, producing mild phreatic eruptions since the beginning of the 19th century. Meakandake is composed of nine overlapping cones. The main cone of Meakandake proper has a triple crater at its summit. Historical eruptions at Meakandake have consisted of minor phreatic explosions, but four major magmatic eruptions including pyroclastic flows have occurred during the Holocene.

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

In September, Arenal's active vent, crater C, continued its regular emission of gases, lava, and sporadic Strombolian eruptions; in addition, there were occasional pyroclastic flows. Lava that began to be emitted in July 1995 followed a course toward the SW and by the end of September had flowed to 1,050-m elevation. By the end of September, lava moving NW reached 800 m elevation. At the 1,400-m elevation a new arm branched off; it trended SW and by the end of September had reached 1,200 m elevation.

Explosions in September sent columns >1 km above the active vent that were typically blown to the NW, W, and SW. Toward the end of the month, some ash also fell on the NE and E flanks. Bombs and blocks arrived at elevations as low as 1,200 m. Crater D was fumarolically active.

During September there were 977 seismic events and 223 hours of tremor (figure 73). The majority of the seismic events were associated with Strombolian eruptions. Some of these eruptions were large enough to register at an outlying station 27 km more distant from Arenal than the one usually used (station JTS, 30 km SW of the crater). The total number of events (figure 73) for February and March were extrapolated based on 9 and 19 days, respectively, of recorded data.

Figure 73. Seismic events and tremor at Arenal, January-September 1995, recorded at Station VACR (2.7 km NE of the main crater). Courtesy of OVSICORI-UNA.

A pulse of expansion detected in May 1995 ceased, and in September distance lines returned to a tendency toward long-term contraction (13 µrad/year). With the exception of one dry tilt instrument, which indicated deflation at 12 µrad/year on the W flank, there were no significant measured changes in tilt.

A brilliant color photograph of Arenal erupting appeared on the front page of the Washington Times on 22 September. Without either objects for scale or accompanying clarifying text, the photo caused considerable short-term confusion about the volcano's status. It was quickly learned that the photo depicted typical conditions at the volcano and observers in Costa Rica had not witnessed increases in activity. The volcano's first chronicled eruption took place in 1968 and many basaltic andesite discharges have followed.

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

Information Contacts: E. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

Throughout September the hot water pool on the floor of Naka-dake Crater 1 frequently ejected mud and water; the highest ejection rose 10 m. Many isolated tremors were recorded at Station A, 800 m W of the crater. The monthly total of isolated tremors was 6,618; only two earthquakes were detected. Continuous tremor with 0.2-0.8 µm amplitude was registered throughout the month.

Mud ejections have been reported since May 1994 (BGVN 19:05). The 24-km-wide Aso Caldera contains 15 central cones. One of these cones, Naka-dake, has erupted more than 165 times since 553 AD, the first documented historical eruption in Japan. Aso is located 75 km E of Unzen and 150 km N of Sakura-jima volcanoes.

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

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

At 0500 on 6 October, regional seismic stations began to record volcanic tremor with a maximum amplitude of 5-6 µm. An ash plume was detected by the Alaska Volcano Observatory (AVO) on a satellite image taken at 0824. The Institute of Volcanology (IV) reported that an eruptive column first appeared over Bezymianny around that time, and by 0900 it was ~8 km high. Weather satellite imagery at 0948 showed that the plume had reached the coastline nearly due E, with a top estimated to be 10 km above sea level.

At 0930, volcanic ash started to fall in Kliuchi, ~50 km NNE. Tremor and ash emission increased up to 1200, followed by 3 hours of intense ashfall; during a period of 140 minutes 700 g/m² of ash fell in Kliuchi. Because the air in Kliuchi was strongly polluted with volcanic gas, a warning was issued for the residents to take precautions. From Kliuchi, E. Zhdanova, a volcanologist from the Institute of Volcanic Geology and Geochemistry (IVGG), reported that ashfall had stopped at about 1700 on 6 October after 2-3 mm of deposition. AVO satellite imagery at 1813 showed the disconnected ash plume ~150 km E. The plume was moving ENE and was over 400 km from the source. By about 0930 on 7 October, the ash plume had undergone significant diffusion and was no longer detectable on satellite images.

As of the morning of 7 October the volcano was obscured, but there was no more tremor. Zhdanova suggested that the explosive phase of the eruption had ended and a lava dome was forming again. This interpretation was confirmed by a large hot spot seen at the vent on AVHRR imagery after the ash cloud had disconnected from the volcano.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Alaska Volcano Observatory; E. Zhdanova and V. Kirianov, Institute of Volcanic Geology & Geochemistry, Piip Avenue 9, Petropavlovsk-Kamchatsky, 683006, Russia; N.A. Zharinov and S.A. Fedotov, Institute of Volcanology, Petropavlovsk-Kamchatsky, 683006, Russia.

A strong episode of black ash emission from Northeast Crater (figure 60) during the late morning of 13 September lasted only a few minutes, sending an ash plume 100 m above the crater rim. Red ash emissions from Bocca Nuova and Northeast Crater continued until about 20 September, but explosions of variable frequency and intensity were heard from both throughout the month. Voragine (Chasm) and Southeast Crater exhibited only weak degassing in September. Poor weather prevented internal crater observations.

Figure 60. Topographic sketch map of Etna's summit craters (stippled), September 1995. Shaded areas within the craters indicate collapsed, degassing pits, and solid points are active boccas producing Strombolian activity. Within Bocca Nuova, the hatched area indicates the deepest part of the crater floor. Courtesy of the Istituto Internazionale di Vulcanologia.

On the evening of 2 October explosive Strombolian activity resumed at Northeast Crater from two small vents, aligned NNE-SSW in the lowest portion of the crater floor, ~150 m below the crater rim (figure 60). During observations the next morning, loud vigorous explosions were almost continuous, throwing scoria above the crater rim. A slight decrease in the frequency and energy of the explosions occurred that afternoon, although some incandescent bombs fell on the outer crater slope. Activity continued to decline during the night, and on the morning of 4 October Strombolian explosions were restricted to a single vent ejecting bombs up to a few tens of meters above the crater floor. By the evening of 5 October only incandescent degassing vents were present. During the same period, Bocca Nuova exhibited frequent red-brown ash emissions alternating with normal degassing. Ash emission was occasionally accompanied by incandescent bomb ejection. The ash puffs, more copious on the morning of 3 October, were produced by the same partially collapsed vent that was the site of Strombolian activity in August (BGVN 20:08).

Bombs collected on the crater rim (the first accessible material since the end of the 1991-93 eruption) were geochemically comparable with the 1991-93 lavas. The bombs were porphyritic hawaiite with phenocrysts of plagioclase (~16 volume %), clinopyroxene (~4%), olivine (~1%) and Ti-magnetite microphenocrysts in an intersertal groundmass.

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

Information Contacts: Sonia Calvari and Massimo Pompilio, CNR Istituto Internazionale di Vulcanologia, Piazza Roma 2, 95123 Catania, Italy.

Irazú's seismic station (IRZ2), located 5 km SW of the active crater, registered a minor increase in seismicity: During August and September there were low-frequency events detected 10 and 14 times, respectively. There were also higher-frequency events only detected locally during August and September; these occurred 30 and 48 times, respectively.

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

Information Contacts: E. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

Between 0019 and 0105 on 15 September, Tohoku University seismometers near Iwate volcano registered intermittent small-amplitude volcanic tremors and low-frequency earthquakes. Four low-frequency earthquakes had epicenters 2 km E of the summit at ~8 km depths.

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

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

On 11-12 and 18 September micro-earthquake swarms occurred offshore near Cape Kawana-zaki, in an area adjacent Ito City on the E coast of the Izu Peninsula (figure 15). After that, few micro-earthquakes took place until late September. An intense swarm began in late September; focal depths shallowed as the swarm shifted N and lay off Cape Shiofuki-zaki (figure 15). Personnel at Ajiro Weather Station, 9 km NNW of the source, felt 33 shocks. Kamata seismic station in Ito City, 5 km SW of the source, registered a total of 3,608 shocks. Two tiltmeters near the coast of Ito Peninsula showed rapid changes in tilt; volume strain meters around the volcano recorded compression.

Figure 15. Izu-Tobu epicenter map (top) and plot of focal depths versus time for September through 16 October 1995 (bottom). Courtesy of JMA.

Geologic Background. The Izu-Tobu volcano group (Higashi-Izu volcano group) is scattered over a broad, plateau-like area of more than 400 km2 on the E side of the Izu Peninsula. Construction of several stratovolcanoes continued throughout much of the Pleistocene and overlapped with growth of smaller monogenetic volcanoes beginning about 300,000 years ago. About 70 subaerial monogenetic volcanoes formed during the last 140,000 years, and chemically similar submarine cones are located offshore. These volcanoes are located on a basement of late-Tertiary volcanic rocks and related sediments and on the flanks of three Quaternary stratovolcanoes: Amagi, Tenshi, and Usami. Some eruptive vents are controlled by fissure systems trending NW-SE or NE-SW. Thirteen eruptive episodes have been documented during the past 32,000 years. Kawagodaira maar produced pyroclastic flows during the largest Holocene eruption about 3,000 years ago. The latest eruption occurred in 1989, when a small submarine crater was formed NE of Ito City.

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

A large lava flow broke out of the E (Kamoamoa) tube on 1 August at 490 m elevation and cascaded down Pulama Pali (a fault scarp); by 3 August the flow had split into three lobes. The E flow was the most voluminous and advanced down the W side of the flow field as an aa/pahoehoe flow with multiple channels. The W flow was a large pahoehoe sheet flow with many active streams. The middle flow was an 1,800-m-long channelized aa/pahoehoe flow, but had stagnated by mid-August. On 17 August the W lobe cascaded over Paliuli and spread out along its base over the next five days. That same day the E lobe reached the coastal plain and on 18 August was within 200 m of the WHA seismometer, which was removed the next day. Two pahoehoe flows were noted upslope on 3 August, originating at ~660 and ~650 m elevation. The upper flow was not active on 11 August; however, the lower flow was still active and burning forest at 590 m elevation. The Highcastle ocean entry was active but variable in August, with mild explosive activity on 7-8 August. During sampling on 11 August, a lava stream visible through a skylight at ~635 m elevation was 15 m wide and 19 m deep. A lava flow 300-400 m long was active close to the 600-m elevation, but all flows and the Highcastle ocean entry stagnated when the eruption paused on the evening of 22 August.

The eruption resumed on 25 August, and the first flows broke out of the tube system at 660 m elevation. Later breakouts were noted at 600 and 510 m elevations. The tube system appeared to be reoccupied only as far as the 510-m elevation, from which point a large aa flow cascaded down the E side of the flow field. By the 28th, flows had advanced to 240 m elevation. By 29 August the Kamoamoa lava tube had been reoccupied as far as the top of Pulama Pali. Several surface breakouts burned kipukas above the pali, and numerous shallow skylights developed. On the slope of Pulama Pali, aa and channelized pahoehoe flows advanced in two major fronts down the E side of the Kamoamoa flow field, burning forest along the edge. The leading edge of these flows reached the base of Pulama Pali on 29 August and advanced as pahoehoe sheet flows toward the coast, entering the ocean on 7 September. Through 11 September, pahoehoe spilled into the ocean at several discrete locations in a zone ~200-300 m wide on the far E margin of the Kamoamoa flow field.

On 12 September, voluminous channelized and sheet flows were observed at the coast and on Pulama Pali; surface flows on the slope were limited to intermittent breakouts. These lava flows continued to burn forest along the E edge of the flow field. Pahoehoe sheet flows several hundred meters wide continued to enter the ocean at the E edge of the Kamoamoa flow field (Kamokuna) in late September. A new ocean entry 1 km W of the other flows (Kamoamoa) was established on 21 September. By 9 October a major ocean entry fed by a tube was well established at Kamokuna and generating a large plume from a diffuse, ~100-m-wide entry zone of surface pahoehoe flows; there was only minor explosive activity. Smaller, intermittent entries were observed farther W at Kamoamoa, where surface pahoehoe flows occasionally reached the ocean. On the slope of Pulama pali, most of the lava was traveling in tubes, though small surface flows were frequent.

The pond at Pu`u `O`o continued to shrink in early August, and a sluggish crust had formed over much of the pond; the only open areas were on the W and N edges. The pond remained locally crusted and fairly small in late August; it was often >95 m below the crater rim, but it rose slightly during the pause. Between 25 August and 1 September the pond rose ~30 m, but subsequently dropped back to around 100 m. During this interval very vesicular tephra were deposited on the crater rim. By 5 September, the lava pond had risen to 60 m, overlapping the old crater floor formed in February 1992, but again receded to ~100 m depth by 12 September. The level of the lava pond then remained unchanged at roughly 80 m below the crater rim through 9 October. Sloshing lava in the ~15-m-diameter circular pond occasionally overflowed onto the adjacent crater floor formed during August.

Eruption tremor levels along the East Rift Zone remained low, with sporadic bursts of higher amplitudes during 6-9 August. Microearthquake counts were high on 1-2 August, but were below average beneath the summit and rift zones through mid-month. Low-level tremor persisted until the evening of 22 August, when it decreased in amplitude. Amplitudes remained at nearly background levels until the morning of the 26th, gradually increasing to nearly 2x background. On 24 August, counts of shallow (LPC-A) and intermediate-depth (LPC-C) long-period earthquakes were high. The counts remained high through the 26th for the LPC-C events and through the 27th for the LPC-A events. On the evening of 24 August a shallow M 3.2 earthquake beneath the upper edge of the East Rift Zone was felt mildly by a few nearby residents. Eruption tremor levels were fairly high until early on 30 August. Tremor amplitudes in early September dropped to nearly background levels with bands of higher amplitudes of one-half to three hours duration.

The number of intermediate, long-period microearthquakes was high during 5-8 September (nearly 500 events). Tremor levels were relatively low during 12-25 September except for isolated higher-amplitude bursts. Eruption tremor amplitudes were ~2x background until 30 September. From 1 October, tremor levels dropped slightly, and by 4 October, banding patterns of low amplitudes alternating with higher amplitudes became apparent. Also, from 1 October, intermediate-depth long-period (LPC-C) earthquake counts began to increase. The most intense days were 5-7 October, with total daily counts of 167, 434, and 214, respectively; many were large enough to locate. Short-period microearthquake activity remained low to moderate beneath the summit and rift zones from August through early October.

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

Information Contacts: Tari Mattox and Paul Okubo, Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, Hawaii Volcanoes National Park, HI 96718, USA.

The total number of earthquakes in September was 182, a significant decreased compared to the 463 recorded in August. On 29 September there were 25 earthquakes recorded at Station A, 1.7 km SW of Shinmoe-dake Crater, the highest daily total of the month.

Geologic Background. Kirishimayama is a large group of more than 20 Quaternary volcanoes located north of Kagoshima Bay. The late-Pleistocene to Holocene dominantly andesitic group consists of stratovolcanoes, pyroclastic cones, maars, and underlying shield volcanoes located over an area of 20 x 30 km. The larger stratovolcanoes are scattered throughout the field, with the centrally located Karakunidake being the highest. Onamiike and Miike, the two largest maars, are located SW of Karakunidake and at its far eastern end, respectively. Holocene eruptions have been concentrated along an E-W line of vents from Miike to Ohachi, and at Shinmoedake to the NE. Frequent small-to-moderate explosive eruptions have been recorded since the 8th century.

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

At 2143 on 6 October, a M 5.6 earthquake occurred near Kozu-shima (figure 2). The earthquake, which had an intensity at Kozu-shima of V on JMA's scale, caused a few landslides there. A M 4.8 earthquake 14 minutes earlier had an intensity of IV. During the next several days, an earthquake swarm continued offshore to the SW of Kozu-shima (figure 2, bottom). The swarm's maximum depth shifted downward with time, reaching 20-25 km. None of this seismicity was thought to have been induced by volcanism.

Figure 2. Kozu-shima epicenter map (top) and plot of focal depths versus time for September through 16 October 1995 (bottom). Courtesy of JMA.

Though obscured by epicenters on figure 2, Kozu-shima island has dimensions of 4 x 6 km and lies 20 km SSW of Nii-jima island and adjacent to the Izu Peninsula. Kozu-shima contains abundant rhyolitic surge deposits and lava domes. Its last eruption was in 838-840 AD. Seismicity near the volcano, and sometimes in vicinity of Nii-jima, has been episodically high in recent years.

Geologic Background. A cluster of rhyolitic lava domes and associated pyroclastic deposits form the 4 x 6 km island of Kozushima in the northern Izu Islands. The island is the exposed summit of a larger submarine edifice more than 20 km long that lies along the Zenisu Ridge, one of several en-echelon ridges oriented NE-SW, transverse to the trend of the northern Izu arc. The youngest and largest of the 18 lava domes, Tenjosan, occupies the central portion of the island. Most of the older domes, some of which are Holocene in age, flank Tenjosan to the north, although late-Pleistocene domes are also found at the southern end of the island. A lava flow may have reached the sea during an eruption in 832 CE. The Tenjosan dome was formed during a major eruption in 838 CE that also produced pyroclastic flows and surges. Earthquake swarms took place during the 20th century.

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

The observation of "smoke" in the Kuju Volcano Group (figure 1) near the Hosho dome (summit elevation 1,762 m, figure 2) at about 1800 on 11 October prompted the local meteorological observatory to issue a volcano alert. Ashfall from the phreatic activity was observed in towns as far as 60 km away, but there was no noticeable seismicity. When observed by the Kyushu Mobile Volcano Observation Team at 1430 on 12 September, steam was rising 400 m from around mid-slope on the dome. At that time there were three vents and fissure vents on the E slope of Hosho in an area ~300 m E-W by 100 m N-S. A photograph published in the Japan Times on 13 October (figure 3) showed steam emissions from numerous points along one ridge of the dome.

Figure 1. Map of central and southern Kyushu Island, showing selected cities and historically active volcanoes. Courtesy of Tokiko Tiba.

Figure 2. Map of the Kuju Volcano Group (modified from Kuno, 1962). Hosho Dome is near the center of the group.

Figure 3. Photograph of steam emissions from the Hosho dome at Kuju. Scanned from a photograph published in the Japan Times, 13 October 1995.

Reference. Kuno, H., 1962, Japan, Taiwan, and Marianas: Catalog of active volcanoes of the world, part 11, p. 54-57.

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

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Tokiko Tiba, Department of Geology, National Science Museum, 3-23-1 Hyakunin-cho, Shinjuku-ku, Tokyo 169, Japan; The Japan Times, Tokyo, Japan.

The overflow of Maughan Lake, the crater lake at Parker volcano, followed heavy rains associated with a passing typhoon and caused flashflooding in NW-flank communities on 6 September. A team from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) was dispatched to determine whether the overflow was caused by volcanic activity. Although no volcanic alert was declared, PHIVOLCS recommended that the crater area should be considered off-limits because of instability of the crater walls. Fieldwork on 8-9 September revealed that the flood was channel-confined along the NW-flank Alah River, which drains the crater lake, from 1,000 m down to 540 m elevation (Barangay New Dumangas, T'boli, South Cotabato Province). Below this point it was transformed into a sheetwash. The floods killed more than 60 people, destroyed 300 homes and nine bridges, and displaced 50,000 people.

Aerial observations on 11 September indicated that two or three landslides, indicated by escarpments, had occurred along the Alah River prior to the crater lake outbreak. The total mass displaced appears to have been sufficient to have dammed the upper reaches of the river. The crater wall was well-vegetated and without landslide scars, although underwater landslides may have contributed to the rise and subsequent overflow of the lake. The overflow breached the blocked river channel, sending an estimated ~10-15 x 10⁶ m³ of lake water down the river, lowering the lake by 1 m. No turbidity or color change was observed in the crater lake, indicating that there had been no volcanic explosion. On 9-10 September PHIVOLCS installed seismometers in T'boli, 12 km NW of the crater at 540 m elevation, and at Tobolok, ~4 km NW of the crater at 1,300 m elevation. No volcanic seismic events were recorded through 11 September.

Geologic Background. Mélébingóy (previously known as Parker) is a low, vegetated stratovolcano overlooking Sarangani Bay near the southern tip of Mindanao Island. The steep andesitic-dacitic stratovolcano is surrounded by extensive, youthful pyroclastic-flow deposits. The summit is truncated by a 2.9-km-wide caldera with steep-sided walls that rise 200-500 m above heart-shaped Maughan Lake. This volcano was the source of a major explosive eruption in 1641 CE that was previously attributed to Awu volcano on Sangihe Island. The eruption caused darkness over the island of Mindanao, deposited voluminous pyroclastic flows and lahars, and resulted in the formation of the summit caldera. This was the last of three major explosive eruptions during the previous 3,800 years.

Information Contacts: Ernesto G. Corpuz, Philippine Institute of Volcanology and Seismology (PHIVOLCS), 5th & 6th Floors Hizon Building, 29 Quezon Avenue, Quezon City, Philippines; United Press International.

Increased seismicity was detected at and up to 15 km around Cerro Negro during 24-28 May. Ash plumes to ~100 m above the crater rim were first observed on 29 May (or the afternoon of 28 May). The eruptions (1-2/hour) correlated with periods of increased seismic activity. On 1 June, the seismicity increased in frequency and intensity, with eruptions occurring about every 15 minutes. Fine-grained ash, consisting primarily of free crystals with minor amounts of basaltic fragments, was deposited N of the cone (figure 7). Bulk density of the ash deposit was measured at 1.3 +- 0.2 g/cm³. Trace amounts of ash (0.5 mm) from eruptions on 2-5 June fell as far N as Malpaisillo (figure 7), with 1-mm ash thicknesses extending 5 km N of the vent. This deposit represents 1 x 10⁴ m³ of ash, equivalent to an eruption rate of 100 m³/hour.

Figure 7. Location map of Cerro Negro and adjacent volcanoes, Nicaragua. Dashed ellipses show the interpreted extent of 0.5- and 1-mm-thick ash deposits for 2-5 June 1995 activity. Courtesy of Brittain Hill.

Eruptive activity was observed and recorded during 2-5 June, 6 June-1 July, and 24 July-16 August. During this time, activity was characterized by discrete explosions occurring on average every 8 +- 5 minutes. There was no apparent periodicity to the explosions, although patterns of increasing and decreasing repose times were apparent. These explosions commonly produced convective columns at least 400 m above the cone, and sometimes rising ~1 km; many produced ballistically transported blocks. Most blocks fell within the crater, but some occasionally impacted as far as half way down the outer slope of the cone. Blocks that reached maximum heights (100 m) and distances had estimated ejection velocities of 100-120 m/second.

The most intense activity was observed just before noon on 2 June, when a small dilute pyroclastic flow formed on the NW flank of the cone during an explosion. Deposits from this flow were found on the NW flank and extended <100 m from the base. The massive, fine-grained (<1 mm median diameter), and very well sorted deposit was ~1 cm thick on the cone slope and covered very fine ash from 29 May-1 June explosions. Although this deposit has the high degree of sorting and grain-size characteristics of surges, sorting is better than commonly observed in basaltic surge deposits (Wohletz, 1983). One explanation for this high degree of sorting is that the deposit was produced from a dilute, relatively low energy surge, which lacks a magmatic component common in most other basaltic surge deposits (Wohletz and Sheridan, 1979). This interpretation is consistent with the generally phreatic character of the eruption.

Seismic activity increased markedly on 24 July, with sustained periods of nearly constant tremor. A small lava flow formed in the E part of the main crater on 24-25 July. Ash eruptions during and after the lava emission continued at the same apparent frequency and magnitude as before. A sporadic increase in seismicity on 3 August was accompanied by increased degassing from within the crater. However, there was no associated increase in the number or magnitude of ash eruptions. Eruptive activity decreased significantly on 15 August, and ceased on 16 August.

Soil radon concentrations were monitored at 28 stations deployed around the base of Cerro Negro during early June. These stations were located near a permanent seismic station on the SSE side of the volcano, and on and near the Cerro La Mula Ridge, which extends NW from Cerro Negro (figure 7). Anomalously high radon concentrations were observed at nearly all of these stations during 2-3 June. At one station (700 m SSE of the crater), the radon concentration was 396 pCi/L on 2-3 June and 146 pCi/L on 3-4 June, compared with 17 pCi/L previously measured (Conway and others, 1994). Anomalous values (100-1,000x background) also were observed on 2-3 June at stations located up to 800 m N of the crater. Radon concentrations had dropped to near background levels at most stations by 4-5 June, but eruptive activity continued with little change. We conclude that a pulse of soil degassing occurred during the initial stages of the eruption, likely associated with dike injection and fracturing of wallrock.

A self-potential anomaly across Cerro La Mula Ridge, 750 m N of the active crater, correlated with the location of low-temperature fumaroles (60-88°C), and radon anomalies. Temperature increases of ~30°C were measured in this area, associated with the 1995 eruptive activity. Continuous monitoring of the self-potential anomaly on 4-5 June revealed amplitude changes corresponding to diurnal changes in air and ground temperatures, and periods of rainfall.

Preliminary ash-leachate studies by M. Navarro show low total S and Cl, along with low S/Cl, consistent with lack of a juvenile component in the ejecta. In addition, the regularity of the eruptions, the consistent fine grain-size of the deposits, and lack of obvious juvenile components support the interpretation of activity as dominantly phreatic. The heat needed to drive this event was provided by the intrusion of a small volume of magma into the shallow subsurface. Evidence for the presence of new magma includes the eruption of lava on 24-25 July, observed patterns of seismicity, increased temperature and radon flux in thermal areas, and the relatively constant periodicity of eruptions over the days of direct observation.

Cerro Negro is the site of the most recent small-volume basaltic eruption at a cinder cone in the western hemisphere, having last erupted in April 1992 (Connor and others, 1993). Cerro Negro first erupted in 1850, with at least 19 documented eruptions occurring up to April 1992. The longest eruption occurred in 1960, when activity persisted for approximately three months. Cerro Negro is characterized by unusually explosive eruptions, and may represent the upper end of basaltic eruption explosivities. Preliminary research suggests that this explosivity may be controlled by relatively high magmatic water contents (>2 weight percent) associated with highly crystalline, viscous magmas (Roggensack and others, 1994).

References. Connor, C.B, Powell, L., Strauch, W., Navarro, M., Urbina, O., and Rose, W.I., 1993, The 1992 eruption of Cerro Negro, Nicaragua: An example of Plinian-style activity at a small basaltic cinder cone: EOS, Transactions of the American Geophysical Union v. 74, no. 43, p. 640.

Conway, F.M., Macfarlane, A.W., Connor, C.B., LaFemina, P.C., and Reimer, M., 1994, Degassing at a young cinder cone: Volcan Cerro Negro: Geological Society of America, 1994 Annual Meeting Abstracts with Program, 26 (7), p. A453.

Roggensack, K., Williams, S.N., Hervig, R.L., McKnight, S.B., Connor, C.B., and Navarro, M., 1994, Evidence of polybaric fractionation: Melt inclusions in 1992 eruption of Cerro Negro volcano, Nicaragua: EOS, Transactions of the American Geophysical Union, v. 75, no. 44, p. 747.

Wohletz, K.H., 1983, Mechanisms of hydrovolcanic pyroclast formation: grainþsize, scanning electron microscopy, and experimental studies: Journal of Volcanology and Geothermal Research, v. 17, p. 31-63.

Wohletz, K.H., and Sheridan, M.F., 1979, A model of pyroclastic surge: Geological Society of America Special Paper 180, Boulder, CO, p. 177-194.

Geologic Background. Nicaragua's youngest volcano, Cerro Negro, was created following an eruption that began in April 1850 about 2 km NW of the summit of Las Pilas volcano. It is the largest, southernmost, and most recent of a group of four youthful cinder cones constructed along a NNW-SSE-trending line in the central Marrabios Range. Strombolian-to-subplinian eruptions at intervals of a few years to several decades have constructed a roughly 250-m-high basaltic cone and an associated lava field constrained by topography to extend primarily NE and SW. Cone and crater morphology have varied significantly during its short eruptive history. Although it lies in a relatively unpopulated area, occasional heavy ashfalls have damaged crops and buildings.

Information Contacts: Martha Navarro, Oscar Canales, and Wilfried Strauch, Instituto Nicaraguense de Estudios Territorales, Managua, Nicaragua; Brittain E. Hill, Charles B. Connor, F. Michael Conway, and Peter LaFemina, Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238-5166 USA.

During September, 9,144 seismic events took place, the most for any month in 1994 or thus far in 1995. These events were predominantly low-frequency (8,854 events, figure 58);

Figure 58. Low-frequency seismicity at Poás, January-September 1995. Data were collected at station POA2 located 2.7 km SW of the active crater. Courtesy of OVSICORI-UNA.

The level of the sky blue lake within the N crater climbed 0.6 m in September with respect to August. The lake's temperature was 34°C.  Fumaroles on the W lake terrace increased their output, but they only generated gas columns <50 m high. Although weaker than these fumaroles, two new fumaroles appeared on the terrace to the NW and SW of the lake. Other new fumaroles were seen along the N wall of the pyroclastic cone; fumarolic gases discharging from the cone reached 93°C. Constant bubbling continued to issue from points in the central and W lake. The fumarolic area on the SW and S wall maintained a 90-95°C temperature and discharged gas columns that rose as high as 100 m. Mass wasting of unstable hydrothermally altered rocks in this area covered some fumarolic vents and opened new ones.

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

Information Contacts: E. Fernandez, E. Duarte, R. Saenz, W. Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

The seismic receiver at the remote Rincón de la Vieja volcanic complex (RIN3) is located 5 km SW of the active crater. During August it registered 42 events at frequencies below 1.5 Hz; during September, 28 events with frequencies below 2.5 Hz.

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

Information Contacts: E. Fernandez, E. Duarte, R. Sáenz, W. Jimenez, and V. Barboza, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

Following noteworthy "vent clearing" eruptions at Ruapehu (figure 17) on 29 June and 3 July, and phreatic eruptions in September, a series of larger eruptions began on 23 September. During the next week Ruapehu discharged plumes that were frequently reported by aviation sources to have reached at least 10 km. The following was compiled from Institute of Geological & Nuclear Sciences (IGNS) reports and aviation notices.

Figure 17. Index map of North Island, New Zealand, showing the location of Ruapehu and other volcanic centers.

Precursory activity and minor eruptions. Many of Ruapehu's frequent small eruptions have been linked to high temperature in the crater lake. Unusually high lake temperatures (as well as other measured changes) also preceded the recent activity. During 1985-95 the surface temperature of Ruapehu's crater lake peaked at >40°C seven times; two of those peaks were in 1995. The early 1995 peak reached 55°C, the highest surface lake temperature recorded in 13 years (BGVN 20:01 and 20:05). The second 1995 peak reached roughly 44°C, the third highest seen in the 1985-95 interval. Key observations, including those from crater lake inspections carried out during visits from 25 May through 23 September (table 7) suggested a build-up in activity.

Table 7. Summary of key observations at Ruapehu, 25 May-23 September 1995. Prior to the larger eruptions observers reported that the lake was generally gray in color, often with sulfur slicks on its shore or surface; the lake began discharging water at Outlet sometime between 4 and 18 July. Courtesy of IGNS (IGNS Immediate Report (25 May-15 Aug 1995); IGNS Science Alert Bulletin (18-21 Sep 1995); Aviation report (23 Sep 1995).

A hydrophone and related acoustical detection components in the crater lake registered unusually high noise levels during late May, consistent with seismic activity. A moderate noise burst took place on 13 June, and relative quiet prevailed through 29 June. These data were communicated via the satellite-relayed ARGOS data system at 2-hour intervals; the last transmission (0800 on 29 June) came just prior to a M 3.2 volcanic earthquake and eruption that destroyed the ARGOS equipment.

Seismicity was at background levels from 15 May until just prior to the 29 June earthquake. The earthquake began at 0802 as a small 2-Hz signal followed by a 1-Hz signal. The main part of the earthquake, which also contained 2-Hz signal, started at 0821, and peaked between 0822 and 0824. After the main part of the earthquake, more signals centered around 1 and 2 Hz prevailed. The 2-Hz signals are common to both volcanic earthquakes and tremor at Ruapehu, suggesting that both may excite the same resonator.

Ruapehu's tremor typically has a dominant frequency of ~2 Hz and occurs almost constantly, often with no clear surface volcanic expression. Although not recorded at all stations, during 1995 and possibly longer, tremor has contained a previously unrecognized dominant frequency of 7 Hz with a consistent amplitude of 1 µm/s. During April, May, and late June, intervals of strong 2-Hz tremor dominated the seismic records. Very strong tremor took place for a few hours on 26 June. Tremor declined thereafter and remained low from early July through much of August.

Lake water increased in Cl and especially Mg ions closer to the eruption. The Mg/Cl ratio rose from values around 0.035 in early 1995 (BGVN 20:05), to the most recently reported value of 0.072 on 15 August (table 8); there was a further increase of unstated magnitude on 20-21 September (table 7). Prior to the eruption, the rise in Mg was thought to represent leaching from unweathered andesites. The increase in Cl, which reached greater levels than seen in at least 9 years, was thought to result from both large-scale evaporation and HCl input. The rise in Mg/Cl ratio represented the largest shifts seen since the large 1971 and 1975 eruptions. Shifts in the concentrations of K, Fe, and SO₄ from samples collected on 18 July suggested increased input of SO₂ into the vent-lake system rather than a water-rock equilibrium process in the vent. Although provisional, results for SO₄ on 18 July suggested a 4.5% increase--the highest ever recorded for the lake.

Table 8. Ruapehu Crater Lake water analyses and temperatures at Outlet, 25 May-4 July 1995. Courtesy of IGNS.

Deformation surveys on 4 July and 15 August confirmed only small measurable changes. This result suggested little or no magmatic movement in the upper part of the vent, in contrast with much of the other data in the same time interval. The limited deformation may have been a consequence of an open vent that allowed a small amount of magma to escape without measurable deformation. Measurable changes were apparently evident later (20-21 September, table 7).

Larger eruptions in late September. Ruapehu produced a series of larger eruptions during 23-30 September and later, continuing into October. Preliminary estimates suggested the eruption plumes reached 8-12 km heights as reported by aviation sources (table 7 and figure 18). The aviation reports and occasional satellite imagery typically noted plumes possibly extending as far as ~270 km from the summit (from an episode of eruptive bursts that were thought to have been more dense and ash-rich beginning at 1600 on 24 September). This particular series of bursts only initially reached low levels, but ash was said to have been lifted higher by induced cumulus convection, ultimately reaching a reported altitude of ~12 km. On subsequent days, the plume's typical maximum lateral extent was given as roughly 60 km.

Figure 18. Histogram summarizing the height of column tops for Ruapehu eruptions, based on available aviation reports and IGNS Science Alert Bulletins. Courtesy of Nick Heffter, NOAA.

For the 24-hour interval ending on 24 September (exact times undisclosed) observers at Ruapehu noted both small- and medium-sized steam-rich ash-bearing explosions, the largest of which had plumes that rose from 500 to over 1,500 m. On 24 September medium-sized explosions yielded a distinctive, though modest seismic signature and lesser explosions were not detectible. Near midnight on 24 September the number of volcanic earthquakes rose significantly; strong tremor roughly doubled in intensity compared to that morning; reflected seismic waves from numerous explosions yielded a confused signal.

Reports for 25 September (at 0900, 1700, and a summary the next day) noted that an eruption column had developed from many moderate-sized eruptions. With its top at 8-10 km altitude, the plume was blown into the E quadrant for several tens of kilometers, dropping ash 18 km E (Desert road; total accumulation, 1 mm), 30 km E (the Kaimanawa mountains), and 120 km E (traces at the coast). The ash deposited at Desert Road contained mainly particles of 10-250 µm size; 30-60% of the particles were juvenile. Significant amounts of ash had accumulated in the vent area but large blocks had been ejected less than ~1 km from the vent. Outlet was dry, but based on later observations, the inner crater still contained a lake.

At 0900 on 25 September a lahar flowed down the Whangaehu valley. The valley forms a key drainage that descends ESE from the crater, ultimately curving S and W to encircle Ruapehu's S flank; downstream parts of the Whangaehu Valley cross the Auckland-Wellington rail line near Tangiwai. Later the lahar declined in size, but it was noted as still continuing and sediment-laden at 1630, having eroded a stream bank upstream of the Tangiwai bridge. Another lahar flowed W of the crater down Mangaturuturu Valley.

At 1700 on 25 September, the volcanism during the previous 30 hours was described as episodic, punctuated by two cycles of increasing then decreasing intensity. Based on seismic data, the second cycle was not quite as vigorous as the first. In the night and morning of 25-26 September minor amounts of ash continued to fall over the volcano's E quadrant. Low-to-moderate tremor continued until at least 1700. Occasional explosions were large enough to be recorded seismically but were smaller than those in the morning of the previous day. Although during much of the day visual observations were hampered by cloud cover, at 0600-0700 on 26 September observers saw the plume drifting ESE. The plume was fed by numerous weak explosions and observers noted that minor amounts of ash fell throughout the night. Observers also noted that lahars flowing down the Whangaehu Valley were smaller than on the previous day. A very small lahar, deposited during an earlier event, was noted in the SE-flank Wahionoa Valley.

A SO₂ flux measurement at 1600 on 26 September indicated an output of 2,600 +- 400 metric tons/day. Such high fluxes confirmed significant magmatic involvement in the eruption. Although cloud cover limited the visibility on much of 26 September, the low seismic activity during the day suggested explosions of modest size. From about 2300 through early the next morning tremor amplitude fluctuated, increasing up to moderate levels. After 0400 tremor coexisted with many volcanic earthquakes.

Visual observations made after sunrise on 27 September correlated tremor and earthquake increases to moderately vigorous eruptive activity. During this period (0600-0700) the earthquakes reached a size equivalent to those on 25 September. By about 0930 on 27 September, however, the earthquakes stopped and the eruption's size dropped. Earthquakes then remained undetected until at least 1700.

Aerial observers on 27 and 28 September saw that Crater Lake had been greatly reduced in size; although indistinct, the steaming surface had clearly dropped by tens of meters. They also saw a previously concealed terrace formed during the 1945 eruption and recognized a new small lahar deposit in a drainage on the NW flank (in the Whakapapaiti Valley). On 27 September observers reported no water in the upper Whangaehu Valley and viewers the next day stated that downstream at the Tangiwai bridge the water level had returned to normal.

During the 24 hours ending at 0930 on 28 September, moderate levels of seismicity prevailed, and three larger volcanic earthquakes took place in the 0215-0340 interval. These earthquakes may have been associated with discrete explosions. Other volcanic earthquakes at 0736 and 0839 were linked to mild puffs of ash-bearing steam rising from the crater.

Ruapehu's alert status was raised to Level 4 (table 9) on 25 September. As late as early October, there had been no reports of death or injury caused by the eruption. Because of potential hazard to aircraft, aviation and meteorological workers have carefully monitored the eruption, producing forecasts of the plume's transport and dispersal ("VAFTAD" modeling program, see BGVN 19:06) as well as the actual visible observations that have confirmed the height of the plume's top (figure 18).

Table 9. Scientific Volcano Alert Level system for New Zealand volcanoes. Note that the frequently active cone volcanoes of New Zealand (White, Ngāuruhoe, and Ruapehu) require definitions different from all other volcanic systems. Because of this, Alert Levels 1-4 are split into two parts: one for the frequently active cones and the other for reawakening systems. Courtesy of the IGNS.

The late September eruption was widely covered in the news. According to Reuters (25 September), "A conservative Australian politician is linking nuclear testing by China and France to a string of earthquakes around the Pacific and volcanic eruptions in Montserrat and New Zealand's Mount Ruapehu." Although this connection was discounted by earth scientists, the accusation did reverberate in the media and parliaments world wide.

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

Information Contacts: C.J.N. Wilson, B.J. Scott, P.M. Otway, and I.A. Nairn, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand; Bureau of Meteorology, Northern Territory Regional Office, POB 735, Darwin NT 0801, Australia; J. Heffter, National Oceanic and Atmospheric Administration (NOAA), Air Resources Laboratory SSMC3, Room 3151, 1315 East West Hwy., Silver Spring, MD 20910 USA; Synoptic Analysis Branch, NOAA/NESDIS, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Following the formation of Vent 3 and significant ashfall on 22 August (20:8), more than 6,000 residents of southern Montserrat were evacuated to safe areas in the N part of the island. Press sources estimated that by late August ~3,000 people had left for neighboring islands. Vent 4 opened on 27 August and produced mainly steam emissions with some minor ash through 30 August. Although seismicity was high from 30 August through 1 September, steam and ash emissions remained low (20:8).

From 0500 on 1 September through 0500 on the 3rd, only 19 shallow earthquakes occurred beneath the volcano. During that same period, 17 episodes of gas venting were recorded; at least six of those episodes produced some ash, and the two events on 2 September each decayed into a long-period signal of ~10 minutes duration. Venting continued to enlarge vents 2 and 3, but emissions from Vent 4 remained low. A helicopter observation flight on the afternoon of 2 September was in progress when an emission episode began at 1606 with increased steaming that developed rapidly into a small steam-and-ash plume. The emission occurred from a narrow part of the main group of vents that extend SE from Vent 1. Mud on the floor of the vent was expelled during the episode, forming a small mudflow that moved down the S side of the moat and over the area of Vent 2. A gas-and-ash emission at 1912 on 2 September, similar in size and duration to emissions in recent days, was widely observed because of clear conditions. Lightning associated with this activity lasted ~1.5 hours, and an SO₂ odor was detected. Installation of a hardened EDM (electronic distance meter) station in the Tar River area was completed on 2 September.

During 3-4 September there were four gas-venting episodes, twelve volcano-tectonic (VT) earthquakes, and four long-period earthquakes. Aerial observations on the morning of 3 September revealed that the area around the S end of the main group of vents had been enlarged. The moat pond in the NW corner was still present, and fragmental material had collapsed into Vent 1. Afternoon observations showed no new mudflows, and the S moat appeared dry.

On the afternoon of 3 September, scientists at the volcano observatory completed an assessment of the current volcanism since 21 August and prospects for future activity. The rate of eruption signals increased slightly after 21 August, but the size of the eruptions did not. No change in the style of eruptions was anticipated, but areas downwind could be subject to ashfall and temporary darkness. Eruptions were thought likely to be concentrated along the linear vent chain on the W side of Castle Peak dome. The amount of shallow seismicity decreased below that prior to 21 August. SO₂ flux remained near detection limits since 21 August. The rate of long-period seismic events showed no clear pattern, although a slight decrease may have occurred. Initial EDM results indicated no movement of the SE flank of Castle Peak dome or at a site in Upper Gages. Electronic tiltmeters have detected no large-scale deformation since they stabilized on 5 August. Ash samples analyzed through 27 August revealed no juvenile material.

The scientists concluded the following: ". . . eruptions to date have been entirely phreatic, with no direct evidence of magmatic involvement. So long as this behavior pattern persists, it only constitutes a significant hazard to areas within 1.5 km of Castle Peak dome and the areas S of White's Bottom ghaut. All ghauts [ephemeral watercourses] that originate on the flanks of the Soufriere Hills volcano are subject to flooding and should be avoided." Based on this advice, the government approved re-occupation of the areas immediately S of the Belham Valley River from which residents were evacuated on 23 August. All other residents from areas closer to the crater, evacuated since 21 August, were required to stay in the northern third of the island. Controlled entry restrictions were relaxed in most areas to allow residents to prepare for an approaching hurricane. Following passage of the hurricane, on 6 September the remaining evacuation orders were lifted.

Activity during 4-8 September was consistent at a low and generally declining level. At about 1530 on 8 September there was a significant steam explosion. Two hours later, at about 1730, two large ash eruptions produced a vertical plume that formed a mushroom cloud, which drifted to St. Peters (~30 km NNW) and to the N. Soufriere Hills continued to have intermittent swarms of earthquakes from the summit and nearby areas, including three events felt in Woodlands on 11 September. Occasional steam eruptions produced falls of fine ash in communities around the volcano, and morphological changes were continuing in the summit area. These developments suggested to volcanologists that magma was close to the surface under the volcano and that a magmatic eruption was still a possibility.

Two weeks later, on 25 September, a lava dome began growing in the W part of the moat near the linear chain of vents. An explosion between 1100 and 1200 on 27 September caused ashfall on the S part of the island, with minor ashfall also reported in the St. Georges area. Minor explosive activity continued through the end of September.

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: Soufriere Hills Volcano Observatory, Plymouth; Seismic Research Unit, UWI; UNDHA; AP; Caribbean News Agency (CANA), Barbados.

No explosions or gas-and-ash emissions occurred from the lava dome between 1 January and 30 September 1995. Seismic activity was still low, but the number of small-magnitude (M

Figure 44. Seismicity at Mount St. Helens, January 1986-September 1995. A high concentration of earthquake activity at

This same zone of seismic activity became active in late 1987, about 2 years before the 1989-91 steam explosions began, and it presumably marks the approximate location of the magma conduit system. Those relatively small explosions hurled dome rocks as large as 30-40 cm in diameter at least 800 m from the dome and produced ash plumes as high as ~6 km above sea level. Detailed study of the 1987-91 seismicity and the 1989-91 explosions suggests that both occurred in response to increased pressure in the conduit system.

One possible cause for the pressure increase is that volcanic gas (primarily water vapor) became concentrated along the conduit system as a consequence of the progressive cooling and crystallization of magma. This increased pressure would likely lead to increased rock fracturing immediately surrounding the conduit system, as well as to intermittent sudden gas release. In addition, downward growth of cracks and fractures in the dome during and immediately after periods of intense precipitation could trigger gas explosions when such fractures intersect pressurized areas; many but not all of the 1989-91 explosions followed periods of heavy rainfall. Another possible cause for the pressure increase is intrusion of new magma into the lower depths of the conduit system. There is no evidence, however, that any magma has approached the surface during 1995. Regardless of the cause, it seems likely that the change in seismicity reflects a renewed increase in pressure along the magma conduit system.

Because the 1989-91 steam explosions were not preceded by any specific short-term warning signs, the similarity of the current seismicity raises concerns that future small dome explosions could occur without additional warning. Experience with the 1989-91 explosions, as well as explosions during the years of dome growth, suggests that they would produce hazards primarily within the crater, to a lesser degree in the stream channels leading from the crater, and to an even smaller degree on the upper flanks of the volcano. These hazards could include the impact of ejected dome rocks and rapidly moving pyroclastic flows sweeping the crater floor. During the 5 February 1991 explosion, a small pyroclastic flow reached the N edge of the crater. Heat from a rock avalanche or pyroclastic flow could also generate a lahar in the crater and in channels leading from the crater. Also, gas explosions could generate dilute but visible ash plumes perhaps as high as 6 km above the volcano and light ashfall as far as ~160 km downwind.

Geologic Background. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: Dan Dzurisin, Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA; Steve Malone, Geophysics Program, University of Washington, Seattle, WA 98195 USA. URL: https://volcanoes.usgs.gov/observatories/cvo/).