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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Robert Dziak at the NOAA/Pacific Marine Environmental Laboratory in Newport, Oregon reported that the large-aperture hydrophone array deployed throughout the north Pacific Ocean basin has been detecting extremely loud, tremor-like signals since May 1998. The best preliminary estimates of the signal sources lie ~1,000 km S of Honshu Island, Japan along the Volcano Island chain (astride the Bonin trench, figure 1).

Figure 1. A sketch map of part of the island of Honshu, and some of the historically active volcanoes of the Izu, Marianas, and Bonin arcs. Data from the NOAA large-aperture hydrophone array indicates a submarine volcano has been erupting in the area within the box shown. Two possible candidate sources for the eruption discussed in the text are Fukutoku-okanoba and Kita-Iwo-jima (Funka-asane). Courtesy of Robert Dziak and Yasuo Otani.

Dziak believes these tremors to be volcanic in origin. The signals are characterized by a high amplitude fundamental around 10 Hz and the next three harmonics (20, 30, and 40 Hz). Typically signals appear as discrete packets lasting 4-5 minutes, with a brief ~30 second quiescence period, followed by the beginning of the next signal packet. For the duration of each signal packet, the spectral peaks typically increase monotonically by 5-10 Hz while maintaining their harmonic spacing. Similar distinctive characteristics have been previously identified in volcanic tremor records from both seismic and airborne acoustic measurements at Arenal Volcano in Costa Rica (Garces et al., 1998) and at Pavlof Volcano, Alaska (Garces and Hansen, 1998).

Unfortunately, the source of these signals is outside the optimum coverage area for the NOAA array, so the estimated locations are not accurate; the best preliminary estimates place the signal source in a box at 22-27°N and 138-141°E that lies W of the Bonin arc (figure 1).

The tremor has been occurring intermittently since May 1998, and was still being recorded as of late December 1999. During this period, intense tremor activity was recorded on 30 different days. The signals have for the most part been occurring continuously (with quiet times ranging from several days to several weeks) since first detected. Specific periods of peak amplitude and duration in 1998 and 1999 are presented in table 1. Signals measured on 10-12 December 1999 were the loudest yet detected.

Table 1. Dates of the strongest hydro-acoustic signals registered on the NOAA large-aperture hydrophone array compared to observation dates of discolored seawater over Fukutoku-okanoba and the Funka-asane vent of Kita-Iwo-jima. Hydro-acoustic data courtesy of R. Dziak; seawater observations courtesy of Yasuo Otani, Japan Maritime Safety Agency and Japan Meteorological Agency.

Yasuo Otani of the Hydrographic Department of Japan has provided subsequent information (courtesy of Yukio Hayakawa) regarding periods of discolored sea water seen over Fukutoku-okanoba (24.3°N, 141.5°E). The latter is a known volcanic area located S of Iwo-Jima (24.75°N, 141.33°E) on the fringes of the area delineated above by Dziak. These dates are also presented in the second column of table 1; however, there does not appear to be an obvious correlation between the two data sets. On the other hand, what is not yet known is the density of visual observations, in effect, the number of observations of these sites when surface discolorations were absent. Without such details, trying to correlate the two data sets could be biased by sampling density.

Japan Meteorological Agency reports provided one other case of sea surface discoloration, at Funka-asane, but this lone observation also failed to show any temporal correlation and has the same limitations of sampling bias mentioned above. Funka-asane, a submarine vent ~2 km NW of Kita-Iwo-jima (25.43°N, 141.23°E), is just E of the preliminary box delineated by the acoustical data.

Olivier Hyvernaud at the Geophysical Laboratory in Tahiti had found no evidence of volcanic T-waves from the region in question through the end of 1999.

The area of the preliminary box is large, and could include many other volcanic centers. Given all of the uncertainty, anyone having possibly related data or comments is urged to contact Robert Dziak or the Smithsonian's Global Volcanism Network.

References. Garces, M.A., Hagerty, M.T., Schwartz, S.Y., 1998, Magma acoustics and time-varying melt properties at Arenal Volcano, Costa Rica: Geophysical Research Letters, v. 25, no. 13, p. 2293-6.

Garces, M.A., Hansen, R.A., 1998, Wave form analysis of seismoacoustic signals radiated during the fall 1996 eruption of Pavlof volcano, Alaska: Geophysical Research Letters, v. 25, no.7, p. 1051-4.

Geologic Background. Reports of floating pumice from an unknown source, hydroacoustic signals, or possible eruption plumes seen in satellite imagery.

Information Contacts: Robert P. Dziak, Oregon State University/NOAA, Hatfield Marine Science Center, 2115 SE OSU Drive, Newport, OR 97365 USA (URL: http://newport.pmel.noaa.gov/); Yasuo Otani, Coastal Surveys and Cartography Division, Hydrographic Department, Maritime Safety Agency, 3-1 Tsukiji, 5-Chome, Chuo-ku, Tokyo 104-0045, Japan; Olivier Hyvernaud, Laboratoire de Géophysique, BP 640 Pamatai, Tahiti, French Polynesia.

Fumarolic plumes generally rising 50-300 m above the volcano were often observed during clear weather in August-December 1999, but views were frequently obscured by meteorological clouds. Weak fumarolic activity without a significant plume was detected on a few other occasions during this period. Plumes were observed on the following days: 9-10, 16, and 20-23 August; 2, 12, 22, 26, and 28 September; 22-24, 25-27, and 29-31 October; 1, 5, 11-12, 19, 22-23, 26, and 29 November; 2-3, 24, 25, and 28 December. Depending on local conditions, the plumes often extended 5-10 km downwind, usually E and SE. Others were blown S, NW, or NE. The longest plume during this period was on 26 August when it extended 15 km NE. No seismicity was registered under the volcano from 10 August through the end of December 1999. On October 6, a shallow earthquake was registered under the volcano.

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

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

Following the gradual reactivation of the summit craters since June 1999 and eruptive episodes at the Voragine on 4 September and at the Bocca Nuova (BN) on 20 September, the activity shifted to the Northeast Crater (NEC) and then to the BN in early October. During the second half of October, the BN crater produced spectacular Strombolian activity, episodes of high lava fountaining, and lava overflows onto the W flank of the volcano, the first flows in that area since 1964. Lava flows on the W flank interrupted two dirt roads and burned a small portion of forest, but presented no threat to inhabited areas downslope. After 3 November, the activity declined to low levels.

The information for the following report, covering October-November 1999, was compiled by Boris Behncke at the University of Catania (DSGUC), Marco Fulle, Roberto Carniel, and Jürg Alean. Additional information was provided by Jean-Claude Tanguy. The compilation is based on personal visits to the summit, observations from Catania, and many other sources cited in the text.

Vigorous Strombolian activity occurred at the NEC during the first week of October. When the summit area was visited by Behncke, Roberto Scandone and Lisetta Giacomelli (Dipartimento di Fisica, Università "Roma Tre"), and Angelo Amara (Catania University) on 1 October, strong explosions ejected bombs up to 100 m above the crater rim, and ash emissions were frequent. Similar activity was observed during a summit visit by Behncke and others on 6 October. Brownish-gray ash plumes were frequent, and some of the Strombolian bursts were densely charged with small bombs.

Eruptive activity resumed within the BN on the afternoon of 5 October, after about two weeks of relative calm. After nightfall, Giuseppe Scarpinati (Italian correspondent of L'Association Volcanologique Européenne, LAVE) observed strong explosions from his home in Acireale (~18 km SE from the summit). Huge incandescent bombs were ejected to halfway down the S flank of the main summit cone. Scarpinati noted fluctuating glow at the NEC and increased effusion at the ESE base of the Southeast Crater (SEC) cone. Powerful explosions from the BN were continuing the next morning as Behncke and two students from the University of Trier visited Piano Provenzana on the N flank (~6 km from the BN). Explosions occurred at intervals of ~10 minutes, with minor activity between the explosions. Many bombs were ejected far beyond the crater rim. The source of this activity was probably at the SE eruptive center, which had been buried under lava on 20-21 September.

Vigorous eruptive activity continued at NEC and BN through 11 October. Dark ash-laden plumes commonly rose every few minutes from the NEC. Bombs were ejected from the BN to a distance of several hundred meters, and some bursts rose more than 300 m above the crater. Eruptive activity resumed within the Voragine and continued at least through the following day (information from Sandro Privitera, DSGUC, and Jean-Claude Tanguy).

On the afternoon of 12 October Behncke and Amara were ~250 m from the W rim of the BN, where activity was vigorous, with ejections of dense jets of bombs to hundreds of meters above the crater rim. Eruptive activity occurred from at least four locations within the crater. At 1830 there was the first in a series of powerful detonations that ejected abundant lithics along with incandescent bombs and a tephra-laden plume to ~500 m above the crater rim. The explosions initiated about 30 minutes of more intense activity from three locations in the W and NW part of the crater.

NEC emitted dark dense ash plumes almost continuously. After nightfall only ~10 percent of the emissions ejected incandescent bombs; other emissions appeared to eject mainly lithics. While near the front of the 22 July 1998 lava flow on the dirt road that connects the N and S routes to the summit (named "summit road" in the following paragraphs), several explosions from the Voragine were heard. At the ESE base of SEC cone lava was still issuing quietly after more than 8 months. The effusion rate was estimated at ~1 m³/s; during the previous four weeks, ~2.5 x 10⁶ m³ had been added to the more than 40 x 10⁶ m³ of lava emitted between 4 February and early September 1999.

Strong ash emission from the NEC on the morning of 13 October continued in a pulsating manner into the early afternoon of the following day. At the BN, however, near-continuous ejections of incandescent bombs caused rapid filling of the crater. On the evening of 15 October, vigorous eruptive activity occurred at the Voragine and loud detonations were audible as far as Catania.

Lava was fountaining in BN on the evening of 16 October, but strong explosions resumed the next morning (17 October). Fulle watched the activity from the summit road and reported that continuous lava jetting to several hundred meters above the crater rim occurred from several vents, and bombs dropped onto the outer flanks of the main summit cone. Sometime around 2015 a small portion of the W rim collapsed, allowing lava to move rapidly down the steep slope, crossing the summit road. On the early morning of 18 October, the farthest flow front had reached ~1,900 m elevation and stopped before reaching the Forestale dirt road (figure 82). Lava was reported to flow vigorously through the breach on the W side of the BN on the evening of 18 October, but the fronts did not extend as far downslope as the first major flows.

Figure 82. Sketch map of the lava flows emitted from the Bocca Nuova during October-November 1999, based on photographs taken after the end of the activity from various locations. Main vents of the Bocca Nova (BN) are shown as dots. The other summit craters are the Northeast Crater (NE), Voragine (V), and Southeast Crater (SE). Inset at upper left shows the entire Etna area with the location of the new lavas and the towns of Bronte and Catania. VDB in the inset is Valle del Bove. Courtesy of Boris Behncke.

At about noon on 19 October, Behncke and Scarpinati reached the summit area and observed near-continuous ejections of large bombs high above the rim of the BN. Movement of the lava flow on the W flank had slowed significantly, and only the central portion of the flow was moving. The lava field had many overlapping flow units with a total width of ~100 m at the summit road crossing. Between 1200 and 1230 activity increased until fountaining from the more southerly of the two vents became virtually continuous; frequent large blasts from the other vent dropped bombs up to 150 m beyond the crater rim. A short time later a new flow with a front ~3 m high advanced rapidly through the central flow channel, on top of the still-moving earlier lava. From points along the N margin of the lava field the summit of a pyroclastic cone growing within the BN could be seen rising above the crater rim. Explosive activity consisted of only a few ash-rich emissions between 1630 and 1730. After sunset the active flows were brightly incandescent over their entire length, and BN produced bursts of huge incandescent bombs every 2-10 seconds.

After continuing vigorously until the early morning of 20 October, the activity from the eruptive vents in the W and NW part of the BN ceased, and the lava overflow through the notch in the W crater rim stopped. Sometime near dawn, forceful expulsions of ash began from the SE vent, which had shown little activity the previous week. The low levels of activity permitted volcanologists from the U.K. to reach the rim of the BN and observe at least three vents with mild Strombolian activity and sizeable pyroclastic cones around them. On 21 October at 0300, intense eruptive activity apparently resumed, with renewed lava overflow onto the W flank. A new lobe on the S margin of the flow-field covered more of the summit road and extended to ~2,400 m elevation.

On the morning of 22 October, Scarpinati, from his home in Acireale, observed mild Strombolian activity (one explosion every 15-20 seconds) at the BN and more vigorous spattering at the vents on the ESE base of the SEC cone. By 1130 another episode of high lava fountaining and overflow from BN was in progress. From Catania jets of incandescent material to several hundred meters above the crater rim were visible, and a dense, ash-poor column of yellowish gas rose at least 4 km above the summit. Fulle witnessed the activity from a distance of a few hundred meters, and reported that a N-S fissure ~200 m long in the W part of the BN ejected a virtually continuous sheet of very fluid lava with jets rising up to 500 m high. A torrent of lava ran halfway down the W flank of the main summit cone at a speed of ~50 m/minute, carrying incandescent blocks more than 10 m across. An overflow may have also occurred on the NNW side of the BN. After 1230 the activity and the volume of overflowing lava diminished, but sporadic explosions threw large bombs hundreds of meters beyond the crater rim until 1700. Between 2000 and 2100 Behncke and Scarpinati visited the ESE base of the SEC cone where lava emission from at least three vents continued, and incandescent gas was emitted forcefully from two large hornitos that had grown earlier that day. Flowing lava was seen ~500 m NE and E from the active vents.

On 23 October another episode of high lava fountaining at the BN and overflow onto the W flank began at about 1000. This activity culminated at about 1045 but was less intense than the episode of the previous day. Relatively mild Strombolian activity persisted through the evening of 24 October, and small volumes of lava flowed onto the W flank. During the afternoon, Fulle and Carniel observed explosions (mostly ash) from four vents on the fissure in BN, and from a vent in the SE sector of the BN. During the night loud explosions at intervals of several minutes rattled windows and doors in towns 24 and 28 km NE.

On the morning of 25 October ash was emitted sporadically from BN until by about 1130 continuous fountaining was in progress. Broad jets of lava generally rose 100-200 m above the crater rim, but occasional jets soared to 500 m height. Lava again descended the W flank. A large pyroclastic cone near the vent that produced most of the fountaining (in the NW part of the BN) was ~30 m above the NW crater rim. Fulle and Carniel observed that the activity occurred from a number of vents along a N-S trending fissure in the W part of the BN. At 1145 Fulle observed that lava was overflowing the rim near the SW vent, covering the southern edge of the previous lava field.

From 1235 to 1300 the flank of the BN was affected by intense deformation, with the opening of several fractures and a series of collapses. Within a few minutes (peaking around 1320) a wide sector of the WNW crater rim was pushed up and out by lava within the crater. Minor collapses occurred for about 30 minutes while vigorous lava fountaining continued. The avalanches resulting from the collapses spilled several hundred meters down the W flank and produced brownish plumes. Movie clips taken by Carniel of the deformation and avalanches are available at Stromboli On-line. Lava flowing through the new breach was repeatedly covered with debris but continued to flow, carrying boulders up to 20 m in diameter. On the N side of the BN the mass of fluid bombs transformed into a rootless lava flow that advanced along the flow emplaced on 22 October, but extended farther downslope. The episode ended by about 1630, but was followed by a series of strong isolated explosions. By 1900, the main vent in the BN produced frequent Strombolian bursts, and lava flow through the breach in the crater rim continued at a reduced rate.

Observations made that evening revealed that a new lava flow with at least seven active branches had descended the W flank, and the farthest flow front had extended to ~1,900 m elevation. By about 1810 the front of the longest branch began moving through a small patch of forest a few hundred meters above the Forestale Road. The new lava flow was slightly N of the flows produced during the preceding week, with the longest branch extending almost 5 km from the BN, thus being one of the longest flows ever produced by a summit eruption.

On the morning of 26 October, the activity consisted mostly of isolated ash-rich explosions from the southernmost fissure vent in the W part of the BN. Towards the evening the activity became more continuous and there was mild Strombolian activity. Fulle and Carniel reported that up to five vents along the fissure were active. Explosions also occurred from two vents in the SE part of the BN where little activity had been observed the previous week.

On 27 October jets of lava rose tens of meters above two main vents in the W part of the BN, and a new large pyroclastic cone was growing around the northernmost vent. Lava continued to overflow on the W side of the crater, with active flow fronts to ~2,600 m elevation. Between 0015 and 1045, Fulle, Carniel, and Tom Pfeiffer (University of Arhus) observed intense activity, mostly in the NW sector of the BN. From 1230 onwards the explosions of the NW vent of the BN became increasingly stronger. Between 1400 and 1415 some of the largest explosions showered bombs over the whole main summit cone, and a scoria fall was noticed at the Torre del Filosofo mountain hut. At 1433 strong explosions of dark ash occurred at the NEC. The activity of the BN remained strong all afternoon. New lava spilled down the W flank, and at about 1700, the farthest flow front cut the Forestale road at about 1,800 m elevation, immediately S of Monte Nunziata (the main scoria cone of the 1843 eruption), and entered a patch of dense forest. Early the next morning the front of the main flow had extended ~200 m below the Forestale road, to ~1,730 m elevation; by 29 October the flow had stopped.

Vigorous lava jetting from the BN was observed at about 0600 on 29 October by Giovanni Sturiale (DSGUC). Activity observed by Sturiale, Behncke, Pfeiffer, and Vincenzo Polizotto (University of Catania) later that day included incandescent bombs from the NW vent, forceful ejections of dark gray ash and blocks from the SE vents, and vigorous Strombolian activity at the NW vent where the top of the new pyroclastic cone was projecting a few tens of meters above the crater rim. A variety of lava flows were seen on the W flank. Vigorous pulsating lava jetting from the NW vent was continuing at about 2230.

On 30 October, Pfeiffer revisited the summit area and reported that relatively mild Strombolian activity continued throughout the day. The entire Voragine area was covered with bombs, and the Voragine itself "had ceased to exist" because the 4 September 1999 crater was filled to within ~40 m of its rim. The active cone at the NW vent in the BN was very close to the location of the former "diaframma," of which no trace was visible. Emission of blocks and ash from the SE vents in the BN continued. During an overflight by Tanguy at about 1300, a bright red vent lay in the middle of the NW-trending BN fissure. Small lava flows were seen on the upper W slopes and a scoria cone was being built around the NW vent. NEC and SEC emitted a moderate white plume. After sunset a large red glow on the W flank indicated renewed strong effusive activity.

On the evening of 31 October, Scarpinati observed from Acireale that vigorous lava spattering had resumed at the ESE base of the SEC cone, while Strombolian activity at the BN was continuing. Scarpinati visited the area on 1 November and described voluminous lava flows running towards the Valle del Bove, and spattering from a group of hornitos. Effusive activity at the ESE base of the SEC cone showed a marked decrease after 2 November. On the 6th, Scarpinati observed trickles of lava flowing from these vents, but none thereafter.

On 1 November, Behncke and others climbed to the SW side of the BN where vigorous Strombolian activity continued from the NW vent, and occasional weak Strombolian bursts occurred from a vent farther S. Lava again extruded from below the uplifted block of 25 October. The southernmost of the three active lava lobes ran along the S margin of the lava field, cutting another 10 m of the summit road. Explosive activity at the NW vent produced jets up to 300 m high, but ~90 percent of the bombs fell back into the crater, enlarging the pyroclastic cone. On the evening of 3 November BN produced continuous jets of lava up to 300 m high, the last major eruptive episode of the sequence initiated on 5 October. Activity ceased after 0400 on 4 November, and after that the BN produced only weak intermittent Strombolian activity through about 15 November.

The volume of lava erupted from the BN between 17 October and 3 November is probably in the range of 15-20 x 10⁶ m³. Tanguy estimated that the lava flows of 27 October alone amounted to ~5 x 10⁶ m³, and similar flows were erupted on at least three other occasions. This places the October-November activity from the BN among the largest summit eruptions recorded at Etna during the past 200 years. The BN, which had been a 400-m-diameter pit about 150 m deep in 1995, was completely filled, and a sizeable pyroclastic cone was built in its N part, partly burying the "diaframma," the former wall separating this crater from the Voragine. Post-eruption collapse and subsidence caused the partial destruction of this cone and the formation of two pits at the main NW and SE vents of the BN, and the lava-covered plateau filling the former crater subsided by several meters towards its center. On the W side of the main summit cone, the accumulation of new lava caused a considerable buildup of this flank. The Voragine was largely filled by pyroclastics from the NW vent of the BN, with only a shallow depression remaining in its central part.

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

Information Contacts: Boris Behncke, Dipartimento di Scienze Geologiche, Palazzo delle Scienze, Università di Catania (DSGUC), Corso Italia 55, 95129 Catania, Italy; Roberto Carniel, Dipartimento di Georisorse e Territorio, Università di Udine, Via Cotonificio 114, 33100 Udine, Italy (URL: http://www.swisseduc.ch/stromboli/); Jürg Alean, Kantonsschule Zürcher Unterland, CH-8180 Bülach, Switzerland; Marco Fulle, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy; Jean-Claude Tanguy, Université Paris 6 and IPGP, Observatoire de Saint-Maur, 4, avenue de Neptune, 94107 Saint-Maur des Fossés Cedex, France.

Following the Ms 7.8 earthquake in Turkey on 17 August (BGVN 24:08) an Email message originating in Turkey was circulated, claiming that volcanic activity was observed coincident with the earthquake and suggesting a new (magmatic) volcano in the Sea of Marmara [40.683°N, 29.1°E]. For reasons outlined below, and in the absence of further evidence, editors of the Bulletin consider this a false report.

The report stated that fishermen near the village of Cinarcik, at the E end of the Sea of Marmara "saw the sea turned red with fireballs" shortly after the onset of the earthquake. They later found dead fish that appeared "fried." Their nets were "burned" while under water and contained samples of rocks alleged to look "magmatic."

No samples of the fish were preserved. A tectonic scientist in Istanbul speculated that hot water released by the earthquake from the many hot springs along the coast in that area may have killed some fish (although they would be boiled rather than fried).

The phenomenon called earthquake lights could explain the "fireballs" reportedly seen by the fishermen. Such effects have been reasonably established associated with large earthquakes, although their origin remains poorly understood. In addition to deformation-triggered piezoelectric effects, earthquake lights have sometimes been explained as due to the release of methane gas in areas of mass wasting (even under water). Omlin and others (1999), for example, found gas hydrate and methane releases associated with mud volcanoes in coastal submarine environments.

The astronomer and author Thomas Gold (Gold, 1998) has a website (Gold, 2000) where he presents a series of alleged quotes from witnesses of earthquakes. We include three such quotes here (along with Gold's dates, attributions, and other comments):

(A) Lima, 30 March 1828. "Water in the bay 'hissed as if hot iron was immersed in it,' bubbles and dead fish rose to the surface, and the anchor chain of HMS Volage was partially fused while lying in the mud on the bottom." (Attributed to Bagnold, 1829; the anchor chain is reported to be on display in the London Navy Museum.)

(B) Romania, 10 November 1940. ". . . a thick layer like a translucid gas above the surface of the soil . . . irregular gas fires . . . flames in rhythm with the movements of the soil . . . flashes like lightning from the floor to the summit of Mt Tampa . . . flames issuing from rocks, which crumbled, with flashes also issuing from non-wooded mountainsides." (Phrases used in eyewitness accounts collected by Demetrescu and Petrescu, 1941).

(C) Sungpan-Pingwu (China), 16, 22, and 23 August 1976. "From March of 1976, various large anomalies were observed over a broad region. . . . At the Wanchia commune of Chungching County, outbursts of natural gas from rock fissures ignited and were difficult to extinguish even by dumping dirt over the fissures. . . . Chu Chieh Cho, of the Provincial Seismological Bureau, related personally seeing a fireball 75 km from the epicenter on the night of 21 July while in the company of three professional seismologists."

Yalciner and others (1999) made a study of coastal areas along the Sea of Marmara after the Izmet earthquake. They found evidence for one or more tsunamis with maximum runups of 2.0-2.5 m. Preliminary modeling of the earthquake's response failed to reproduce the observed runups; the areas of maximum runup instead appeared to correspond most closely with several local mass-failure events. This observation together with the magnitude of the earthquake, and bottom soundings from marine geophysical teams, suggested mass wasting may have been fairly common on the floor of the Sea of Marmara.

Despite a wide range of poorly understood, dramatic processes associated with earthquakes (Izmet 1999 apparently included), there remains little evidence for volcanism around the time of the earthquake. The nearest Holocene volcano lies ~200 km SW of the report location. Neither Turkish geologists nor scientists from other countries in Turkey to study the 17 August earthquake reported any volcanism. The report said the fisherman found "magmatic" rocks; it is unlikely they would be familiar with this term.

The motivation and credibility of the report's originator, Erol Erkmen, are unknown. Certainly, the difficulty in translating from Turkish to English may have caused some problems in understanding. Erkmen is associated with a website devoted to reporting UFO activity in Turkey. Photographs of a "magmatic rock" sample were sent to the Bulletin, but they only showed dark rocks photographed devoid of a scale on a featureless background. The rocks shown did not appear to be vesicular or glassy. What was most significant to Bulletin editors was the report author's progressive reluctance to provide samples or encourage follow-up investigation with local scientists. Without the collaboration of trained scientists on the scene this report cannot be validated.

References. Omlin, A, Damm, E., Mienert, J., and Lukas, D., 1999, In-situ detection of methane releases adjacent to gas hydrate fields on the Norwegian margin: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Yalciner, A.C., Borrero, J., Kukano, U., Watts, P., Synolakis, C. E., and Imamura, F., 1999, Field survey of 1999 Izmit tsunami and modeling effort of new tsunami generation mechanism: (Abstract) Fall AGU meeting 1999, Eos, American Geophysical Union.

Gold, T., 1998, The deep hot biosphere: Springer Verlag, 256 p., ISBN: 0387985468.

Gold, T., 2000, Eye-witness accounts of several major earthquakes (URL: http://www.people.cornell.edu/ pages/tg21/eyewit.html).

Geologic Background. False or otherwise incorrect reports of volcanic activity.

Information Contacts: Erol Erkmen, Tuvpo Project Alp.

At 1832 on 22 October, a 10-minute series of shallow earthquakes was recorded at the volcano. The last Gorely eruptive activity occurred in 1980-81 (SEAN 05:07) and 1984-86 (SEAN 10:01).

Geologic Background. Gorely volcano consists of five small overlapping stratovolcanoes constructed along a WNW-ESE line within a large 9 x 13 km caldera. The caldera formed about 38,000-40,000 years ago accompanied by the eruption of about 100 km3 of tephra. The massive complex includes about 40 cinder cones, some of which contain acid or freshwater crater lakes; three major rift zones cut the complex. Another Holocene stratovolcano is located on the SW flank. Activity during the Holocene was characterized by frequent mild-to-moderate explosive eruptions along with a half dozen episodes of major lava extrusion. Early Holocene explosive activity, along with lava flows filled in much of the caldera. Quiescent periods became longer between 6,000 and 2,000 years ago, after which the activity was mainly explosive. About 600-650 years ago intermittent strong explosions and lava flow effusion accompanied frequent eruptions. Historical eruptions have consisted of moderate Vulcanian and phreatic explosions.

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

This report covers 22 November through 24 December 1999, an interval when long-period earthquakes increased precipitously. The dome in the caldera's western sector continued to produce explosions, lava extrusions, and rockfalls. November 1999 marked the 32nd month since the unrest began; occasional ashfalls and associated disruptions (minor ashfall, airport closures, hundreds of evacuated refugees) have had a significant impact on Quito residents.

Seismicity. Earthquake hypocenter maps appearing on the Geophysical Institute's website showed the vast majority of earthquakes clustering beneath the crater area; in some cases these clusters also spread W with gradually decreasing density. The website also included a diagrammatic cross section through the crater (figure 20) illustrating the inferred plumbing system, including some typical depths for various kinds of earthquakes. On the inset, the diagram shows an inferred shallow aquifer within the edifice that intersects the active conduit and presumably contributes to the repeated phreatic eruptions.

Figure 20. A diagrammatic E-W cross-section through the crater at Guagua Pichincha. The cross-section is intended to show the overall internal structure and the zones where the main kinds of earthquakes seen during the crisis have typically originated. The scale across the bottom of the main diagram corresponds to a local coordinate system; the one along the left side of the main diagram indicates depth with respect to sea level (0 km). The inset contains an enlarged view of the crater area. Courtesy of the Geophysical Institute.

During November 1999 phreatic explosions took place 41 times. Many months during the crisis had fewer than 20 explosions per month, and the November 1999 value was the second highest of the crisis. The highest monthly total occurred during October 1999, a count of 53 explosions.

Seismicity had been escalating rapidly during September-October 1999 (see plot, BGVN 24:10). A precipitous climb in long-period (LP) earthquakes continued during November, reaching dramatic levels (table 7); in September long-period earthquakes occurred ~12,000 times, in October ~15,000 times, and in November ~44,000 times. For another comparison, LP counts earlier in the crisis (July 1998-August 1999) generally remained below 200 earthquakes per month. Thus, compared to this broader interval, the November 1999 count of LP events reflected more than a 200-fold increase. In addition, November's LP earthquakes exceeded the sum for LP events during the previous 16 months.

Table 7. Monthly earthquake counts at Guagua Pichincha representing two key time intervals. The "upper threshold" refers to the highest values registered during the earlier parts of the crisis, July 1998-August 1999. The next three columns indicate the monthly counts during September-November 1999, an interval with the highest numbers of earthquakes yet seen during the current crisis. Courtesy of the Geophysical Institute.

A change in the relative numbers of events appears to have occurred beginning in September 1999. From then on, LP events occurred with either similar abundance to MP events, or in some cases LP events became dominant. The total of MP plus LP events (table 2) continued to increase through November 1999.

On the other hand, the escalation in Multiphase (MP) and volcano-tectonic (VT) earthquakes has diminished since the anomalously high values seen in September and October 1999 (table 7, and BGVN 24:10). Compared to earlier in this crisis, MP earthquake counts underwent a sudden peak in October at ~15,000 events; in November there were ~6,000 MP events. VT earthquake counts underwent a less pronounced peak in September and October with ~1,300 and ~1,700 respective events. November VT earthquakes totaled only 104, a value still within the upper end of the monthly counts seen for the bulk of the crisis.

As a result of ongoing dome growth, rockfall-associated seismicity increased. The highest days in September-November had daily LP counts of 250-300 per day. Peaks in dome-growth events approached or exceeded 100 events/day for sustained intervals both during early October and late November 1999.

Daily observations. Tens of daily phreatic explosions were common. Counted seismically, these events appeared so numerous that generally only large ones received much mention in the daily reports (summarized in table 8). On many days visibility into the caldera remained limited because of clouds and fog.

Table 8. Summary of the more important explosions reported at Guagua Pichincha during 22 November-22 December 1999. The explosions discussed here were selected by choosing the Institute's daily reports where the seismically determined parameter of reduced displacement (RD) was reported. Courtesy of the Geophysical Institute.

Two explosions on 24 November resulted in significant ashfall on inhabited areas. The latter explosion, around noon, sent a plume to 10 km altitude. Fine ash fell in areas N of Quito, blanketing zones that included the airport, which closed. The ash also affected numerous settlements within a few tens of kilometers N to NE of the summit (including Carcelén, 14.5 km NE; Cotocollao, 9.4 km N; Quito Tenis, 13.5 km NE; and at locations not found on available maps, at la Roldós, La Carolina, Mariscal, and el Ejido). The greatest thicknesses of ash reportedly fell between Jipijapa (unlocated) and la Mariana de Jesús (20.9 km NE).

More events took place the next day, and in the morning ashfalls were reported in Quito's northwestern neighborhoods. The ash lingered in the air well into the next day as a result of disturbances by traffic and cleanup.

An inspection of the W flank on 24 November revealed that during the past week the Cristal river had been inundated by lahars 400 m wide and 10 m deep, although the point of measurement was at an unstated distance from the summit. They were still hot, at least in places, and contained some component of pyroclastic flows bearing carbonized tree-trunks in addition to blocks from the dome. On 30 November observers visiting the Cristal river noted a 1-day-old block-and-ashflow deposit. In the same sector on 8 and 10 December field crews again linked observed zones of burned and singed leaves to probable pyroclastic flows.

On 17 December a white mushroom cloud preceded a dark, ash-bearing one that rose 8-9 km above the volcano. On 18 December, light ash again fell on Quito landing mainly in its central and northern zones. Portions of the cone's flanks received pumice 2-5 cm in diameter. Strong sulfur smells were noted by S-flank residents in Lloa.

An overflight on 21 December enabled the dome height to be estimated at 50-100 m from the base of the caldera. On the dome's W side observers identified a spine, possibly the same one as seen in November. Dark coffee-colored rocks were observed along the E margin of the new dome.

GOES-8 satellite imagery captured plumes on several occasions. For example, it recorded an explosion at about 1140 on 29 November. NOAA analysts estimated the ash plume rose to an altitude of 10-12 km and drifted S toward Tungurahua volcano (which was also producing a faint ash plume). The same ash plume was noted using the "split window" technique, wherein infrared channel 5 (13 µm) is subtracted from infrared channel 4 (11 µm), which often discriminates airborne silicates such as dust and volcanic ash from other features in an image.

During comparatively passive intervals with adequate visibility, daily reports typically described several distinct plumes emitted from the following sources: a) the "aligned" fumaroles (in Spanish, "las alineadas"), b) the fumaroles on the caldera's W border near the head of the Cristal river, c) fumaroles escaping from the 1981 crater, and d) emissions from the top of the new dome. Fumaroles designated as "a" and "b" had plumes that typically reached several hundred meters from base to top; "c" fumaroles typically had plumes that reached tens of meter from base to top.

Radiosondes. According to the Washington Volcanic Ash Advisory Center at NOAA's Satellite Analysis Branch (SAB), during late 1999 and early 2000 authorities in Quito have been launching weather balloons twice a day. The resulting upper atmospheric air movements generally appear on the Geophysical Institute's website. Because these data have been occasionally internally inconsistent in azimuth, they have not yet been incorporated into the modeled data nor the plume trajectory modeling. The SAB has repeatedly seen highly variable winds in the region.

News reports. A brief review of news reports during the past few months revealed numerous stories, some of which were listed on an Ecuadorian Embassy website. ABC News discussed the effects on the explosions of 5-7 October (BGVN 24:09); previously unmentioned in the Bulletin was that the explosion of 5 October caused respiratory problems for many area residents and the death of one man. Four others were injured clearing ash from the roofs of their homes. Quito's Marshal Sucre airport closed for multiple days during the crisis. This not only causes travel problems, but inevitably some commercial aircraft that remain on the ground require cleaning to regain flight worthiness. ABC News also reported that the 24-26 November eruptions that forced one closure of the airport had also caused the Ministry of Education to shut down schools for a few days.

A series of 17-22 November articles in the online Diario Hoy newspaper discussed conditions confronted by 500 refugees from Lloa and neighboring areas living in the largest of several tent cities in a pass above their town. The tent city's amenities included electrical power, water, bathroom facilities, and trash collection; tents came equipped with stoves and beds. The city also provided medical and dental services. Other tent cities provided refuge for ~300 more people. Guards limited access into Lloa, and the town itself was patrolled by the military.

Hoy Digital reported that Quito's mayor, Roque Sevilla, delivered Motorola radios to each one of the leaders of the 35 neighborhoods located on the volcano's slopes as a means of maintaining constant communication with the emergency system locally referred to as "911." The article also mentioned a project developed with the support of the German embassy and the firm Siemens that consists of a system of warning sirens intended to alert citizens of impending danger.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately W of Ecuador's capital city, Quito. A lava dome grew at the head of a 6-km-wide scarp formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the lava dome. Many minor eruptions have been recorded since the mid-1500's; the largest took place in 1660, when ash fell over a 1,000 km radius and accumulated to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity.

Information Contacts: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Embassy of Ecuador, 2535 15th Street NW, Washington, D.C. 20009 USA (URL: http://www.ecuador.org/); Washington Volcanic Ash Advisory Center, NOAA Satellite Services Division, NESDIS E/SP23, NOAA Science Center, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); ABC News (URL: http://abcnews.go.com/); Diario Hoy, Ecuador (URL: http://www.hoy.com.ec/).

The low-level strombolian eruptive activity that has characterized the volcano for more than three years gradually decreased after August until seismicity returned to background levels, and by late December there were no explosions. The eruption began on 2 January 1996 (BGVN 21:01) following an eruption from the Akademia Nauk caldera lake the previous day.

During the week of 9-15 August, steam-and-ash plumes were observed in satellite imagery extending as far as 75 km downwind at an altitude of 500-1,000 m above the crater. The number of gas-and-ash explosions was still more than 300/day the next week, with the plume rising 300-600 m above the volcano. During the last week of August through 5 September, the number of explosions was more than 75/day, with plumes to heights of 300-1,000 m above the volcano. Visual observations by KVERT staff on 1 and 5 September confirmed that explosive activity occurred every 10-20 minutes.

The number of gas-and-ash explosions decreased from 130 on 6 September to 80 on the 12th, but the plumes continued to rise 300-1,000 m above the volcano. That rate continued until the week of 20-26 September, when the average number of daily explosions decreased to 60. The number of explosions was 60-75/day during the next two-week reporting periods, through 10 October. During the week of 11-17 October the explosion rate decreased once again, to 20-35/day, although plume heights remained at 300-1,000 m. The number of explosions increased slightly, to 20-50/day, during 5-18 November, but then dropped the following week to 10-20/day and then only 2-5/day. During the week ending on 2 December, gas and ash explosions numbered 1-10/day.

The nearest seismic station (KRY) was out of order during 4-18 December. According to the regional seismic network, no strong events occurred during that period. The station was restored to operation on 19 December. As of 30 December seismicity at the volcano had decreased to background levels. About 1-2 local earthquakes occur every day and the volcano has returned to its normal state. At the end of December seismicity was at background levels of about 1-2 local earthquakes/day.

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

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

Highly variable activity continued throughout August-December 1999. Typical daily activity observed during clear weather consisted of a small fumarolic plume rising 50-200 m above the crater and extending a few kilometers downwind, usually E or SE. Seismicity was generally at background levels, consisting of shallow earthquakes with some periods of tremor. However, higher gas-and-steam plumes were frequently seen and two episodes of increased seismicity were detected. The volcano was frequently obscured by clouds.

Tremors and shallow earthquakes were registered during 9-15 August. Typical small fumarolic plumes were seen on 9-10, 13-14, 16, 21-26, and 28 August, and 2, 4-5, 7-8, and 12 September. On 30-31 August a gas-and-steam plume rose 500-1,500 m above the crater. On 15 September a gas-and-steam plume rose 600 m, and on 16 September the plume rose 200 m extending 5 km E. Mainly shallow earthquakes were registered from 19 September through 24 October. Gas-and-steam plumes rose up to 500 m during 19-26 and 28 September, and 3, 5, 7, 11, 20-21, and 24 October, extending as far as 5 km E or SE. During the afternoon of 15 October there was a 6.5-hour-long series of shallow earthquakes. On 22-23 October a fumarolic plume rose 700-1,000 m and extended 5-20 km to the E and SE.

Seismicity, consisting of shallow earthquakes and tremor, was above background levels during much of the period from 25 October until 17 December. Only small fumarolic plumes 50-300 m high were seen on 25 and 27 October, but on 26 October a plume rose 1,000 m above the volcano and extended 40 km NE. Small fumarolic plumes to 300 m extending 5 km SE were seen on 29-31 October and 4 November, with smaller typical plumes on 5, 7-8, and 10-11 November. Shallow earthquakes and volcanic tremor were recorded especially on 15, 21, and 25 November, when a gas-and-steam plume rose 1,000 m and extended more than 7 km NE. Typical smaller fumarolic plumes were seen on 12, 16, 18-19, 22-24, 26, and 28 November, and on 1, 3, and 10 December. On 29 November and 1 December gas-and-steam plumes rose 1,500 m above the volcano and extended more than 20 km SE. A fumarolic plume on 8 December rose 2,500 m.

During December 17-29 seismicity at the volcano returned to background levels. Small plumes were recorded on 17, 19-21, 25, and 28 December. Another plume on the 23rd rose 700 m.

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

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

The following report resulted from a visit to the crater of Ol Doinyo Lengai during 23 July-7 August. Prior to the visit and according to a local source (Burra Ami Gadiye), lava breaching the NW crater rim on 18 July flowed down the flank of the volcano and was visible at night from Ngare Sero village, ~10 km N. When the visitor's crater observations began at 1100 on 23 July, this lava flow from the NW crater rim breach had cooled and was becoming white from weathering, but it was clearly the most recent lava in the crater. Its source was hornito T40 (figure 63) based on comparisons of 1998 and 1999 photographs by C. Weber. From 2 to 6 August, an intermittent lava lake 3 m in diameter also existed inside T37N1 at a depth of 20 m.

Figure 63. Sketch map of the crater at Ol Doinyo Lengai for the period 23 July-7 August 1999. Courtesy of Christoph Weber.

The conical part of T40 was 85 m around at its base and 12 m tall. The N side of the hornito's cone was walled by a low overhanging rim and its S side was covered by a high half-dome. The hornito also included a large, 6-m-deep crater. A small lava pond at the N end of the crater ejected 16-20 spatters per minute through 24-25 July. Twice on 26 July parts of the half-dome and the cone's summit collapsed into the crater.

During 27-28 July lava gradually rose inside the crater of T40 and formed a 4 x 6 m lake and several ponds. By 29 July the lake was ~12 m long and 7 m wide. In a pattern repeating every 15-20 minutes a surge of fresh lava boiled up from the NE corner of the lake, raising the level by 0.5 m. Lava flowed out of the lake to the NW through a subterranean tunnel but did not escape onto the main crater floor.

Although this pattern persisted for some time, at 1400 on 30 July an abrupt increase in activity produced high lava spatters that fell on the N flank of T40. Fresh lava swept into the lake from the N like breaking ocean waves and strong ground tremor shook the N flank of the cone. This activity continued through 31 July, when the lake rose to ~60 cm below the lowest point along the vent rim. Spatter gradually built up the N wall of the crater by more than 1 m and formed a large hood overhanging the area of most intense degassing.

At 0045 on 1 August, a hole developed in the hornito's new crater wall. Lava escaped and moved N as short aa flows up to 60 cm thick. Lava ceased to escape by 0600 but similar eruptions recurred through 1300 on 2 August. Intense degassing later destroyed the hood covering the N part of the lake, but splashing built a thick covering of spatter on the N flank of the cone and reconstructed the hood. Around 0300 on 3 August a new vent opened low on the NW flank of T40 where the strongest tremor had been during the previous few days. An aa lava flow 20 cm thick moved 73 m NW. By 0800 the eruption had ended and the lake level dropped by 2 m. By 0600 on 4 August the lake temporarily disappeared, leaving a solid crater floor 2.5 m below the rim. Lava reappeared about noon but only occupied a 2 m² area at the crater's N end; the lava frequently overflowed from the pond and produced many small lava flows that covered most of the hornito's crater floor. At 2345 solid lava covering the new vent on the NW flank of T40 blew off; explosions occurred at a rate of 18-20/minute and constructed a new spatter cone. During repose periods, the activity shifted to the lava lake, creating high spatters that reached the summit of T40. After explosions ended at 0800 on 5 August, the new cone was 3 m tall with a circular summit vent 60 cm in diameter. Lava was bubbling in the vent at a depth of 1 m (figure 64).

Figure 64. Photograph taken in the crater at Ol Doinyo Lengai showing a local guide in front of T40 during formation of the new spatter cone taken at about 0700 on 5 August 1999. Courtesy of Frederick Belton.

At 2000 on 5 August pahoehoe lava flowed rapidly across the NE rim of T40 and moved E for 55 m. At 0645 the next morning, more lava escaped the lake through a hole in the NE rim of T40 and covered much of the previous night's flow. Beginning at 1800 on 6 August the lake repeatedly overflowed the hornito's NE rim, later overflowing the NW rim. Around 0400 on 7 August a hole that opened 1 m below the NE rim of T40 gradually enlarged and drained ~60 m³ of lava from the lake forming an open NE-directed lava channel 60 cm wide. By 0800 on 7 August the hole was 1 m high and 0.5 m wide. When observations ended at 0815, lava was nearing the NE crater wall and subsequent reporting noted that lava never reached the breach in the E crater rim, stopping short by 70 m. It was later learned from Guillaume Delpech, a French geology student, that during his visit to T40 on 9 August, the lava lake level inside the hornito varied between 3 and 4 m below its rim. No lava flowed outside of T40 and the spatter cone was inactive.

Christoph Weber made temperature measurements using a digital thermometer (TM 9¹⁴C with a stab feeler standard K-type) during the crater visit (table 2). The instrument was used in the 0-1200 Celsius mode, taking readings by inserting the feeler 15 cm into the lava. Calibration was made by the Delta-T method: values are ± 6°C in the 0-750°C range. Most values shown were maxima recorded from a series of at least five repeat measurements.

Table 2. Temperature estimates from 60 measurements at Ol Doinyo Lengai made during 23 July-7 August 1999. See text for method used. Courtesy of Christoph Weber.

Activity during early September 1999. Bruno Hermier visited the crater in early September and made the following observations. On the afternoon of 6 September only the northernmost hornito (T40) seemed to be active. A lava flow was estimated to be about two days old. Two E-W fissures cross the western half of the crater emitting fumaroles that deposit sulfur. The fissures are perpendicular to the N-S trend of the volcano and radiate from the hornitos. On 7 September at 0900 some spatter came from the top of the 7-m-high T40 hornito. The spatter became larger, creating a pond of lava visible at the top of the hornito. It began to overflow on all sides of the hornito for 15 minutes before the lava level dropped. This cycle repeated until 1300, after which only a low hissing noise was perceptible. Interestingly, a foam filled the hornito. The spatter that splashed on the sides of the chimney and the fluid that overflowed the rim instantaneously lost 75 percent of their volume as gas exsolved. The remaining 25 percent cooled or flowed as black carbonatite. The extremely fluid flows (consistency of oil or hot tar) were only a few centimeters thick, but extended 50-100 m. No additional activity was seen through the evening of 9 September.

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

Information Contacts: Frederick Belton, 3555 Philsdale Ave., Memphis, TN 38111 USA (URL: http://oldoinyolengai.pbworks.com/); Celia Nyamweru, Department of Anthropology, St. Lawrence University, Canton, NY 13617 USA (URL: http://blogs.stlawu.edu/lengai/); Christoph Weber, Friesenstrasse 20, 42107 Wuppertal, Germany; Bruno Hermier, France.

During the night of 4-5 August 1999, strong seismic activity occurred near Cerro Negro and the earthquakes with magnitudes up to 4.8 were felt throughout NW Nicaragua, especially in the big cities of León (20 km away, where many people could not sleep because of the seismic events) and Chinandega (40 km away). The strongest event was even felt 70 km away in Managua. The Nicaraguan seismic network recorded hundreds of earthquakes and strong seismic tremor at the seismic station at the volcano and at the MIRAMAR station (7 km away).

Three notices were received from the GOES alarm network concerning Cerro Negro. Distinct hot spots, indicating small plumes over the volcano, were detected on infrared satellite imagery at 0055, 0155, and 0235 on 5 August.

Explosive eruptions began at about 1000 on 5 August 1999. Ash clouds at heights of about 7,000 m were reported by aircraft. Ashfall was reported from some places SW of the volcano. The activity issued from four new vents outside the main crater, very near to the parasitic crater Cristo Rey, on the S flank of Cerro Negro. The vents formed cones ~40 m high during the day.

Wilfried Strauch visited the volcano that afternoon and observed explosions every few seconds, sometimes generating lava fountains ~300 m high. The activity alternated among the different new cones. No significant amounts of volcanic ash were emitted at this time. Local residents ~1 km from the volcano reported that seismicity was extremely strong during the night. Fissures appeared in the soil near their houses, releasing vapor.

INETER informed Civil Defense and other institutions on the night of 4 August about the seismic activity. Civil Defense officers visited the volcano early in the morning of 5 August, but could not yet detect signs of volcanic activity. When they got the information about the beginning of the eruption they proceeded with the evacuation of nearby villages, involving several hundreds of people.

Volcanic ash advisory statements on 6 August indicated that well-defined hot spots were still occasionally visible on GOES-8 multi-spectral imagery through 1615. No ash was visible in the imagery at that time, and thick clouds moved over the area later in the day. Imagery obtained under clear skies at 1015 on 7 August revealed no ash or hot spot.

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

Information Contacts: Wilfried Strauch, Instituto Nicaraguense de Estudios Territoriales (INETER), Division of Geophysics, Apartado 2110, Managua, Nicaragua; Benjamin van Wyk de Vries, Magmas et volcans Observatoire du Physique du Globe, Departement des Sciences de la Terre, Université Blaise Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France (URL: http://modis.higp.hawaii.edu/); Washington Volcanic Ash Advisory Center, NOAA Satellite Services Division, NESDIS E/SP23, NOAA Science Center, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

In August, several stations of the seismic network at San Salvador volcano recorded a few volcano-tectonic events 5 km from the crater. Local scientists investigated a fumarolic field, but nothing abnormal was found.

Geologic Background. The massive compound San Salvador volcano dominates the landscape W of El Salvador's capital city of San Salvador. The dominantly andesitic Boquerón stratovolcano has grown within a 6-km-wide caldera whose rim is partially exposed at Picacho and Jabalí peaks, which themselves were formed by collapse of an older edifice about 40,000 years ago. The summit of Boquerón is truncated by a steep-walled crater 1.5 km wide and ~500 m deep that formed during a major eruption around 800 years ago. It contained a crater lake prior to an eruption during 1917 that formed a small cinder cone on the crater floor; a major N-flank lava flow also erupted in this year. Three fracture zones that extend beyond the base of the volcano have been the locus for numerous flank eruptions, including two that formed maars on the WNW and SE sides. Most of the four historical eruptions recorded since the 16th century have originated from flank vents, including two in the 17th century from the NW-flank cone of El Playón, during which explosions and a lava flow damaged inhabited areas.

Information Contacts: Douglas Hernandez, Centro de Investigaciones Geotecnicas, Apartado Postal 109, San Salvador, El Salvador.

The volcano was frequently obscured by clouds during August-December 1999, but small fumarolic gas-and-steam plumes rising 50-200 m were often observed during clear weather. Higher fumarolic plumes were seen on three days in late November-early December. Four short explosions generated ash-bearing plumes during August-December that were confirmed visually. As many as five additional dome explosions were identified seismically.

On 11 and 13-14 August, fumarolic plumes rose 50-200 m above the crater. On 15 August a 5-minute ash explosion sent a plume to 800 m above the crater. On 17 and 23 August, fumarolic plumes rose 200-600 m; on the 30th a similar plume rose 1,200 m. On 4-5, 12, and 23-25 September, fumarolic plumes rose 50-200 m, extending 5 km E or SE. Similar plumes were seen on 7, 11, 23, and 25-26 October. On the morning of 27 October a short-lived ash explosion was observed, with an accompanying 20-minute burst of seismic activity. According to a Japanese satellite image taken about 3.5 hours later, an ash plume extended NE at an altitude of 6,900 m. Overall seismicity remained about at background levels until the end of October.

Seismicity was above background levels in late October through mid-November, when the hazard status was increased to "Yellow." On the morning of 31 October a 20-minute series of shallow earthquakes and tremor may have been associated with explosions on the dome; however, at daylight only a small fumarolic plume was seen. According to visual reports from Klyuchi town, on the late morning of 1 November a short explosive eruption sent an ash plume to an altitude of 5.5-6.0 km and extended S; an accompanying increase in seismicity occurred. On 2 November a fumarolic plume rose 50 m. On 8 and 10 November, three 20-50-minute-long series of shallow earthquakes and tremor were recorded that may have been associated with dome explosions. On 11 November a fumarolic plume rose 200 m.

A 5-minute-long series of shallow earthquakes and tremor was recorded on the morning of 17 November that may have been associated with an explosion on the dome. On 12, 16, 19, and 22 November fumarolic plumes rose 200 m. On the morning of 24 November a gas-and-ash plume rose 3 km above the crater. Plumes rising 1-2 km above the crater were also observed on the evening of 27 November and the afternoon of 2 December. All three of these larger plumes disappeared within one hour. Smaller fumarolic plumes, to 50-200 m above the crater, were seen again on 26 and 29-30 November, and 1-2, 10, 17, and 20-21 December. On the morning of 27 December a possible gas-and-ash plume was registered.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

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

Frequent explosive eruptions continued at Tungurahua volcano through 30 November (figure 1 and table 2). Ash plumes rose to maximum heights of about 5 km above the summit. Daily explosions increased during the month, reaching a peak during 16-25 November before decreasing slightly (figures 2 and 3). On 19 November 0.5 mm of ash fell on Baños, 9 km NNE of the summit at an elevation of ~1,850 m. Two millimeters of ash fell on the town of Runtún farther up slope at ~2,350 m elevation and ~6.2 km NE of the summit.

Figure 1. An aerial oblique photograph of Tungurahua taken from the W during July 1974 shows the morphology of the snow-and-ice-covered summit crater prior to the current eruption. Courtesy of the Geophysical Institute.

Table 2. Explosions and other activity at Tungurahua as described in daily reports, 31 October to 30 November 1999. Courtesy of the Geophysical Institute.

Figure 2. A dark ash plume rises from Tungurahua's formerly snow-covered summit crater on 16 November 1999. Courtesy of the Geophysical Institute.

Figure 3. A histogram indicating the number of daily explosions at Tungurahua during 24 October to 30 December 1999. Explosions were most frequent during 22-25 November. Courtesy of the Geophysical Institute.

A pronounced peak in monthly earthquakes during August-September diminished rapidly in October and still farther in November (figure 4). The greatest number of monthly earthquakes were volcano-tectonic, in a pattern that became prominent in September 1998 and prevailed until October 1999. The ratio of multiphase to long-period earthquakes showed significant variability. In some months (eg., February, March, May, June, and September 1999) the multiphase events dominated. August 1999 showed the extreme reversal of this pattern with 436 long-period and 58 multiphase events. The last two months shown on figure 4, October and November, portrayed a similar though less pronounced reversal in their relative abundance of the multiphase events. These months also displayed a comparative scarcity of volcano-tectonic events.

Figure 4. A histogram for Tungurahua showing three types of monthly earthquakes occurring between April 1998 and November 1999. For any given month, from left to right the earthquakes shown are long-period (LP), hybrid or multiphase (MP), and volcano-tectonic (VT). All three types plot on the same scale, shown on the left side of the histogram. Courtesy of the Geophysical Institute.

SO₂ flux during the crisis (figure 5) showed wide variability. Comparatively high fluxes were measured prior to the eruption. On the eruptions first day, 5 October, measured SO₂-flux values reached 9,000-10,000 metric tons/day (t/d) (BGVN 24:09). The highest fluxes, seen during mid-September through early November, also showed rough, though inexact correlations with the seismic and explosion patterns.

Figure 5. SO2 flux measured at Tungurahua during 11 July-8 December 1999. Although error bars were not provided they are typically on the order of plus or minus 10-20%. Courtesy of the Geophysical Institute.

Two mud flows were reported on 20 November. They occurred after a strong rain that washed large tree trunks and rocks into a main highway in Baños. One of these mudflows was 20 m wide; another earlier in the day blocked part of a different highway in Baños.

1998-99 activity divided into five stages. In January 2000 the Geophysical Institute issued a summary report that divided 1998-99 activity into five stages. The first stage, December-May 1998, included swarms of small predominantly volcano-tectonic earthquakes. Tremor also continued, presumably associated with a phreatic source; this kind of tremor has been detected since 1993 and is thus here referred to as persistent or long-lived tremor.

The second stage, May 1998-15 July 1999, was an interval when seismic swarms (including volcano-tectonic (VT), long period (LP), and hybrid or multiphase (MP) earthquakes) became more energetic. Small explosion signals began to register from greater-than-shallow depths. The preponderance of VT earthquakes was interpreted as a result of stress beneath the edifice due to intruding magma. Stable-frequency tremor at that time underwent a slight increase in amplitude.

In the third stage, which began after 15 July 1999, tremor included higher frequency signals. Geophysicists noted a series of many small earthquakes of all kinds that continued until mid-December. At the end of July came the first reports of strong sulfurous odors in the vicinity of the crater. In the meantime, SO₂ fluxes rose from essentially zero to 3,200 t/d (figure 5).

During 24-28 July and 8-10 September LP earthquake swarms struck with significant energy. Seismicity continued to rise considerably during August and early September. An alert was declared on 8 September 1999.

The fourth stage began 14 September 1999 when low-frequency tremor appeared, presumably associated with degassing and ascending magma. The persistent tremor increased in amplitude. On 14 September a column of vapor 2 km tall was observed. On 15 September the alert status rose to yellow. Later and until 25 October tremor reached extraordinarily high amplitudes and contained three dominant frequencies: 1, 1.7, and 2-2.5 Hz.

The first explosive activity was reported on 5 October (BGVN 24:09), when blocks and ash were ejected at 0721, 0738, and 0743 hours. This emission was associated with a comparatively big explosive seismic signal with a reduced displacement of 25 cm² and high SO₂ fluxes. The next day an ash plume rose to 2 km above the summit; small airfall ash deposits were found in Quero, Bilbao (where the thickness was given as 2 mm), and probably in Ambato. Subsequent Geophysical Institute reports described small ash-bearing or "dark" plumes to 0.5-5 km above the summit.

On 13 October observers first noted incandescence. SO₂ fluxes rose to over 10,000 t/d (figure 5). Deformation at one of the tilt stations on the SW underwent significant changes. Activity increased on 16 October when an ash plume reached ~5 km above the summit and blew W. During the previous night's darkness observers saw incandescent ash and blocks deposited on the upper flanks of the volcano. On 16 October the alert status was raised from yellow to orange, prompting evacuations of Baños and settlements along Tungurahua's W and SW flanks (see below).

During the fifth stage, which began after 25 October, the persistent tremor remained near the levels seen in the third stage. Low-frequency tremor also continued. SO₂ fluxes dropped to 3,500-4,000 tons/day in mid-November. Magmatic explosions became common in this stage. At night observers saw pyroclasts descending 1-2 km below the summit. Ash-charged plumes rose 3-5 km above the summit. During 1999 the Geophysical Institute tallied 2,030 explosions and emissions, 2,542 VT earthquakes, 4,086 LP earthquakes, and 1,038 MP earthquakes.

Geography and hazards. Baños sits in a narrow valley on the N margin of the volcano 75 miles S of Quito and 9 km NE of Tungurahua's summit. Baños lies along the Pastaza river (draining the N flanks) below the Chambo river (draining the W flanks over the NW to SW sector). This geography leaves Baños open to "high hazard for directed blasts and fallback pyroclastic flows" as well as lahars (Hall and others, 1999). Within this hazard zone, ~4.5 km downstream, sits the Agoyan dam, an important source of hydroelectric power.

Tungurahua is very dangerous because it has 3 km of vertical relief, 30°slopes, a record of previous sector collapses and a comparatively high propensity for future collapses, a pre-evacuation at-risk population of ~25,000 people, a major hydroelectric dam on its NNE margin, and a record of relatively violent, sudden andesitic eruptions with pyroclastic flows (Hall and others, 1999). The same authors noted that the volume of magma emitted by Tungurahua during the last 2,300 years has been ~3.45 km³. This gives it a magma flux rate similar to that at Merapi during the last century and 2- to 3-fold larger than the estimated rates seen in the Central Andes during the Late Cenozoic.

Evacuations. The newspaper El Universo reported that on 16 October when Tungurahua's volcanic activity increased and its hazard status first rose to orange, evacuations followed at cities closest to the volcano, including Baños. On 21 October the United Nations (UN) reported that the evacuations relocated "22,000 persons from some 60 locations." El Universo noted that at one point nearing the end of the evacuation one hundred buses were used.

As of late October some of the residents had moved to Ambato, 32 km NW of the volcano. Official sources indicated that 1,200-1,500 evacuees went to temporary shelters in the provinces of Tungurahua, Chimborazo, and Pastaza. Besides Ambato, individual cities that took refugees included Puyo (45 km E of the summit) and Shell (41 km E). About 100 families found shelter in a religious foundation and 200 families on a farm belonging to the Polytechnic Institute of Chimborazo. The UN further reported that ~600 military police and personnel have been deployed to the affected region to protect abandoned property. Access into this area was to be strictly prohibited.

The UN reported that 4,000 livestock, 100,000 fowl, and the animals from the zoological garden in Baños had also been evacuated. According to the Associated Press, a government census found that 40,000 chickens died from respiratory infections during early October.

According to the Associated Press, Baños had been evacuated for two months when on 13 December a caravan of residents briefly returned. During this brief visit, one resident entered his home and found it intact, although most parts of the house lay covered in ash. Residents faced an uncertain future because they did not know exactly when they would be able to return. The governor of Tungurahua province, Ignacio Vargas said, "This won't be permanent. We will have to wait until the eruption ends so that everyone can return to his normal activities."

Because of economic problems associated with leaving their homes and livelihoods, Baños area residents have been bypassing the military to plant crops and tend their farms. According to early January ABC News reports there have even been skirmishes between residents and the military. The eruptions are occurring in the context of tension and conflict between the military and some Unions and other groups as the country's economy has worsened.

Reference.: Hall, M., Robin, C., Beate, B., Mothes, P., Monzier, M., 1999. Tungurahua Volcano, Ecuador: structure, eruptive history and hazards: Journal of Volcanology and Geothermal Research, v. 91, p. 1-21.

Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Embassy of Ecuador, 2535 15th Street NW, Washington, DC 20009 USA (URL: http//www.ecuador.org/); United Nations Office for the Coordination of Humanitarian Affairs (OCHA), Palais des Nations, 1211 Geneva 10, Switzerland; El Universo, Quito, Ecuador (URL: http://www.eluniverso.com/); Associated Press, International Headquarters, 50 Rockefeller Plaza, New York, NY 10020 USA (URL: http://www.ap.org/); ABC News (URL: http://abcnews.go.com/).

No eruptions have occurred at White Island since the minor ash emissions in July-August 1999 from the PeeJay vent area. This report includes observations following a visit on 23 November to service the seismic installation, conduct a deformation survey, collect volcanic gas samples, and assess the general status of volcanic activity on the island.

During the visit a weak steam-and-gas plume was rising 300-500 m. This plume originated from fumarolic vents NW of the former PeeJay vents. Since the last surveillance visit in July a crater lake has developed on the floor of 1978/90 Crater Complex, inundating Metra Crater and parts of the PeeJay vent area. A series of strand lines around the crater lake edge indicated a recent drop in the lake level. Small collapse pits had recently formed near the lakeshore, below the Sag area, and may have accompanied the recent drop in lake level. The lake is a lime green color, with minor convection evident. A temperature of 45°C was measured, down slightly from the previous measurements.

The strongest fumarolic vents were on the NW side of the PeeJay vents area, emerging from the vent wall, which is ~10-15 m high. There were three prominent vents, which were emitting steam and gas that were weakly transparent at the vent. At times the steam and gas plume appeared a yellow color. The emissions were audible from 2-300 m distance. Temperatures of Main Crater fumaroles ranged from 103-115°C, and are similar to previous measurements this year.

A ground-deformation survey was also made. Eight pegs were replaced, as these were damaged during the April-July 1999 eruptions. The survey results showed that subsidence continued at the E-SE margin of the 1978/90 Crater Complex, but at a lesser rate than observed in 1998. Over the remainder of the Main Crater floor weak subsidence was also apparent at many of the marks.

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: Brad Scott, Wairakei Research Center, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand (URL: http://www.gns.cri.nz/).