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/).

Persistent eruptive activity since April 2008 at Ibu, a stratovolcano on Indonesian’s Halmahera Island, has consisted of daily explosive ash emissions and plumes, along with observations of thermal anomalies (BGVN 47:04). The current eruption continued during October 2022-May 2023, described below, based on advisories issued by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), daily reports by MAGMA Indonesia (a PVMBG platform), and the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data. The Alert Level during the reporting period remained at 2 (on a scale of 1-4), except raised briefly to 3 on 27 May, and the public was warned to stay at least 2 km away from the active crater and 3.5 km away on the N side of the volcano.

According to MAGMA Indonesia, during October 2022-May 2023, daily gray-and-white ash plumes of variable densities rose 200-1,000 m above the summit and drifted in multiple directions. On 30 October and 11 November, plumes rose a maximum of 2 km and 1.5 km above the summit, respectively (figures 42 and 43). According to the Darwin VAAC, discrete ash emissions on 13 November rose to 2.1 km altitude, or 800 m above the summit, and drifted W, and multiple ash emissions on 15 November rose 1.4 km above the summit and drifted NE. Occasional larger ash explosions through May 2023 prompted PVMBG to issue Volcano Observatory Notice for Aviation (VONA) alerts (table 6); the Aviation Color Code remained at Orange throughout this period.

Figure 42. Larger explosion from Ibu’s summit crater on 30 October 2022 that generated a plume that rose 2 km above the summit. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Figure 43. Larger explosion from Ibu’s summit crater on 11 November 2022 that generated a plume that rose 1.5 km above the summit. Courtesy of MAGMA Indonesia.

Table 6. Volcano Observatory Notice for Aviation (VONA) ash plume alerts for Ibu issued by PVMBG during October 2022-May 2023. Maximum height above the summit was estimated by a ground observer. VONAs in January-May 2023 all described the ash plumes as dense.

Sentinel-2 L1C satellite images throughout the reporting period show two, sometimes three persistent thermal anomalies in the summit crater, with the most prominent hotspot from the top of a cone within the crater. Clear views were more common during March-April 2023, when a vent and lava flows on the NE flank of the intra-crater cone could be distinguished (figure 44). White-to-grayish emissions were also observed during brief periods when weather clouds allowed clear views.

Figure 44. Sentinel-2 L2A satellite images of Ibu on 10 April 2023. The central cone within the summit crater (1.3 km diameter) and lava flows (gray) can be seen in the true color image (left, bands 4, 3, 2). Thermal anomalies from the small crater of the intra-crater cone, a NE-flank vent, and the end of the lava flow are apparent in the infrared image (right, bands 12, 11, 8A). Courtesy of Copernicus Browser.

The MIROVA space-based volcano hotspot detection system recorded almost daily thermal anomalies throughout the reporting period, though cloud cover often interfered with detections. Data from imaging spectroradiometers aboard NASA’s Aqua and Terra satellites and processed using the MODVOLC algorithm (MODIS-MODVOLC) recorded hotspots on one day during October 2022 and December 2022, two days in April 2023, three days in November 2022 and May 2023, and four days in March 2023.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in Indonesia), Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); 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/).

Dukono, a remote volcano on Indonesia’s Halmahera Island, has been erupting continuously since 1933, with frequent ash explosions and sulfur dioxide plumes (BGVN 46:11, 47:10). This activity continued during October 2022 through May 2023, based on reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG; also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data. During this period, the Alert Level remained at 2 (on a scale of 1-4) and the public was warned to remain outside of the 2-km exclusion zone. The highest reported plume of the period reached 9.4 km above the summit on 14 November 2022.

According to MAGMA Indonesia (a platform developed by PVMBG), white, gray, or dark plumes of variable densities were observed almost every day during the reporting period, except when fog obscured the volcano (figure 33). Plumes generally rose 25-450 m above the summit, but rose as high as 700-800 m on several days, somewhat lower than the maximum heights reached earlier in 2022 when plumes reached as high as 1 km. However, the Darwin VAAC reported that on 14 November 2022, a discrete ash plume rose 9.4 km above the summit (10.7 km altitude), accompanied by a strong hotspot and a sulfur dioxide signal observed in satellite imagery; a continuous ash plume that day and through the 15th rose to 2.1-2.4 km altitude and drifted NE.

Figure 33. Webcam photo of a gas-and-steam plume rising from Dukono on the morning of 28 January 2023. Courtesy of MAGMA Indonesia.

Sentinel-2 images were obscured by weather clouds almost every viewing day during the reporting period. However, the few reasonably clear images showed a hotspot and white or gray emissions and plumes. Strong SO₂ plumes from Dukono were present on many days during October 2022-May 2023, as detected using the TROPOMI instrument on the Sentinel-5P satellite (figure 34).

Figure 34. A strong SO2 signal from Dukono on 23 April 2023 was the most extensive plume detected during the reporting period. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Reports from this remote volcano in northernmost Halmahera are rare, but Dukono has been one of Indonesia's most active volcanoes. More-or-less continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the N-flank Gunung Mamuya cone. This complex volcano presents a broad, low profile with multiple summit peaks and overlapping craters. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also been active during historical time.

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 (Multiplatform Application for Geohazard Mitigation and Assessment in 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Sabancaya is located in Peru, NE of Ampato and SE of Hualca Hualca. Eruptions date back to 1750 and have been characterized by explosions, phreatic activity, ash plumes, and ashfall. The current eruption period began in November 2016 and has more recently consisted of daily explosions, gas-and-ash plumes, and thermal activity (BGVN 47:11). This report updates activity during November 2022 through April 2023 using information from Instituto Geophysico del Peru (IGP) that use weekly activity reports and various satellite data.

Intermittent low-to-moderate power thermal anomalies were reported by the MIROVA project during November 2022 through April 2023 (figure 119). There were few short gaps in thermal activity during mid-December 2022, late December-to-early January 2023, late January to mid-February, and late February. According to data recorded by the MODVOLC thermal algorithm, there were a total of eight thermal hotspots: three in November 2022, three in February 2023, one in March, and one in April. On clear weather days, some of this thermal anomaly was visible in infrared satellite imagery showing the active lava dome in the summit crater (figure 120). Almost daily moderate-to-strong sulfur dioxide plumes were recorded during the reporting period by the TROPOMI instrument on the Sentinel-5P satellite (figure 121). Many of these plumes exceeded 2 Dobson Units (DU) and drifted in multiple directions.

Figure 119. Intermittent low-to-moderate thermal anomalies were detected during November 2022 through April 2023 at Sabancaya, as shown in this MIROVA graph (Log Radiative Power). There were brief gaps in thermal activity during mid-December 2022, late December-to-early January 2023, late January to mid-February, and late February. Courtesy of MIROVA.

Figure 120. Infrared (bands 12, 11, 8A) satellite images showed a constant thermal anomaly in the summit crater of Sabancaya on 14 January 2023 (top left), 28 February 2023 (top right), 5 March 2023 (bottom left), and 19 April 2023 (bottom right), represented by the active lava dome. Sometimes gas-and-steam and ash emissions also accompanied this activity. Courtesy of Copernicus Browser.

Figure 121. Moderate-to-strong sulfur dioxide plumes were detected almost every day, rising from Sabancaya by the TROPOMI instrument on the Sentinel-5P satellite throughout the reporting period; the DU (Dobson Unit) density values were often greater than 2. Plumes from 23 November 2022 (top left), 26 December 2022 (top middle), 10 January 2023 (top right), 15 February 2023 (bottom left), 13 March 2023 (bottom middle), and 21 April 2023 (bottom right) that drifted SW, SW, W, SE, W, and SW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

IGP reported that moderate activity during November and December 2022 continued; during November, an average number of explosions were reported each week: 30, 33, 36, and 35, and during December, it was 32, 40, 47, 52, and 67. Gas-and-ash plumes in November rose 3-3.5 km above the summit and drifted E, NE, SE, S, N, W, and SW. During December the gas-and-ash plumes rose 2-4 km above the summit and drifted in different directions. There were 1,259 volcanic earthquakes recorded during November and 1,693 during December. Seismicity also included volcano-tectonic-type events that indicate rock fracturing events. Slight inflation was observed in the N part of the volcano near Hualca Hualca (4 km N). Thermal activity was frequently reported in the crater at the active lava dome (figure 120).

Explosive activity continued during January and February 2023. The average number of explosions were reported each week during January (51, 50, 60, and 59) and February (43, 54, 51, and 50). Gas-and-ash plumes rose 1.6-2.9 km above the summit and drifted NW, SW, and W during January and rose 1.4-2.8 above the summit and drifted W, SW, E, SE, N, S, NW, and NE during February. IGP also detected 1,881 volcanic earthquakes during January and 1,661 during February. VT-type earthquakes were also reported. Minor inflation persisted near Hualca Hualca. Satellite imagery showed continuous thermal activity in the crater at the lava dome (figure 120).

During March, the average number of explosions each week was 46, 48, 31, 35, and 22 and during April, it was 29, 41, 31, and 27. Accompanying gas-and-ash plumes rose 1.7-2.6 km above the summit crater and drifted W, SW, NW, S, and SE during March. According to a Buenos Aires Volcano Ash Advisory Center (VAAC) notice, on 22 March at 1800 through 23 March an ash plume rose to 7 km altitude and drifted NW. By 0430 an ash plume rose to 7.6 km altitude and drifted W. On 24 and 26 March continuous ash emissions rose to 7.3 km altitude and drifted SW and on 28 March ash emissions rose to 7.6 km altitude. During April, gas-and-ash plumes rose 1.6-2.5 km above the summit and drifted W, SW, S, NW, NE, and E. Frequent volcanic earthquakes were recorded, with 1,828 in March and 1,077 in April, in addition to VT-type events. Thermal activity continued to be reported in the summit crater at the lava dome (figure 120).

Geologic Background. Sabancaya, located in the saddle NE of Ampato and SE of Hualca Hualca volcanoes, is the youngest of these volcanic centers and the only one to have erupted in historical time. The oldest of the three, Nevado Hualca Hualca, is of probable late-Pliocene to early Pleistocene age. The name Sabancaya (meaning "tongue of fire" in the Quechua language) first appeared in records in 1595 CE, suggesting activity prior to that date. Holocene activity has consisted of Plinian eruptions followed by emission of voluminous andesitic and dacitic lava flows, which form an extensive apron around the volcano on all sides but the south. Records of historical eruptions date back to 1750.

Information Contacts: Instituto Geofisico del Peru (IGP), Centro Vulcanológico Nacional (CENVUL), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URL: https://www.igp.gob.pe/servicios/centro-vulcanologico-nacional/inicio); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); 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 MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Sheveluch (also spelled Shiveluch) in Kamchatka, has had at least 60 large eruptions during the last 10,000 years. The summit is truncated by a broad 9-km-wide caldera that is breached to the S, and many lava domes occur on the outer flanks. The lava dome complex was constructed within the large open caldera. Frequent collapses of the dome complex have produced debris avalanches; the resulting deposits cover much of the caldera floor. A major south-flank collapse during a 1964 Plinian explosion produced a scarp in which a “Young Sheveluch” dome began to form in 1980. Repeated episodes of dome formation and destruction since then have produced major and minor ash plumes, pyroclastic flows, block-and-ash flows, and “whaleback domes” of spine-like extrusions in 1993 and 2020 (BGVN 45:11). The current eruption period began in August 1999 and has more recently consisted of lava dome growth, explosions, ash plumes, and avalanches (BGVN 48:01). This report covers a significant explosive eruption during early-to-mid-April 2023 that generated a 20 km altitude ash plume, produced a strong sulfur dioxide plume, and destroyed part of the lava-dome complex; activity described during January through April 2023 use information primarily from the Kamchatka Volcanic Eruptions Response Team (KVERT) and various satellite data.

Satellite data. Activity during the majority of this reporting period was characterized by continued lava dome growth, strong fumarole activity, explosions, and hot avalanches. According to the MODVOLC Thermal Alerts System, 140 hotspots were detected through the reporting period, with 33 recorded in January 2023, 29 in February, 44 in March, and 34 in April. Frequent strong thermal activity was recorded during January 2023 through April, according to the MIROVA (Middle InfraRed Observation of Volcanic Activity) graph and resulted from the continuously growing lava dome (figure 94). A slightly stronger pulse in thermal activity was detected in early-to-mid-April, which represented the significant eruption that destroyed part of the lava-dome complex. Thermal anomalies were also visible in infrared satellite imagery at the summit crater (figure 95).

Figure 94. Strong and frequent thermal activity was detected at Sheveluch during January through April 2023, according to this MIROVA graph (Log Radiative Power). These thermal anomalies represented the continuously growing lava dome and frequent hot avalanches that affected the flanks. During early-to-mid-April a slightly stronger pulse represented the notable explosive eruption. Courtesy of MIROVA.

Figure 95. Infrared (bands B12, B11, B4) satellite imagery showed persistent thermal anomalies at the lava dome of Sheveluch on 14 January 2023 (top left), 26 February 2023 (top right), and 15 March 2023 (bottom left). The true color image on 12 April 2023 (bottom right) showed a strong ash plume that drifted SW; this activity was a result of the strong explosive eruption during 11-12 April 2023. Courtesy of Copernicus Browser.

During January 2023 KVERT reported continued growth of the lava dome, accompanied by strong fumarolic activity, incandescence from the lava dome, explosions, ash plumes, and avalanches. Satellite data showed a daily thermal anomaly over the volcano. Video data showed ash plumes associated with collapses at the dome that generated avalanches that in turn produced ash plumes rising to 3.5 km altitude and drifting 40 km W on 4 January and rising to 7-7.5 km altitude and drifting 15 km SW on 5 January. A gas-and-steam plume containing some ash that was associated with avalanches rose to 5-6 km altitude and extended 52-92 km W on 7 January. Explosions that same day produced ash plumes that rose to 7-7.5 km altitude and drifted 10 km W. According to a Volcano Observatory Notice for Aviation (VONA) issued at 1344 on 19 January, explosions produced an ash cloud that was 15 x 25 km in size and rose to 9.6-10 km altitude, drifting 21-25 km W; as a result, the Aviation Color Code (ACC) was raised to Red (the highest level on a four-color scale). Another VONA issued at 1635 reported that no more ash plumes were observed, and the ACC was lowered to Orange (the second highest level on a four-color scale). On 22 January an ash plume from collapses and avalanches rose to 5 km altitude and drifted 25 km NE and SW; ash plumes associated with collapses extended 70 km NE on 27 and 31 January.

Lava dome growth, fumarolic activity, dome incandescence, and occasional explosions and avalanches continued during February and March. A daily thermal anomaly was visible in satellite data. Explosions on 1 February generated ash plumes that rose to 6.3-6.5 km altitude and extended 15 km NE. Video data showed an ash cloud from avalanches rising to 5.5 km altitude and drifting 5 km SE on 2 February. Satellite data showed gas-and-steam plumes containing some ash rose to 5-5.5 km altitude and drifted 68-110 km ENE and NE on 6 February, to 4.5-5 km altitude and drifted 35 km WNW on 22 February, and to 3.7-4 km altitude and drifted 47 km NE on 28 February. Scientists from the Kamchatka Volcanological Station (KVS) went on a field excursion on 25 February to document the growing lava dome, and although it was cloudy most of the day, nighttime incandescence was visible. Satellite data showed an ash plume extending up to 118 km E during 4-5 March. Video data from 1150 showed an ash cloud from avalanches rose to 3.7-5.5 km altitude and drifted 5-10 km ENE and E on 5 March. On 11 March an ash plume drifted 62 km E. On 27 March ash plumes rose to 3.5 km altitude and drifted 100 km E. Avalanches and constant incandescence at the lava dome was focused on the E and NE slopes on 28 March. A gas-and-steam plume containing some ash rose to 3.5 km altitude and moved 40 km E on 29 March. Ash plumes on 30 March rose to 3.5-3.7 km altitude and drifted 70 km NE.

Similar activity continued during April, with lava dome growth, strong fumarolic activity, incandescence in the dome, occasional explosions, and avalanches. A thermal anomaly persisted throughout the month. During 1-4 April weak ash plumes rose to 2.5-3 km altitude and extended 13-65 km SE and E.

Activity during 11 April 2023. The Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS) reported a significant increase in seismicity around 0054 on 11 April, as reported by strong explosions detected on 11 April beginning at 0110 that sent ash plumes up to 7-10 km altitude and extended 100-435 km W, WNW, NNW, WSW, and SW. According to a Tokyo VAAC report the ash plume rose to 15.8 km altitude. By 0158 the plume extended over a 75 x 100 km area. According to an IVS FEB RAS report, the eruptive column was not vertical: the initial plume at 0120 on 11 April deviated to the NNE, at 0000 on 12 April, it drifted NW, and by 1900 it drifted SW. KVS reported that significant pulses of activity occurred at around 0200, 0320, and then a stronger phase around 0600. Levin Dmitry took a video from near Békés (3 km away) at around 0600 showing a rising plume; he also reported that a pyroclastic flow traveled across the road behind him as he left the area. According to IVS FEB RAS, the pyroclastic flow traveled several kilometers SSE, stopping a few hundred meters from a bridge on the road between Klyuchi and Petropavlovsk-Kamchatsky.

Ashfall was first observed in Klyuchi (45 km SW) at 0630, and a large, black ash plume blocked light by 0700. At 0729 KVERT issued a Volcano Observatory Notice for Aviation (VONA) raising the Aviation Color Code to Red (the highest level on a four-color scale). It also stated that a large ash plume had risen to 10 km altitude and drifted 100 km W. Near-constant lightning strikes were reported in the plume and sounds like thunderclaps were heard until about 1000. According to IVS FEB RAS the cloud was 200 km long and 76 km wide by 0830, and was spreading W at altitudes of 6-12 km. In the Klyuchi Village, the layer of both ash and snow reached 8.5 cm (figure 96); ashfall was also reported in Kozyrevsk (112 km SW) at 0930, Mayskoye, Anavgay, Atlasovo, Lazo, and Esso. Residents in Klyuchi reported continued darkness and ashfall at 1100. In some areas, ashfall was 6 cm deep and some residents reported dirty water coming from their plumbing. According to IVS FEB RAS, an ash cloud at 1150 rose to 5-20 km altitude and was 400 km long and 250 km wide, extending W. A VONA issued at 1155 reported that ash had risen to 10 km and drifted 340 km NNW and 240 km WSW. According to Simon Carn (Michigan Technological University), about 0.2 Tg of sulfur dioxide in the plume was measured in a satellite image from the TROPOMI instrument on the Sentinel-5P satellite acquired at 1343 that covered an area of about 189,000 km² (figure 97). Satellite data at 1748 showed an ash plume that rose to 8 km altitude and drifted 430 km WSW and S, according to a VONA.

Figure 96. Photo of ash deposited in Klyuchi village on 11 April 2023 by the eruption of Sheveluch. About 8.5 cm of ash was measured. Courtesy of Kam 24 News Agency.

Figure 97. A strong sulfur dioxide plume from the 11 April 2023 eruption at Sheveluch was visible in satellite data from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of Simon Carn, MTU.

Activity during 12-15 April 2023. On 12 April at 0730 satellite images showed ash plumes rose to 7-8 km altitude and extended 600 km SW, 1,050 km ESE, and 1,300-3,000 km E. By 1710 that day, the explosions weakened. According to news sources, the ash-and-gas plumes drifted E toward the Aleutian Islands and reached the Gulf of Alaska by 13 April, causing flight disruptions. More than 100 flights involving Alaska airspace were cancelled due to the plume. Satellite data showed ash plumes rising to 4-5.5 km altitude and drifted 400-415 km SE and ESE on 13 April. KVS volcanologists observed the pyroclastic flow deposits and noted that steam rose from downed, smoldering trees. They also noted that the deposits were thin with very few large fragments, which differed from previous flows. The ash clouds traveled across the Pacific Ocean. Flight cancellations were also reported in NW Canada (British Columbia) during 13-14 April. During 14-15 April ash plumes rose to 6 km altitude and drifted 700 km NW.

Alaskan flight schedules were mostly back to normal by 15 April, with only minor delays and far less cancellations; a few cancellations continued to be reported in Canada. Clear weather on 15 April showed that most of the previous lava-dome complex was gone and a new crater roughly 1 km in diameter was observed (figure 98); gas-and-steam emissions were rising from this crater. Evidence suggested that there had been a directed blast to the SE, and pyroclastic flows traveled more than 20 km. An ash plume rose to 4.5-5.2 km altitude and drifted 93-870 km NW on 15 April.

Figure 98. A comparison of the crater at Sheveluch showing the previous lava dome (top) taken on 29 November 2022 and a large crater in place of the dome (bottom) due to strong explosions during 10-13 April 2023, accompanied by gas-and-ash plumes. The bottom photo was taken on 15 April 2023. Photos has been color corrected. Both photos are courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Activity during 16-30 April 2023. Resuspended ash was lifted by the wind from the slopes and rose to 4 km altitude and drifted 224 km NW on 17 April. KVERT reported a plume of resuspended ash from the activity during 10-13 April on 19 April that rose to 3.5-4 km altitude and drifted 146-204 km WNW. During 21-22 April a plume stretched over the Scandinavian Peninsula. A gas-and-steam plume containing some ash rose to 3-3.5 km altitude and drifted 60 km SE on 30 April. A possible new lava dome was visible on the W slope of the volcano on 29-30 April (figure 99); satellite data showed two thermal anomalies, a bright one over the existing lava dome and a weaker one over the possible new one.

Figure 99. Photo showing new lava dome growth at Sheveluch after a previous explosion destroyed much of the complex, accompanied by a white gas-and-steam plume. Photo has been color corrected. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

References. Girina, O., Loupian, E., Horvath, A., Melnikov, D., Manevich, A., Nuzhdaev, A., Bril, A., Ozerov, A., Kramareva, L., Sorokin, A., 2023, Analysis of the development of the paroxysmal eruption of Sheveluch volcano on April 10–13, 2023, based on data from various satellite systems, ??????????? ???????? ??? ?? ???????, 20(2).

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: 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/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); 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/); 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/); Kam 24 News Agency, 683032, Kamchatka Territory, Petropavlovsk-Kamchatsky, Vysotnaya St., 2A (URL: https://kam24.ru/news/main/20230411/96657.html#.Cj5Jrky6.dpuf); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn).

Bezymianny is located on the Kamchatka Peninsula of Russia as part of the Klyuchevskoy volcano group. Historic eruptions began in 1955 and have been characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. During the 1955-56 eruption a large open crater 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. The current eruption period began in December 2016 and more recent activity has consisted of strong explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers activity during November 2022 through April 2023, based on weekly and daily reports from the Kamchatka Volcano Eruptions Response Team (KVERT) and satellite data.

Activity during November and March 2023 was relatively low and mostly consisted of gas-and-steam emissions, occasional small collapses that generated avalanches along the lava dome slopes, and a persistent thermal anomaly over the volcano that was observed in satellite data on clear weather days. According to the Tokyo VAAC and KVERT, an explosion produced an ash plume that rose to 6 km altitude and drifted 25 km NE at 1825 on 29 March.

Gas-and-steam emissions, collapses generating avalanches, and thermal activity continued during April. According to two Volcano Observatory Notice for Aviation (VONA) issued on 2 and 6 April (local time) ash plumes rose to 3 km and 3.5-3.8 km altitude and drifted 35 km E and 140 km E, respectively. Satellite data from KVERT showed weak ash plumes extending up to 550 km E on 2 and 5-6 April.

A VONA issued at 0843 on 7 April described an ash plume that rose to 4.5-5 km altitude and drifted 250 km ESE. Later that day at 1326 satellite data showed an ash plume that rose to 5.5-6 km altitude and drifted 150 km ESE. A satellite image from 1600 showed an ash plume extending as far as 230 km ESE; KVERT noted that ash emissions were intensifying, likely due to avalanches from the growing lava dome. The Aviation Color Code (ACC) was raised to Red (the highest level on a four-color scale). At 1520 satellite data showed an ash plume rising to 5-5.5 km altitude and drifting 230 km ESE. That same day, Kamchatka Volcanological Station (KVS) volcanologists traveled to Ambon to collect ash; they reported that a notable eruption began at 1730, and within 20 minutes a large ash plume rose to 10 km altitude and drifted NW. KVERT reported that the strong explosive phase began at 1738. Video and satellite data taken at 1738 showed an ash plume that rose to 10-12 km altitude and drifted up to 2,800 km SE and E. Explosions were clearly audible 20 km away for 90 minutes, according to KVS. Significant amounts of ash fell at the Apakhonchich station, which turned the snow gray; ash continued to fall until the morning of 8 April. In a VONA issued at 0906 on 8 April, KVERT stated that the explosive eruption had ended; ash plumes had drifted 2,000 km E. The ACC was lowered to Orange (the third highest level on a four-color scale). The KVS team saw a lava flow on the active dome once the conditions were clear that same day (figure 53). On 20 April lava dome extrusion was reported; lava flows were noted on the flanks of the dome, and according to KVERT satellite data, a thermal anomaly was observed in the area. The ACC was lowered to Yellow (the second lowest on a four-color scale).

Figure 53. Photo showing an active lava flow descending the SE flank of Bezymianny from the lava dome on 8 April 2023. Courtesy of Yu. Demyanchuk, IVS FEB RAS, KVERT.

Satellite data showed an increase in thermal activity beginning in early April 2023. A total of 31 thermal hotspots were detected by the MODVOLC thermal algorithm on 4, 5, 7, and 12 April 2023. The elevated thermal activity resulted from an increase in explosive activity and the start of an active lava flow. The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on the analysis of MODIS data also showed a pulse in thermal activity during the same time (figure 54). Infrared satellite imagery captured a continuous thermal anomaly at the summit crater, often accompanied by white gas-and-steam emissions (figure 55). On 4 April 2023 an active lava flow was observed descending the SE flank.

Figure 54. Intermittent and low-power thermal anomalies were detected at Bezymianny during December 2022 through mid-March 2023, according to this MIROVA graph (Log Radiative Power). In early April 2023, an increase in explosive activity and eruption of a lava flow resulted in a marked increase in thermal activity. Courtesy of MIROVA.

Figure 55. Infrared satellite images of Bezymianny showed a persistent thermal anomaly over the lava dome on 18 November 2022 (top left), 28 December 2022 (top right), 15 March 2023 (bottom left), and 4 April 2023 (bottom right), often accompanied by white gas-and-steam plumes. On 4 April a lava flow was active and descending the SE flank. Images using infrared (bands 12, 11, 8a). Courtesy of Copernicus Browser.

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: 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/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); 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/); 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/).

Chikurachki, located on Paramushir Island in the northern Kuriles, has had Plinian eruptions during the Holocene. Lava flows have reached the sea and formed capes on the NW coast; several young lava flows are also present on the E flank beneath a scoria deposit. Reported eruptions date back to 1690, with the most recent eruption period occurring during January through October 2022, characterized by occasional explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers a new eruptive period during January through February 2023 that consisted of ash explosions and ash plumes, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

According to reports from KVERT, an explosive eruption began around 0630 on 29 January. Explosions generated ash plumes that rose to 3-3.5 km altitude and drifted 6-75 km SE and E, based on satellite data. As a result, the Aviation Color Code (ACC) was raised to Orange (the second highest level on a four-color scale). At 1406 and 1720 ash plumes were identified in satellite images that rose to 4.3 km altitude and extended 70 km E. By 2320 the ash plume had dissipated. A thermal anomaly was visible at the volcano on 31 January, according to a satellite image, and an ash plume was observed drifting 66 km NE.

Occasional explosions and ash plumes continued during early February. At 0850 on 1 February an ash plume rose to 3.5 km altitude and drifted 35 km NE. Satellite data showed an ash plume that rose to 3.2-3.5 km altitude and drifted 50 km NE at 1222 later that day (figure 22). A thermal anomaly was detected over the volcano during 5-6 February and ash plumes drifted as far as 125 km SE, E, and NE. Explosive events were reported at 0330 on 6 February that produced ash plumes rising to 4-4.5 km altitude and drifting 72-90 km N, NE, and ENE. KVERT noted that the last gas-and steam plume that contained some ash was observed on 8 February and drifted 55 km NE before the explosive eruption ended. The ACC was lowered to Yellow and then Green (the lowest level on a four-color scale) on 18 February.

Figure 22. Satellite image showing a true color view of a strong ash plume rising above Chikurachki on 1 February 2023. The plume drifted NE and ash deposits (dark brown-to-gray) are visible on the NE flank due to explosive activity. Courtesy of Copernicus Browser.

Geologic Background. Chikurachki, the highest volcano on Paramushir Island in the northern Kuriles, is a relatively small cone constructed on a high Pleistocene edifice. Oxidized basaltic-to-andesitic scoria deposits covering the upper part of the young cone give it a distinctive red color. Frequent basaltic Plinian eruptions have occurred during the Holocene. Lava flows have reached the sea and formed capes on the NW coast; several young lava flows are also present on the E flank beneath a scoria deposit. The Tatarinov group of six volcanic centers is located immediately to the south, and the Lomonosov cinder cone group, the source of an early Holocene lava flow that reached the saddle between it and Fuss Peak to the west, lies at the southern end of the N-S-trending Chikurachki-Tatarinov complex. In contrast to the frequently active Chikurachki, the Tatarinov centers are extensively modified by erosion and have a more complex structure. Tephrochronology gives evidence of an eruption around 1690 CE from Tatarinov, although its southern cone contains a sulfur-encrusted crater with fumaroles that were active along the margin of a crater lake until 1959.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far East Division, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

The JMA Ocean Bottom Seismograph off the Boso Peninsula (E of Tokyo) detected T-phase-like signals after 22 January, and clear T-phase signals on 27 January (figure 1). According to tentative analyses of arrival times at the detectors, the signals were interpreted to have propagated from the S. As of mid-February, JMA had not determined a specific source for these signals. However, discolored seawater was observed above two submarine volcanoes in the Volcano Islands during January: Minami-Hiyoshi on 12 January, and Fukutoku-okanoba on 12, 22, and 23 January.

Figure 1. Example of T-phase signals (spikes) detected by the Ocean Bottom Seismograph off the Boso Peninsula, Japan, 27 January 1996. Courtesy of JMA.

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

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

Minami-dake crater was slightly active throughout January. The monthly total number of eruptions was 60, including 42 explosive ones. At the seismic station 2.3 km NW of Minami-dake crater (Station B), 601 earthquakes and 684 tremors were recorded. The highest ash plume of the month rose 2,300 m above the summit crater on the 21st. Ashfall measured at the Kagoshima Local Meteorological Observatory, 10 km W form the crater, was 41 g/m².

The VRC noted that there were more than 200 eruptions in 1995; total amount of erupted material was estimated at 3-4 million tons by the Sakurajima Volcanological Observatory, Kyoto University. The latter has been observing continuous uplift on the N side of the volcano, implying accumulation of magma beneath the volcano.

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

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

Periods of seismic unrest have occurred several times in the past 12 months, including one episode in April 1995 (BGVN 19:05), and the volcano usually emits a continuous steam plume. Based on recorded seismic activity, an eruption apparently began during 1700-1900 on 1 January. Russian aviation sources reported an ash plume to 7 km altitude at approximately 1130 the next day. A satellite image at 1400 on 2 January showed that the plume had extended at least 200 km SE and S of the volcano. Several aviation notices (SIGMETs) were issued concerning the ash plume. GMS satellite imagery revealed multiple ash emissions on 2 January, with the cloud height estimated at ~7 km. Satellite data on 3 January continued to show multiple low-level (below 5,400 m) ash bursts of short duration that drifted S and dissipated within an hour.

When the volcano was visited by Vladimir Kirianov and Yuri Doubik of the Institute of Volcanic Geology and Geochemistry between 1330 and 1630 on 3 January, they discovered that the initial eruption had vented from the N end of Karymsky Lake. The lake occupies the 5-km-diameter late-Pleistocene Akademia Nauk caldera, ~5 km S of Karymsky volcano proper. However, by the time of their visit activity had shifted to Karymsky volcano where a new crater had formed on the SSW side of the summit, adjacent to the old crater. The new crater, approximately the same size as the old crater, produced explosions every 1-5 minutes that fed a thick black ash plume to an altitude of ~2.5 km moving E. Fresh ashfall was widespread throughout the 5-km-wide Karymsky caldera and for a considerable area to the E and N. Karymsky Lake was yellow-gray in color and mostly covered by steam and vapor. The Karymsky River, which drains the lake to the N, was completely buried in ash and no longer visible; a new beach with numerous fumaroles marked the former source of the river. Very strong seismic activity associated with the eruption included one M 6.5 earthquake on the first day of the eruption. Seismic stations as far as 110 km from the volcano recorded the activity.

By 5 January the new summit crater was over twice the size of the old crater. A thick black ash plume had been observed the previous two days erupting explosively from the new crater to altitudes ranging of 2,400-5,500 m. Seismicity on 6 January indicated continued explosions every 1-3 minutes. Karymsky Lake remained yellow-gray and covered by steam and vapor. Seismicity through 12 January was interpreted to reflect continued, but less explosive, eruptive activity.

Karymsky, ~110 km NW of Petropavlovsk-Kamchatsky, is the most active volcano of eastern Kamchatka. The latest eruptive period began ~500 years ago; much of the cone is mantled by lava flows less than 200 years old. It has been quiescent since frequent eruptive activity during 1970-82. The volcano is capable of explosive eruptions which can send ash to over 10 km and continue sporadically for days or weeks; short lava flows are also common.

Geologic Background. The lake-filled Akademia Nauk caldera is one of three volcanoes constructed within the mid-Pleistocene, 15-km-wide Polovinka caldera. The eroded Beliankin stratovolcano, in the SW part of Polovinka caldera, has been active in postglacial time (Sviatlovsky, 1959). Two nested calderas, 5 x 4 km Odnoboky and 3 x 5 km Akademia Nauk (also known as Karymsky Lake or Academii Nauk), were formed during the late Pleistocene, the latter about 30,000 years ago. Eruptive products varied from initial basaltic-andesite lava flows to late-stage rhyodacitic lava domes. Two maars, Akademia Nauk and Karymsky, subsequently formed at the southern and northern margins of the caldera lake, respectively. The northern maar, Karymsky, erupted about 6,500 radiocarbon years ago and formed a small bay. The first recorded eruption from Akademia Nauk took place on 2 January 1996, when a day-long explosive eruption of unusual basaltic and rhyolitic composition occurred from vents beneath the NNW part of the caldera lake near Karymsky maar.

Information Contacts: Tom Miller, Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, 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; Vladimir Kirianov and Yuri Doubik, Institute of Volcanic Geology and Geochemistry, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia; NOAA/NESDIS Synoptic Analysis Branch (SAB) , Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

During December 1995 and January 1996, Crater C continued its ongoing emission of gases, lava flows, and sporadic Strombolian eruptions. The intensity of Strombolian activity remained high, but showed some decline during this interval. Ash columns reached up to 1 km above the crater.

Lava that began extruding in July 1995 ceased advancing during much of January 1996. In contrast, new lava began extruding during January; it followed previously established channels and reached the 1,100-m contour.

During some past years, a general tendency toward deflation of the edifice had been noted. For the W and N sectors, inclinometers detected a 1995 deflation averaging 15.4 µrad/year. In harmony with this observed deflation, 1995 distance surveys over the local net registered an overall contraction of 21.2 ppm/year.

During December and January, seismic station VACR (2.7 km NE of the main crater) registered a moderate number of local events (figure 75). The majority of these events were thought to be associated with Strombolian eruptions; some were sufficiently strong to be recognized at station JTS (30 km SW of the crater). Tremor during these same two months registered for 306 and 420 hours, respectively (figure 75).

Figure 75. Arenal monthly low-frequency seismicity (<3.5 hz) and tremor, January 1995-January 1996. Data courtesy of OVSICORI-UNA.

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

Information Contacts: Rodolfo Van der Laat, Vilma Barboza, Erick Fernández, Jorge Barquero, Franklin de Obaldia, Tomás Marino, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

On 12, 22, and 23 January, an aviator from the Japan Marine Safety Agency (JMSA) reported distinct discoloration of seawater to yellowish green. Similar discoloration was seen during 25-28 November 1995 (BGVN 20:11/12). Prior to that, discolored seawater was last seen at this location in September 1993.

Geologic Background. Fukutoku-Oka-no-ba is a submarine volcano located 5 km NE of the island of Minami-Ioto. Water discoloration is frequently observed, and several ephemeral islands have formed in the 20th century. The first of these formed Shin-Ioto ("New Sulfur Island") in 1904, and the most recent island was formed in 1986. The volcano is part of an elongated edifice with two major topographic highs trending NNW-SSE, and is a trachyandesitic volcano geochemically similar to Ioto.

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

When visited in January, the crater lake's dark-yellow water remained high, covering the entire crater floor. Prominent fumaroles continued to bubble along the lake's N, NW, and SE shores. Slides continued to take place along the N, SW, and W walls. Seismic station IRZ2 (5 km SW) registered a swarm of 29 local earthquakes between 1512 and 2111 on 30 January; one was detected at a more distant station.

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

Information Contacts: Rodolfo Van der Laat, Vilma Barboza, Erick Fernández, Jorge Barquero, Franklin de Obaldia, Tomás Marino, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

On 9 and 16 November "thunderclaps" were heard from the summit. A gray plume 500 m high was observed, and incandescent ejecta rose 10-50 m above the summit at night. On 17 December thunderclaps were heard again and ejecta rose 100 m above the summit. Seismicity increased from 26 October until the end of 1995. Daily counts of deep volcanic (A-type) earthquakes fluctuated up to 116 (figure 3).

Figure 3. A-type seismicity at Karangetang, September-December 1995. Courtesy of VSI.

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: Wimpy S. Tjetjep (Director), Volcanological Survey of Indonesia (VSI), Jalan Diponegoro 57, Bandung, Indonesia.

Periods of seismic unrest have occurred several times in the past 12 months, including one episode in April 1995, and the volcano usually emits a continuous steam plume. Based on recorded seismic activity, an eruption apparently began during 1700-1900 on 1 January. Russian aviation sources reported an ash plume to 7 km altitude at approximately 1130 the next day. A satellite image at 1400 on 2 January showed that the plume had extended at least 200 km SE and S of the volcano. Several aviation notices (SIGMETs) were issued concerning the ash plume. GMS satellite imagery revealed multiple ash emissions on 2 January, with the cloud height estimated at ~7 km. Satellite data on 3 January continued to show multiple low-level (below 5,400 m) ash bursts of short duration that drifted S and dissipated within an hour.

When the volcano was visited by Vladimir Kirianov and Yuri Doubik of the Institute of Volcanic Geology and Geochemistry between 1330 and 1630 on 3 January, they discovered that the initial eruption had vented from the N end of Karymsky Lake. The lake occupies the 5-km-diameter late-Pleistocene Akademia Nauk caldera, ~5 km S of Karymsky volcano proper. However, by the time of their visit activity had shifted to Karymsky volcano where a new crater had formed on the SSW side of the summit, adjacent to the old crater. The new crater, approximately the same size as the old crater, produced explosions every 1-5 minutes that fed a thick black ash plume to an altitude of ~2.5 km moving E. Fresh ashfall was widespread throughout the 5-km-wide Karymsky caldera and for a considerable area to the E and N. Karymsky Lake was yellow-gray in color and mostly covered by steam and vapor. The Karymsky River, which drains the lake to the N, was completely buried in ash and no longer visible; a new beach with numerous fumaroles marked the former source of the river. Very strong seismic activity associated with the eruption included one M 6.5 earthquake on the first day of the eruption. Seismic stations as far as 110 km from the volcano recorded the activity.

By 5 January the new summit crater was over twice the size of the old crater. A thick black ash plume had been observed the previous two days erupting explosively from the new crater to altitudes ranging of 2,400-5,500 m. Seismicity on 6 January indicated continued explosions every 1-3 minutes. Karymsky Lake remained yellow-gray and covered by steam and vapor. Seismicity through 12 January was interpreted to reflect continued, but less explosive, eruptive activity.

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: Tom Miller, Alaska Volcano Observatory; Vladimir Kirianov and Yuri Doubik, Institute of Volcanic Geology and Geochemistry; Synoptic Analysis Branch, NOAA/NESDIS, USA.

The eruption on the East Rift Zone continued in January with the usual activity consisting of surface lava flows and lava traveling through tubes to enter the ocean. However, an unusual event on 1 February almost resulted in a summit eruption. Following this event an eruptive pause began that continued through mid-February.

Normal activity in January. Surface flows during the first two weeks of January covered ~4 km² on the E side of the Kamoamoa flow field. On the slopes of Pulama pali, lava breakouts followed the E margin of older flows as far as 1 km, sometimes burning adjacent forest and grassland. On the coastal plain, inflating sheet flows were widespread. At the West Kamokuna entry the bench was rebuilt following the collapse of 1 January (BGVN 20:11/12), and two littoral cones were created.

The Great Pit, a collapse pit on the SW slope of Pu`u `O`o cinder cone, was enlarged by several collapses since late December, expanding toward a second collapse pit over an Episode-51 vent. This vent, which has continued to issue lava during the current Episode 53, is connected to the Pu`u `O`o lava pond; a series of collapses in recent months defined a line from this pit across the rim of the cone to the lava pond inside and probably resulted from breakdown of the roof in the underlying plumbing system.

Almost daily explosive activity at the Kamokuna bench in the second half of January included bubble bursts and spattering to 50 m height, as well as steam and ash jetting to 150 m. On 25 January a small collapse removed a 40 × 100 m area of the lower bench, along with part of a littoral cone. The collapsed area was rebuilt within three days, and explosive activity constructed a new littoral cone. On the night of 30 January a large bench collapse at Kamokuna occurred over a period of six hours, during which time approximately 800 × 200 m of the bench slid into the ocean accompanied by frequent explosions that threw blocks as far as 300 m inland. Lava flows were continually active on the slope of Pulama pali, particularly around the base and on the coastal plain within 1.5 km of the ocean entry. Pahoehoe flows burning into forest and brush caused loud and abundant methane explosions. Lava flows on the coastal plain advanced E to within 500 m of the Royal Gardens access road.

Eruption tremor remained low with minor, irregular variations. Tremor amplitudes in the second half of January measured ~2x the background level on the STC seismic station near Pu`u `O`o. Shallow, long-period activity at the summit was about average. Short-period microearthquake counts were low beneath the summit and rift zones.

Eruptive pulse of 1 February. Unusual and dramatic activity started on the morning of 1 February with a large increase in shallow tremor at the summit caldera and rapid inflation of the summit, suggesting that an eruption from the summit or upper East Rift Zone might be imminent. As a result, most of Hawaii Volcanoes National Park, including the local airspace, was closed for several hours. However, no new ground cracks or eruptive fissures formed. Instead, the increase in tremor was followed three hours later by an order-of-magnitude increase in the volume of lava flowing into tubes from the vent on the flank of Pu`u `O`o cone.

Excess pressure in the tube system caused the lava to gush out of four skylights, forming dome fountains at 735, 720, 695, and 675 m elevations. Prior to this event, no surface flows had occurred above 675 m since early 1993. Heights of the fountains in mid-afternoon were 1.8-7.6 m. Lava from the largest fountain formed a channelized flow that ran E into the forest, and then descended Pulama pali as a fast-moving aa flow. Inside Pu`u `O`o, the actively circulating lava pond rose to at least 60 m below the rim, and sloshed another 15-20 m higher. Spatter from the pond was thrown 50 m beyond the S crater rim.

Over the next three days, the level of activity gradually decreased. The Pu`u `O`o lava pond exhibited cycles of influx and drainback, but remained relatively deep (65-80 m below the rim). Lava flows from the skylights started and stopped, as did surface flows on the coastal plain. The Kamokuna ocean entry paused for 11 hours on 2 February, probably because of blockage in the tube system, and then flowed at diminished volume on 3-4 February. Late on 4 February the eruption stopped. Sometime during the next night, an explosion in the Great Pit blasted out fragments (up to fist-size) of mostly reddish oxidized rock. The smaller fragments were carried ~9.5 km NE by Kona winds. The pause continued through 12 February.

The change in lava effusion rate was attended by an episodic increase in gas release. Fumaroles at the summit and East Rift Zone showed abnormally high CO₂ levels starting on 1 February, possibly reflecting a pulse of gas-rich magma. Ambient SO₂ in the HVO parking lot topped 4 ppm on 2 February and was greater than 1 ppm near the NPS Visitors Center. SO₂ emission rates measured near the eruption site were as high as 2,000 metric tons/day on 2 February. SO₂ emission rates and fumarole chemistry from the summit and east rift were approaching near-normal for an eruptive pause by 12 February.

Seismicity and deformation. Tremor amplitudes remained at 2-3x background level until the morning of 1 February. Shortly after 0800, tremor on the STC station gradually increased; summit seismicity increased at the same time. Tremor, as well as a swarm of shallow, short-period earthquakes, registered on the summit seismic stations as a result of the intrusive episode. Several hundred microearthquakes were counted. Seismicity at the summit began to subside shortly after noon with a notable decrease in summit tremor and short-period earthquakes. An increase of shallow and intermediate long-period earthquakes followed. By 1600, tremor amplitudes on the STC station were high and remained relatively steady until about 1200 on 2 February. Tremor amplitudes again increased on 3 February at 0700 and remained at elevated levels until 0200 on the 4th. From then through 12 February amplitudes fluctuated in banded patterns of a few minutes to a few hours of increased tremor, alternating with low- level tremor.

The Uwekahuna short-base water-tube tiltmeter recorded summit inflation, with ~11 µrads of N-S tilt and ~15 µrads of E-W tilt during the seismic swarm of 1 February; the maximum tilt was read at 1246. Summit deflation followed at a rate of ~1.5 µrads/hour, coincident with the increase in activity at Pu`u `O`o. By 0830 on 2 February, the E-W component had recovered to the pre-swarm level of deflation and the N-S component to ~2/3 of the pre-swarm level. A permanent-glass EDM monitor spanning the summit caldera of Kīlauea from the flank of Mauna Loa to the Kalanaokuaiki fault recorded the maximum extension between the N caldera rim and a site 1 km S of Halemaumau. This was also the area in which the earthquake swarm was localized. Measurements on 6 February showed contraction to pre-swarm values in conjunction with the summit deflation.

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

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

During an approved visit on 6 November, the volcano was steaming but not erupting. A large sulfur-stained plug of lava, ~20% the diameter of the summit cone, was bulging out of a black cinder cone just below and W of the summit; it appeared to be inside the SW margin of a broad depression. A smaller sulfur-stained plug was farther S in another depression. The landing site on the SE shore was a black-sand beach with tiny dunes of white pumice. While climbing the SE slope of the older cone, the party crossed water-eroded fields of pyroclastic material dotted with volcanic bombs. The ascent to the summit went through deep cinder deposits covered with a blanket of loose breadloaf-sized stones. From the summit complicated internal crater structures could be seen. One symmetrical cone (~100 m across) had a sulfur-lined inner rim that was fuming. The largest and most active fumaroles were inside this cone's S rim. A smaller cone within the larger one was almost horseshoe-shaped and steeper to the S. Bombs on the summit cone were as large as 1-2 m in diameter.

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: Steve O'Meara, PO Box 218, Volcano, HI 96785, USA.

Frequent earthquakes during the night of 13 January and through the next morning were centered 3-4 km NW of the Hosho dome near Sujiyu spa; eruptions caused minor ashfall around the volcano. Instruments recorded 526 earthquakes during the 13-14 January episode, some of which were felt by local residents. Some earthquakes on 27 January were centered SW of the active dome. Overall, there were 861 earthquakes detected in January, but no tremor. The plume height remained at 100-300 m throughout most of the month. Scientists at the University of Tokyo noted that vesiculated glass was again observed in the 13 January material, and deflation near the crater area was continuing.

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

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

Vapor emission was observed during November-December 1995 and whitish-gray plumes rose to 100 m above the active crater. Seismicity during September-October 1995 consisted of 1- 8 A-type events/day. On 1 November there were 46 A-type events recorded, followed by very low seismicity over the next ten days. Activity then increased from 12 November through 31 December, but was highly variable with 4-21 events/day. B-type events remained at 0-8 events/day.

The present activity is located at Tompaluan crater, in the saddle between the peaks of Lokon (1,579 m) and Empung (1,340 m). About 10,000 people evacuated following an explosion in October 1991 accompanied by a pyroclastic flow; the eruption ended in January 1992.

Geologic Background. The Lokong-Empung volcanic complex, rising above the plain of Tondano in North Sulawesi, includes four peaks and an active crater. Lokon, the highest peak, has a flat craterless top. The morphologically younger Empung cone 2 km NE has a 400-m-wide, 150-m-deep crater that erupted last in the 18th century. A ridge extending 3 km WNW from Lokon includes the Tatawiran and Tetempangan peaks. All eruptions since 1829 have originated from Tompaluan, a 150 x 250 m crater in the saddle between Lokon and Empung. These eruptions have primarily produced small-to-moderate ash plumes that sometimes damaged croplands and houses, but lava-dome growth and pyroclastic flows have also occurred.

Information Contacts: Wimpy S. Tjetjep (Director), Volcanological Survey of Indonesia (VSI), Jalan Diponegoro 57, Bandung, Indonesia.

During November-December 1995 glowing avalanches down the Boyong, Krasak, and Bebeng rivers reached up to 2.5 km from the source. Seismic activity was dominated by multiphase earthquakes, low-frequency earthquakes, and lava avalanches (rockfalls). The number of multiphase earthquakes decreased from 924 in November to 152 in December; low-frequency earthquakes also decreased from 74 in November to 42 in December. Seismicity associated with lava avalanches and rock falls increased from 816 events in November to 1,078 in December (figure 17). A deep volcanic earthquake (A- type) and two tremor events were recorded in November, three shallow volcanic earthquakes (B-type) occurred in December.

Figure 17. Monthly number of rockfall, multiphase, and low-frequency earthquakes at Merapi, June-December 1995. Courtesy of VSI.

Inflation increased since 17 November from 2.5 to 10.8 µrad/day. Tilt data collected at two stations in the summit area during November and December indicated inflation of 60 and 320 µrad, respectively. The geomagnetic intensity in early December was -14.5 nTs; it then decreased to -1.5 nTs by the end of the month. The emission rate of SO₂ during November fluctuated between 27 and 275 t/d, averaging 95 t/d, and the plume velocity was ~3.2-3.5 m/s. In December the emission rate decreased to 70 t/d, fluctuating between 18 and 156 t/d; plume speed was slightly higher at 3.3-3.6 m/s.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: Wimpy S. Tjetjep (Director), Volcanological Survey of Indonesia (VSI), Jalan Diponegoro 57, Bandung, Indonesia; Steve O'Meara, PO Box 218, Volcano, HI 96785, USA .

On 12 January, an aviator from the JMSA observed seawater discolored to yellowish green in an area 500 m wide and 6 km long, flowing from Minami-Hiyoshi seamount to the S. Discolored water was last observed in February 1992 (BGVN 17:02).

Periodic water discoloration and water-spouting has been reported over this submarine volcano since 1975, when detonations and an explosion were also reported. Located 90 km SE of Minami-Iwo-Jima, Minami-Hiyoshi lies near the SE end of a coalescing chain of youthful seamounts, and is the only historically active vent. The morphologically youthful seamounts Kita-Hiyoshi and Naka-Hiyoshi lie to the NW, and Ko-Hiyoshi to the SE.

Geologic Background. Periodic water discoloration and water-spouting have been reported over the Minami-Hiyoshi submarine volcano since 1975, when detonation sounds and an explosion were also reported. It lies near the SE end of a coalescing chain of youthful seamounts, and is the only vent with recorded activity. The reported depth of the summit of the trachyandesitic volcano has varied between 274 and 30 m. The morphologically youthful seamounts Kita-Hiyoshi and Naka-Hiyoshi lie to the NW, and Ko-Hiyoshi to the SE.

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

Volcanic seismicity in the Nyamuragira/Nyiragongo area during 15-30 September was very high (figure 15). This activity was characterized by A-type (high-frequency) and C-type (low-frequency) events. Hypocenters were principally concentrated ESE of the summit of Nyamuragira, NE of Nyiragongo, at depths of 0-20 km. Depths for all of the earthquakes decreased from W to E and from S to N, suggesting a volcanic conduit rising in a generally NE direction towards the surface on the ESE flank of Nyamuragira. The level of volcanic tremor was very low. The low tremor level does not signify an overall reduction in the Nyiragongo lava lake activity, but no fresh lava was apparent in the crater.

Figure 15. Seismicity in the Nyamuragira/Nyiragongo area, 1 September-15 October 1995.

Seismic recordings in the first half of October were severely impaired by frequent power disruptions to the observatory and the regular discharge of the batteries. The number of earthquakes decreased significantly compared to September; events were mainly centered at greater depths (5-30 km) SE of Nyiragongo. Problems with the transmitter at the Kunene seismic station prevented acquisition of better data; scientists were unable to visit the station regularly due to a lack of tires for their vehicle.

Seismicity during 15 November-2 December remained high, mainly consisting of A- and C-type earthquakes, and volcanic tremor. The distribution of hypocenters was similar to that observed during September. Each series of earthquakes was followed by up to several hours of tremor. Five seismic stations were operating during this period, four of them (Kibati, Rusayo, Buhimba, and Kunene) telemetered to the Goma observatory, and the fifth (Mt. Goma) connected by cable. However, the transmitter from Kunene was intermittent. Tremor amplitude remained low.

Monitoring of both active Virunga volcanoes is done from a small observatory building located in Goma, ~18 km S of the Nyiragongo crater. Goma is the city where the major encampment of Rwandan civil-war refugees is located. A previous lava lake in the deep summit crater of Nyiragongo, active since 1894, drained suddenly on 10 January 1977, killing about 70 people. Lava lake activity resumed in June 1982, but had ceased by early 1983. The lava lake was again activated after an eruption that began in June 1994 (BGVN 19:06-19:08). Historical eruptions from Nyamuragira (14 km NW of Nyiragongo) have occurred within the summit caldera and from numerous flank fissures and cinder cones. Twentieth century flank lava flows extend >30 km from the summit. Nyamuragira also began erupting in July 1994, producing lava fountaining, lava flows, and ash emission.

Geologic Background. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Information Contacts: Wafula Mifundi and Mahinda Kasereka, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, Zaire.

Volcanic seismicity in the Nyamuragira/Nyiragongo area during 15-30 September was very high (figure 8). This activity was characterized by A-type (high-frequency) and C-type (low-frequency) events. Hypocenters were principally concentrated ESE of the summit of Nyamuragira, NE of Nyiragongo, at depths of 0-20 km. Depths for all of the earthquakes decreased from W to E and from S to N, suggesting a volcanic conduit rising in a generally NE direction towards the surface on the ESE flank of Nyamuragira. The level of volcanic tremor was very low. The low tremor level does not signify an overall reduction in the Nyiragongo lava lake activity, but no fresh lava was apparent in the crater.

Figure 8. Seismicity in the Nyamuragira/Nyiragongo area, 1 September-15 October 1995. Courtesy of the Goma Volcano Observatory.

Seismic recordings in the first half of October were severely impaired by frequent power disruptions to the observatory and the regular discharge of the batteries. The number of earthquakes decreased significantly compared to September; events were mainly centered at greater depths (5-30 km) SE of Nyiragongo. Problems with the transmitter at the Kunene seismic station prevented acquisition of better data; scientists were unable to visit the station regularly due to a lack of tires for their vehicle.

Seismicity during 15 November-2 December remained high, mainly consisting of A- and C-type earthquakes, and volcanic tremor. The distribution of hypocenters was similar to that observed during September. Each series of earthquakes was followed by up to several hours of tremor. Five seismic stations were operating during this period, four of them (Kibati, Rusayo, Buhimba, and Kunene) telemetered to the Goma observatory, and the fifth (Mt. Goma) connected by cable. However, the transmitter from Kunene was intermittent. Tremor amplitude remained low.

Monitoring of both active Virunga volcanoes is done from a small observatory building located in Goma, ~18 km S of the Nyiragongo crater. Goma is the city where the major encampment of Rwandan civil-war refugees is located. A previous lava lake in the deep summit crater of Nyiragongo, active since 1894, drained suddenly on 10 January 1977, killing about 70 people. Lava lake activity resumed in June 1982, but had ceased by early 1983. The lava lake was again activated after an eruption that began in June 1994 (BGVN 19:06-19:08). Historical eruptions from Nyamuragira (14 km NW of Nyiragongo) have occurred within the summit caldera and from numerous flank fissures and cinder cones. Twentieth century flank lava flows extend >30 km from the summit. Nyamuragira also began erupting in July 1994, producing lava fountaining, lava flows, and ash emission.

Geologic Background. The Nyiragongo stratovolcano contained a lava lake in its deep summit crater that was active for half a century before draining catastrophically through its outer flanks in 1977. The steep slopes contrast to the low profile of its neighboring shield volcano, Nyamuragira. Benches in the steep-walled, 1.2-km-wide summit crater mark levels of former lava lakes, which have been observed since the late-19th century. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south. About 100 cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a NE-SW zone extending as far as Lake Kivu. Many cones are buried by voluminous lava flows that extend long distances down the flanks, which is characterized by the eruption of foiditic rocks. The extremely fluid 1977 lava flows caused many fatalities, as did lava flows that inundated portions of the major city of Goma in January 2002.

Information Contacts: Wafula Mifundi and Mahinda Kasereka, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, Zaire.

When visited during January, the surface of the turquoise-green crater lake had risen 45 cm with respect to its December level. At the dome's N margin a small bubbling spring discharged relatively clear water. Lake water measured at the shoreline near the spring had a temperature of 29°C and on the N shore, a temperature of 27°C. The ongoing collapse of the N wall continued, as did reports of sulfur odors at the park entrance station, ~1 km away. Within the crater a small landslide had partially covered the W terrace and local fumaroles there, but fumaroles had migrated to the NW terrace where escaping gases made loud noises. SW-terrace fumaroles also remained active.

The SE-SW portion of the crater wall contained new fumaroles. These chiefly had temperatures of 73-91°C, with the maximum fumarole temperature, 94°C, found along the S wall. During the January visit, the intracrater cone displayed an increased number of gas emission points. Although the majority were inaccessible, gas columns emanating from the crater margins gave temperatures of 94°C.

During 1995, two distance-survey lines across the active crater underwent an expansion of 19.5 ppm. During the same interval, the distance-survey network outside the crater and the tilt network both lacked significant changes.

During December 1995 and January 1996, the total number of seismic events was relatively high, 5,960 and 4,187, respectively (figure 60). These events were predominantly of low frequency. During December and January, tremor registered for a total of 259 and 6 hours, respectively.

Figure 60. The monthly number of seismic events (top) and hours of tremor (bottom) at Poás for January 1995-January 1996. The December 1995 data were adjusted because the local seismic station (poa2, 2.7 km SW of the active crater) did not operate on the 16th-21st of the month. Frequency designations are as follows: intermediate (medium), 2.0-3.0 Hz; and low,

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

Information Contacts: Rodolfo Van der Laat, Vilma Barboza, Erick Fernández, Jorge Barquero, Franklin de Obaldia, Tomás Marino, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA).

Several months of explosive activity began at Popocatépetl on 21 December 1994. The eruption followed over a year of increased seismicity, SO₂ flux, and fumarolic activity.

Beginning at 1140 on 6 January 1996, observers in Puebla saw a steam column reaching ~6.1 km (20,000 ft). Meteorologists at the Synoptic Analysis Branch of NOAA were unable to see this steam column in either the GOES visible or infrared satellite imagery. On 7 January, as viewed from towns at the volcano s base (such as Amecameca and Atlautla), strong fumarolic activity continued from the crater.

Three days later, on 10 January, a helicopter from the Procuraduria General de la Republica flew L. Cardenas, H. Delgado, C. Siebe, and others over the volcano. They noted that fumaroles in the crater appeared rather weak, including those in the field on the volcano s NW side first noticed in April 1995. New fumaroles had sprung up on the N crater rim. No emanations were seen coming from the E-flank fumarolic field.

Later, when observed in conditions of good visibility from surrounding towns on 13-14 and 17-18 January, fumarolic activity was absent. Around this time a climb to the crater rim by Cardenas and Delgado enabled them to look inside; they again saw fumarolic activity that was weaker compared to that seen during most of 1994 and 1995. Despite this weak activity, two boccas on the rim of the old inner lava dome each contained an intensely hissing fumarole. The rocks cradling the fumarole glowed a reddish color visible in daylight, attesting to very high temperatures. In addition, fresh rockslide rubble that had probably sloughed off the crater s N walls lay on the crater floor. The crater s inner E wall looked extremely altered, suggesting that it may be susceptible to additional mass wasting in the near future.

They saw more intense fumarolic activity from the crater rim than they had ever seen before, possibly indicating that the main conduit below the crater floor was in the process of sealing. In this context they advised against mountaineers climbing to the summit area, because small Vulcanian explosions could occur at any time without warning.

C. Valdés-González, G. González-Pomposo, and A. Arciniega-Ceballos (UNAM) reported on seismic activity from November 1995 through early January 1996. Activity was monitored using seismic station PPM, a part of the Mexican National Seismic Network, located on the north flank at 3,900 m elevation. Seismic events were classified as type-A, -B, and -AB (BGVN 19:01).

Type-B events dominated during 1 November-8 January, an interval when 617 were recorded. Where data are available in the interval 1 November-8 January (figure 11), the number of type-B events ranged between 3 and 18 events/day. During this same interval, type-A and -AB events registered only 7 and 6 times, respectively. Fewer than 13 type-B events/day registered during late November through early January, ending on 5 and 6 January 17 and 18 events/day were recorded, respectively.

Figure 11. Type-B events (1.0-1.6 Hz) at Popocatépetl, 1 November 1995 to 8 January 1996. Blank where data were unavailable. Courtesy of UNAM.

Galindo and others (1995) summarized the available SO₂ flux estimates for 2 January 1994-28 January 1995. Other reports in the same volume described different facets of the volcano s behavior, including those relevant to public health (e.g. ash- aerosol dispersion).

Reference. Galindo, I., González, and Ayala, R., 1995, Emisiones de Bioxido de Azurfre del Volcan Popocatépetl, Mexico durante la erupcion de Diciembre 1994-Enero 1995; Comite Cientifico Asesor CENAPRED-UNAM, 1995, Volcan Popocatépetl Estudios Realizados Durante la crisis de 1994-1995; Capitulo VI, Aspectos GeoQuimicos y de Impacto Atmosferico, p. 245-256.

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

Information Contacts: Claus Siebe and Hugo Delgado, Instituto de Geofisíca, UNAM, Cuidad Univ., 04510 D.F., México; Guillermo González- Pomposo, Carlos Valdés González, and Ana Lillian Martin del Pozzo, Departamento de Sismología y Volcanología, Instituto de Geofísica, UNAM, Cd. Universitaría, 04510, D.F., México; NOAA/NESDIS Synoptic Analysis Branch, Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA.

Low-level activity from Tavurvur throughout January continued with plumes containing low to moderate ash contents. The plumes were released at intervals of 3-10 minutes, sometimes accompanied by weak roaring or explosion sounds. Incandescent lava fragments were ejected during some ash emissions. Large explosions occurred on 3 (2), 17 (2), 19 (1), and 24 (1) January. These explosions deposited lava blocks, as large as 60-80 cm in diameter, 1-1.5 km from the vent. The plumes rose 400-1,000 m above Tavurvur and were generally blown SE, but sometimes in a broad arc extending from the SW to the N. Ashfalls were recorded at Talwat village, in the Kokopo area, and in Rabaul Town. No vapor emissions were observed from Vulcan.

Seismicity was higher in January compared to December, with three high-frequency earthquakes, 2,401 low-frequency earthquakes, and 1,404 explosion events. Discontinuous non-harmonic tremor also occurred during the month. High-frequency earthquakes that could be located occurred NE of the caldera. Except for three low-frequency earthquakes, which originated NW of the caldera, all the other seismicity was associated with eruptive activity at Tavurvur. The increase in the number of both types of events was also accompanied by an increase in their amplitudes.

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: Ben Talai, Rabaul Volcano Observatory, P.O. Box 386, Rabaul, Papua New Guinea.

The eruption that began on [6] November had ended by 13 November (BGVN 20:11/12), yet somewhat elevated seismicity (4 events/day) prevailed through late November. Although the seismic system (RIN3, 5 km SW of the active crater) later failed (all or partly inoperative, 3 December-3 January), it received low-frequency events during most of 1-10 December at the rate of 1-3 events/day, and on 6 December it recorded eight events. During January, RIN3 registered near background levels of seismicity: 8 events/month, consisting of two at low frequencies and six at high frequencies.

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

Information Contacts: Rodolfo Van der Laat, Vilma Barboza, Erick Fernández, Jorge Barquero, Franklin de Obaldia, Tomás Marino, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.

No significant eruptions are known to have occurred at Ruapehu since November 1995. During a crater lake inspection on 18 January 1996, in conditions of limited visibility, the crater floor was occupied by a 70-m diameter lake whose surface lay around 80 m below the overflow level. The lake was chiefly green-gray in color; the E part of the lake, beneath Pyramid Peak, was mostly clear. Vigorous fumaroles were present above the lake s W and N shores, and beneath Pyramid Peak. At the Peak s foot, new crater wall exposures revealed 20 m of steep fans composed of ash and deposits tentatively identified as lake sediments. Both of these deposits were being dissected and eroded into mudflows. A zone of 1995 ejecta, 5 m thick, included a scoria-and-block unit (probably erupted on 11 October) and fine-medium ash layers. Glaciers in the crater basin and Whangaehu continued their rapid, and in the experience of the IGNS observers, unprecedented, retreat.

A tilt-leveling survey was conducted using four Dome benchmarks falling along a 45-m-long line radial to the crater. The benchmarks had moved since last measured. Assuming site instability at these benchmarks had developed since the latest pre-eruption survey (April 1994), the Dome s deformation averaged 30 ± 10 µrad deflation.

Both earthquakes and tremor started to decline in mid- October. From 31 October-8 November, long-duration events almost completely ceased, while short-duration events continued unchanged. During the month of November through 18 January, the previously seen 2-Hz tremor ceased, replaced mainly by higher-frequency tremor. This 7-Hz tremor had amplitudes 3-4x higher than seen prior to the commencement of activity in September 1995. Numerous, short-duration high-frequency earthquakes continued to be recorded at the Dome station through December, January, and into February 1996, defining Ruapehu s most recent background seismicity.

Maximum fumarole temperatures in one area (station A, beneath peg I) had fallen from 281°C (December 6, 1995) to 92 °C (18 January). Despite the lower temperatures, the discharges were still vigorous but notably water-rich. H2/Ar temperatures fell only slightly over this period (from 397 to 392°C), suggesting that the cooling reflects near-surface quenching by shallow groundwater.

Analytical results for lake water samples are given in Table 10. These include samples taken on 6 December (by sampler suspended from a helicopter), and on 20 December 1995, and 18 January 1996 (from the lakeshore). A dilution trend with time is evident in the data, but it is not clear whether this represents an artifact of sampling or decreased magmatic input into the lake. So far, ratios of SO₄/Cl and Mg/Cl show inconsistent trends. Much of the volcano s discharged heat and gases evaded the lake, to be released instead beyond the lake s NE and SW margins. A COSPEC flight on 31 December measured an SO₂ flux of 1,295 ± 160 tons/day; this value was presumed to indicate continuing, steady state, open vent degassing.

Table 10. Lake water temperature and chemical analyses from Ruapehu, 6 and 20 December 1995, and 18 January 1996. Courtesy of IGNS.

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

Information Contacts: P.M. Otway, S. Sherburn, and I.A. Nairn, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.

Volcanic activity continued until the end of 1995 at a level of intensity comparable to August-October (BGVN 20:09). During November and December, small to moderate explosions and avalanches occurred from the Jonggring Seloko summit crater. The average plume height was 300-500 m. Pyroclastic avalanches descended along the Kembar river to a distance of 500-1,000 m from the summit. The number of lava avalanches increased during November, traveling down the Kembar river up to 300 m from the summit. On 27 December an incandescent lava flow traveled 500 m; during this event volcanic tremor was recorded with a maximum amplitude of 3 mm.

The frequency of volcanic earthquakes (both A- and B- types) during the November- December period ranged from 1 to 10 events/day. Volcanic tremor was recorded on 1, 2, 17, and 18 November; tremor from 15 to 18 December increased to 3-8 events/day (figure 9). Explosion earthquakes were variable (31-136 events/day), with two minima on 5 and 25 December (24 and 19 events/day, respectively) (figure 9).

Figure 9. Eruptive activity at Semeru as detected by seismograph, November - December 1995: Daily number of tremor and volcanic earthquakes (top), explosions and avalanches (bottom). Courtesy of VSI.

Typical activity for this volcano was observed by Steve O'Meara on 13 November from the rim of Tengger Caldera. Thick cauliflower-shaped columns of gray ash rose up to 200 m high every 15-20 minutes, and the sky to the SW was spotted with ash clouds from previous eruptions. One event lasted ~5 minutes, generating a dark gray ash cloud that caused ashfall on the S slopes before detaching from the summit. Another eruption cloud spilled over the SW rim and flowed downslope.

Geologic Background. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: Wimpy S. Tjetjep (Director), Volcanological Survey of Indonesia (VSI), Jalan Diponegoro 57, Bandung, Indonesia; Steve O'Meara, PO Box 218, Volcano, HI 96785, USA.

An eruption on 23 December was first reported by pilots who observed an ash plume as high as 10.5 km altitude (BGVN 20:11/12). Possible very light ashfall was reported a few hours later 90 km away. Large steam plumes were reported in early January, and an intermittent "hot spot" was detected on AVHRR satellite imagery during 5-26 January, in the vicinity of the summit crater. This thermal anomaly may reflect unusually high temperatures in the vicinity of the continuously active fumaroles in the summit crater. AVO received no additional pilot reports or other observations of eruptive activity after 26 January.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory.

Activity in late 1995 consisted of whitish vapor emission to 25-100 m above the summit. During November occasional volcanic tremors were recorded with a maximum amplitude of 1.5 mm. Aviation reports on 7 November indicated increased eruptive activity with an ash cloud rising as high as 4.5 km altitude. Satellite imagery showed a possible ash cloud extending 90 km to the SW.

On 5 December, an increase in tremor amplitude up to 5 mm followed a tectonic earthquake felt throughout the Mimahassa Peninsula on Sulawesi. The same day maximum tremor amplitude reached 200 mm and glow was observed from three points on the lava dome. About an hour later tremor reached a maximum amplitude of 40 mm. On 6 December, tremor was still being recorded, but maximum amplitude had decreased to 2 mm.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is the only active cone in the Sempu-Soputan volcanic complex, which includes the Soputan caldera, Rindengan, and Manimporok (3.5 km ESE). Kawah Masem maar was formed in the W part of the caldera and contains a crater lake; sulfur has been extracted from fumarolic areas in the maar since 1938. Recent eruptions have originated at both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Information Contacts: Wimpy S. Tjetjep (Director), Volcanological Survey of Indonesia (VSI), Jalan Diponegoro 57, Bandung, Indonesia; Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin, NT 0801 Australia.

The dome continued to extrude in the breached summit crater. During January, subtle to dramatic variations occurred in the location, style, and rate of growth (with some areas undergoing up to 1-m vertical rise per day). Numerous spines grew, fell, and shattered. Besides obtaining the first samples of the new dome, fieldworkers established that the emplacement of the old dome (Castle Peak) was accompanied by one or more pyroclastic flows and lahars. The total seismic energy release for the last week of January was the highest since early December. Cumulative deformation measurements suggested inflation of the edifice.

Dome growth and visible observations. On 28-30 December 1995, the dome's E side grew 3 m upwards. Local avalanches accompanied this growth, but by 1 January both the growth and avalanches in this area temporarily slowed to a stop. Adjacent Castle Peak, on the new dome's S side where a small spine had formed on 26 December, volume increased without vertical growth in the week ending on 3 January. During that same week, significant steam escaped near the N part of the dome; the nearly continuous steam plume was sometimes charged with small amounts of ash.

Although dome growth appeared slow during part of the week ending on 10 January, it did not cease. Observers noted local avalanches (off both the dome's N and E sides) coupled with suggested swelling and new lava extrusions in the dome's central region. During this same week, the September spine appeared to move and tilt, and in the subsequent week the spine was pushed S by new dome growth. A new spine was identified on 10 January in the dome's center; this spine grew relatively rapidly until it fell down on 13 or 14 January. Other spines on the new dome also appeared to undergo a growth spurt.

Another spine appeared just after 14 January in the N dome area. Large parts of this spine had fallen by 18 January, possibly contributing to airborne ash seen on two occasions that day; the next day large blocks of the broken spine lay on the talus slope. Yet another spine appeared around 18 January along the S edge of the dome; it grew for two days prior to fall and breakup. Spalling material created a substantial talus pile in the S moat. On 20-21 January another spine grew in roughly the same spot. During the next two days this spine reached 25 m in height and 15 m in basal diameter prior to its partial collapse (an event correlated with significant ash emission on 23 January). Late in the week ending on 24 January, growth of the dome's S edge included growth of spines, spalling debris, slow swelling, and vertical growth.

Steam emissions were generally high during mid-January, and observers first saw new dome material piling up against the crater's W wall at the base of Chances Peak. The other side of the new dome extended a formidable distance up Castle Peak.

The most pronounced dome growth in the final week of January took the form of swelling on the dome's steeply sloping N margin. Although early in the week mass wasting repeatedly sent debris into the adjacent moat, later in the week this took place less frequently. On 26 January a spine was again noted in the dome's S area, but growth there on 28 January was manifested as swelling. Beginning on about 29 January on the N and S ends of the new dome observers saw two elongate ridges trending NE-SW. These appeared as rough mirror images of each other, their forms resembling whale backs.

Lofted ash and mass wasting. Airborne ash seen during January was mainly attributed to mass wasting. For example, a small amount of ash fell in Plymouth early on 4 January; the source was thought to have been a crater-confined rock avalanche off the dome. Minor ash fell in Plymouth four times on 12 January, and one time on both 15 and 16 January. Four ash emission events on 24 January were all associated with major rock-fall events on the lava dome. In some cases very minor ash emissions also issued directly from the dome and some of the material involved in mass wasting may have been dislodged by small explosions.

Visual observations into the crater have enabled good correlation between seismic signals and rock-fall events. During the week ending on 24 January, heat production from the dome appeared somewhat higher than in the past. During January dome incandescence was reported and some material within the rock falls was hot. Rockfalls and avalanches remained confined within the crater area, although the moat continued to gradually fill with debris.

Field studies. Good conditions on 8 January enabled the collection of a sample from the part of the new dome located within the 18 July vent, an area thought to have been extruded in late November. The crystal-rich sample contained dominant plagioclase, subordinate pyroxene and hornblende; parts of the sample were sent to four labs for further analysis.

Around the same time, other fieldwork in flanking drainages (Hot River and Fort Ghaut) found new exposures and established that several charcoal-bearing pyroclastic units (including at least one pyroclastic flow unit) were erupted during the growth of Castle Peak dome. These were also found adjacent to deposits having the character of lahars.

Deformation. EDM lines composed a network consisting of four surveyed triangles around the volcano. The lines continued to be measured routinely. Dry-tilt sites at Amersham and Brodericks on the volcano's W side were re-occupied during the early part of the week ending 10 January; neither showed any change since their last occupation in October. During January, the Spring Hill tiltmeter failed and was moved to a new site, but the Long Ground tiltmeter continued to indicate angular stability.

Looked at in the short term, EDM measurements taken during the first half of January did not show any changes in slant distance (above the error of the method); however, during the week ending 17 January it was reported that a slow shortening had occurred on many of the lines towards the volcano. The shortening indicated swelling of the edifice.

In the week ending on 24 January, it was reported that slant line distance in the NE sector (Tar River to Castle Peak area) underwent a 2.5-cm shortening over the period of a month. During the shorter interval of the final week of January, no changes above the error of the method were detected in slant distance measurements on two deformation triangles in the volcano's S to SW and NE sectors (the Galways-Chance's Peak-O Garra's and Long Ground-White's Yard-Castle Peak triangles).

Seismicity. During January, broadband tremor commonly registered on the Gages station. Tremor was generally absent at the other stations, although the Long Ground station also registered some tremor in mid-January. The number of daily earthquakes typically measured in the thousands (eg. 5,000 to 6,000 events at the closest station on 27 January), too numerous to count on a real-time basis. Instead, MVO often quantified seismicity for rapid dissemination by using located events. These are events for which a hypocenter (the earthquake focus, the point at which the first motion originates) was calculated based on one S-wave and the records from four stations.

An MVO report on 3 January stated that long-period events recorded at most seismic stations had been occurring at a rate of 10-15/day. The hypocenters for these events could not be found but they were thought to be at very shallow depths in the crater area. Later reports in January did not quantify the rate of occurrence for long-period events.

Late on 5 January, broadband tremor picked up slightly in amplitude at Gages station. Then, small long-period events occurred for about the next 12 hours. This was followed by an 8-hour swarm of >300 hybrid events with virtually identical waveforms <3 km beneath the volcano. Lower amplitude, regular hybrid events occurred every 1-2 minutes until 8 January. A smaller series of similar hybrid events took place on 12-15 January. Some initially small hybrid events that first appeared on 23 January grew in amplitude and rate of occurrence (to 6-7/minute) and continued until at least 31 January. During the last week of January these hybrid events formed the dominant seismic activity. repeated shallow hybrid events in early January within the crater preceded new dome growth by a few days. This had happened on at least two previous occassions.

During the first week of January, shallow (0-7 km depth) volcano-tectonic earthquakes with epicenters scattered around the volcano continued at a rate of 2-3/day. An exception was 1 January, when a cluster of 17 volcano-tectonic earthquakes took place just N of the active crater at 1-3 km depths. They occurred in an eight-hour period following an M 5.0 earthquake that struck at a depth of 25 km, centered ~55 km N of Port of Spain, Trinidad. This larger earthquake may have been the trigger for the 1 January seismicity.

Three small, 1-3 km deep, volcano-tectonic earthquakes struck SW of the island during mid-January. Very occasional, small, long-period earthquakes started to appear on the evening of 28 January and a solitary volcano-tectonic earthquake took place early on the 29th. This M 2-2.5 earthquake was located beneath the crater area at a depth of 2.8 km; it was felt by Long Ground area residents.

Crisis management. The eruption driving the current crisis began on 18 July 1995 (BGVN 20:06). According to the Montserrat Government Information Unit (on 13 February 1996), during the crisis there has been no official off-island evacuation. However, a phased relocation of 6,000 residents from the southern half of the island to the northern half immediately followed a large phreatic eruption on the morning of 21 August 1995. Ash from that eruption's cloud, and from a density current that flowed down the flanks of the volcano, caused darkness in the capital (Plymouth) and surrounding areas, and ultimately deposited several millimeters of ash there. The relocation order was partially lifted on 3 September, a day before the passage of Hurricane Luis.

A change in eruptive style in mid-November ultimately lead to the extrusion of lava at the surface. On 1 December 1995 a second relocation of 4,000 residents took place. The relocation lasted a month for residents on the island's SW side and about a month-and-a-half for those on the island's SE side. Some preparatory steps for future emergencies included the continued relocation of Glendon Hospital and newly acquired school buses to move residents.

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), c/o Chief Minister's Office, PO Box 292, Plymouth, Montserrat.

Monitoring from the Sakurajima Volcanological Observatory revealed nine explosions from Suwanose-jima in 1995. According to the Japan Meteorological Agency and the Kagoshima Prefectural Government, small eruptions during 10-13 January 1996 sent plumes 300-600 m above the volcano and caused ashfall to the S. Activity has been high since 1950, with 1-2 ash emissions every month, and some Strombolian explosions.

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).

During January, observers witnessed weak fumarolic activity continuing along the NE, N, NW, and W walls. Fumarole temperatures measured 83-89 °C. Mass wasting had taken place mainly on the S and W walls.

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

Information Contacts: Rodolfo Van der Laat, Vilma Barboza, Erick Fernández, Jorge Barquero, Franklin de Obaldia, Tomás Marino, and Rodrigo Sáenz, Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica.