Manam is a 10-km-wide island that consists of two active summit craters: the Main summit crater and the South summit crater and is located 13 km off the northern coast of mainland Papua New Guinea. Frequent mild-to-moderate eruptions have been recorded since 1616. The current eruption period began during June 2014 and has more recently been characterized by intermittent ash plumes and thermal activity (BGVN 47:11). This report updates activity that occurred from November 2022 through May 2023 based on information from the Darwin Volcanic Ash Advisory Center (VAAC) and various satellite data.

Ash plumes were reported during November and December 2022 by the Darwin VAAC. On 7 November an ash plume rose to 2.1 km altitude and drifted NE based on satellite images and weather models. On 14 November an ash plume rose to 2.1 km altitude and drifted W based on RVO webcam images. On 20 November ash plumes rose to 1.8 km altitude and drifted NW. On 26 December an ash plume rose to 3 km altitude and drifted S and SSE.

Intermittent sulfur dioxide plumes were detected using the TROPOMI instrument on the Sentinel-5P satellite, some of which exceeded at least two Dobson Units (DU) and drifted in different directions (figure 93). Occasional low-to-moderate power thermal anomalies were recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system; less than five anomalies were recorded each month during November 2022 through May 2023 (figure 94). Two thermal hotspots were detected by the MODVOLC thermal alerts system on 10 December 2022. On clear weather days, thermal activity was also captured in infrared satellite imagery in both the Main and South summit craters, accompanied by gas-and-steam emissions (figure 95).

Figure 93. Distinct sulfur dioxide plumes were captured, rising from Manam based on data from the TROPOMI instrument on the Sentinel-5P satellite on 16 November 2022 (top left), 6 December 2022 (top right), 14 January 2023 (bottom left), and 23 March 2023 (bottom right). Plumes generally drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 94. Occasional low-to-moderate power thermal anomalies were detected at Manam during November 2022 through May 2023, as shown in this MIROVA graph (Log Radiative Power). Only three anomalies were detected during late November, one in early December, two during January 2023, one in late March, four during April, and one during late May. Courtesy of MIROVA.

Figure 95. Infrared (bands B12, B11, B4) satellite images show a consistent thermal anomaly (bright yellow-orange) in both the Main (the northern crater) and South summit craters on 10 November 2022 (top left), 15 December 2022 (top right), 3 February 2023 (bottom left), and 24 April 2023 (bottom right). Gas-and-steam emissions occasionally accompanied the thermal activity. Courtesy of Copernicus Browser.

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

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers activity during November 2022 through April 2023 based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

Activity was relatively low during November and December 2022. Daily white gas-and-steam plumes rose 25-100 m above the summit and drifted in different directions. Gray ash plumes rose 200 m above the summit and drifted NE at 1047 and at 2343 on 11 November. On 14 November at 0933 ash plumes rose 300 m above the summit and drifted E. An ash plume was reported at 0935 on 15 December that rose 100 m above the summit and drifted NE. An eruptive event at 1031 later that day generated an ash plume that rose 700 m above the summit and drifted NE. A gray ash plume at 1910 rose 100 m above the summit and drifted E. Incandescent material was ejected above the vent based on an image taken at 1936.

During January 2023 daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions. Gray-to-brown ash plumes were reported at 1638 on 3 January, at 1410 and 1509 on 4 January, and at 0013 on 5 January that rose 100-750 m above the summit and drifted NE and E; the gray-to-black ash plume at 1509 on 4 January rose as high as 3 km above the summit and drifted E. Gray ash plumes were recorded at 1754, 2241, and 2325 on 11 January and at 0046 on 12 January and rose 200-300 m above the summit and drifted NE. Toward the end of January, PVMBG reported that activity had intensified; Strombolian activity was visible in webcam images taken at 0041, 0043, and 0450 on 23 January. Multiple gray ash plumes throughout the day rose 200-500 m above the summit and drifted E and SE (figure 135). Webcam images showed progressively intensifying Strombolian activity at 1919, 1958, and 2113 on 24 January; a gray ash plume at 1957 rose 300 m above the summit and drifted E (figure 135). Eruptive events at 0231 and 2256 on 25 January and at 0003 on 26 January ejected incandescent material from the vent, based on webcam images. Gray ash plumes observed during 26-27 January rose 300-500 m above the summit and drifted NE, E, and SE.

Figure 135. Webcam images of a strong, gray ash plume (left) and Strombolian activity (right) captured at Krakatau at 0802 on 23 January 2023 (left) and at 2116 on 24 January 2023 (right). Courtesy of PVMBG and MAGMA Indonesia.

Low levels of activity were reported during February and March. Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in different directions. The Darwin VAAC reported that continuous ash emissions rose to 1.5-1.8 km altitude and drifted W and NW during 1240-1300 on 10 March, based on satellite images, weather models, and PVMBG webcams. White-and-gray ash plumes rose 500 m and 300 m above the summit and drifted SW at 1446 and 1846 on 18 March, respectively. An eruptive event was recorded at 2143, though it was not visible due to darkness. Multiple ash plumes were reported during 27-29 March that rose as high as 2.5 km above the summit and drifted NE, W, and SW (figure 136). Webcam images captured incandescent ejecta above the vent at 0415 and around the summit area at 2003 on 28 March and at 0047 above the vent on 29 March.

Figure 136. Webcam image of a strong ash plume rising above Krakatau at 1522 on 28 March 2023. Courtesy of PVMBG and MAGMA Indonesia.

Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions during April and May. White-and-gray and black plumes rose 50-300 m above the summit on 2 and 9 April. On 11 May at 1241 a gray ash plume rose 1-3 km above the summit and drifted SW. On 12 May at 0920 a gray ash plume rose 2.5 km above the summit and drifted SW and at 2320 an ash plume rose 1.5 km above the summit and drifted SW. An accompanying webcam image showed incandescent ejecta. On 13 May at 0710 a gray ash plume rose 2 km above the summit and drifted SW (figure 137).

Figure 137. Webcam image of an ash plume rising 2 km above the summit of Krakatau at 0715 on 13 May 2023. Courtesy of PVMBG and MAGMA Indonesia.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during November 2022 through April 2023 (figure 138). Some of this thermal activity was also visible in infrared satellite imagery at the crater, accompanied by gas-and-steam and ash plumes that drifted in different directions (figure 139).

Figure 138. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during November 2022 through April 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 139. A thermal anomaly (bright yellow-orange) was visible at Krakatau in infrared (bands B12, B11, B4) satellite images on clear weather days during November 2022 through May 2023. Occasional gas-and-steam and ash plumes accompanied the thermal activity, which drifted in different directions. Images were captured on 25 November 2022 (top left), 15 December 2022 (top right), 27 January 2023 (bottom left), and 12 May 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The renowned Krakatau (frequently mis-named as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 350 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of frequent Strombolian explosions and lava flows (BGVN 48:02). This report updates activity during January through April 2023 primarily characterized by Strombolian explosions and lava flows based on reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 253, table 16). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 254). According to data collected by the MODVOLC thermal algorithm, a total of 9 thermal alerts were detected: one on 2 January 2023, one on 1 February, five on 24 March, and two on 26 March. The stronger pulses of thermal activity likely reflected lava flow events. Infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 8 March (figure 255).

Figure 253. Explosive activity persisted at Stromboli during January through April 2023, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during January through April (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 17/2023, Stromboli, Bollettino Settimanale, 18/04/2023 - 24/04/2023).

Table 16. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during January-April 2023; information from webcam observations. Courtesy of INGV weekly reports.

Figure 254. Intermittent thermal activity at Stromboli was detected during January through April 2023 and varied in strength, as shown in this MIROVA graph (Log Radiative Power). A pulse of activity was captured during late March. Courtesy of MIROVA.

Figure 255. Infrared (bands B12, B11, B4) satellite images showing persistent thermal anomalies at both summit crater on 1 February 2023 (top left), 23 March 2023 (top right), 8 March 2023 (bottom left), and 27 April 2023. A particularly strong thermal anomaly was visible on 8 March. Courtesy of Copernicus Browser.

Activity during January-February 2023. Strombolian explosions were reported in the N crater area, as well as lava effusion. Explosive activity in the N crater area ejected coarse material (bombs and lapilli). Intense spattering was observed in both the N1 and N2 craters. In the CS crater area, explosions generally ejected fine material (ash), sometimes to heights greater than 250 m. The intensity of the explosions was characterized as low-to-medium in the N crater and medium-to-high in the CS crater. After intense spattering activity from the N crater area, a lava overflow began at 2136 on 2 January that flowed part way down the Sciara del Fuoco, possibly moving down the drainage that formed in October, out of view from webcams. The flow remained active for a couple of hours before stopping and beginning to cool. A second lava flow was reported at 0224 on 4 January that similarly remained active for a few hours before stopping and cooling. Intense spattering was observed on 11 and 13 January from the N1 crater. After intense spattering activity at the N2 crater at 1052 on 17 January another lava flow started to flow into the upper part of the Sciara del Fuoco (figure 256), dividing into two: one that traveled in the direction of the drainage formed in October, and the other one moving parallel to the point of emission. By the afternoon, the rate of the flow began to decrease, and at 1900 it started to cool. A lava flow was reported at 1519 on 24 January following intense spattering in the N2 area, which began to flow into the upper part of the Sciara del Fuoco. By the morning of 25 January, the lava flow had begun to cool. During 27 January the frequency of eruption in the CS crater area increased to 6-7 events/hour compared to the typical 1-7 events/hour; the following two days showed a decrease in frequency to less than 1 event/hour. Starting at 1007 on 30 January a high-energy explosive sequence was produced by vents in the CS crater area. The sequence began with an initial energetic pulse that lasted 45 seconds, ejecting predominantly coarse products 300 m above the crater that fell in an ESE direction. Subsequent and less intense explosions ejected material 100 m above the crater. The total duration of this event lasted approximately two minutes. During 31 January through 6, 13, and 24 February spattering activity was particularly intense for short periods in the N2 crater.

Figure 256. Webcam images of the lava flow development at Stromboli during 17 January 2023 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 04/2023, Stromboli, Bollettino Settimanale, 16/01/2023 - 22/01/2023).

An explosive sequence was reported on 16 February that was characterized by a major explosion in the CS crater area (figure 257). The sequence began at 1817 near the S2 crater that ejected material radially. A few seconds later, lava fountains were observed in the central part of the crater. Three explosions of medium intensity (material was ejected less than 150 m high) were recorded at the S2 crater. The first part of this sequence lasted approximately one minute, according to INGV, and material rose 300 m above the crater and then was deposited along the Sciara del Fuoco. The second phase began at 1818 at the S1 crater; it lasted seven seconds and material was ejected 150 m above the crater. Another event 20 seconds later lasted 12 seconds, also ejecting material 150 m above the crater. The sequence ended with at least three explosions of mostly fine material from the S1 crater. The total duration of this sequence was about two minutes.

Figure 257. Webcam images of the explosive sequence at Stromboli on 16 February 2023 taken by the SCT and SCV infrared and visible cameras. The lava appears light yellow-green in the infrared images. Courtesy of INGV (Report 08/2023, Stromboli, Bollettino Settimanale, 13/02/2023 - 19/02/2023).

Short, intense spattering activity was noted above the N1 crater on 27 and 28 February. A lava overflow was first reported at 0657 from the N2 crater on 27 February that flowed into the October 2022 drainage. By 1900 the flow had stopped. A second lava overflow also in the N crater area occurred at 2149, which overlapped the first flow and then stopped by 0150 on 28 February. Material detached from both the lava overflows rolled down the Sciara del Fuoco, some of which was visible in webcam images.

Activity during March-April 2023. Strombolian activity continued with spattering activity and lava overflows in the N crater area during March. Explosive activity at the N crater area varied from low (less than 80 m high) to medium (less than 150 m high) and ejected coarse material, such as bombs and lapilli. Spattering was observed above the N1 crater, while explosive activity at the CS crater area varied from medium to high (greater than 150 m high) and ejected coarse material. Intense spattering activity was observed for short periods on 6 March above the N1 crater. At approximately 0610 a lava overflow was reported around the N2 crater on 8 March, which then flowed into the October 2022 drainage. By 1700 the flow started to cool. A second overflow began at 1712 on 9 March and overlapped the previous flow. It had stopped by 2100. Material from both flows was deposited along the Sciara del Fuoco, though much of the activity was not visible in webcam images. On 11 March a lava overflow was observed at 0215 that overlapped the two previous flows in the October 2022 drainage. By late afternoon on 12 March, it had stopped.

During a field excursion on 16 March, scientists noted that a vent in the central crater area was degassing. Another vent showed occasional Strombolian activity that emitted ash and lapilli. During 1200-1430 low-to-medium intense activity was reported; the N1 crater emitted ash emissions and the N2 crater emitted both ash and coarse material. Some explosions also occurred in the CS crater area that ejected coarse material. The C crater in the CS crater area occasionally showed gas jetting and low intensity explosions on 17 and 22 March; no activity was observed at the S1 crater. Intense, longer periods of spattering were reported in the N1 crater on 19, 24, and 25 March. Around 2242 on 23 March a lava overflow began from the N1 crater that, after about an hour, began moving down the October 2022 drainage and flow along the Sciara del Fuoco (figure 258). Between 0200 and 0400 on 26 March the flow rate increased, which generated avalanches of material from collapses at the advancing flow front. By early afternoon, the flow began to cool. On 25 March at 1548 an explosive sequence began from one of the vents at S2 in the CS crater area (figure 258). Fine ash mixed with coarse material was ejected 300 m above the crater rim and drifted SSE. Some modest explosions around Vent C were detected at 1549 on 25 March, which included an explosion at 1551 that ejected coarse material. The entire explosive sequence lasted approximately three minutes.

Figure 258. Webcam images of the lava overflow in the N1 crater area of Stromboli on 23 March 2023 taken by the SCT infrared camera. The lava appears light yellow-green in the infrared images. The start of the explosive sequence was also captured on 25 March 2023 accompanied by an eruption plume (e) captured by the SCT and SPT infrared webcams. Courtesy of INGV (Report 13/2023, Stromboli, Bollettino Settimanale, 20/03/2023 - 26/03/2023).

During April explosions persisted in both the N and CS crater areas. Fine material was ejected less than 80 m above the N crater rim until 6 April, followed by ejection of coarser material. Fine material was also ejected less than 80 m above the CS crater rim. The C and S2 crater did not show significant eruptive activity. On 7 April an explosive sequence was detected in the CS crater area at 1203 (figure 259). The first explosion lasted approximately 18 seconds and ejected material 400 m above the crater rim, depositing pyroclastic material in the upper part of the Sciara del Fuoco. At 1204 a second, less intense explosion lasted approximately four seconds and deposited pyroclastic products outside the crater area and near Pizzo Sopra La Fossa. A third explosion at 1205 was mainly composed of ash that rose about 150 m above the crater and lasted roughly 20 seconds. A fourth explosion occurred at 1205 about 28 seconds after the third explosion and ejected a mixture of coarse and fine material about 200 m above the crater; the explosion lasted roughly seven seconds. Overall, the entire explosive sequence lasted about two minutes and 20 seconds. After the explosive sequence on 7 April, explosions in both the N and CS crater areas ejected material as high as 150 m above the crater.

Figure 259. Webcam images of the explosive sequence at Stromboli during 1203-1205 (local time) on 7 April 2023 taken by the SCT infrared camera. Strong eruption plumes are visible, accompanied by deposits on the nearby flanks. Courtesy of INGV (Report 15/2023, Stromboli, Bollettino Settimanale, 03/04/2023 - 09/04/2023).

On 21 April research scientists from INGV made field observations in the summit area of Stromboli, and some lapilli samples were collected. In the N crater area near the N1 crater, a small cone was observed with at least two active vents, one of which was characterized by Strombolian explosions. The other vent produced explosions that ejected ash and chunks of cooled lava. At the N2 crater at least one vent was active and frequently emitted ash. In the CS crater area, a small cone contained 2-3 degassing vents and a smaller, possible fissure area also showed signs of degassing close to the Pizzo Sopra La Fossa. In the S part of the crater, three vents were active: a small hornito was characterized by modest and rare explosions, a vent that intermittently produced weak Strombolian explosions, and a vent at the end of the terrace that produced frequent ash emissions. Near the S1 crater there was a hornito that generally emitted weak gas-and-steam emissions, sometimes associated with “gas rings”. On 22 April another field inspection was carried out that reported two large sliding surfaces on the Sciara del Fuoco that showed where blocks frequently descended toward the sea. A thermal anomaly was detected at 0150 on 29 April.

Geologic Background. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Nishinoshima is a small island located about 1,000 km S of Tokyo in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. Eruptions date back to 1973; the most recent eruption period began in October 2022 and was characterized by ash plumes and fumarolic activity (BGVN 47:12). This report describes ash plumes and fumarolic activity during November 2022 through April 2023 based on monthly reports from the Japan Meteorological Agency (JMA) monthly reports and satellite data.

The most recent eruptive activity prior to the reporting internal occurred on 12 October 2022, when an ash plume rose 3.5 km above the crater rim. An aerial observation conducted by the Japan Coast Guard (JCG) on 25 November reported that white fumaroles rose approximately 200 m above the central crater of a pyroclastic cone (figure 119), and multiple plumes were observed on the ESE flank of the cone. Discolored water ranging from reddish-brown to brown and yellowish-green were visible around the perimeter of the island (figure 119). No significant activity was reported in December.

Figure 119. Aerial photo of gas-and-steam plumes rising 200 m above Nishinoshima on 25 November 2022. Reddish brown to brown and yellowish-green discolored water was visible around the perimeter of the island. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, November 2022).

During an overflight conducted by JCG on 25 January 2023 intermittent activity and small, blackish-gray plumes rose 900 m above the central part of the crater were observed (figure 120). The fumarolic zone of the E flank and base of the cone had expanded and emissions had intensified. Dark brown discolored water was visible around the perimeter of the island.

Figure 120. Aerial photo of a black-gray ash plume rising approximately 900 m above the crater rim of Nishinoshima on 25 January 2023. White fumaroles were visible on the E slope of the pyroclastic cone. Dense brown to brown discolored water was observed surrounding the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, January, 2023).

No significant activity was reported during February through March. Ash plumes at 1050 and 1420 on 11 April rose 1.9 km above the crater rim and drifted NW and N. These were the first ash plumes observed since 12 October 2022. On 14 April JCG carried out an overflight and reported that no further eruptive activity was visible, although white gas-and-steam plumes were visible from the central crater and rose 900 m high (figure 121). Brownish and yellow-green discolored water surrounded the island.

Figure 121. Aerial photo of white gas-and-steam plumes rising 900 m above Nishinoshima on 14 April 2023. Brown and yellow-green discolored water is visible around the perimeter of the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, April, 2023).

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during November 2022 through April 2023 (figure 123). A cluster of six to eight anomalies were detected during November while a smaller number were detected during the following months: two to three during December, one during mid-January 2023, one during February, five during March, and two during April. Thermal activity was also reflected in infrared satellite data at the summit crater, accompanied by occasional gas-and-steam plumes (figure 124).

Figure 123. Intermittent low-to-moderate thermal anomalies were detected at Nishinoshima during November 2022 through April 2023, according to this MIROVA graph (Log Radiative Power). A cluster of anomalies occurred throughout November, while fewer anomalies were detected during the following months. Courtesy of MIROVA.

Figure 124. Infrared (bands B12, B11, B4) satellite images show a small thermal anomaly at the summit crater of Nishinoshima on 9 January 2023 (left) and 8 February 2023 (right). Gas-and-steam plumes accompanied this activity and extended S and SE, respectively. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Karangetang (also known as Api Siau), at the northern end of the island of Siau, Indonesia, contains five summit craters along a N-S line. More than 40 eruptions have been recorded since 1675; recent eruptions have included frequent explosive activity, sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters and collapses of lava flow fronts have produced pyroclastic flows. The two active summit craters are Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). The most recent eruption began in late November 2018 and has more recently consisted of weak thermal activity and gas-and-steam emissions (BGVN 48:01). This report updates activity characterized by lava flows, incandescent avalanches, and ash plumes during January through June 2023 using reports from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin VAAC (Volcano Ash Advisory Center), and satellite data.

Activity during January was relatively low and mainly consisted of white gas-and-steam emissions that rose 25-150 m above Main Crater (S crater) and drifted in different directions. Incandescence was visible from the lava dome in Kawah Dua (the N crater). Weather conditions often prevented clear views of the summit. On 18 January the number of seismic signals that indicated avalanches of material began to increase. In addition, there were a total of 71 earthquakes detected during the month.

Activity continued to increase during the first week of February. Material from Main Crater traveled as far as 800 m down the Batuawang (S) and Batang (W) drainages and as far as 1 km W down the Beha (W) drainage on 4 February. On 6 February 43 earthquake events were recorded, and on 7 February, 62 events were recorded. White gas-and-steam emissions rose 25-250 m above both summit craters throughout the month. PVMBG reported an eruption began during the evening of 8 February around 1700. Photos showed incandescent material at Main Crater. Incandescent material had also descended the flank in at least two unconfirmed directions as far as 2 km from Main Crater, accompanied by ash plumes (figure 60). As a result, PVMBG increased the Volcano Alert Level (VAL) to 3 (the second highest level on a 1-4 scale).

Figure 60. Photos of the eruption at Karangetang on 8 February 2023 that consisted of incandescent material descending the flanks (top left), ash plumes (top right and bottom left), and summit crater incandescence (bottom right). Courtesy of IDN Times.

Occasional nighttime webcam images showed three main incandescent lava flows of differing lengths traveling down the S, SW, and W flanks (figure 61). Incandescent rocks were visible on the upper flanks, possibly from ejected or collapsed material from the crater, and incandescence was the most intense at the summit. Based on analyses of satellite imagery and weather models, the Darwin VAAC reported that daily ash plumes during 16-20 February rose to 2.1-3 km altitude and drifted NNE, E, and SE. BNPB reported on 16 February that as many as 77 people were evacuated and relocated to the East Siau Museum. A webcam image taken at 2156 on 17 February possibly showed incandescent material descending the SE flank. Ash plumes rose to 2.1 km altitude and drifted SE during 22-23 February, according to the Darwin VAAC.

Figure 61. Webcam image of summit incandescence and lava flows descending the S, SW, and W flanks of Karangetang on 13 February 2023. Courtesy of MAGMA Indonesia.

Incandescent avalanches of material and summit incandescence at Main Crater continued during March. White gas-and-steam emissions during March generally rose 25-150 m above the summit crater; on 31 March gas-and-steam emissions rose 200-400 m high. An ash plume rose to 2.4 km altitude and drifted S at 1710 on 9 March and a large thermal anomaly was visible in images taken at 0550 and 0930 on 10 March. Incandescent material was visible at the summit and on the flanks based on webcam images taken at 0007 and 2345 on 16 March, at 1828 on 17 March, at 1940 on 18 March, at 2311 on 19 March, and at 2351 on 20 March. Incandescence was most intense on 18 and 20 March and webcam images showed possible Strombolian explosions (figure 62). An ash plume rose to 2.4 km altitude and drifted SW on 18 March, accompanied by a thermal anomaly.

Figure 62. Webcam image of intense summit incandescence and incandescent avalanches descending the flanks of Karangetang on 18 March 2023. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Summit crater incandescence at Main Crater and on the flanks persisted during April. Incandescent material at the S crater and on the flanks was reported at 0016 on 1 April. The lava flows had stopped by 1 April according to PVMBG, although incandescence was still visible up to 10 m high. Seismic signals indicating effusion decreased and by 6 April they were no longer detected. Incandescence was visible from both summit craters. On 26 April the VAL was lowered to 2 (the second lowest level on a 1-4 scale). White gas-and-steam emissions rose 25-200 m above the summit crater.

During May white gas-and-steam emissions generally rose 50-250 m above the summit, though it was often cloudy, which prevented clear views; on 21 May gas-and-steam emissions rose 50-400 m high. Nighttime N summit crater incandescence rose 10-25 m above the lava dome, and less intense incandescence was noted above Main Crater, which reached about 10 m above the dome. Sounds of falling rocks at Main Crater were heard on 15 May and the seismic network recorded 32 rockfall events in the crater on 17 May. Avalanches traveled as far as 1.5 km down the SW and S flanks, accompanied by rumbling sounds on 18 May. Incandescent material descending the flanks was captured in a webcam image at 2025 on 19 May (figure 63) and on 29 May; summit crater incandescence was observed in webcam images at 2332 on 26 May and at 2304 on 29 May. On 19 May the VAL was again raised to 3.

Figure 63. Webcam image showing incandescent material descending the flanks of Karangetang on 19 May 2023. Courtesy of MAGMA Indonesia.

Occasional Main Crater incandescence was reported during June, as well as incandescent material on the flanks. White gas-and-steam emissions rose 10-200 m above the summit crater. Ash plumes rose to 2.1 km altitude and drifted SE and E during 2-4 June, according to the Darwin VAAC. Material on the flanks of Main Crater were observed at 2225 on 7 June, at 2051 on 9 June, at 0007 on 17 June, and at 0440 on 18 June. Webcam images taken on 21, 25, and 27 June showed incandescence at Main Crater and from material on the flanks.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed strong thermal activity during mid-February through March and mid-May through June, which represented incandescent avalanches and lava flows (figure 64). During April through mid-May the power of the anomalies decreased but frequent anomalies were still detected. Brief gaps in activity occurred during late March through early April and during mid-June. Infrared satellite images showed strong lava flows mainly affecting the SW and S flanks, accompanied by gas-and-steam emissions (figure 65). According to data recorded by the MODVOLC thermal algorithm, there were a total of 79 thermal hotspots detected: 28 during February, 24 during March, one during April, five during May, and 21 during June.

Figure 64. Strong thermal activity was detected during mid-February 2023 through March and mid-May through June at Karangetang during January through June 2023, as recorded by this MIROVA graph (Log Radiative Power). During April through mid-May the power of the anomalies decreased, but the frequency at which they occurred was still relatively high. A brief gap in activity was shown during mid-June. Courtesy of MIROVA.

Figure 65. Incandescent avalanches of material and summit crater incandescence was visible in infrared satellite images (bands 12, 11, 8A) at both the N and S summit crater of Karangetang on 17 February 2023 (top left), 13 April 2023 (top right), 28 May 2023 (bottom left), and 7 June 2023 (bottom right), as shown in these infrared (bands 12, 11, 8A) satellite images. The incandescent avalanches mainly affected the SW and S flanks. Sometimes gas-and-steam plumes accompanied the thermal activity. Courtesy of Copernicus Browser.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); IDN Times, Jl. Jend. Gatot Subroto Kav. 27 3rd Floor Kuningan, Jakarta, Indonesia 12950, Status of Karangetang Volcano in Sitaro Islands Increases (URL: https://sulsel.idntimes.com/news/indonesia/savi/status-gunung-api-karangetang-di-kepulauan-sitaro-meningkat?page=all).

Ahyi seamount is a large, conical submarine volcano that rises to within 75 m of the ocean surface about 18 km SE of the island of Farallon de Pajaros in the Northern Marianas. The remote location of the seamount has made eruptions difficult to document, but seismic stations installed in the region confirmed an eruption in the vicinity in 2001. No new activity was detected until April-May 2014 when an eruption was detected by NOAA (National Oceanic and Atmospheric Administration) divers, hydroacoustic sensors, and seismic stations (BGVN 42:04). New activity was first detected on 15 November by hydroacoustic sensors that were consistent with submarine volcanic activity. This report covers activity during November 2022 through June 2023 based on daily and weekly reports from the US Geological Survey.

Starting in mid-October, hydroacoustic sensors at Wake Island (2.2 km E) recorded signals consistent with submarine volcanic activity, according to a report from the USGS issued on 15 November 2022. A combined analysis of the hydroacoustic signals and seismic stations located at Guam and Chichijima Island, Japan, suggested that the source of this activity was at or near the Ahyi seamount. After a re-analysis of a satellite image of the area that was captured on 6 November, USGS confirmed that there was no evidence of discoloration at the ocean surface. Few hydroacoustic and seismic signals continued through November, including on 18 November, which USGS suggested signified a decline or pause in unrest. A VONA (Volcano Observatory Notice for Aviation) reported that a discolored water plume was persistently visible in satellite data starting on 18 November (figure 6). Though clouds often obscured clear views of the volcano, another discolored water plume was captured in a satellite image on 26 November. The Aviation Color Code (ACC) was raised to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level (VAL) was raised to Advisory (the second lowest level on a four-level scale) on 29 November.

Figure 6. A clear, true color satellite image showed a yellow-green discolored water plume extending NW from the Ahyi seamount (white arrow) on 21 November 2022. Courtesy of Copernicus Browser.

During December, occasional detections were recorded on the Wake Island hydrophone sensors and discolored water over the seamount remained visible. During 2-7, 10-12, and 16-31 December possible explosion signals were detected. A small area of discolored water was observed in high-resolution Sentinel-2 satellite images during 1-6 December (figure 7). High-resolution satellite images recorded discolored water plumes on 13 December that originated from the summit region; no observations indicated that activity breached the ocean surface. A possible underwater plume was visible in satellite images on 18 December, and during 19-20 December a definite but diffuse underwater plume located SSE from the main vent was reported. An underwater plume was visible in a satellite image taken on 26 December (figure 7).

Figure 7. Clear, true color satellite images showed yellow-green discolored water plumes extending NE and W from Ahyi (white arrows) on 1 (left) and 26 (right) December 2022, respectively. Courtesy of Copernicus Browser.

Hydrophone sensors continued to detect signals consistent with possible explosions during 1-8 January 2023. USGS reported that the number of detections decreased during 4-5 January. The hydrophone sensors experienced a data outage that started at 0118 on 8 January and continued through 10 January, though according to USGS, possible explosions were recorded prior to the data outage and likely continued during the outage. A discolored water plume originating from the summit region was detected in a partly cloudy satellite image on 8 January. On 11-12 and 15-17 January possible explosion signals were recorded again. One small signal was detected during 22-23 January and several signals were recorded on 25 and 31 January. During 27-31 January a plume of discolored water was observed above the seamount in satellite imagery (figure 8).

Figure 8. True color satellite images showed intermittent yellow-green discolored water plumes of various sizes extending N on 5 January 2023 (top left), SE on 30 January 2023 (top right), W on 4 February 2023 (bottom left), and SW on 1 March 2023 (bottom right) from Ahyi (white arrows). Courtesy of Copernicus Browser.

Low levels of activity continued during February and March, based on data from pressure sensors on Wake Island. During 1 and 4-6 February activity was reported, and a submarine plume was observed on 4 February (figure 8). Possible explosion signals were detected during 7-8, 10, 13-14, and 24 February. During 1-2 and 3-5 March a plume of discolored water was observed in satellite imagery (figure 8). Almost continuous hydroacoustic signals were detected in remote pressure sensor data on Wake Island 2,270 km E from the volcano during 7-13 March. During 12-13 March water discoloration around the seamount was observed in satellite imagery, despite cloudy weather. By 14 March discolored water extended about 35 km, but no direction was noted. USGS reported that the continuous hydroacoustic signals detected during 13-14 March stopped abruptly on 14 March and no new detections were observed. Three 30 second hydroacoustic detections were reported during 17-19 March, but no activity was visible due to cloudy weather. A data outage was reported during 21-22 March, making pressure sensor data unavailable; a discolored water plume was, however, visible in satellite data. A possible underwater explosion signal was detected by pressure sensors at Wake Island on 26, 29, and 31 March, though the cause and origin of these events were unclear.

Similar low activity continued during April, May, and June. Several signals were detected during 1-3 April in pressure sensors at Wake Island. USGS suggested that these may be related to underwater explosions or earthquakes at the volcano, but no underwater plumes were visible in clear satellite images. The pressure sensors had data outages during 12-13 April and no data were recorded; no underwater plumes were visible in satellite images, although cloudy weather obscured most clear views. Eruptive activity was reported starting at 2210 on 21 May. On 22 May a discolored water plume that extended 4 km was visible in satellite images, though no direction was recorded. During 23-24 May some signals were detected by the underwater pressure sensors. Possible hydroacoustic signals were detected during 2-3 and 6-8 June. Multiple hydroacoustic signals were detected during 9-11 and 16-17 June, although no activity was visible in satellite images. One hydroacoustic signal was detected during 23-24 June, but there was some uncertainty about its association with volcanic activity. A single possible hydroacoustic signal was detected during 30 June to 1 July.

Geologic Background. Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the ocean surface ~18 km SE of the island of Farallon de Pajaros in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Information Contacts: US Geological Survey, Volcano Hazards Program (USGS-VHP), 12201 Sunrise Valley Drive, Reston, VA, USA, https://volcanoes.usgs.gov/index.html; Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kadovar is a 2-km-wide island that is the emergent summit of a Bismarck Sea stratovolcano. It lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the volcano, filling an arcuate landslide scarp open to the S. Submarine debris-avalanche deposits occur to the S of the island. The current eruption began in January 2018 and has comprised lava effusion from vents at the summit and at the E coast; more recent activity has consisted of ash plumes, weak thermal activity, and gas-and-steam plumes (BGVN 48:02). This report covers activity during February through May 2023 using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

Activity during the reporting period was relatively low and mainly consisted of white gas-and-steam plumes that were visible in natural color satellite images on clear weather days (figure 67). According to a Darwin VAAC report, at 2040 on 6 May an ash plume rose to 4.6 km altitude and drifted W; by 2300 the plume had dissipated. MODIS satellite instruments using the MODVOLC thermal algorithm detected a single thermal hotspot on the SE side of the island on 7 May. Weak thermal activity was also detected in a satellite image on the E side of the island on 14 May, accompanied by a white gas-and-steam plume that drifted SE (figure 68).

Figure 67. True color satellite images showing a white gas-and-steam plume rising from Kadovar on 28 February 2023 (left) and 30 March 2023 (right) and drifting SE and S, respectively. Courtesy of Copernicus Browser.

Figure 68. Infrared (bands B12, B11, B4) image showing weak thermal activity on the E side of the island, accompanied by a gas-and-steam plume that drifted SE from Kadovar on 14 May 2023. Courtesy of Copernicus Browser.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

San Miguel in El Salvador is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century and consists of the summit known locally as Chaparrastique. Flank eruptions have produced lava flows that extended to the N, NE, and SE during the 17-19th centuries. The most recent activity has consisted of minor ash eruptions from the summit crater. The current eruption period began in November 2022 and has been characterized by frequent phreatic explosions, gas-and-ash emissions, and sulfur dioxide plumes (BGVN 47:12). This report describes small gas-and-ash explosions during December 2022 through May 2023 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

Activity has been relatively low since the last recorded explosions on 29 November 2022. Seismicity recorded by the San Miguel Volcano Station (VSM) located on the N flank at 1.7 km elevation had decreased by 7 December. Sulfur dioxide gas measurements taken with DOAS (Differential Optical Absorption Spectroscopy) mobile equipment were below typical previously recorded values: 300 tons per day (t/d). During December, small explosions were recorded by the seismic network and manifested as gas-and-steam emissions.

Gas-and-ash explosions in the crater occurred during January 2023, which were recorded by the seismic network. Sulfur dioxide values remained low, between 300-400 t/d through 10 March. At 0817 on 14 January a gas-and-ash emission was visible in webcam images, rising just above the crater rim. Some mornings during February, small gas-and-steam plumes were visible in the crater. On 7 March at 2252 MARN noted an increase in degassing from the central crater; gas emissions were constantly observed through the early morning hours on 8 March. During the early morning of 8 March through the afternoon on 9 March, 12 emissions were registered, some accompanied by ash. The last gas-and-ash emission was recorded at 1210 on 9 March; very fine ashfall was reported in El Tránsito (10 km S), La Morita (6 km W), and La Piedrita (3 km W). The smell of sulfur was reported in Piedra Azul (5 km SW). On 16 March MARN reported that gas-and-steam emissions decreased.

Low degassing and very low seismicity were reported during April; no explosions have been detected between 9 March and 27 May. The sulfur dioxide emissions remained between 350-400 t/d; during 13-20 April sulfur dioxide values fluctuated between 30-300 t/d. Activity remained low through most of May; on 23 May seismicity increased. An explosion was detected at 1647 on 27 May generated a gas-and-ash plume that rose 700 m high (figure 32); a decrease in seismicity and gas emissions followed. The DOAS station installed on the W flank recorded sulfur dioxide values that reached 400 t/d on 27 May; subsequent measurements showed a decrease to 268 t/d on 28 May and 100 t/d on 29 May.

Figure 32. Webcam image of a gas-and-ash plume rising 700 m above San Miguel at 1652 on 27 May 2023. Courtesy of MARN.

Geologic Background. The symmetrical cone of San Miguel, one of the most active volcanoes in El Salvador, rises from near sea level to form one of the country's most prominent landmarks. A broad, deep, crater complex that has been frequently modified by eruptions recorded since the early 16th century caps the truncated unvegetated summit, also known locally as Chaparrastique. Flanks eruptions of the basaltic-andesitic volcano have produced many lava flows, including several during the 17th-19th centuries that extended to the N, NE, and SE. The SE-flank flows are the largest and form broad, sparsely vegetated lava fields crossed by highways and a railroad skirting the base of the volcano. Flank vent locations have migrated higher on the edifice during historical time, and the most recent activity has consisted of minor ash eruptions from the summit crater.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia).

Semisopochnoi is located in the western Aleutians, is 20-km-wide at sea level, and contains an 8-km-wide caldera. The three-peaked Mount Young (formerly Cerberus) was constructed within the caldera during the Holocene. Each of these peaks contains a summit crater; the lava flows on the N flank appear younger than those on the S side. The current eruption period began in early February 2021 and has more recently consisted of intermittent explosions and ash emissions (BGVN 47:12). This report updates activity during December 2022 through May 2023 using daily, weekly, and special reports from the Alaska Volcano Observatory (AVO). AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

Activity during most of December 2022 was relatively quiet; according to AVO no eruptive or explosive activity was observed since 7 November 2022. Intermittent tremor and occasional small earthquakes were observed in geophysical data. Continuous gas-and-steam emissions were observed from the N crater of Mount Young in webcam images on clear weather days (figure 25). On 24 December, there was a slight increase in earthquake activity and several small possible explosion signals were detected in infrasound data. Eruptive activity resumed on 27 December at the N crater of Mount Young; AVO issued a Volcano Activity Notice (VAN) that reported minor ash deposits on the flanks of Mount Young that extended as far as 1 km from the vent, according to webcam images taken during 27-28 December (figure 26). No ash plumes were observed in webcam or satellite imagery, but a persistent gas-and-steam plume that might have contained some ash rose to 1.5 km altitude. As a result, AVO raised the Aviation Color Code (ACC) to Orange (the second highest level on a four-color scale) and the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale). Possible explosions were detected during 21 December 2022 through 1 January 2023 and seismic tremor was recorded during 30-31 December.

Figure 25. Webcam image of a gas-and-steam plume rising above Semisopochnoi from Mount Young on 21 December 2022. Courtesy of AVO.

Figure 26. Webcam image showing fresh ash deposits (black color) at the summit and on the flanks of Mount Young at Semisopochnoi, extending up to 1 km from the N crater. Image was taken on 27 December 2022. Image has been color corrected. Courtesy of AVO.

During January 2023 eruptive activity continued at the active N crater of Mount Young. Minor ash deposits were observed on the flanks, extending about 2 km SSW, based on webcam images from 1 and 3 January. A possible explosion occurred during 1-2 January based on elevated seismicity recorded on local seismometers and an infrasound signal recorded minutes later by an array at Adak. Though no ash plumes were observed in webcam or satellite imagery, a persistent gas-and-steam plume rose to 1.5 km altitude that might have carried minor traces of ash. Ash deposits were accompanied by periods of elevated seismicity and infrasound signals from the local geophysical network, which AVO reported were likely due to weak explosive activity. Low-level explosive activity was also detected during 2-3 January, with minor gas-and-steam emissions and a new ash deposit that was visible in webcam images. Low-level explosive activity was detected in geophysical data during 4-5 January, with elevated seismicity and infrasound signals observed on local stations. Volcanic tremor was detected during 7-9 January and very weak explosive activity was detected in seismic and infrasound data on 9 January. Weak seismic and infrasound signals were recorded on 17 January, which indicated minor explosive activity, but no ash emissions were observed in clear webcam images; a gas-and-steam plume continued to rise to 1.5 km altitude. During 29-30 January, ash deposits near the summit were observed on fresh snow, according to webcam images.

The active N cone at Mount Young continued to produce a gas-and-steam plume during February, but no ash emissions or explosive events were detected. Seismicity remained elevated with faint tremor during early February. Gas-and-steam emissions from the N crater were observed in clear webcam images on 11-13 and 16 February; no explosive activity was detected in seismic, infrasound, or satellite data. Seismicity has also decreased, with no significant seismic tremor observed since 25 January. Therefore, the ACC was lowered to Yellow (the second lowest level on a four-color scale) and the VAL was lowered to Advisory (the second lowest level on a four-color scale) on 22 February.

Gas-and-steam emissions persisted during March from the N cone of Mount Young, based on clear webcam images. A few brief episodes of weak tremor were detected in seismic data, although seismicity decreased over the month. A gas-and-steam plume detected in satellite data extended 150 km on 18 March. Low-level ash emissions from the N cone at Mount Young were observed in several webcam images during 18-19 March, in addition to small explosions and volcanic tremor. The ACC was raised to Orange and the VAL increased to Watch on 19 March. A small explosion was detected in seismic and infrasound data on 21 March.

Low-level unrest continued during April, although cloudy weather often obscured views of the summit; periods of seismic tremor and local earthquakes were recorded. During 3-4 April a gas-and-steam plume was visible traveling more than 200 km overnight; no ash was evident in the plume, according to AVO. A gas-and-steam plume was observed during 4-6 April that extended 400 km but did not seem to contain ash. Small explosions were detected in seismic and infrasound data on 5 April. Occasional clear webcam images showed continuing gas-and-steam emissions rose from Mount Young, but no ash deposits were observed on the snow. On 19 April small explosions and tremor were detected in seismic and infrasound data. A period of seismic tremor was detected during 22-25 April, with possible weak explosions on 25 April. Ash deposits were visible near the crater rim, but it was unclear if these deposits were recent or due to older deposits.

Occasional small earthquakes were recorded during May, but there were no signs of explosive activity seen in geophysical data. Gas-and-steam emissions continued from the N crater of Mount Young, based on webcam images, and seismicity remained slightly elevated. A new, light ash deposit was visible during the morning of 5 May on fresh snow on the NW flank of Mount Young. During 10 May periods of volcanic tremor were observed. The ACC was lowered to Yellow and the VAL to Advisory on 17 May due to no additional evidence of activity.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).

Ebeko, located on the N end of Paramushir Island in the Kuril Islands, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruption period began in June 2022 and has recently consisted of frequent explosions, ash plumes, and thermal activity (BGVN 47:10). This report covers similar activity during October 2022 through May 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Activity during October consisted of explosive activity, ash plumes, and occasional thermal anomalies. Visual data by volcanologists from Severo-Kurilsk showed explosions producing ash clouds up to 2.1-3 km altitude which drifted E, N, NE, and SE during 1-8, 10, 16, and 18 October. KVERT issued several Volcano Observatory Notices for Aviation (VONA) on 7, 13-15, and 27 October 2022, stating that explosions generated ash plumes that rose to 2.3-4 km altitude and drifted 5 km E, NE, and SE. Ashfall was reported in Severo-Kurilsk (Paramushir Island, about 7 km E) on 7 and 13 October. Satellite data showed a thermal anomaly over the volcano on 15-16 October. Visual data showed ash plumes rising to 2.5-3.6 km altitude on 22, 25-29, and 31 October and moving NE due to constant explosions.

Similar activity continued during November, with explosions, ash plumes, and ashfall occurring. KVERT issued VONAs on 1-2, 4, 6-7, 9, 13, and 16 November that reported explosions and resulting ash plumes that rose to 1.7-3.6 km altitude and drifted 3-5 km SE, ESE, E, and NE. On 1 November ash plumes extended as far as 110 km SE. On 5, 8, 12, and 24-25 November explosions and ash plumes rose to 2-3.1 km altitude and drifted N and E. Ashfall was observed in Severo-Kurilsk on 7 and 16 November. A thermal anomaly was visible during 1-4, 16, and 20 November. Explosions during 26 November rose as high as 2.7 km altitude and drifted NE (figure 45).

Figure 45. Photo of an ash plume rising to 2.7 km altitude above Ebeko on 26 November 2022. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

Explosions and ash plumes continued to occur in December. During 1-2 and 4 December volcanologists from Severo-Kurilsk observed explosions that sent ash to 1.9-2.5 km altitude and drifted NE and SE (figure 46). VONAs were issued on 5, 9, and 16 December reporting that explosions generated ash plumes rising to 1.9 km, 2.6 km, and 2.4 km altitude and drifted 5 km SE, E, and NE, respectively. A thermal anomaly was visible in satellite imagery on 16 December. On 18 and 27-28 December explosions produced ash plumes that rose to 2.5 km altitude and drifted NE and SE. On 31 December an ash plume rose to 2 km altitude and drifted NE.

Figure 46. Photo of an explosive event at Ebeko at 1109 on 2 December 2022. Photo has been color corrected. Photo by S. Lakomov, IVS FEB RAS.

Explosions continued during January 2023, based on visual observations by volcanologists from Severo-Kurilsk. During 1-7 January explosions generated ash plumes that rose to 4 km altitude and drifted NE, E, W, and SE. According to VONAs issued by KVERT on 2, 4, 10, and 23 January, explosions produced ash plumes that rose to 2-4 km altitude and drifted 5 km N, NE, E, and ENE; the ash plume that rose to 4 km altitude occurred on 10 January (figure 47). Satellite data showed a thermal anomaly during 3-4, 10, 13, 16, 21, 22, and 31 January. KVERT reported that an ash cloud on 4 January moved 12 km NE. On 6 and 9-11 January explosions sent ash plumes to 4.5 km altitude and drifted W and ESE. On 13 January an ash plume rose to 3 km altitude and drifted SE. During 20-24 January ash plumes from explosions rose to 3.7 km altitude and drifted SE, N, and NE. On 21 January the ash plume drifted as far as 40 km NE. During 28-29 and 31 January and 1 February ash plumes rose to 4 km altitude and drifted NE.

Figure 47. Photo of a strong ash plume rising to 4 km altitude from an explosive event on 10 January 2023 (local time). Photo by L. Kotenko, IVS FEB RAS.

During February, explosions, ash plumes, and ashfall were reported. During 1, 4-5 and 7-8 February explosions generated ash plumes that rose to 4.5 km altitude and drifted E and NE; ashfall was observed on 5 and 8 February. On 6 February an explosion produced an ash plume that rose to 3 km altitude and drifted 7 km E, causing ashfall in Severo-Kurilsk. A thermal anomaly was visible in satellite data on 8, 9, 13, and 21 February. Explosions on 9 and 12-13 February produced ash plumes that rose to 4 km altitude and drifted E and NE; the ash cloud on 12 February extended as far as 45 km E. On 22 February explosions sent ash to 3 km altitude that drifted E. During 24 and 26-27 February ash plumes rose to 4 km altitude and drifted E. On 28 February an explosion sent ash to 2.5-3 km altitude and drifted 5 km E; ashfall was observed in Severo-Kurilsk.

Activity continued during March; visual observations showed that explosions generated ash plumes that rose to 3.6 km altitude on 3, 5-7, and 9-12 March and drifted E, NE, and NW. Thermal anomalies were visible on 10, 13, and 29-30 March in satellite imagery. On 18, 21-23, 26, and 29-30 March explosions produced ash plumes that rose to 2.8 km altitude and drifted NE and E; the ash plumes during 22-23 March extended up to 76 km E. A VONA issued on 21 March reported an explosion that produced an ash plume that rose to 2.8 km altitude and drifted 5 km E. Another VONA issued on 23 March reported that satellite data showed an ash plume rising to 3 km altitude and drifted 14 km E.

Explosions during April continued to generate ash plumes. On 1 and 4 April an ash plume rose to 2.8-3.5 km altitude and drifted SE and NE. A thermal anomaly was visible in satellite imagery during 1-6 April. Satellite data showed ash plumes and clouds rising to 2-3 km altitude and drifting up to 12 km SW and E on 3 and 6 April (figure 48). KVERT issued VONAs on 3, 5, 14, 16 April describing explosions that produced ash plumes rising to 3 km, 3.5 km, 3.5 km, and 3 km altitude and drifting 5 km S, 5 km NE and SE, 72 km NNE, and 5 km NE, respectively. According to satellite data, the resulting ash cloud from the explosion on 14 April was 25 x 7 km in size and drifted 72-104 km NNE during 14-15 April. According to visual data by volcanologists from Severo-Kurilsk explosions sent ash up to 3.5 km altitude that drifted NE and E during 15-16, 22, 25-26, and 29 April.

Figure 48. Photo of an ash cloud rising to 3.5 km altitude at Ebeko on 6 April 2023. The cloud extended up to 12 km SW and E. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

The explosive eruption continued during May. Explosions during 3-4, 6-7, and 9-10 May generated ash plumes that rose to 4 km altitude and drifted SW and E. Satellite data showed a thermal anomaly on 3, 9, 13-14, and 24 May. During 12-16, 23-25, and 27-28 May ash plumes rose to 3.5 km altitude and drifted in different directions due to explosions. Two VONA notices were issued on 16 and 25 May, describing explosions that generated ash plumes rising to 3 km and 3.5 km altitude, respectively and extending 5 km E. The ash cloud on 25 May drifted 75 km SE.

Thermal activity in the summit crater, occasionally accompanied by ash plumes and ash deposits on the SE and E flanks due to frequent explosions, were visible in infrared and true color satellite images (figure 49).

Figure 49. Infrared (bands B12, B11, B4) and true color satellite images of Ebeko showing occasional small thermal anomalies at the summit crater on 4 October 2022 (top left), 30 April 2023 (bottom left), and 27 May 2023 (bottom right). On 1 November (top right) ash deposits (light-to-dark gray) were visible on the SE flank. An ash plume drifted NE on 30 April, and ash deposits were also visible to the E on both 30 April and 27 May. Courtesy of Copernicus Browser.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Home Reef is a submarine volcano located in the central Tonga islands between Lateiki (Metis Shoal) and Late Island. The first recorded eruption occurred in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, a large volume of floating pumice, and an ephemeral island 500 x 1,500 m wide, with cliffs 30-50 m high that enclosed a water-filled crater. Another island-forming eruption in 2006 produced widespread pumice rafts that drifted as far as Australia; by 2008 the island had eroded below sea level. The previous eruption occurred during October 2022 and was characterized by a new island-forming eruption, lava effusion, ash plumes, discolored water, and gas-and-steam plumes (BGVN 47:11). This report covers discolored water plumes during November 2022 through April 2023 using satellite data.

Discolored plumes continued during the reporting period and were observed in true color satellite images on clear weather days. Satellite images show light green-yellow discolored water extending W on 8 and 28 November 2022 (figure 31), and SW on 18 November. Light green-yellow plumes extended W on 3 December, S on 13 December, SW on 18 December, and W and S on 23 December (figure 31). On 12 January 2023 discolored green-yellow plumes extended to the NE, E, SE, and N. The plume moved SE on 17 January and NW on 22 January. Faint discolored water in February was visible moving NE on 1 February. A discolored plume extended NW on 8 and 28 March and NW on 13 March (figure 31). During April, clear weather showed green-blue discolored plumes moving S on 2 April, W on 7 April, and NE and S on 12 April. A strong green-yellow discolored plume extended E and NE on 22 April for several kilometers (figure 31).

Figure 31. Visual (true color) satellite images showing continued green-yellow discolored plumes at Home Reef (black circle) that extended W on 28 November 2022 (top left), W and S on 23 December 2022 (top right), NW on 13 March 2023 (bottom left), and E and NE on 22 April 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. Home Reef, a submarine volcano midway between Metis Shoal and Late Island in the central Tonga islands, was first reported active in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, large amounts of floating pumice, and an ephemeral 500 x 1,500 m island, with cliffs 30-50 m high that enclosed a water-filled crater. In 2006 an island-forming eruption produced widespread dacitic pumice rafts that drifted as far as Australia. Another island was built during a September-October 2022 eruption.

Information Contacts: Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ambae, also known as Aoba, is a large basaltic shield volcano in Vanuatu. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas. Periodic phreatic and pyroclastic explosions have been reported since the 16th century. A large eruption more than 400 years ago resulted in a volcanic cone within the summit crater that is now filled by Lake Voui; the similarly sized Lake Manaro fills the western third of the caldera. The previous eruption ended in August 2022 that was characterized by gas-and-steam and ash emissions and explosions of wet tephra (BGVN 47:10). This report covers a new eruption during February through May 2023 that consisted of a new lava flow, ash plumes, and sulfur dioxide emissions, using information from the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and satellite data.

During the reporting period, the Alert Level remained at a 2 (on a scale of 0-5), which has been in place since December 2021. Activity during October 2022 through March 2023 remained relatively low and mostly consisted of gas-and-steam emissions in Lake Voui. VMGD reported that at 1300 on 15 November a satellite image captured a strong amount of sulfur dioxide rising above the volcano (figure 99), and that seismicity slightly increased. The southern and northern part of the island reported a strong sulfur dioxide smell and heard explosions. On 20 February 2023 a gas-and-ash plume rose 1.3 km above the summit and drifted SSW, according to a webcam image (figure 100). Gas-and-steam and possibly ash emissions continued on 23 February and volcanic earthquakes were recorded by the seismic network.

Figure 99. Satellite image of the strong sulfur dioxide plume above Ambae taken on 15 November 2022. The Dobson Units (DU) exceeded 12. Courtesy of VMGD.

Figure 100. Webcam image of a gas-and-ash plume rising above Ambae at 1745 on 20 February 2023. The plume drifted SSW. Courtesy of VMGD.

During April, volcanic earthquakes and gas-and-steam and ash emissions were reported from the cone in Lake Voui. VMGD reported that activity increased during 5-7 April; high gas-and-steam and ash plumes were visible, accompanied by nighttime incandescence. According to a Wellington VAAC report, a low-level ash plume rose as high as 2.5 km above the summit and drifted W and SW on 5 April, based on satellite imagery. Reports in Saratamata stated that a dark ash plume drifted to the WSW, but no loud explosion was heard. Webcam images from 2100 showed incandescence above the crater and reflected in the clouds. According to an aerial survey, field observations, and satellite data, water was no longer present in the lake. A lava flow was reported effusing from the vent and traveling N into the dry Lake Voui, which lasted three days. The next morning at 0745 on 6 April a gas-and-steam and ash plume rose 5.4 km above the summit and drifted ESE, based on information from VMGD (figure 101). The Wellington VAAC also reported that light ashfall was observed on the island. Intermittent gas-and-steam and ash emissions were visible on 7 April, some of which rose to an estimated 3 km above the summit and drifted E. Webcam images during 0107-0730 on 7 April showed continuing ash emissions. A gas-and-steam and ash plume rose 695 m above the summit crater at 0730 on 19 April and drifted ESE, based on a webcam image (figure 102).

Figure 101. Webcam image showing a gas-and-ash plume rising 5.4 km above the summit of Ambae at 0745 on 6 April 2023. Courtesy of VMGD.

Figure 102. Webcam image showing a gas-and-ash plume rising 695 m above the summit of Ambae at 0730 on 19 April 2023. Courtesy of VMGD.

According to visual and infrared satellite data, water was visible in Lake Voui as late as 24 March 2023 (figure 103). The vent in the caldera showed a gas-and-steam plume drifted SE. On 3 April thermal activity was first detected, accompanied by a gas-and-ash plume that drifted W (figure 103). The lava flow moved N within the dry lake and was shown cooling by 8 April. By 23 April much of the water in the lake had returned. Occasional sulfur dioxide plumes were detected by the TROPOMI instrument on the Sentinel-5P satellite that exceeded 2 Dobson Units (DU) and drifted in different directions (figure 104).

Figure 103. Satellite images showing both visual (true color) and infrared (bands B12, B11, B4) views on 24 March 2023 (top left), 3 April 2023 (top left), 8 April 2023 (bottom left), and 23 April 2023 (bottom right). In the image on 24 March, water filled Lake Voui around the small northern lake. A gas-and-steam plume drifted SE. Thermal activity (bright yellow-orange) was first detected in infrared data on 3 April 2023, accompanied by a gas-and-ash plume that drifted W. The lava flow slowly filled the northern part of the then-dry lake and remained hot on 8 April. By 23 April, the water in Lake Voui had returned. Courtesy of Copernicus Browser.

Figure 104. Images showing sulfur dioxide plumes rising from Ambae on 26 December 2022 (top left), 25 February 2023 (top right), 23 March 2023 (bottom left), and 5 April 2023 (bottom right), as detected by the TROPOMI instrument on the Sentinel-5P satellite. These plumes exceeded at least 2 Dobson Units (DU) and drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone with numerous scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Gaua awoke in 2009 (BGVN 34:10) and has continued sporadic eruptions and seismic unrest into 2012. Our last Bulletin report discussed events at Gaua (and island of the same name) into late 2010, with some later seismic and thermal data (BGVN 35:05). A new report from the Vanuatu Geohazards Observatory (VGO) issued in October 2011 contains a new hazards map (figure 21).

Figure 21. An updated hazard map for Gaua ("Gaua Volkeno Denja Map" in local parlance). Note the crescent-shaped Lake Letas (blue, and overlain with other colors) wrapping around the N and E sides of the active center's ~800-m-tall summit (Mt. Garat). An earlier map appeared in BGVN 34:12. From the VGO Bulletin issued on 26 October 2011. [Note: This image is very low resolution; a higher resolution version of this map and explanation of symbols will be posted if it becomes available.]

In addition, a new geosciences publication, Globe Magazine, contained photos (figures 22 and 23) and a brief discussion of Gaua's behavior as late as early 2010 (Scott and others, 2010). The report included the following statements on events at the volcano and efforts to bolster instrumental observations.

Figure 22. Undated photo of Gaua in the course of a modest ash-bearing eruption at Mt. Garat. The water in the foreground is Lake Letas, which surrounds the N to SE flanks. From Scott and others (2010).

Figure 23. Vanuatu Geohazards Unit staff member Jimmy Loic checking one of the GNS Science seismic stations installed on Gaua. From Scott and others (2010).

"Mount Garet [Garat] on Gaua, a 20-km-diameter island 400 km N of the capital Port Vila, started erupting in September 2009, and by late November there were signs that eruptions might become larger and more explosive. Because of its remoteness and the vulnerability of its population of about 3,000 to volcanic ash, the Vanuatu government decided immediate action was needed. The main concerns are volcanic ash contaminating water supplies and anxiety caused by the erratic behaviour of the volcano.

"The volcano has been erupting mostly steam and fine ash. However, in early 2010 several more explosive eruptions threw scoria bombs up to 2 km from the crater. The ash has been falling mostly on villages and fields W and NW of the volcano, and more than 200 people living in those areas have been relocated.

"Based on their observations and the recent history of eruptions on Gaua, volcanologists from GNS Science and Vanuatu concluded that the eruptive activity is most likely to continue for some months at a level similar to that seen so far. The New Zealand government's international aid and development agency, NZAID, has funded the visits by GNS Science. NZAID has subsequently asked GNS Science to provide the Vanuatu government with three seismographs and to train local staff in their use, and in data analysis and interpretation."

2011 activity. VGO reported on 10 October 2011 that data collected by the Gaua monitoring system showed the existence of earthquakes caused by volcanic activity in August 2011. OMI satellite images clearly showed degassing during 17 and 27-28 September 2011, indicating ongoing activity. According to VGO, on 10 October local authorities reported ashfall on the NE and W sides of Gaua Island.

VGO issued a report on 26 October 2011 that described an activity assessment made during 17-18 October 2011. The report confirmed Gaua's ash emissions since September 2011, with ash distribution dictated by trade winds. Seismic data suggested eruptive activity since June 2011, but the intensity of the activity was lower than during 2009-2010.

VGO indicated that two scenarios were envisaged for Gaua. Activity could intensify with little or no warning and then cease. On the other hand, activity could continue more regularly, causing ashfall in the neighboring communities, especially those on the W side of the island that are exposed to trade winds. With this analysis, the Alert Level of Gaua remained at level 1 (on a scale from 0-4), meaning that activity had slightly increased, with the risk remaining near the volcano crater, within the red zone (see figure 21).

Reference. Scott, B., Jolly, A., Sherburn, S., and Jolly, G., 2010, Expert advice on Vanuatu volcano, Globe Magazine, Issue 1 (July 2010); pp. 12-13. Published by GNS Science (New Zealand; Editor, John Callan; Chief Executive, Alex Malahoff); ISSN 1179-7177 (Print); ISSN 1179-7185 (Online)

Geologic Background. The roughly 20-km-diameter Gaua Island, also known as Santa Maria, consists of a basaltic-to-andesitic stratovolcano with an 6 x 9 km summit caldera. Small vents near the caldera rim fed Pleistocene lava flows that reached the coast on several sides of the island; littoral cones were formed where these lava flows reached the ocean. Quiet collapse that formed the roughly 700-m-deep caldera was followed by extensive ash eruptions. The active Mount Garet (or Garat) cone in the SW part of the caldera has three pit craters across the summit area. Construction of Garet and other small cinder cones has left a crescent-shaped lake. The onset of eruptive activity from a vent high on the SE flank in 1962 ended a long period of dormancy.

Information Contacts: Vanuatu Geohazards Observatory (VGO), Department of Geology, Mines and Water Resources (DGMWR), Vanuatu (URL: http://www.vmgd.gov.vu/vmgd/).

Since our last report covering Masaya's seismic activity and emissions from November 2011 through March 2012, the Instituto Nicaragüense de Estudios Territoriales (INETER) has maintained monitoring efforts including site visits in April and May 2012. Here we discuss regular gas emissions (SO₂ and CO₂) and seismic monitoring efforts and highlight events preceding the 30 April 2012 explosion from Santiago crater that ejected ash and incandescent blocks within the bounds of the National Park. That event began a series of explosions; more than 68 explosions occurred between 30 April and 17 May 2012.

On 21 April 2012 INETER conducted routine site visits and made field measurements at Masaya. Maximum temperatures recorded with an infrared sensor found temperatures between 98.7°C and 102°C within Santiago crater. Some jetting sounds were heard from the depths of the crater, cracks were observed on the E wall that emitted abundant gases, and the W interior wall showed signs of rockfalls. INETER field teams also visited Comalito cone, located on the NE flank, and measured maximum temperatures of 72°C to 77°C.

During field investigations on 25 April 2012, INETER volcanologists measured diffuse CO₂ emissions from Comalito cone. At night on 26 April, the National Park guards reported incandescence within the crater; the last report of incandescence was in October 2010 (BGVN 36:11). SO₂ was measured with Mobile DOAS on 27 April on a traverse between the towns Ticuantepe and La Concha (see map for location in figure 25 from BGVN 36:11).

INETER reported that, on 27 April 2012 at approximately 0500 volcanic tremor appeared in Masaya's seismic records (figure 34). Tremor slowly increased to 70 RSAM that day, and civil defense authorities released notices to officials that significant seismic unrest was detected at Masaya.

Figure 34. RSAM (averaged seismic amplitude) record from Masaya volcano during April 2012, an interval leading up to and including a 30 April eruption. Tremor drove a notable increase in RSAM on 27 April, diminishing slightly as monochromatic tremor prevailed over the following days. After an abrupt decrease in RSAM, the eruption occurred on 30 April. Courtesy of INETER.

On 28 April 2012, authorities, including the Masaya Volcano National Park, released a public announcement about the unusual seismic activity. Three hours following that announcement, the tremor signal became monochromatic near 15 Hz (figure 35). INETER suggested that this signal arose from magma moving beneath the edifice. RSAM reached 100 units with spectral analysis indicating frequencies oscillating between ~1.26 Hz and ~18.84 Hz. The strongest frequency during one particular time window (figure 35) was centered near 15.8 Hz, with a smaller peak at ~1.5 Hz.

Figure 35. (Upper panel) Seismic signal dominated by ongoing tremor recorded at Masaya on 28 April 2012 on a seismogram (amplitude, y-axis, and time (hours : minutes), x-axis). (Lower panel) A spectral analysis made for the interval shown above (frequency, in Hz, along x-axis). Courtesy of INETER.

INETER noted that before the onset of tremor on 27 April, an average of 35 seismic events per day were recorded. These were low frequency earthquakes that included signals reaching 16 Hz and interpreted as rupture events beneath Masaya. The depths of the earthquakes were determined by the P- and S-wave arrival times indicating a depth range between 3 and 4 km.

On 28 April, tremor continued at 70 RSAM and monochromatic tremor occurred again, reaching 90 RSAM. Up to 40 earthquakes were detected that day.

On 29 April, seismic tremor was slightly lower at 65 RSAM and monochromatic tremor was recorded. A total of 45 earthquakes were recorded. Signals were again monochromatic at peak frequencies of 15.8 Hz.

On 30 April at 0045, the tremor signal dramatically decreased to 30 RSAM. INETER commented that this was abnormal since tremor was often recorded between 40 and 50 RSAM during times of quiescence. Seven hours later, a strong explosion was recorded by seismic instruments and observers within the National Park witnessed a blast of gas and ash from Santiago crater (figure 36).

Figure 36. Ash explosions began on 30 April 2012 from Masaya's Santiago crater. (A) A large explosion occurred at 0829 on 30 April and was photographed by National Park staff. (B) Later in the day a smaller explosion released a small ash plume. Courtesy of INETER and the Masaya Volcano National Park.

Due to the explosions, the Plaza de Oviedo, an overlook at the edge of Santiago crater, was covered with sand-sized pink and yellow ash and lapilli with some rocks up to 10 cm in diameter. Some of the clasts were incandescent and damaged the roofs of structures near the crater and also burned the asphalt of the plaza (figure 37). Small brush fires were ignited on the N flank of the volcano due to hot blocks falling onto the dry plants. Local firefighters worked with the National Park and Civil Defense for most of the day in order to contain and extinguish the fires. The national park was closed due to the hazardous conditions.

Figure 37. (A) The roofs of several structures near Santiago crater were damaged by volcanic bombs during the 30 April 2012 explosions. (B) Some of the bombs ejected during the primary explosion were incandescent and burned the asphalt of the plaza when they landed. Courtesy of INETER.

INETER reported the explosion ejected a column of ash, gas, and blocks reaching 1,000 m above the summit and the initial explosion was followed by 24 smaller explosions that reached 500 m. Ballistic ejecta covered an area with a 300 m radius to the SSE of the crater and ash fell as far as 3 km to the SE of the crater. Blocks measured from this area had maximum dimensions of 50 x 40 x 30 cm. Ash fell to a thickness of 2 mm in some areas and INETER calculated a total volume of 736 cubic meters of ejecta.

INETER measured temperatures from Santiago crater on 30 April with an infrared thermal camera and detected a maximum of 165°C. During the night of 30 April, 23 explosions were recorded by the seismic network.

Between 30 April and 3 May, a collaborative effort among INETER, Civil Defense, local fire fighters, and the National Park succeeded in maintaining a 24-hour watch of Santiago crater. Over four days, the teams recorded observations and determined that 68 explosions had occurred and the maximum detected crater temperature was 162°C.

On 1 May 2012 at 0223 a small explosion was recorded by the INETER seismic network. This event produced ash and volcanic bombs that fell across the NE-SE sectors including the flanks of Nindirí cone (see figure 30 in BGVN 37:04 for site names). The dimensions of the largest blocks were 60 x 50 x 40 cm.

On 3 May there were two small explosions at 0008 and 0022 with abundant gas and ash emissions. Throughout these events, tremor was constant at 1.5 Hz. On 4 May no earthquakes were recorded but tremor remained between 45 and 50 RSAM; explosions of gas and light ash were observed. On 5 May a total of 19 earthquakes were recorded and RSAM varied between 45 and 58 RSAM; ash and gas explosions were reported by National Park staff. On 6 May between 0700 and 1030 a total of 45 earthquakes were recorded and RSAM increased to 70 units.

Sporadic explosions continued until mid-May (figure 38). INETER noted that in May, RSAM averaged 60 units and a significant increase occurred on 18 May. RSAM reached 120 units and was maintained at that level until 21 May. Low tremor was recorded up to 75 RSAM units after 21 May and two days later reached 85 RSAM units with frequencies in the range 1.5-3.0 Hz. Tremor decreased and remained between 65 and 70 RSAM units until the end of the month. A total of 266 earthquakes were recorded in May.

Figure 38. RSAM record from Masaya volcano during May 2012. Courtesy of INETER.

Long-term gas monitoring. Long-term records of Masaya's gas emissions (SO₂ and CO₂) and fumarole temperatures have been developed by INETER. On 2 May, SO₂ flux was measured during traverses between Ticuantepe and La Concha (table 5). INETER commented that they observed increasing SO₂ flux since December 2011 (648 tons per day) that peaked in March 2012 (1002 tons per day). Flux was decreasing at the time of the explosion on 30 April 2012. INETER noted that overall trends in SO₂ flux did not correlate with trends in seismicity, however, they emphasized that difficult-to-constrain variables such as wind speed and direction should be factored into the SO₂ data interpretations.

Table 5. SO₂ flux detected at Masaya from January 2011 through May 2012 during traverses with a Mobile DOAS. Courtesy of INETER.

Since 7 December 2008, INETER measured CO₂ emissions from Comalito cone, an active fumarolic site on the NE flank of Masaya. Diffuse CO₂ was measured from a 9 hectare sector of soil as recently as 1 May 2012 (table 6). INETER reported the highest CO₂ emissions were detected in 2008 and decreased between 2010 and 2011. Emissions recorded on 25 April 2012 (before the eruption) were considered low, however, there was a small peak on 1 May that may have been related to the explosive activity.

Table 6. The long-term record of diffuse CO₂ analyses from Comalito cone measured from September 2008 through May 2012. Courtesy of INETER.

On 17 May, INETER conducted fieldwork at Santiago crater and determined a maximum temperature of 162°C. While in the field, INETER staff observed two small explosions from the crater. Temperatures were also measured at Comalito cone (figure 39); the maximum recorded temperature was from Fumarole 2, 78.2°C, the highest temperature reading at Comalito cone since February 2012.

Figure 39. Temperatures measured at Comalito cone from January through May 2012. Courtesy of INETER.

New monitoring efforts and installations. Two seismic stations were installed in May 2012. One station, called La Azucena, was installed by INETER on 1 May. This site was located ~4 km N of the active crater and was considered temporary. A second station, called El Comalito, was installed on 15 May; located within the National Park at Comalito cone. INETER recognized potential contributions of background noise from the fumarolic sites close to the station and planned to reevaluate the location after reviewing the results from this station. Both stations transmitted realtime data through radio repeaters.

On 4 May a web camera was installed within the town of La Azucena on a short tower; the camera was programmed to send images through a wireless network every 5 minutes. A second camera was installed in the town of Masaya at the office building of the Center of Disaster Operations (CODE); this camera also captured images every 5 minutes. The camera at CODE suffered malfunctions after installation due to overexposure from direct sunlight. Future fieldwork was planned to fix these problems.

Geologic Background. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of observed eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); La Prensa (URL: http://www.laprensa.com.ni/).

Monowai volcano, located 1,000 km NE of New Zealand's North Island, is one of the most active submarine volcanoes identified in the Tonga-Kermadec arc, a 2,500-km-long chain of submarine volcanoes stretching from New Zealand to just N of Tonga (figure 22). Bradley Scott, a volcanologist at New Zealand's GNS Science, reported that seismic activity recorded by GeoNet on the seismograph at Rarotonga, Cook Islands, had shown there were several days of eruptive activity at Monowai starting on 3 August 2012. A large pumice raft, first spotted on 19 July 2012, was suspected to have a source in Monowai; however, that was later discounted (see a report on Havre seamount in a subsequent issue). The most recent previous eruptions of Monowai began on 8 February 2008 and 14 May 2011.

Figure 22. Regional bathymetric map of the Monowai Volcanic Centre (MVC), comprising the Monowai cone to the SW and the 10-km-wide Monowai caldera to the NE. Grey and red lines show the tracks of R/V Sonne on 14 May and during 1-2 June 2011, respectively. The yellow star with the red border shows the SW caldera hydrothermal site (from Leybourne and others, 2010). The letter 'V' indicates regions of active venting. The dashed black square around the cone shows the location of the maps in figure 23. The inset shows the location of the MVC in relation to historically active volcanoes in the Kermadec Trench area (red triangles; from Smithsonian Global Volcanism Program web site), Tonga and Kermadec Trench (blue lines), and the Louisville Ridge seamount chain (dashed black line). The depth scale along the right-hand side of the figure keys the colors in the figure to the appropriate depths, in meters. Courtesy of Watts and others (2012).

All previous Bulletin reports on Monowai, including most of the latest one in 2008 (BGVN 33:03), describe eruptive activity as measured remotely by the Polynesian Seismic Network (Réseau Sismique Polynésien, or RSP). In contrast, this report will emphasize recent oceanographic surveys conducted over the volcanic complex that help define the features of the area.

Background. According to a recent publication by Leybourne and others (2010), the MVC comprises a large, elongate caldera (7.9 x 5.7 km; 35 km²; floor depth = 1,590 m) to the NE, formed within an older caldera (84 km²). Associated is a large active stratovolcano to the SW, which rises to within ~100 m of the sea surface. Mafic rocks dominate MVC, with only rare andesites. Plume mapping shows at least four hydrothermal systems with venting from the summit of Monowai cone and its N flank. Monowai caldera has a major hydrothermal vent system associated with the SW wall of the caldera (figure 22).

Wright and others (2008) wrote that "The first recorded eruptions at Monowai date from between 1877 and 1928 (Mastin and Witter, 2000), and subsequently reported as a shoal in 1944 (Royal Australian Navy, written communication, 1944). More recent eruptions were first observed by maritime aircraft patrols in October 1977 (Davey, 1980). A bathymetric survey undertaken in July, 1978, and towed-sonar array surveys, undertaken in March and July 1978 and March, April, and June 1979, recorded periods of volcanic activity that included discolored water and vigorous gas emissions at the sea surface (Davey, 1980). A single-beam bathymetric survey recorded a conical edifice with a summit shoal of 117 m (velocity uncorrected) in September 1978 (Davey, 1980). A reconnaissance multibeam survey in 1986 by R/V Thomas Washington identified a shoal at a depth of 115 ± 5 m (Scripps Institute of Oceanography, unpublished data, 1986)."

Bathymetry. Multibeam surveys by RV Sonne in 1998 (SO-135 voyage) and RV Tangaroa in 2004 showed the Monowai stratovolcano cone (10-12 km in diameter, rising 965 m from the 1,100-m isobath) to be the largest of a number of postcollapse cones sited around the rim of the newly discovered Monowai caldera (part of the larger volcanic complex; Graham and others, 2008). The elongate caldera was 11 x 8.5 km in size and showed evidence of at least two phases of caldera formation. Monowai cone forms a relatively simple edifice on the S caldera rim, with near constant 13-18° slopes that were interpreted by the investigators of these cruises as angles of repose of volcaniclastic deposits generated at the summit. Prominent radial dikes and small aligned vents protruded up to 50 m above the edifice slopes, especially on the N and W flanks. The S flank showed evidence of repeated sector collapse. A single video-grab transect during the 1998 RV Sonne survey across the then-shallowest vent showed that it comprised coarse scoriaceous blocks with a lapilli sand matrix. Sampled rocks from Monowai cone comprise highly vesicular, plagioclase-clinopyroxene basalts (Brothers and others, 1980; Haase and othres, 2002).

Table 2 shows the various depths of the summit of the Monowai cone as measured by multiple bathymetyric surveys conducted since 1978. Figures 23 and 24 show regions of bathymetric changes.

Table 2. Summit depth measurements of Monowai cone since 1978. Courtesy of Watts and others (2012) and references listed.

Figure 23. Detailed bathymetric maps of Monowai cone as it appeared in September 2004, May 2007 and May-June 2011. The map area is outlined in figure 22 (dashed black square). 'SC' denotes sector collapses. (a) Swath bathymetry acquired by R/V Tangaroa in September 2004, contoured at 100 m intervals, with thick contours at 500 m intervals. (b) Swath bathymetry, R/V Sonne, May 2007. (c) Swath bathymetry, R/V Sonne, 14 May and 1-2 June 2011 merged into a single grid. The dashed black rectangle shows the view area in figure 24. (d) Difference in bathymetry between the 2007 and 2004 surveys colored to indicate depth changes from -125 to +125 m. Shades of blue indicate depth increase (collapse), and shades of red, depth decrease (growth). (e) Difference in bathymetry between the 2011 and 2007 surveys. Colors as in (d). Colored scales indicate depths as in (a)-(c) and differences in bathymetry (as in (e) and (f)) between two dated surveys. Courtesy of Watts and others (2012).

Figure 24. Perspective view from the SW (azimuth 240°, view angle 14° above horizontal) showing the bathymetry of the summit of Monowai cone in September 2004, May 2007, and May-June 2011. The view area is outlined by the dashed black rectangle in figure 23c. The bathymetry data have been artificially shaded by a sun located in the NW to enhance topography. The negative numbers in brackets to the right of each profile indicate the depth below sea level of the shallowest point on the summit. Colored scale shows key for bathymetry. Courtesy of Watts and others (2012).

Mid-May to early June 2011. Watts and others (2012) reported the results of two recent bathymetric surveys of MVC conducted within a period of 14 days (14 May and 1-2 June 2011). They found marked differences in bathymetry between the surveys. New growth structures, probably due to new lava cones and debris flows, caused decreases in depth of up to 71.9 m, while collapse of the volcano summit region caused increases in depth of up to 18.8 m.

Hydro-acoustic T-wave data revealed a 5-day-long swarm of seismic events with unusually high amplitude between the two 2011 surveys, which link the depth changes to explosive activity (figures MON4, MON5, and MON6). [Note: According to NOAA (Chadwick, 2001), "A 'T-phase' or 'T-wave' is an acoustic phase from an earthquake that travels through the ocean. The 'T' stands for 'tertiary', as in: P-waves are 'primary', S-waves are 'secondary', and T-waves are 'tertiary', because they travel the slowest and so arrive third. Basically, when an earthquake occurs in the earth's crust under the ocean, the usual crustal phases are generated (P and S waves), but in addition part of the energy goes into the ocean as acoustic energy, and that is the T-wave. Not all earthquakes generate T-waves (since they need to be near water)...T-waves are typically recorded by hydrophones, but on some islands seismometers sometimes record T-wave signals that have been converted to crustal phases when they hit the island."]

Figure 25. Time-series plots of hydro-acoustic T-wave data recorded at Rarotonga (IRIS station RAR, IU network) spanning the R/V Sonne repeat swath bathymetric surveys of 14 May and 1-2 June 2011. (a) Number of T-wave events per day (gray bars, left axis) and cumulative number of events (red line, right axis) versus time. An event is defined as one with a peak-to-peak amplitude in ground velocity >1,200 nm/s that is separated from another event by at least 1 min of quiescence. Note the abrupt increase in the number of events observed during the 5-day-long period between 17 to 22 May. (b) Peak-to-peak amplitude of individual events versus time. Red arrows mark the time of the 14 May and 1-2 June swath surveys. The full waveform of the event highlighted by the red circle is shown in the inset in (c). (c) Plot of ground velocity versus time. Courtesy of Watts and others (2012).

Figure 26. Swath bathymetry of the summit of Monowai cone as it appeared on 14 May and 1-2 June 2011. (a and b) Swath bathymetry acquired on R/V Sonne on 14 May (a) and 1-2 June (b) 2011. Open triangles with dates show the sequential position of the summit at selected times since 1978. 'SC3' indicates sector collapse 3 (see figures 23 and 24). Solid black lines show the profiles plotted in figure 27. The contour interval is 20 m. (c) Difference in the swath bathymetry between 14 May and 1-2 June colored to show depth decreases (blue) and increases (red). (d) Perspective view from the SSE (azimuth 168°, view angle 16° above horizontal) of the difference in swath bathymetry between 14 May and 1-2 June. Colored scales indicate depth (a, b, and d) and depth differences (c) in bathymetry between two dated surveys. Courtesy of Watts and others (2012).

Figure 27. Progressive southward growth of the S flank of Monowai cone and the rate of volcanism. (a and b) Bathymetry profiles 1 (a) and 2 (b) from figure 26 of the summit of Monowai cone, shown with no vertical exaggeration. Black arrows highlight the 14 May and 1-2 June summits. The S flank shows progressive southward growth since 1977, contrasting with the more stable N flank. (c) Plot of eruptive volume versus duration of magmatism at Monowai, compared to other selected oceanic volcanoes. Symbols: red/orange diamond, 2011 survey (filled, cone only; unfilled, all data); blue triangles, previous repeat surveys in 1998, 2004 and 2007; small blue filled circles, selected seamounts and ocean islands from Chrisp (1984); green square, Vailulu'u (Staudigel and others, 2006); large light blue circles, data from >9,000 seamounts (Watts and others, 2006) that formed during 0-30 Myr, 95-125 Myr, and 105-110 Myr; small open brown circles, Montserrat (Sparks and others, 1998). Courtesy of Watts and others (2012).

References. Brothers, R.N., Heming, R.F., Hawke, M.M., and Davey, F.J., 1980, Tholeiitic basalt from the Monowai seamount, Tonga-Kermadec ridge, New Zealand Journal of Geology and Geophysics, v. 23, no. 4, p. 537-539.

Chadwick, W.W., Jr., 2001, What is a T-phase?, URL: http://www.pmel.noaa.gov/vents/geology/tphase.html; posted 9 November 2001, accessed 14 August 2012.

Chadwick, W.W., Jr., Wright, I.C., Schwarz-Schampera, U., Hyvernaud O., Reymond, D., and de Ronde, C.E.J., 2008, Cyclic eruptions and sector collapses at Monowai submarine volcano, Kermadec arc: 1998-2007, GeochemistryGeophysicsGeosystemsG3, v. 9, p. 1-17 (DOI: 10.1029/2008GC002113).

Chrisp, J.A., 1984, Rates of magma emplacement and volcanic output, Journal of Volcanology and Geothermal Research, v. 20, pp. 177-211.

Davey, F.J., 1980, The Monowai seamount: An active submarine volcanic centre on the Tonga-Kermadec ridge (note), New Zealand Journal of Geology and Geophysics, v. 23, no. 4, p. 533-536.

Haase, K.M., Worthington, T.J., Stoffers, P., G-Schonberg, D., and Wright, I., 2002, Mantle dynamics, element recycling, and magma genesis beneath the Kermadec Arc-Havre Trough, GeochemistryGeophysicsGeosystemsG3, v. 3, no. 11. p. 1071 (DOI: 10.1029/2002GC000335).

Leybourne, M.I., de Ronde, C.E.J., Baker, E.T., Faure, K., Walker, S.L., Resing, J., and Massoth, G.J., 2010, Submarine magmatic-hydrothermal systems at the Monowai Volcanic Centre, Kermadec Arc, Goldschmidt Conference Abstracts 2010, Abstract A587.

Sparks, R.S.J., Young, S.R., Barclay, J., Calder, E.S., Cole, P., Darroux, B., Davies, M.A., Druitt, T.H., Harford, C., Herd, R., James, M., Lejeune, A.M., Loughlin, S., Norton, G., Skerrit, G., Stasiuk, M.V., Stevens, N.S., Toothill, J., Wadge, G., and Watts, R., 1998, Magma production and growth of the lava dome of the Soufriére Hills volcano, Montserrat, West Indies: November 1995 to December 1997, Geophysical Research Letters, v. 25, no. 18, pp. 3421-3424 (DOI: 10.1029/98GL00639).

Staudigel, H., Hart, S.R., Pile, A., Bailey, B.E., Baker, E.T., Brooke, S., Connelly, D.P., Haucke, L., German, C.R., Hudson, I., Jones, D., Koppers, A.A.P., Konter, J., Lee, R., Pietsch, T.W., Tebo, B.M., Templeton, A.S., Zierenberg, R., and Young, C.M., 2006, Vailulu'u Seamount, Samoa: Life and death of an active submarine volcano, Procedures of the National Academy of Science, USA, v. 103, pp. 6448-6453 (DOI: 10.1073/pnas.0600830103).

Watts, A.B., Sandwell, D.T., Smith, W.H.F., and Wessel, P., 2006, Global gravity, bathymetry, and the distribution of submarine volcanism through space and time, Journal of Geophysical Research, v. 111 (DOI: 10.1029/2005JB004083).

Watts, A.B., Peirce, C., Grevemeyer, I., Paulatto, M., Stratford, W., Bassett, D., Hunter, J.A., Kalnins, L.M., and de Ronde, C.E.J., 2012 (13 May), Rapid rates of growth and collapse of Monowai submarine volcano in the Kermadec Arc, Nature Geoscience, v. 5, p. 510-515 (DOI: 10.1038/ngeo1473).

Wright I.C., Chadwick, W.W., Jr, de Ronde, C.E.J., Reymond, D., Hyvernaud, O., Gennerich, H., Stoffers, P., Mackay, K., Dunkin, M.A., and Bannister, S.C., 2008, Collapse and reconstruction of Monowai submarine volcano, Kermadec arc, 1998-2004, Journal of Geophysical Research, v. 113, p. 1-13 (DOI: 10.1029/2007JB005138).

Geologic Background. Monowai, also known as Orion seamount, is a basaltic stratovolcano that rises from a depth of about 1,500 to within 100 m of the ocean surface about halfway between the Kermadec and Tonga island groups, at the southern end of the Tonga Ridge. Small cones occur on the N and W flanks, and an 8.5 x 11 km submarine caldera with a depth of more than 1,500 m lies to the NNE. Numerous eruptions have been identified using submarine acoustic signals since it was first recognized as a volcano in 1977. A shoal that had been reported in 1944 may have been a pumice raft or water disturbance due to degassing. Surface observations have included water discoloration, vigorous gas bubbling, and areas of upwelling water, sometimes accompanied by rumbling noises. It was named for one of the New Zealand Navy bathymetric survey ships that documented its morphology.

Information Contacts: Bradley J. Scott, GNS Science, Wainakel Research Centre, Taupo, New Zealand (URL: http://www.gns.cn.nz); GeoNet, New Zealand (URL: http://www.geonet.org.nz)

Minor seismic activity and fumarolic plumes at Papandayan occurred in July 2005, July and August 2007, and April 2008 (BGVN 33:06; figure 9). This report covers a seismic swarm reported in July and August 2011. According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM), Papandayan is monitored by eight seismic stations (three permanent and five temporary).

Figure 9. A map showing the location of Papandayan relative to many other Indonesian volcanoes of Holocene age. Courtesy of USGS.

Since April 2008, reports on seismicity were sparse. Then, in July 2011, seismicity increased; several hundred earthquakes were detected per month, and the occurrence of deep earthquakes nearly tripled. (figure 10, table 4).

Figure 10. Papandayan crater as seen from the trail to Pondok Salada in August 2011. Courtesy of Daniel Quinn.

Table 4. The occurrence of various types of seismicity at Papandayan during July-24 August 2011. '--' indicates data not reported. Data from CVGHM.

According to CVGHM, sulfur-dioxide (SO₂) plumes rose 20-75 m above the vents between 1 June and at least 12 August 2011. Between 12-23 August, SO₂ emissions ranged from 3-8 tons per day. Carbon dioxide (CO₂) levels measured in the soil at 1 m depth in multiple areas did not increase. The temperature in the Manuk thermal area increased during 29 June to 12 August, and deformation measurements indicated inflation from 4 July to 10 August. On 13 August 2011, CVGHM announced that the Alert Level for Papandayan had been increased to 3 (on a scale of 1-4) based on seismicity, deformation, geochemistry, and visual observations. Visitors and residents were warned not to venture within 2 km of the active crater. The increase spurred multiple news reports.

On 14 August 2011, the Jakarta Globe reported that Sutopo Purwo Nugroho, a spokesman for the National Disaster Mitigation Agency, had stated that gas was emanating from three craters - Walirang, Manuk and Balagadama. The same report quoted Surono, who heads CVGHM, as saying: "For now, we are not too worried about a major eruption. We are more concerned by the toxic gas."

According to other news reports, by mid-August 2011 local officials had completed evacuation planning, especially for three vulnerable villages within 7 km of the active crater. The report also mentioned that as of 19 August, residents near the volcano were continuing their normal activities, but that tourist visitation had dropped sharply at the popular destination.

On 26 August 2011, CVGHM reported that Papandayan's activity had not increased during the previous few days. Seismicity remained high, but stable, and was dominated by shallow volcanic earthquakes. Deformation measurements (such as leveling and Electronic Distance Measurement - EDM) showed no change, and water temperatures in multiple fumarolic areas and lakes remained relatively constant.

On 31 January 2012, CVGHM lowered the Alert Level from 3 to 2, without indication of eruption details or reasons for the change. As of 30 June 2012, the Alert Level remained at 2.

Crater emission videos. Video clips of crater emissions taken at Papandayan in October 2009, and at an uncertain other date can be found on YouTube:

Pwarr3n, 2009, YouTube (URL: http://www.youtube.com/watch?feature=endscreen&NR=1&v=H_GIwMdkWT8).

Sweetmarias, undated, posted 13 August 2010, YouTube (URL: http://www.youtube.com/watch?v=tSFoybapqe0).

Geologic Background. Papandayan is a complex stratovolcano with four large summit craters, the youngest of which was breached to the NE by collapse during a brief eruption in 1772 and contains active fumarole fields. The broad 1.1-km-wide, flat-floored Alun-Alun crater truncates the summit of Papandayan, and Gunung Puntang to the north gives a twin-peaked appearance. Several episodes of collapse have created an irregular profile and produced debris avalanches that have impacted lowland areas. A sulfur-encrusted fumarole field occupies historically active Kawah Mas ("Golden Crater"). After its first historical eruption in 1772, in which collapse of the NE flank produced a catastrophic debris avalanche that destroyed 40 villages and killed nearly 3000 people, only small phreatic eruptions had occurred prior to an explosive eruption that began in November 2002.

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

Since our recent brief report on Tinakula (BGVN 37:02), the Bulletin received an informal report from Timothy McConachy of Neptune Minerals, Inc., containing observations of Tinakula volcano made 10 May 2012 (Cook and others, 2012). Most of the following information in the next few paragraphs was extracted from that report.

The location of Tinakula with respect to other islands in the Santa Cruz Islands is shown in figure 12; figure 13 shows geological details of the Tinakula volcanic island.

Figure 12. The location of Tinakula in the Santa Cruz Islands; inset area shows location of Santa Cruz Islands with respect to New Guinea and Australia. Courtesy of McCoy and Cleghorn (1988). This map previously appeared in BGVN 36:08.

Figure 13. Sketch map of Tinakula island based on work and publications by G.W. Hughes (1972) and colleagues, and summarized by Eissen and others (1991). This figure previously appeared in BGVN 28:01 and 36:08.

Visit to Tinakula. Cook and others (2012) twice circumnavigated Tinakula clockwise in a banana boat with a 40-horse-power engine in the afternoon on Thursday, 10 May 2012. The day was sunny and clear with minor clouds and a NE breeze which stiffened during the afternoon; cloud cover increased during the afternoon. During the 2 transits they observed recent land slides, the NW collapse area (shown on Figure 13), and steam/gas plumes. A highlight of the visit was when red incandescent boulders of lava bounced down the large scree slope (up to 200-m-wide and 600- to 800-m-long) in the NW collapse sector. As they bounced, the boulders broke into smaller fragments and puffs of stream/gas were seen making white dotted tracks, or 'vapour trails' (figure 14). A number of the fragments from the larger boulders made their way into the sea, and plumes of steam rose along with the splash. When the larger boulders rolled into the sea, the authors could hear thudding sounds as they hit the water, followed by a hissing sound. At times the splash would rise 2 m or higher when the boulders hit the sea. Some of the boulders and fragments did not roll into the sea, but sat on the edge of the water, steaming and hissing for some time (between 3-5 min) before they cooled off.

Figure 14. The main scree slope in the NW collapse sector of the volcano, photographed at 1416 hours on 10 May 2012. White patches of steam/gas ('vapour trails') were caused by boulders bouncing down the slope. Courtesy of Cook and others (2012).

To the naked eye, there appeared to be a steady cloud above Tinakula (figure 15), quite visible even from the town of Lata (~35 km S of Tinakula, located on Graciosa Bay, Nendö Island - aka Ndende Island, the provincial capital of Temotu Province in the far eastern Solomon Islands). It was difficult for Cook and others (2012) to photograph the incandescent color of the boulders and it only became apparent on the second time around the volcano in the later part of the afternoon when the area was backlit by the sun. The boulders originated from an area obscured by steam and gas. When the authors turned the outboard motor off, they could hear rumbling and small explosions at times. The size of the boulders was difficult to judge, but they thought that the larger ones were the size of a small car. They were surprised to see coconut palms growing up the slopes on most sides of the volcano, up to 50 m above sea level, possibly planted by locals.

Figure 15. Cloud covering the summit of Tinakula at 1358 on 10 May 2012. The top of the volcano is virtually deforested. Courtesy of Cook and others (2012).

Other comments. MODVOLC satellite thermal imagery continued to measure several thermal alerts almost daily.

References. Cook, H.J., Koraua, B.L., and McConachy, T.F., 2012, Observations of Tinakula Volcano, 10 May 2012, Solomon Islands (-10.38°S / 165.8°E), Informal report, 12 pp.

Eissen, J-P., Blot, C., and Louat, R., 1991, Chronology of the historic volcanic activity of the New Hebrides island arc from 1595 to 1991: Rapports Scientifiques et Technique, Sciences de la Terre, No. 2, ORSTOM, France.

Hughes, G.W., 1972, Geological map of Tinakula: Nendö sheet EOI 1, Soloman Geol. Survey, Honiara.

McCoy, P.C., and Cleghorn, 1988, Archaeological Excavations on Santa Cruz (Nendö), Southeast Solomon Islands: Summary Report, pp. 104-115; in Archaeology in Oceania.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. It has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The Mendana cone is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Recorded eruptions have frequently originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

Information Contacts: Timothy F. McConachy, Neptune Minerals, Inc. (URL: http://www.neptuneminerals.com); Brent McInnes, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia (URL: http://www.csiro.au); MODVOLC, Hawai’i Institute of Geophysics and Planetology (HIGP) 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/).

Turrialba is the eastern-most of Costa Rica's active volcanoes, located 65 km E of the capitol, San Jose. The previous Bulletin report discussed frequent degassing and occasional ashfall between March 2010-June 2011 (BGVN 36:09). This report discusses activity between July 2011 and May 2012.

A recent comprehensive report prepared by Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA) provides an excellent background: "Since May 1996, Turrialba volcano has shown an important increase in activity, which can possibly be interpreted as precursory of a new eruptive phase. The volcano-tectonic activity and degassing increase is particularly noticeable since 2007, and even more since the opening of the first fumarolic vent in the W crater [the main crater 'pLa Quemada'] in January 2010, which suggested a magmatic intrusion between 2005 and 2007 as well as the beginning of a new eruptive phase. A new vent opened on January 12th, 2012 (Boca 2012 or 2012 vent) on the southeast external flank of the W crater, with few hours of ash emission, followed by a second ash emission from the same vent on January 18th, 2012." A chronology of events leading up to the 12 January 2012 event is shown in table 6.

Table 6. Events since 1996 leading up to the 12 January 2012 vent opening event, and associated previous Bulletin coverage. Dates and event descriptions courtesy of OVSICORI-UNA.

Seismicity at Turrialba from early November through December 2011 was variable with event frequency ranging from as low as 20 events per day to an occasional high of 80 events per day. The frequency of events dropped significantly in early December to generally less than 60 per day until there was a dramatic increase on 31 December when 155 seismic events were recorded. Event frequency in early January 2012 showed a steady increase from 40 events per day reaching about 80-100 events per day between 6 and 13 January.

Eruption on 12 January 2012. After midnight on 9 January 2012, residents of the Central Valley heard booming and crashing sounds. Investigators at OVSICORI-UNA reviewed the seismic records but did not find associated seismic or volcanic activity. On 11 January, residents again reported several instances of rumbling. On 12 January, OVSICORI-UNA reported that a new vent, located on the SE flank of the volcano's W crater had opened. According to OVSICORI-UNA, the new vent exhibited "a vigorous output of bluish gas at high temperature (T > 592°C) that generated a jet-like sound audible from the visitor lookout." This activity included a few hours of ash emission. A second ash emission from the same vent occurred on 18 January (see subsection below). Seismic recordings, deformation, and diffused gas flux measurements allowed the conclusion that the opening of the 2012 fumarolic vent is not due to a change in the magmatic activity but to an excessive shallow accumulation of gas. This conclusion is substantiated by information obtained from a network of Electronic Distance Measurement (EDM) equipment using five reference points (prisms) which have been taking measurements since 2009. No significant variations of the distance relationships that would coincide with the ash emissions of 2010 and 2012 had been noted. EDM data after March 2011 showed a decrease in measured distances, mainly in the N direction with small variations in the other directions. This information is considered corroborated by Global Position System (GPS) data provided by two GPS stations which show a small but continuous trend of decreasing distance observed during April 2010-January 2012.

Similar vent openings occurred at Turrialba prior to the 1864-66 eruption and at Irazú volcano prior to its 1963-65 eruption. Hence, other openings of fumarolic vents can be expected in the future, especially along the fractures and weak zones aligned in a SW-NE direction that passes by the three upper craters of Turrialba.

The activity of 12 January was a pressure release on the SE flank of the W crater. OVSICORI-UNA considered the release to have penetrated weakened rock, not a magmatic or phreatic (steam-driven) eruption. (The rock at the summit of Turrialba is considered to be very weak due to the intense rainfall and the persistent hydrothermal activity at the summit. This weakness facilitates the development of vents.) An ash plume rose ~500 m above the crater and drifted NNE and NNW, rising to an altitude of ~4 km. Later that day residents reported a dark plume coming from the main crater and a white vapor plume that rose from the fumarolic vent which had formed in the main crater on 5 January 2010. The emissions caused OVSICORI-UNA to raise the Alert Level to Yellow in the communities of La Central (34 km SW), Santa Cruz (7 km SE), and around the perimeter of the crater. Towns of Jiménez (21 km N), Oreamuno (45 km SW), Alvarado (38 km SW), and Cartago (25 km SW) remained at Alert Level Green. Ashfall was reported in Tres Ríos (27 km SW).

Gas emission analysis the day before the opening of the 2012 vent (11 January) showed high values of CO₂ and H₂S over the entire E flank of the W crater. A 115-m-long liquid sulfur flow was observed in the main crater from the E side of W crater.

Eruption on 18 January 2012. During the evening of 18 January 2012, scientists observed gas emissions and ejection of tephra from the vent. They also observed reddish flames from combusting gas, estimated to be ~700°C. Degassing of Turrialba is considered a normal ongoing activity. An OVSICORI-UNA pilot observed an ash plume that rose to altitudes of ~4.3-6.1 km.

The seismogram from the 18 January eruption (figure 26) showed strong tremor coincident with the tephra and gas emissions. The tremor, which started at 1455, was most intense between 1502 and 1610 according to OVSICORI-UNA. Figure 26a shows >5,000 seconds of the most intense part of the tremor having significant variations in amplitude, especially at the beginning of the activity. Figure 26c shows the signal's frequency content over the same interval, with the highest normalized amplitudes having peaks between 5 and 15 Hz.

Figure 26. (a) A seismic recording for Turrialba on 18 January 2012 at station VTUN showing the most intense phase of the tremor that prevailed during the eruption that day. (b) Spectrogram of the seismicity shown in (a). (c) Normalized frequency spectrum of the seismic signal; the main peaks are between 5 and 15 Hz. Courtesy of OVSICORI-UNA.

A false color satellite image of Turrialba taken on 21 January 2012 highlights ongoing impacts to vegetation from high gas emissions (figure 27). One of the concerns of the government is the amount of acid rain that has fallen on the region surrounding Turrialba. The acid rain, with a pH as low as 3.2, has degraded the local agricultural and livestock economy.

Figure 27. A false-color satellite image of Turrialba (a combination of near infrared, red, and green light) acquired on 21 January 2012. Healthy vegetation appears bright red, while vegetation damaged by years of acidic gas emissions is brown. Bare ground in the summit craters is brown or gray. This image was acquired by NASA's Advanced Spaceborne Thermal Reflecton and Emission Radiometer (ASTER) instrument aboard the TERRA satellite. Courtesy of NASA Earth Observatory.

Vent incandescence in February 2012. A nocturnal visit to the W crater by volcanologists from OVSICORI-UNA on 2 February revealed several incandescent spots. Figure 28 (a view from the overlook taken on 9 February), shows a panoramic view of vent locations in relation to the West, Central, and East Craters. Each vent had different gas and vapor output, and different incandescence intensities. The 2012 vent, which opened on 12 January, registered temperatures above 700°C on 22 February. Continued degassing was noted in conjunction with incandescent spots at several locations on the W crater (figure 29).

Figure 28. A panoramic view of the relative locations of the three vents which have been the sites of activity since 2010. Courtesy of OVSICORI-UNA.

Figure 29. A view of the 2012 vent from the overlook taken on 9 February 2012. The insert on the right is the second ash emission from the 2012 vent on 18 January. Courtesy of G. A. Avard, OVSICORI-UNA.

Geologic Background. Turrialba, the easternmost of Costa Rica's Holocene volcanoes, is a large vegetated basaltic-to-dacitic stratovolcano located across a broad saddle NE of Irazú volcano overlooking the city of Cartago. The massive edifice covers an area of 500 km2. Three well-defined craters occur at the upper SW end of a broad 800 x 2200 m summit depression that is breached to the NE. Most activity originated from the summit vent complex, but two pyroclastic cones are located on the SW flank. Five major explosive eruptions have occurred during the past 3500 years. A series of explosive eruptions during the 19th century were sometimes accompanied by pyroclastic flows. Fumarolic activity continues at the central and SW summit craters.

Information Contacts: Avard G., Pacheco J., Fernández E., Martínez M., Menjívar E., Brenes J., van der Laat R., Duarte E., Sáenz W., Observatorio Vulcanológico y Sismológico de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Tico Times (URL: http://www.ticotimes.net/); Reuters (URL: http://www.reuters.com/); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/).

After evaporating during 2011 and early 2012, White Island's crater lake rapidly rose on 28 July. Within two weeks, the first ash emissions from White Island in ~10 years occurred. This report summarizes GeoNet Alert Bulletins and provides selected photos of what "may represent the start of a new phase of activity at White Island."

Lake-level rise. During 2011-July 2012, White Island's crater lake slowly evaporated, exposing steam vents and leaving large mud pools on the lake floor (figure 52a). GeoNet reported intermittent volcanic tremor in early July 2012. One period of tremor lasted several hours in the early morning on 28 July; GeoNet stated that it may have been an indication than an eruption had occurred. Later that day, field observations revealed that the lake-level had rapidly risen 3-5 m sometime during the previous night or early morning (figure 52b). According to Brad Scott of GNS Science, rain and water derived from condensation within plumes were the sources of the lake-level rise.

Figure 52. Photos of White Island's crater lake taken on 6 March (a) and 28 July 2012 (b) illustrating the nearly dry lake floor during a period of evaporation in 2011-early 2012 and the newly refilled lake containing 3-5 m of water. The lake-level rose suddenly during 27-28 July 2012 (see text). The white asterisk marks the same location in each photograph. Courtesy of GeoNet.

The lake-level rise was accompanied by significant gas-and-steam emissions rising from the water. Gas measurements indicated an increase in SO₂ emissions compared to the last measurement three months prior, but CO₂ emissions were about the same. Ground surveys indicated that subsidence of the crater floor had stopped, and that the floor may have been slowly rising prior to the lake-level rise. Tremor was more continuous after 28 July 2012. As a result of the increased activity, the Aviation Colour Code was increased to Yellow (on a increasing scale of Green-Yellow-Orange-Red) on 2 August; the Alert Level remained at 1 (on a scale from 0-5).

First ash eruption in more than 10 years. An overnight episode of stronger tremor ended in a volcanic earthquake at 0454 on 5 August. Webcam images during the few minutes following revealed an accompanying plume rising from the crater lake (figure 53). As a result, the Alert Level/Aviation Colour Code was raised to 2/Orange.

Figure 53. An early morning webcam image of an eruptive plume at White Island on 5 August 2012. This was the first observed plume since the onset of the new episode of unrest in White Island's 1978/90 Crater Complex. Courtesy of GeoNet.

Two days later, on 7 August, tremor sharply decreased to levels seen prior to July 2012. A few hours later, however, the plume rising from the crater lake changed color from white to light brown, indicating the first observed ash erupted from White Island since February 2001 (BGVN 26:09). During a visit to the crater area, GeoNet volcanologists confirmed the ash emissions, and photographed the newly formed vent emerging in an area near the SW corner of the 1978/90 Crater Complex (figure 54). They described a 40-50-m-wide tuff cone forming around the vent and isolating the vent from the lake water. Impact craters around the tuff cone were the result of falling ejecta from explosions. The impact craters were confined to the 1978/90 Crater Complex.

Figure 54. A photograph of the new eruptive vent in the SW corner of White Island's 1978/90 Crater Complex. In this photograph, an ash laden plume is rising from the vent, and a 40-50-m-wide tuff cone is forming around the vent. Courtesy of GeoNet.

Through 13 August, weak volcanic tremor continued, along with steam-and-gas plumes that rose to 200-300 m above the crater and intermittently contained ash. A GeoNet Alert Bulletin released on the afternoon of 13 August announced the lowering of the Aviation Colour Code to Yellow "as a result of generally reduced ash emission." Four days later, on 17 August, the Alert Level was lowered to 1. GeoNet stated that "minor eruptive activity, which is required for Volcanic Alert Level 2, is no longer occurring and the Volcanic Alert Level is consequently reduced from 2 to 1." They noted that little-to-no ash was contained in steam-and-gas plumes, seismicity was low, and typical SO₂ levels were emitted during the previous week.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: GeoNet, a collaboration between the Earthquake Commission and GNS Science (URL: http://www.geonet.org.nz/); Brad Scott, GNS Science, Wairakei Research Center, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Earthquake Commission (EQC), PO Box 790, Wellington, New Zealand (URL: http://www.eqc.govt.nz/).