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).

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/).

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/).

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/).

Since 1968, Arenal experienced periods of moderate-to-robust volcanic activity that continued through September 2010, when activity declined (BGVN 35:07 and 36:04). This report discusses events between December 2010 and October 2012, a period of continued relative tranquility.

Although sporadic Strombolian explosions were reported in December 2010, they soon ceased; since then, no explosions had occurred through as late as October 2012. According to the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI), activity was limited to weak gas emissions, primarily through the NE vent in Crater C and through fumaroles in Crater D (figure 113).

Figure 113. A photograph of Arenal's summit taken on 20 February 2011, featuring the volcano's two peaks, both showing weak fumaroles. To the right is crater C, which has been active since 1968; to the left is crater D. Courtesy of Jairo Murillo Solís.

During the reporting period, the pH of rain-water gradually increased near the volcano. According to OVSICORI, the gradual decrease in rainfall acidity was associated with reduced magmatic activity.

According to OVSICORI, 2012 was one of the years of lowest activity for Arenal since 1968. No volcano-tectonic earthquakes, volcanic earthquakes, or tremors were recorded during the year, and no magmatic activity was detected. OVSICORI (citing Muller and others, 2011) reported that the Electronic Distance Measurement (EDM) network on the W flank of Arenal showed some subsidence from 2008 to near the end of 2011, but then the rate of subsidence decreased and no deformation occurred in 2012.

In June 2012, OVSICORI reported that night observations and long-exposure photographs of the summit revealed no incandescence. According to OVSICORI, the lack of incandescence indicated that gas emissions were of low temperature (probably <300°C), allowing water vapor to condense rapidly upon contact with the atmosphere. Hydrothermal activity remained low with only a few diffuse fumaroles rising from the N flank of Crater C (figure 113).

According to OVSICORI, an M_w 7.6 earthquake on 5 September 2012 centered on the Nicoya Peninsula (Costa Rica) caused moderate rock avalanches at Arenal, mainly dislodging unstable blocks on the active crater's N and NW rim. However, no changes were noted either in the hot springs around the volcano or in surficial expressions of volcanism.

A special issue of Journal of Volcanology and Geothermal Research was devoted to Arenal volcano (see Reference subsection below).

References. Marsh, B. (ed.), 2006, Arenal volcano, Costa Rica: Magma genesis and volcanological processes, Journal of Volcanology and Geothermal Research, v. 157, issues 1-3.

Muller, C., del Potro, R., Gottsmann, J., Biggs, J., and Van der Laat, R., 2011, Combined GPS, EDM and triangulation surveys of the rapid down-slope motion of the western flank of Arenal Volcano, Costa Rica, American Geophysical Union, Fall Meeting 2011, abstract ## V53C-2639 (Poster).

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: Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); CostaRica21 (URL: http://www.costarica21.com/).

This report discusses a series of small but punctuated eruptions on 15-17 September 2012 associated with the return of seismicity at Gamalama. Fog obscured visibility but ash fell on inhabited areas. The eruptions were judged similar to those seen 4 December 2011 (BGVN 36:12).

As we noted previously, heavy rains after the 4 December 2011 eruptions led to lahars on 27-28 December that killed four people, injured dozens, and displaced thousands (BGVN 36:12). Photos showed that these lahars had carried many meter-diameter blocks into inhabited areas on the lower flanks. Videos from helicopter flights confirmed that in the upslope region, chutes and drainages had also fed finer ash into the lahars.

According to the Center for Volcanology and Geological Hazard Mitigation (CVGHM), on 24 January 2012, after witnessing an interval of generally reduced seismicity, an absence of significant ash-bearing plumes, and weak steam plumes rising only ~100 m above the summit, they lowered the Alert Level from 3 to 2 (on a scale from 1-4).

As geographic background, Gamalama volcano emerges from the sea to form the near-conical 76 km² Ternate island. The island is situated in the Molucca (Maluku) islands in NE Indonesia about midway between the islands of Borneo and New Guinea (figure 5).

Figure 5. (A) An index map of Indonesia, including the Molucca islands and regional landmarks. Courtesy of U.S. Department of State. (B) A map of the Molucca (Maluku) islands, highlighting Gamalama (Ternate). Courtesy of Indonesia Explore.

Seismicity and eruptions of September 2012. Significant seismicity and other activity at Gamalama remained low from early 2012 until September. During 1-14 September white plumes were sometimes observed rising ~10 m above the crater. When visibility allowed, these plumes were observed from the local obseratory post at Marikuruba and from the W coast of the island, but fog and clouds generally obscured the view.

The telemetered seismograph system (PS-2) recorded deep volcanic earthquakes, shallow volcanic earthquakes, and local tectonic earthquakes, each occurring fewer than five times during 1-14 September. During that same period, there were 63 long-distance tectonic earthquakes and 42 hot air blasts recorded; once they began, signals interpreted as the hot air blasts amounted to 8 occurrences per day. Visual observations and tremor during this time period appeared similar to this volcano's past behavior.

On 15 September 2012 the following seismic events were recorded: 6 long distance tectonic earthquakes, 9 deep volcanic earthquakes, 2 shallow volcanic earthquakes, 14 hot air blasts accompanied by rumbling sounds, and an interval of tremor began with amplitudes reaching 3-4 mm. Six minutes after the tremor, eruption signals occurred with a maximum amplitude of 40 mm. A phreatic explosion produced ash fall and debris fall. Fog obscured the visibility.

On 16 September 2012, CVGHM reported low-amplitude tremor continuing during 0000-1200 (with 1.5-2.5 mm amplitudes). Medium-to-heavy rain fell at the summit around 1200. At 1358 tremor amplitudes increased to 28 mm, followed 17 min later by a "severe eruption."

That eruption drove an ash-laden plume to ~1 km above the crater. The plume drifted S and SE (figure 6A), and 5 min later ash fell at the observation post. The Alert Level was raised to 3 and visitors and residents were warned not to come within 2.5 km of the crater. CVGHM suggested that the eruption vented at the same location as those of December 2011.

Figure 6. (A) Photo of the Gamalama eruption on 16 September 2012 viewed from the NW. The ash plume is immediately blown to the S and SE with almost no vertical development. (B) The 17 September 2012 eruption of Gamalama viewed from the ESE. Both photos courtesy of Associated Press and The Jakarta Globe.

An eruption on 17 September 2012 produced a white-and-gray plume that rose 300 m above the crater and drifted E and SE (figure 6B). Ashfall was reported in the S, SE, and E parts of the island.

Calm prevailed for at least a few weeks after the eruption. Seismicity decreased in early October; on 8 October white plumes rose a mere 10-50 m. The Alert Level was lowered to 2 on 9 October, and the resulting exclusionary zone extended 1.5 km from the crater.

Geologic Background. Gamalama is a near-conical stratovolcano that comprises the entire island of Ternate off the western coast of Halmahera, and is one of Indonesia's most active volcanoes. The island was a major regional center in the Portuguese and Dutch spice trade for several centuries, which contributed to the extensive documentation of activity. Three cones, progressively younger to the north, form the summit. Several maars and vents define a rift zone, parallel to the Halmahera island arc, that cuts the volcano; the S-flank Ngade maar formed after about 14,500–13,000 cal. BP (Faral et al., 2022). Eruptions, recorded frequently since the 16th century, typically originated from the summit craters, although flank eruptions have occurred in 1763, 1770, 1775, and 1962-63.

Information Contacts: Center for Volcanology and Geological Hazard Mitigation (CVGHM), Jl. Diponegoro 57, Bandung, West Java, Indonesia, 40 122 (URL: http://www.vsi.esdm.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/); The Jakarta Post, Jl. Palmerah Barat 142-143, Jakarta 10270, Indonesia (URL: http://www.thejakartapost.com/); Associated Press (AP) (URL: http://www.apimages.com/); USA Today, 7950 Jones Branch Road, McLean, VA 22102 (URL: http://www.usatoday.com/); BBC News (URL: http://www.bbc.co.uk/); United States Department of State - Bureau of Consular Affairs (URL: http://travel.state.gov/); Indonesia Explore (URL: http://indonesiaexplore.com/).

Our last report on Kasatochi discussed the eruption of 7-8 August 2008 (BGVN 33:07). Since the 2008 eruption, the volcano has remained quiet except for gas emissions. Erosion and deposition of erupted pyroclastic material are rapidly altering slopes and beaches on the island (Scott and others, 2010). This report highlights studies conducted during 2008-2009 of the uninhabited island. Alaska Volcano Observatory (AVO) still monitors Kasatochi (figure 8) indirectly from the Great Sitkin Island seismic network located 42 km away and from satellite imagery. After the 2008 eruption, and the associated almost total biosystem extinction in 2009, Kasatochi Island became a site for monitoring ecosystem succession.

Figure 8. Map showing the location of Kasatochi in the Aleutian Islands. The extent of ash fall from the 7-8 August 2008 eruption is represented by dots with unverified areas indicated by question marks. Courtesy of Alaska Science Center (DeGange, 2010).

The terrestrial and surrounding marine environments of Kasatochi Island examined in June and July of 2009 saw changes in abundance or distribution of the ecosystem when compared to patterns observed on earlier surveys conducted in 1996 through June 2008. The largest direct effect of the eruption to individual animals was probably mortality of young birds. Indirect effects on wildlife consisted of the loss of suitable foraging habitats for species that relied on former terrestrial, intertidal, or nearshore-subtidal habitats and the near-total destruction of all former nesting habitats for most species. Although several species attempted to breed in 2009, all except Steller's sea lions failed due to the lack of suitable breeding sites.

The 7-8 August 2008 eruption. One or more of six remote International Monitoring System (IMS) infrasound arrays (figure 9) detected three well-defined eruption pulses of the 7 August 2008 eruption. The first was an infrasonic very long period (IVLP) acoustic pulse (pulse 1) that began at 21:59:44 UTC on 7 August with a gradual onset and duration of ~123 min and a peak RMS pressure of 0.22 Pa. The acoustic origin time was consistent with that computed for seismic signals (22:01 UTC). Pulse 2 began at 01:34:44 UTC on 8 August with a more impulsive onset, a duration of ~59 min and a peak RMS pressure of 0.46 Pa. Pulse 3 started at 04:20:34 on 8 August with an RMS pressure slightly higher than pulse 1 but lower than pulse 2 and a duration of ~33 min.

Figure 9. Kasatochi's 2008 eruption generated infrasonic signals detected by at least one of these six International Monitoring System (IMS) numbered stations (Fee and others, 2010).

The formerly steep and rugged island which previously had dense low-growing vegetation similar to other Aleutian Islands (figure 10a), became visibly devoid of vegetation after the 7-8 August 2008 eruption (figure 10b). In brief, the island habitat appeared to have been destroyed.

Figure 10. Kasatochi island as viewed before and after the 7-8 August 2008 eruption. (a) Aerial image from 9 July 2008 looking S showed extensive vegetation. (b) Aerial image from 23 October 2008 looking E showed pervasive pyroclastic material mantling the island. By this time, a shallow, gray, acidic lake had reformed in the widened summit crater. Photographs taken by Jerry Morris, Security Aviation; (from Waythomas and others, 2010).

Table 1 compares physical measurements of the island on 9 April 2004 (4 years prior to the 7-8 August 2008 eruption) to those taken on 17 September 2008 (nearly 6 weeks after the eruption). The aerial extent of the island increased by 40% after the eruption, the crater area increased by 25%, and the lake surface area enlarged by 73%. The accumulation of pyroclastic debris (most visible to the right in figure 10b) resulted in the seaward extension of the entire coastline by about 400 m, thus increasing the diameter of the island by about 800 m.

Table 1. Kasatochi Island's physiographic changes resulting from the 7-8 August 2008 eruption. *Data from 18 April 2009 Quickbird image. Reproduced from Waythomas and others (2010).

Post-eruption geology - eruptive deposit studies. Waythomas and others (2010) performed tephra studies in summer 2009 and reported that the bulk of the eruptive products from the 2008 eruption were pyroclastic-flow deposits, produced mainly by phreatomagmatic activity. The eruption lasted ~24 hours and included two initial explosive pulses and pauses over a 6-hr period that produced ash-poor eruption clouds, a 10-hr period of continuous ash-rich emissions initiated by an explosive pulse and punctuated by two others, and a final 8-hr period of nearly continuous ash emission and intermittent phreatic and phreatomagmatic activity. The authors reported that the eruption "...resulted in the accumulation of a uniform cover of medium gray-brown fine ash and pyroclastic-surge deposits over all flanks of the volcano. These deposits are 2-3 m thick and consist of silt, fine sand, and granules that are easily eroded by channelized water flows, and turn to sticky muck when wet." The deposits included a basal muddy tephra from eruptions through the shallow crater lake and accidental lithic debris derived from pre-existing lava flows in the crater. The juvenile material, which accounts for about 20-50% of the volume of the deposits, is pumiceous andesite (58-59% SiO₂).

Surface erosion on the slopes of Kasatochi volcano determined the transfer of sediment to the marine environment and is largely a function of the local hydrologic conditions. Analysis of satellite images and field studies in 2008 and 2009 have shown that within about one year of the 7-8 August 2008 eruption, significant geomorphic changes associated with surface and coastal erosion occurred (figure 11).

Figure 11. Cliffs eroded by wave action on an ENE shoreline of Kasatochi, photographed on 12 June 2009. Courtesy of AVO.

Although technically, sizes of rills and gullies differ, Waythomas and others (2010), using 1 m resolution imagery, could not resolve the size difference; thus they defined both as a narrow, relatively deep, v-shaped or rectangular gully on a hillside formed by flowing water. They observed extensive gully erosion beginning shortly after the eruption and continuing thereafter. Gully erosion removed 300,000 to 600,000 m³ of mostly fine-grained volcanic sediment from the flanks of the volcano, much of which reached the ocean (figure 12).

Figure 12. Images of erosion into pyroclastic deposits from the 7-8 eruption of Kasatochi. (A) The gully pattern that developed on the SW flank (person for scale indicated by arrow). (B) Looking in an E flank gully; maximum gully depth is ~3 m. Courtesy of Waythomas/AVO.

As seen during the summer of 2009 (Scott and others, 2010), the 2008 volcanic deposits that mantle much of the island mainly consisted of decimeter-thick veneers. Veneers greater than 10 m were found locally on middle-to-upper flanks. Broad aprons and fans up to several tens of meters thick were found along much of the lower flanks below former sea cliffs.

Fans originally extended out to 460 m from the former sea cliffs, but by the summer of 2009, fans on the W, N, and E flanks had been truncated to about half that distance or less by coastal erosion. They terminated in active sea cliffs about 15-20 m high. Fans on the S-side of the island either terminated in low cliffs or, more typically, were buried by post-eruption fans of alluvium and debris-flow deposits or by accreting beach sediments that displaced the shoreline an additional 150-250 m seaward.

Post-eruption habitat - vegetation studies. Talbot and others (2010) searched Kasatochi Island for remnant vegetation and signs of re-vegetation at pre-eruption sampling sites. Plants that apparently survived the eruption dominated early plant communities. The most diverse post-eruption community resembled a widespread pre-eruption community. Figure 13 shows a representative plot containing 11 species assigned to bluff ridge vegetation type that inhabited wave-cut cliffs prior to the eruption. Although this ridge vegetation type is nominally species-poor, in this sampling, the mean-species diversity was generally higher than the other post eruption types (Talbot and others, 2010).

Figure 13. A representative plot of 11 species assigned to bluff ridge vegetation type that existed on wave-cut cliffs prior to the 7-8 August 2008 eruption. Photo by Lawrence Walker, UNLV; courtesy of Talbot and others (2010).

Jewett and others (2010) examined the subtidal zone and reported that algal and faunal communities as well as rocky substrates were buried with volcanic deposits from the Kasatochi 2008 eruption. Existing plants were buried and the former stable rocky habitat was buried well into the subtidal zone. The loss of this rocky habitat may constrain kelp recolonization. However, little information is known regarding ocean current directions and velocities that may ultimately help erode soft-sediments and expose the hard rocky substrates necessary for kelp bed recolonization. Higher trophic marine organisms (for example, phytoplankton, the photosynthesizers that provide energy for a vast number of primary consumers, which in turn provide energy for secondary consumers and decomposers) were also affected by the eruption.

Post-eruption habitat - arthropod studies. A 2009 field campaign recorded 17 post-eruption insect species presumed to be non-breeding survivors and 4-9 breeding species. By 2010, 7 of the species seen in 2009 were lost while 18 post-eruption species survived, most of which were breeding (Ridling, 2012). The arthropod, Agyrtidae: Lyrosoma opacum Mannerheim (figure 14) was found to be the only breeding beetle among the 4-9 species found on post-eruption Kasatochi during the 2009 campaign.

Figure 14. During the 2009 field campaign one beetle species remained breeding on Kasatochi (Agyrtidae: Lyrosoma opacum Mannerheim) as seen the on remains of an unidentified bird species. From Ridling (2012).

Post-eruption habitat - avian and mammalian studies. Birds have been studied on Kasatochi by the U.S. Fish and Wildlife Service continually since 1996, providing a critical data base to evaluate ecosystem impact and long-term recovery. The pre-eruption avifauna on Kasatochi was dominated by over 200,000 crested and least auklets. Williams and others (2010) determined that most, if not all, of the auklet nesting habitat was covered by the eruption products (figure 15).

Figure 15. Kasatochi Island auklet rookery seen (a) before the 7-8 August 2012 eruption and (b) after the 2008 eruption, when the auklet hatch had failed completely. Photographs taken by G. Drew, courtesy of Alaska Park Science.

The largest direct effect of the eruption on individual animals was likely the mortality of chicks, with an estimated total 20,000-40,000 young birds lost during and shortly after the August 2008 eruption. Drew and others (2010) found that surviving older least auklets around Kasatochi Island showed little change in densities which ranged from 26 to 34 birds per km². Similar to the least auklet finding, numbers crested auklets were not significantly reduced by the initial explosion. They also returned to attempt breeding in 2009, even though their nesting habitat had been rendered unusable.

Although seven species of birds and mammals attempted to breed in 2009, all but one specie failed due to lack of suitable breeding sites. The one successful breeding specie identified was Steller's sea lions. Williams and others (2010) noted the abundance of sea lions and many seabird species in 2009 was comparable to pre-eruption estimates, suggesting that adult mortality was low for these species. In contrast, shorebirds and passerines, commonly called perching birds, that formerly bred on the island were no longer observed in 2009 and probably perished in the eruption.

Drew and others (2010) also surveyed the marine environment surrounding Kasatochi in June and July of 2009 to document changes, including nutrient abundance, compared to patterns observed in 1996 and 2003. Analysis of SeaWiFS satellite imagery indicated that a large marine chlorophyll-a anomaly may have been the result of ash fertilization during the eruption. Drew and others (2010) found no evidence of continuing marine fertilization from terrestrial runoff 10 months after the eruption.

Post-eruption habitat - volcanic degassing and the landscape. Kasatochi remained quiet except for gas emissions after the 7-8 August 2008 eruption while erosion and deposition have altered the slopes and beaches (figure 16). By April 2009 the level of the crater lake had risen and the lake surface area was 67% larger than it was before the eruption due to an increase in crater diameter (Scott and others, 2010). Fieldwork in summer 2009 determined the locations of various rills and gullies at representative locations on the island. As the gully system on Kasatochi Island began to stabilize and sediment yield declined accordingly, wave action was expected to become the dominant process affecting the landscape (Waythomas and others, 2010).

Figure 16. The prominent cliff-like feature (arrow) seen here in this view from the SW sits well inboard of Kasatochi's present coastline. The cliff was the island's former (pre-eruptive) shoreline. At the post-eruptive coastline, surge deposits are 1-2 m thick, and are much thicker higher up on the flanks. This image, taken 23 August 2008, shows gas emitted from the crater, drifting over the crater rim. Lithic clasts up to 2 m in diameter have been eroded out of the pyroclastic flow deposits by the sea and form a boulder-lag deposit along the coastline. Courtesy of Waythomas/AVO.

Post-eruptive landscape - drainage density. As stated by Waythomas and others (2010), "A fundamental landscape property that describes the degree of dissection by gullies and stream channels is drainage density... Drainage density is the ratio of total channel length to drainage-basin area [km/km²]. Changes in drainage density with time indicate that the threshold for erosion by runoff has been exceeded during individual rainfall events, and that the drainage system has yet to reach a state of quasi-equilibrium where routine rainfall events no longer bring about appreciable changes in drainage density. Time-dependent changes in drainage density also are surrogate measures of erosion because an increase in channel length must reflect channel head processes such as landsliding or gullying... Eventually the rates of gully development will decline and drainage density will approach a steady-state value or perhaps decrease. This is commonly due to the stabilizing effects of vegetation growth... We note that prior to the 2008 eruption of Kasatochi, the flanks of the volcano were covered with a nearly continuous mantle of herbaceous tundra, and no surface streams or drainages were present. Thus, prior to the eruption, the drainage density was very low, if not zero, and over time, we expect that the island will return to this condition."

Based on Waythomas and others (2010) and additional satellite image data from years 2008, 2009, and 2011, Julie Herrick calculated two Kasatochi surface drainage parameters: change in drainage density and change in gully volume. These two calculations used vector images to locate gully lines. These lines were superimposed as vectors on the rasterized (bit digitized) images and then a density analysis was performed. Comparisons of the three years by raster calculations (a form of bit analysis) determined the drainage line density as shown in figure 17A. Spatial analysis determined relative increase, decrease and unchanged surface volumes throughout the island as shown in figure 17B.

Figure 17. Two surface models of drainage trends at Kasatochi developed by Julie Herrick. (A) 3D visualization of 9 March 2011 sedimentation drainage line density in units of km/km2 (see text). Colors represent drainage density as shown in the key (bottom left). Notice that the SE and NE sectors have relatively higher densities. (B) Map (N at top of image) showing volume change of 2008-2011 tephra superimposed on a topographic image (legend at right). The relative net loss of volume areas (blue), are mainly on the island's northerly shorelines. The relatively unchanged areas (gray) are near or on the crater rim. The S shorelines have expanded, as shown by the net gain volume areas (red).

The recovery of habitats at Kasatochi will depend on erosion of the tephra layer blanketing the island to re-expose former breeding habitats as well as anecdotal introduction of various species.

References. DeGange, A.R., Byrd, G.V., Walker, L.R., and Waythomas, C.F., 2010, Introduction-The Impacts of the 2008 Eruption of Kasatochi Volcano on Terrestrial and Marine Ecosystems in the Aleutian Islands, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 245-249.

Drew, G.S., Dragoo, D.E., Renner, M., and Piatt, J.F., 2010, At-sea Observations of Marine Birds and Their Habitats before and after the 2008 Eruption of Kasatochi Volcano, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 306-314.

Fee, D., Steffke A., and Garces, M., 2010, Characterization of the 2008 Kasatochi and Okmok eruptions using remote infrasound arrays, Journal of Geophysical Research, 115, D00L10 (DOI: 10.1029/2009JD013621).

Jewett, S.C., Bodkin, J.L., Chenelot, H., Esslinger, G.G., and Hoberg, M.K., 2010, The nearshore Benthic Community of Kasatochi Island, One Year after the 2008 Eruption, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 315-324.

Neal, C.A., McGimsey, R.G., Dixon, J.P., Cameron, C.E., Nuzhdaev, A.A., and Chibisova, M., 2011, 2008 Volcanic activity in Alaska, Kamchatka, and the Kurile Islands: Summary of events and response of the Alaska Volcano Observatory, U.S. Geological Survey Scientific Investigations Report 2010-5243, 94 p.

Ridling, S., 2012, Origins of Post-Eruption Insect Populations on the Volcanic Aleutian Island of Kasatochi (Presentation, URL: www.akentsoc.org/doc/Ridling_S_2012.pptx).

Scott, W.E., Nye, C.J., Waythomas, C.F., and Neal, C.A., 2010, August 2008 Eruption of Kasatochi Volcano, Aleutian Islands, Alaska-Resetting an Island Landscape, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 250-259.

Talbot, S.S., Talbot, S.L., and Walker, L.R., 2010, Post-eruption Legacy Effects and Their Implications for Long-Term Recovery of the Vegetation on Kasatochi Island, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 285-296.

Wang, B., Michaelson, G., Ping, C.L., Plumlee, G., and Hageman, P., 2010, Characterization of Pyroclastic Deposits and Pre-eruptive Soils following the 2008 Eruption of Kasatochi Island Volcano, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 276-284.

Waythomas, C.F., Scott, W.E., and Nye, C.J., 2010, The Geomorphology of an Aleutian Volcano following a Major Eruption: the 7-8 August 2008 Eruption of Kasatochi Volcano, Alaska, and Its Aftermath, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 260-275.

Williams, J.C., Drummond, B.A., and Buxton, R.T., 2010, Initial effects of the August 2008 volcanic eruption on breeding birds and marine mammals at Kasatochi Island, Alaska, Arctic, Antarctic, and Alpine Research, Vol. 42, No. 3, pp. 306-314.

Geologic Background. Located at the northern end of a shallow submarine ridge trending perpendicular to the Aleutian arc, Kasatochi is small 2.7 x 3.3 km island volcano with a 750-m-wide summit crater lake. The summit reaches only about 300 m elevation, and the lake surface lies less than about 60 m above the sea. A lava dome is located on the NW flank at about 150 m elevation. The asymmetrical island is steeper on the northern side than the southern, and the crater lies north of the center of the island. Reports of activity from the heavily eroded Koniuji volcano to the east probably refer to eruptions from Kasatochi. A lava flow may have been emplaced during the first recorded eruption in 1760. A major explosive eruption in 2008 produced pyroclastic flows and surges that swept into the sea, extending the island's shoreline.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA; Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA; and Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.avo.alaska.edu/); Julie Herrick, Global Volcanism Program, Smithsonian National Museum of Natural History, Washington, DC 20560.

Our previous report (BGVN 36:08) discussed two eruption episodes: one from 25 October 2010 to March 2011, and another from August 2011 to about 1 October 2011. During the last two weeks of September 2011, the volcano produced persistent volcanic earthquake swarms and thin emissions (BGVN 36:08). This report discusses two visits to the volcano in 2011. Scientists that visited on 8 October 2011 reported degassing and an ongoing seismic swarm then consisting chiefly of M ~1 and smaller earthquakes. During 12-13 November 2011 a photographer noted steady degassing, then observed the start of a 12-hour interval of minor but repeated Stombolian eruptions (see next section).

2011 visits by Øystein Lund Andersen. The photographer and guide Øystein Lund Andersen lives in Jakarta, Indonesia and visits Anak Krakatau often. His website... shows one photo of a seismograph at CVGHM's Pasauran Observatory recording part of a prolonged swarm of small earthquakes from 8 October 2011; Youtube features a video he took on the same subject.

His visit... during 12-13 November 2011 took place during an interval of gas emissions devoid of ash. He stayed up all night to observe Anak Krakatau emit a steady, white, ash-free plume. At dusk on 12 November he noticed that the crater glowed bright red and after a few hours a series of mild Strombolian eruptions occurred in a sequence that lasted 12 hours (figure 29). The time between the eruptions was from 30 seconds to a few minutes. Some of Andersen's photos captured glowing pyroclasts arcing tens of meters above the crater rim (figure 29b, c). Andersen saw ash lava bombs in the plume during these eruptions. He noted that the lava bombs ejected over the crater mainly fell back into the crater. During the night the crater remained almost constantly illuminated by the glowing bombs and the fragments they created when they landed. The eruptions were often accompanied by loud sounds from the volcano.

Figure 29. Three photos of Anak Krakatau associated with mild Stombolian eruptions taken during 12-13 November 2011 amid unusually clear conditions. Provided to Bulletin editors by Øystein Lund Andersen.

References. Lockwood, J. and Hazlett, R.W., 2010, Volcanoes: global perspectives. Wiley-Blackwell.

Simkin, T. and Fiske, R.S., 1983, Krakatau, 1883--the volcanic eruption and its effects, Smithsonian Institution Press.

Self, S., Rampino, M.R., 1981, The 1883 eruption of Krakatau, Nature, 294, pp. 699-704.

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: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Øystein Lund Andersen (URL: http://www.oysteinlundandersen.com/).

Ol Doinyo Lengai, located close to the N border of Tanzania (figure 153), is both accessible and monitored closely.

Figure 153. Map of Tanzania showing Ol Doinyo Lengai's proximity to Meru and Kilimanjaro. Courtesy of USGS/CVO.

Frederick Belton's Ol Doinyo Lengai web site has provided many interesting photos from the expeditions that he has taken to the volcano since his initial visit in 1997, including annual visits until 2006, followed by his last expedition in 2008. In addition, Belton has included in his web site observations, photographs, and other graphics provided by many visitors to Ol Doinyo Lengai; these descripions have been the primary source of reports found in the Bulletin. Recently, Belton informed Bulletin editors that he rarely gets any updates on visits to Ol Doinyo Lengai for his web site, but he did receive one in September 2012. Bulletin editors wrote to a number of past contributors to BGVN; some of their comments are included below.

Belton's web site reported that Frank Möckel and Wendy Blank visited Ol Doinyo Lengai's summit area (figure 154) in September 2012. They climbed the volcano during 14-15 September and camped in the S Crater during the nights of 15 and 16 September (figure 155). Figures 155-160 contain some views they captured at the summit of the volcano. Figures 157 and 158 show what appear to be active spatter cones inside the N Crater. According to Belton, the activity looks very typical of the type frequently seen prior to the last explosive eruption in 2007-2008 (BGVN 32:11). Möckel and Blank reported that on the bottom of the active N Crater they saw fresh black natrocarbonatite lava and active vents, and they heard boiling noises from the bottom of the N Crater. They reported a strong smell of hydrogen sulfide (H₂S) everywhere in the area.

Figure 154. Topographic map modified from the Ol Doinyo Lengai quadrangle of 1990, with a contour interval of 20 m. The original map was modified in the N Crater area from observations made on 12 March 2010. Courtesy of Sherrod and others (2010).

Figure 155. Ol Doinyo Lengai's S Crater, seen with tents for scale (center). Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 156. View of Ol Doinyo Lengai's N Crater as seen from the summit. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 157. Ol Doinyo Lengai's active N Crater as seen from an unidentified point on the crater rim. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 158. View looking down into Ol Doinyo Lengai's N Crater at interior spatter cones. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 159. A closer view of Ol Doinyo Lengai's N Crater floor showing several vent openings (black) and an area of fresh spatter covering a region downhill of a vent. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Figure 160. Climbers on the NE rim of Ol Doinyo Lengai's N crater. Photo taken 14-15 September 2012. Courtesy of Frank Möckel and Wendy Blank.

Abigail Church, who studied Ol Doinyo Lengai for her PhD dissertation in 1996 and has published several articles on the petrogensis of the natrocarbonatite lavas, now lives in Nairobi, Kenya, and regularly flies over Ol Doinyo Lengai in chartered aircraft. She has flown over and landed on Ol Doinyo Lengai in a helicopter probably 5 times in the last few years. She camped close to the volcano in November 2012 and flew around the summit, however, it was cloudy. On many occasions when she was able to see into the crater, she observed what appeared to be small-scale activity continuing in the base of the deep pit. There are normally 1 or 2 active vents in which one can see very dark material which she assumed was fresh natrocarbonatite lava. She also has good contacts with people in the area and with pilots who fly the routes between Arusha and the Serengeti. On recent flights over Ol Doinyo Lengai, Church has observed that the sides of the central crater within the N Crater are collapsing inwardly, reducing the depth of the crater hole, and that small scale activity in the crater continues. Figures 161-165 show some photographs from 2011 and 2012 of the inside of Ol Doinyo Lengai's N crater.

Figure 161. Ol Doinyo Lengai's N crater floor showing a vent and some spatter. Photo taken 2 December 2011. Courtesy of Phil Mathews and Abigail Church.

Figure 162. A collapse in the floor of Ol Doinyo Lengai's N crater, showing natrocarbonatite lava flows. Photo taken 2 December 2011. Courtesy of Phil Mathews and Abigail Church.

Figure 163. Small active vent in the N Crater floor of Ol Doinyo Lengai. Photo taken 2 December 2011. Courtesy of Phil Mathews and Abigail Church.

Figure 164. A view of Ol Doinyo Lengai's N Crater from over the rim. Photo taken in August 2012. Courtesy of Phil Mathews and Abigail Church.

Figure 165. A recently active vent on Ol Doinyo Lengai's N Crater floor. Photo taken in August 2012. Courtesy of Phil Mathews and Abigail Church.

Hannes Mattsson reported to Bulletin staff that he has 3 PhD projects running on different aspects of Ol Doinyo Lengai volcanism, but he has not been at the volcano for about 1.5 years. He is planning a 6-week field campaign scheduled in mid 2013. He noted that very little current or recent information on Ol Doinyo Lengai is currently available.

Joerg Keller also noted that there are not many recent reports about activity in the crater area. According to reports of Möckel's visit in February 2010 (BGVN 35:05), the access route to and from Ol Doinyo Lengai's summit seems much more difficult to negotiate than before the 2007 eruption. Keller believes that another possible factor leading to less observations of Ol Doinyo Lengai is the change in the crater formations since the eruption of 2007. There seems to be little change since 2008, a stable situation with the new ash cone dominating the entire N crater area completely (see figures in BGVN 32:11 and 33:02, as well as the above photos). The unique crater landscape seen before 2007, with accessible hornitos and lava flows of different ages, and the chance to see active spatter cones, lava pools and flowing lava, was an attraction to visiters. The logistical problems for visiting and climbing the volcano since 2008, incluing safety and political factors, have resulted in greatly diminished numbers of visitors.

Keller reported that Elias Danner, a teenage photographer, filmer, and designer, started Ol Doinyo Lengai photo documentation that shows the ash cone, its deep pit, and, in particular, looks inside the pit with fresh, overlapping lava lobes and vigorously boiling lava pools. Keller received from Danner a video of the boiling lava pools which was so typical and so impressive that he wrote in a recent paper (Keller and Zaitsev, 2012) the following: "The present vertically sided, almost 100 m deep pit crater formed by the 2007-2008 explosive activity is inaccessible. However, since 2008 frequent overflights and reports and photographs by visitors climbing the mountain (Belton, 2012) suggest that new natrocarbonatite effusions are occurring at the bottom of the deep pit. This is indicated from a distance by the typical morphological features of natrocarbonatite appearing as small hornitos and gray pahoehoe flows on the floor of the crater. On 26th June 2011, Elias Danner ... filmed a vigorously boiling and splashing, obviously carbonatitic lava pool at the bottom of the pit, with features very reminiscent of Figs. 5 and 6 in Keller and Krafft (1990)."

MODIS/MODVOLC Satellite Thermal Alerts. Table 26 gives an update of MODVOLC satellite thermal alerts at the Ol Doinyo Lengai summit since a similar update found in BGVN 33:06. It is not uncommon to find thermal alerts down and beyond the sides of the volcano, probably caused by fires. It is possible that fewer thermal alerts are measured by the MODIS satellites because the current deep crater (since the 2007-2008 eruptions) shields some of the hotter areas from the satellite sensors.

Table 26. MODVOLC thermal alerts measured at Ol Doinyo Lengai from 3 April 2008 to December 2012. Courtesy of the Hawai`i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System.

References. Belton, F., 2012, Oldoinyo Lengai, The Mountain of God (URL: www.oldoinyolengai.pbworks.com).

Keller, J., and Krafft, M., 1990, Effusive natrocarbonatite activity of Oldoinyo Lengai, June 1988, Bulletin of Volcanology v. 52, pp. 629-645.

Keller, J., and Zaitsev, A.N., 2012, Geochemistry and petrogenetic significance of natrocarbonatites at Oldoinyo Lengai, Tanzania: Composition of lavas from 1988 to 2007, Lithos, v.148, pp. 45-53.

Sherrod, D., Mollel, K., and Nantatwa, O., 2010, Oldoinyo Lengai: Trip Report, March 12-14, 2010, informal report (URL: http:/Sherrod_OldonyoLengai_March12_20106-1.pdf).

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: Frederick Belton, University Studies Department, Middle Tennessee State University, Murfreesboro, TN (URL: http://oldoinyolengai.pbworks.com/); Sonja Bosshard, Institute of Geochemistry and Petrology, Swiss Federal Institute of Technology Zürich (ETH Zürich), Zürich, Switzerland; Laura Carmody, Planetary Geoscience Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN; Abigail Church, The Ker & Downey Safari Tradition, P.O. Box 86, Karen 00502, Kenya; Elias Danner, Elias Danner Productions (URL: http://www.mammut-studios.com/); Joerg Keller, Institut für Geowissenschaften/Mineralogie-Geochemie, Universität Freiburg, Albertstrasse 23b, 79104 Freiburg, Germany; Hannes B. Mattsson, Institute of Geochemistry and Petrology, Swiss Federal Institute of Technology Zürich (ETH Zürich), Zürich, Switzerland; Frank Möckel; Celia Nyamweru, St. Lawrence University; David Sherrod, Cascades Volcano Observatory (CVO), U.S. Geological Survey, Vancouver, WA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Christoph Weber, Volcano Expeditions International (VEI), Muehlweg 11, 74199 Untergruppenbach, Germany (URL: http://www.v-e-i.de/); Ben Wilhelmi, commercial pilot (URL: http://benwilhelmi.typepad.com/benwilhelmi/).

Elevated seismicity during January 2011 was discussed in our last report on Cerro Machín volcano (BGVN 36:04). Between September 2010 and January 2011, more than 800 volcano-tectonic (VT) earthquakes were detected per month and local residents reported shaking from these events, particularly during November 2010-February 2011. Here we describe trends in seismicity at Machín from January 2011 to November 2012 and the frequency of seismic swarms. We also include descriptions of monitoring efforts by the Volcanic and Seismological Observatory of Manizales at the Colombia Institute of Geology and Mining (INGEOMINAS) including two field campaigns focused on CO₂ emissions from the crater.

Geophysical monitoring. Since January 2011, INGEOMINAS had been monitoring Cerro Machín with a network that included broadband and short-period seismometers, magnetometers, self potential, and an acoustic monitoring system (acoustic flow detection for early flood warning). The deformation network included electronic and dry tilt (longterm monitoring since 2005), and starting in November 2012, three GPS stations were also operating (figure 4). Electronic-distance measurements (EDM) were conducted in 2012 at seven stations (EDM data was available since 2008). Data from these monitoring efforts were available in the INGEOMINAS online technical reports.

Figure 4. The deformation monitoring network at Cerro Machín in 2012 included three GPS stations and five electronic tilt stations. EDM measurements in September 2011 used three base stations (San Lorenzo, "SLOR;" La Palma, "PALM;" and Anillo, "ANIL") while measurements in October 2012 relied on one base station (San Lorenzo, "SLOR"). Courtesy of INGEOMINAS.

Geochemical monitoring. Geochemical monitoring at Cerro Machín has been conducted within the circular crater and the central dome complex (figures 5 and 6). During 2011-2012, geochemical monitoring included diffuse CO₂ detection, alkaline traps, and radon monitoring from soil emissions (13 stations were online in November 2012) as well as regular testing at fumarolic and hot spring locations.

Figure 5. Geochemical monitoring during 2011-2012 at Cerro Machín included several dozen sampling sites mainly spread across the ~2-km diameter crater. INGEOMINAS released long-term datasets from radon-gas traps, an alkaline trap, and a fumarole in their monthly technical bulletins. Courtesy of INGEOMINAS.

Figure 6. Two aerial views of Cerro Machín were captured during an overflight on 16 November 2011. (Top) In this view of the NE flank of Machín, the crater lake is visible near the left-hand side of the image within a moat-like region surrounding the dome. (Bottom) This view shows an access road along the breached, SW edge of the dome complex (lower center). This view also reveals a glimpse of the crater lake (appears gray) in the distant portion of the moat (right, from center). Courtesy of INGEOMINAS.

In May and September 2012, INGEOMINAS conducted field surveys to measure diffuse carbon dioxide emissions (figure 7). With a mobile LICOR 820 monitoring device, INGEOMINAS technicians traversed the interior crater rim detecting CO₂, air temperature, and pressure. The survey on 28 May determined baseline levels of CO₂ flux at 28 points within the crater. The survey conducted during 19 and 20 September 2012 detected relatively high CO₂ emissions from seven locations along a traverse within the crater. The highest CO₂ fluxes ranged between 739 and 8,077 mol·m^-2·day^-1, and in their technical report, INGEOMINAS noted that future gas monitoring should focus on those sites with peak values.

Figure 7. On 28 May 2012, INGEOMINAS conducted a CO2 campaign within the crater of Cerro Machín. Courtesy of INGEOMINAS.

Seismicity in 2011. Elevated seismicity in late 2010 continued through early 2011 (BGVN 36:04) and local communities reported shaking in January and February 2011 (figure 8). For many months after May 2011, earthquakes per month had declined to below 400 per month. The clear exception to that trend took place during September 2011, a month with over 1,200 earthquakes.

Figure 8. Volcano-tectonic (VT) seismicity at Cerro Machín abruptly increased in September 2010. This histogram shows time on the x-axis and number of earthquakes on the y-axis. Earthquake count per month decreased in January and February 2011 and reached low levels in August. Except for peaks in September and December 2011, the number of earthquakes remained below 400 events per month until November 2012. Courtesy of INGEOMINAS.

Compared with 2010 activity, fewer seismic swarms were detected in 2011 and in the available record for 2012 (table 2). In 2011, swarms tended to cluster beneath the dome complex and in areas ~2 km S and SE. INGEOMINAS frequently noted earthquake epicenters in an area known as Moralito, a location SE of the volcano near the MRAL GPS station (see figure 4). Deeper earthquakes (frequently at depths between 7 and 18 km) were detected in that region and were attributed to displacements along a fault zone.

Table 2. Seismic swarms detected at Machín during 2010-2012. Days were counted and tallied based on whether one or more swarms occurred. For example, during January-February 2010 there were six swarms recorded. Courtesy of INGEOMINAS.

Local residents felt shaking from earthquakes in September 2011 when six occurred with magnitudes greater than 2.5. INGEOMINAS reported that this month had the largest combined free-energy release that year. The largest magnitude event of that group was an M 3.6 volcano-tectonic (VT) earthquake detected at 2013 on 12 September. The average depth of the earthquakes was 4.5 km with some events as deep as 13 km. Epicenters were primarily clustered in the area of Moralito (near the MORA seismic station, see figure 9).

Figure 9. During September 2011, several moderate-sized earthquakes were located in an area SE of Cerro Machín. Seismic stations are labeled and located at purple squares. Note that the summit of Machín sits ~4 km NW of the clustered earthquakes, near the CIMA seismic station. Courtesy of INGEOMINAS.

In December 2011, INGEOMINAS reported that rockfall-type seismic signals were detected within the area. A total of 19 signatures were counted on 11 December; some events had durations up to 73 seconds. The largest earthquake that month was an M 2.32 that occurred at 0542 on 1 December.

Seismicity from January to November 2012. Rockfall-type signatures were also recorded in January 2012. These events occurred on 10 January at 1556 and lasted up to 64 seconds. As frequently observed during previous months, VT earthquakes tended to occur beneath the dome, S, and SE in the area of Moralito.

From January to August 2012, seismic swarms occurred intermittently (table 2). Elevated seismicity occurred during April 2012 and was felt by local residents. During this time period, the largest earthquake was an M 2.8 VT detected on 11 April at 0655. In April, VT earthquakes clustered ~1 km S of the dome complex and were ~4 km deep.

During May-August 2012, earthquakes were rarely clustered and occurred at a wide range of depths (0-16 km). In August, several earthquakes were located ~8 km SE of the CIMA station at depths between 12-15 km. The largest earthquake that month was an M 1.45 detected at 2026 on 9 August.

During September-October, seismic swarms occurred on six days (table 2). Local residents in the municipalities of Cajamarca and Ibagué (locations appear in figure 2 of BGVN 36:04) as well as the nearby departments of Quindio, Risaralda, and Caldas reported shaking due to these earthquakes (locations of these districts appear in the regional map of figure 5 in this report). These events were clustered beneath the dome complex at depths between 2 and 5 km. In October, however, relatively large earthquakes were detected in an area ~8 km SE of the dome at depths around 13 km. The largest earthquakes were on 9 September (M 3.6) and on 7 October (M 4.6) prompting INGEOMINAS staff to visit residences and investigate the impact of the events (figure 10). The M 4.6 earthquake was one of several located SE of the dome (near the TAPI seismic station, see figure 9).

Figure 10. A visit to areas around Machín by INGEOMINAS staff in order to evaluate the possible damage from seismic unrest that was detected on 7 October 2012. Courtesy of INGEOMINAS.

In November, INGEOMINAS reported that VT earthquakes continued to occur beneath the dome although at a reduced rate compared to October. Earthquakes tended to occur 2-5 km beneath the dome, and deeper events were detected to the SE at depths between 9 and 15 km. The largest earthquake detected was an M 2.8 on 20 November at 1754. This earthquake was located at a depth of 2.75 km and was ~2 km SW of the dome complex.

Geologic Background. The small Cerro Machín stratovolcano lies at the southern end of the Ruiz-Tolima massif about 20 km WNW of the city of Ibagué. A 3-km-wide caldera is breached to the south and contains three forested dacitic lava domes. Voluminous pyroclastic flows traveled up to 40 km away during eruptions in the mid-to-late Holocene, perhaps associated with formation of the caldera. Late-Holocene eruptions produced dacitic block-and-ash flows that traveled through the breach in the caldera rim to the west and south. The latest known eruption of took place about 800 years ago.

Information Contacts: Instituto Colombiano de Geologia y Mineria (INGEOMINAS), Observatorio Vulcanológico y Sismológico de Manizales, Manizales, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html).

During April 2010-June 2012 the Japan Meteorological Agency (JMA) maintained the hazard status for Miyake-jima at Alert Level 2, where it had stood since 31 March 2008. Our last report (BGVN 34:06) mentioned a minor eruption at Miyake-jima on 1 April 2009 which produced an ash plume that rose ~600 m above the crater. Since that time, activity was relatively low with up to four minor eruptions occurring during April-July 2010, as reported by the Tokyo Volcanic Ash Advisory Center (VAAC) based on information from JMA.

Eruptions occurred on 11 April, 4 July (two possible eruptions during the early morning), and 21 July 2010; the 21 July eruption was the only eruption for which the Tokyo VAAC issued an altitude and drift direction for the plume (~1.2 km altitude with E drift; table 5). The eruptions were characterized by gas and steam emissions lacking significant ash content (e.g. figure 23).

Table 5. Summary of detailed activity reports for Miyakejima during April 2010-June 2012; '--' indicates data not reported. Courtesy of JMA and Tokyo VAAC.

Figure 23. A S-looking photograph of Miyake-jima's crater taken from a flight on 17 March 2011 showing an apparent small gas-and-steam emission. Miyake Jima Airport is located along the coast, just out of view to the E. Courtesy of Flickr user R. Forrest.

JMA reported low levels of seismicity centered just beneath the crater during the reporting interval. Occasional episodes of volcanic tremor occurred, but were not correlated with other data indicating emissions or eruptions (table 5). Sources in Miyakemura village reported that high SO₂ concentrations were occasionally detected in some inhabited flank areas.

GPS data revealed contraction in some parts of the edifice, a process that, although diminishing, had continued since 2000. Over the same time period, long-term extension of the baseline along the N-S section of Miyake-jima had been observed since 2006, indicating inflation in deeper portions of the volcano.

Geologic Background. The circular, 8-km-wide island of Miyakejima forms a low-angle stratovolcano that rises about 1,100 m from the sea floor in the northern Izu Islands about 200 km SSW of Tokyo. The basaltic volcano is truncated by small summit calderas, one of which, 3.5 km wide, was formed during a major eruption about 2,500 years ago. Numerous craters and vents, including maars near the coast and radially oriented fissure vents, are present on the flanks. Frequent eruptions have been recorded since 1085 CE at vents ranging from the summit to below sea level, causing much damage on this small populated island. After a three-century-long hiatus ending in 1469 CE, activity has been dominated by flank fissure eruptions sometimes accompanied by minor summit eruptions. A 1.6-km-wide summit crater was slowly formed by subsidence during an eruption in 2000.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/).

Our most recent report on Tangkubanparahu (also known as Tangkuban Perahu) described increased seismicity during April 2005, consisting primarily of volcanic earthquakes and tremor (BGVN 30:12). This report describes elevated seismicity during August-September 2012. The Center of Volcanology and Geological Hazard Mitigation (CVGHM) notes that at least three magmatic eruptions and four phreatic eruptions had occurred at Ratu Crater, the most active vent, during 1829-1994. Ratu Crater is about 30 km N of Bandung in W Java. Figure 1 indicates the general location of the volcano.

Figure 1. Sketch maps of Indonesia and W Java indicating the location of Tangkubanparahu (Tangkuban Perahu). Courtesy of Kartadinata and others (2002).

The next report we received on Tangkubanparahu described activity starting in August 2012. According to CVGHM, the frequency of earthquakes and tremor increased on both 13 and 23 August. Around this time, hot blasts of sulfuric gases, white in color, rose from Ratu Crater to heights of 50-400 m above the crater's floor. CVGHM reported that the temperature of emissions from Ratu Crater on 24 August was 246°C, compared to a measurement of 111°C on 18 August. On 23 August, the Alert Level was raised to 2 (on a scale of 1-4), and visitors and residents were prohibited within a 1.5-km radius of the active crater.

Seismic activity declined on 23 August; shallow volcanic earthquakes continued to be recorded but were less frequent through 21 September (table 2 provides data through 20 September). Hypocenters of volcanic tremors during this period were located beneath an area W of Ratu Crater at depths of 4-12 km. Soil temperatures at Ratu Crater were 30.5°C on 26 August, then were 35°C on 30 August, but then gradually declined during 31 August-21 September to ~34°C.

Table 2. Type and occurrence of earthquakes at Tangkubanparahu between 24 August and 20 September 2012. Courtesy of CVGHM.

Between 5-11 September, sulfur dioxide gas emissions were elevated in an area NW of the crater associated with the plume, but in the latter part of September 2012 concentrations averaged4in Ratu Crater increased from 0.11 in December 2011 to ~4 on 24 August 2012 and remained at that level on 11 September 2012, which suggested to CVGHM that hot fluid was rising to the surface.

Based on seismicity, visual observations, deformation data, gas measurements, and soil and crater lake water temperatures, the Alert Level was lowered to 1 on 21 September 2012.

The eruptive history of Tangkubanparahu was described by Kartadinata and others (2002).

Reference. Kartadinata, M., Okuno, M., Nakamura, T., and Kobayashi, T., 2002, Eruptive history of Tangkuban Perahu volcano, West Java, Indonesia: A preliminary report, Journal of Geography, v. 111, issue 3, p. 404-409.

Geologic Background. Gunung Tangkuban Parahu is a broad stratovolcano overlooking Indonesia's former capital city of Bandung. The volcano was constructed within the 6 x 8 km Pleistocene Sunda caldera, which formed about 190,000 years ago. The volcano's low profile is the subject of legends referring to the mountain of the "upturned boat." The Sunda caldera rim forms a prominent ridge on the western side; elsewhere the rim is largely buried by deposits of the current volcano. The dominantly small phreatic eruptions recorded since the 19th century have originated from several nested craters within an elliptical 1 x 1.5 km summit depression.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).