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/).

Volcanic activity has remained low since the last explosive eruption on 20 February. However, a non-explosive eruption generated an ash plume to 1,400 m altitude on 3 April (19:04). The highest ash plume of the month rose to 1,800 m above sea level at 1506 on 1 May . . . . Two explosions on 30 May caused no damage. Explosive activity has increased since then, with frequent explosions in June.

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: JMA.

During May, Crater C continued its continuous emission of gases, lava flows, and sporadic Strombolian-style eruptions. The lava flows that began to exit in late December (1993) and late April (1994) both continued to move, but some of the smaller lobes had stopped. Though not erupting, Crater D maintained fumarolic activity.

During May, Strombolian eruptions remained low in number and magnitude. As in April, erupted ash reached 100-200 m above Crater C, but no explosive noises were evident ("mute" events). In late June, ICE geologists saw an average of one eruption every half hour, ejecting ash plumes up to 1,200 m above the crater.

During May, the OVSICORI seismic station ("VACR," located 2.7 km NE of the main crater) registered 831 events with frequencies of 1.7-2.3 Hz; the majority of these were associated with eruption of gas and pyroclastics (figure 69). The number of hours of harmonic tremor received for the month was relatively low (a total of 29 hours, figure 69c). Several peaks and troughs in seismic activity took place during the course of the month (figure 69c). The greatest duration of tremor took place around the 12th, when seismicity was moderate to low. A comparison with May data collected at the ICE seismic station ("La Fortuna," 3.5 km E of Crater C) shows good agreement in terms of seismic events and tremor near the middle of the month, but less agreement early and late in the month.

Figure 69. Seismicity and duration of tremor at Arenal, as follows: (a and b) monthly summary for January-May 1994, (c and d) daily summary for May 1994. Courtesy of OVSICORI.

During April and May, surveys of both a W-flank trigonometric-leveling line and the distance-measurement network showed no significant changes.

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: G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, Univ de Costa Rica; E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI.

Activity at [Crater 1] has been moderate since an explosion on 20 February 1993 ejected scoriae 100 m above the vent. During the daily rim visit on 2 May 1994, mud ejection was observed for the first time since 10 June 1993. However, the crater floor has been covered by water and frequent water ejections have been observed. Continuous tremor was registered at a seismic station 800 m W of the crater. Average amplitude of continuous tremor had been 0.2 µm through May, but on 7-9 June the average amplitude suddenly increased to >6 µm.

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

Information Contacts: JMA.

A vigorous steam plume was observed by pilots on 29 April and by AVO observers on 10 May. No ash was observed on 10 May either in the plume or on the flanks of the volcano. A single ash burst on 25 May generated a plume that rose to ~10.5 km altitude according to two pilot reports between 1700 and 1800 in the afternoon. The plume was described as dark gray and moderately dense by one pilot. Weather clouds obscured the view from satellites immediately following the eruption, but NWS satellite imagery later showed a small volcanic cloud drifting NE over the Bering Sea at ~5 km altitude. Apparently the activity consisted of a single burst without a sustained eruption; no additional eruptive activity was reported through mid-June.

Geologic Background. The beautifully symmetrical Mount Cleveland stratovolcano is situated at the western end of the uninhabited Chuginadak Island. It lies SE across Carlisle Pass strait from Carlisle volcano and NE across Chuginadak Pass strait from Herbert volcano. Joined to the rest of Chuginadak Island by a low isthmus, Cleveland is the highest of the Islands of the Four Mountains group and is one of the most active of the Aleutian Islands. The native name, Chuginadak, refers to the Aleut goddess of fire, who was thought to reside on the volcano. Numerous large lava flows descend the steep-sided flanks. It is possible that some 18th-to-19th century eruptions attributed to Carlisle should be ascribed to Cleveland (Miller et al., 1998). In 1944 it produced the only known fatality from an Aleutian eruption. Recent eruptions have been characterized by short-lived explosive ash emissions, at times accompanied by lava fountaining and lava flows down the flanks.

Information Contacts: AVO; J. Lynch, SAB.

Activity remained at low levels through April and May, similar to January-March of this year. Seismicity was characterized by small-magnitude "butterfly-type" events near the active cone, principally shallow earthquakes associated with rock fractures and fluid movement. It is possible that this activity is influenced by the gravitational field associated with tides (lunar-solar attraction) and by external agents such as rain. Sporadic long-period events are associated with fluid movement, and high-frequency events are associated with rock fractures.

Shallow "butterfly-type" earthquakes were frequent until mid-April, then decreased during May to an average of <10 earthquakes/day toward the middle of the month. High-frequency earthquakes reached a maximum of 3/day and were located mainly 3-4 km W and N of the summit at depths of 2-7 km. On 12 May, one of these earthquakes (M 1.9), was felt in Jenoy, 8 km N of the volcano. Five small-magnitude "screw-type" events were registered from 1 to 12 May. A tremor pulse on 27 May that lasted for ~15 minutes was possibly caused by magma-water interaction; it occurred during a time of strong rains in the region.

Electronic tiltmeters installed on the volcanic structure did not register any deformation in April or May. The SO₂ measurements taken from the gas column during April revealed continued low emission levels. COSPEC measurements of SO₂ in May were also low, with a variation of 50-798 t/d. Most fumarolic activity was toward the W side of the main crater.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS, Pasto.

Following its May 1993 eruption . . . activity remained high. An explosion in January 1994 at the main crater produced a dark ash cloud 750-1,000 m tall. Small gas explosions were common during February 1994, they often rose 200-400 m above the crater. One or more ash eruptions took place 25-27 March, dusting the village of Rua on the volcano's eastern slopes with thin ash.

Tectonic earthquakes were numerous, especially following the Halmahera earthquake of 21 January, 1994. Prior to the earthquake there were typically 10-25 events/day, following it there were 40 events/day. Volcanic earthquakes remained at normal levels, 3-5 events/day.

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

Information Contacts: W. Tjetjep, VSI; BOM Darwin, Australia; S. Matthews, Univ of Bristol; UPI; Antara News Agency.

. . . earthquake-triggered mudflows swept down steep-walled valleys engulfing multiple villages and settlements (figure 1). The M 6.4 earthquake . . . took place at 1547 on 6 June, apparently falling along the Cauca Romeral fault. It disturbed a wide area, causing minor structural damage in Bogota, but more significant damage to 10 buildings in Cali (100 km W of the epicenter; see inset, figure 1). Near the epicenter, located 10-30 km W of the volcano, the earthquake destroyed at least 40 homes. The most catastrophic damage caused by the earthquake took place when Nevado del Huila released gravitationally unstable rock, snow, and ice down the volcano's slopes. These mudflows are the main focus of the rest of this report.

Figure 1. A 500-m contour interval topographic map (map coordinates approximate) of the Paez river basin, the primary drainage from Nevado del Huila. The map shows villages (large dots), roads (heavy lines), and rivers (broken lines). The index map of SW Colombia shows the epicenter, large rivers, and the chain of active volcanoes (solid triangles) along the Andes as far south as the international border (heavy broken line). After Cepeda (1989).

A . . . topographic map from a published hazard study (Cepeda, 1989) shows the rugged local geography (figure 1, note the contour interval, 500 m). The study also contains a second map that outlines areas of likely risk from lava flows and mudflows. To avoid confusion with the actual event we have omitted this second map, however, it shows the mudflows along drainages down the mountain continuing toward the SSE into the channel of the Paez river. The region of mudflow risk extends all the way to the map's margin near Paical (in the SE corner). Available information suggests the mudflows did basically follow the Paez river as anticipated.

According to a 9 June Reuters news report, "Graphic video images shot by a tourist . . . captured the moment when the huge brown-grey mass of mud roared down the valley, sweeping away trees, rocks, and houses in its path." According to witnesses, the mudflow reached 30-m high. In the wake of the mudflow, access to the area was cut off. Roads and bridges were damaged or blocked by mud, necessitating the use of helicopters. News reports repeatedly cited damage and casualties in the villages of Irlanda, Toez, Talaga, and Paez Belalcazar (figure 1).

A 7 June, UPI report quoted the archbishop of Paez Belalcazar, Jorge Garcia. On a flight over the area, he observed that the village of Toez had been "buried in mud," and "only the roof of the school can be seen." The same news report noted "There were no immediate reports of how many Toez residents managed to escape before the village was smothered, although some 500 people were thought to have been buried." The news report also related that in Paez Belalcazar ". . . 12 people were washed away by the rushing waters."

Overall, the number affected by the widely felt earthquake and the more restricted mudflows was estimated at 50,000. In terms of the mudflows alone, fatality estimates ranged from 253 to over 1,200 people. About 250 people, including many severely injured children, were evacuated by helicopter to hospitals in the provincial capital Neiva. Some 2,500 survivors were brought by helicopters to tent camps in La Plata.

A 6 June Reuters news report told of people hearing a "strong explosion" leading to initial confusion about whether the mudflows were triggered by an eruption or seismic loading. It was reported that geologists monitoring the volcano suggested the explosion may have come from an avalanche in the area.

Problems apparently went beyond the damage from the initial mudflows and subsequent limited access. For example, the 6 June news report stated that at one point: ". . . the river burst through a natural dam created by a mud and rock slide caused earlier by the quake." Other reports cited aftershocks and heavy rains contributing to ground instability, conditions that in some cases injured both survivors and rescue workers.

Reference. Cepeda, H., 1989, Catálogo de los volcanes activos de Colombia: Bol. Geol., v. 30, no. 3.

Geologic Background. Nevado del Huila, the highest peak in the Colombian Andes, is an elongated N-S-trending volcanic chain mantled by a glacier icecap. The andesitic-dacitic volcano was constructed within a 10-km-wide caldera. Volcanism at Nevado del Huila has produced six volcanic cones whose ages in general migrated from south to north. The high point of the complex is Pico Central. Two glacier-free lava domes lie at the southern end of the volcanic complex. The first historical activity was an explosive eruption in the mid-16th century. Long-term, persistent steam columns had risen from Pico Central prior to the next eruption in 2007, when explosive activity was accompanied by damaging mudflows.

Information Contacts: T. Casadevall, USGS; UPI; Reuters.

Phreatic eruptions in July 1993 were preceded by increasing seismicity, but caused no damage. The following report, summarized from . . . VSI (1993a and b), provides additional details about this activity.

The number of volcanic earthquakes started to increase at the end of June 1993. Continuous tremor recorded on 21 June had a maximum amplitude of 0.5-2 mm. The next day, 37 shallow volcanic earthquakes were detected. Tremor amplitude gradually increased from 23 to 30 June. On 26 June, 4 deep volcanic earthquakes occurred. The number of volcanic earthquakes increased until 1 July when a gradual decrease began. However, by 1 July the maximum tremor amplitude was 7-10 mm. Because of the seismic activity, a warning was issued to the local population, to tourists, and to workers at the sulfur mine, saying that the area around the crater was closed.

Water temperature in the crater lake on 2 July was normal (36°C). The lake water was a pale green color, and the surface was covered by dense white vapor to a height of 10 m. Yellowish white vapor was being emitted from the solfatara field, and a very strong sulfur odor could be smelled.

A phreatic eruption at 0845 on 3 July from the center of the crater lake was accompanied by loud eruption sounds. The cloud released from the lake was 10-15 m high and 60-80 m in diameter. Lake water became brownish green, and the surface was dark. Two more phreatic eruptions the next morning (at 0835 and 1045) were smaller than the first; the early morning cloud rose 8-10 m, and no sounds were heard during the second of the 4 July eruptions. Rockfalls occurred at 1000 on 5 July from the S inner crater wall. A rumbling noise indicative of another phreatic eruption was heard at 0215 on 7 July at the sulfur weighing station, ~750 m from the crater.

During the period from 8 to 31 July, seismicity was variable, but there were no phreatic eruptions. Maximum tremor amplitude decreased to 0.5-4 mm. The number of deep volcanic earthquakes fluctuated in the 1-13 events/day range while shallow volcanic earthquakes occurred at a rate of 3-22/day. The temperature of water in the crater lake rose from 39 to 40°C.

Two phreatic eruptions occurred on 1 August starting at 1635; the sound could be heard at the sulfur weighing station. These eruptions were preceded by a tectonic earthquake with an amplitude >46 mm. There were no reports of injuries during any of the phreatic eruptions in July or August.

Seismic activity gradually decreased during 2-21 August when 0-2 deep and 5-23 shallow volcanic earthquakes were recorded each day. Crater lake water temperature through most of August was 39-41°C, and the pH was 1. Maximum tremor amplitude was 1-6 mm until 22 August when tremor was no longer continuous and maximum amplitude decreased to 1 mm. Between 22 August and 9 September deep volcanic earthquakes were recorded at a rate of 1-2/day; shallow events varied from 2 to 17/day. By 10 September, seismic data and visual observations indicated that the volcano had returned to a "normal" level of activity.

References. Volcanological Survey of Indonesia, 1993a, Ijen Volcano: Journal of Volcanic Activity in Indonesia, v. 1, no. 1/2, p. 14.

Volcanological Survey of Indonesia, 1993b, Ijen Volcano: Journal of Volcanic Activity in Indonesia, v. 1, no. 3/4, p. 8-12.

Geologic Background. The Ijen volcano complex at the eastern end of Java consists of a group of small stratovolcanoes constructed within the 20-km-wide Ijen (Kendeng) caldera. The north caldera wall forms a prominent arcuate ridge, but elsewhere the rim was buried by post-caldera volcanoes, including Gunung Merapi, which forms the high point of the complex. Immediately west of the Gunung Merapi stratovolcano is the historically active Kawah Ijen crater, which contains a nearly 1-km-wide, turquoise-colored, acid lake. Kawah Ijen is the site of a labor-intensive mining operation in which baskets of sulfur are hand-carried from the crater floor. Many other post-caldera cones and craters are located within the caldera or along its rim. The largest concentration of cones forms an E-W zone across the southern side of the caldera. Coffee plantations cover much of the caldera floor; nearby waterfalls and hot springs are tourist destinations.

Information Contacts: W. Tjetjep, VSI; BOM Darwin, Australia; S. Matthews, Univ of Bristol; UPI; ANS.

An ICE report for May stated that fumarolic activity continued in the bottom of the main crater. The warm grass-green-colored lake remained at the same level as in January and March. Water temperature was in the range 20-24.5°C (temperature of the inner lake, 21.4°C), and the minimum pH was 5.5. Fumarole temperatures reached as high as 86°C, and subaqueous fumarolic activity, which involved mainly CO₂, maintained the same vigor as seen in January and March. Fumarolic activity on the NW flank was unchanged. In May, the OVSICORI deformation network did not register significant changes.

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: G. Soto, Guillermo E. Alvarado, and Francisco (Chico) Arias, ICE; Héctor (Chopo) Flores, Escuela Centroamericana de Geologia, Univ de Costa Rica; E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI.

Low-level steam and ash emissions continued through late May and the first half of June, although poor weather frequently prevented observations. On several occasions in late May a vigorous steam plume was observed rising through scattered clouds above the volcano. Observers in Adak . . . saw a steam plume over the volcano on 31 May and a gray plume rising 1,000-1,200 m on 9 June. Aerial photographs of the summit area taken by U.S. Navy personnel in late January show that the vent system extends beyond the summit onto the upper W flank, corroborating reports by ground observers during the last several months.

Geologic Background. Symmetrical Kanaga stratovolcano is situated within the Kanaton caldera at the northern tip of Kanaga Island. The caldera rim forms a 760-m-high arcuate ridge south and east of Kanaga; a lake occupies part of the SE caldera floor. The volume of subaerial dacitic tuff is smaller than would typically be associated with caldera collapse, and deposits of a massive submarine debris avalanche associated with edifice collapse extend nearly 30 km to the NNW. Several fresh lava flows from historical or late prehistorical time descend the flanks of Kanaga, in some cases to the sea. Historical eruptions, most of which are poorly documented, have been recorded since 1763. Kanaga is also noted petrologically for ultramafic inclusions within an outcrop of alkaline basalt SW of the volcano. Fumarolic activity occurs in a circular, 200-m-wide, 60-m-deep summit crater and produces vapor plumes sometimes seen on clear days from Adak, 50 km to the east.

Information Contacts: AVO.

"The . . . eruption continued this month with lava entering the ocean along a 500-m-long front between the Kamoamoa and Lae Apuki areas in Hawaii Volcanoes National Park. Explosive activity was reported on 8 May, and continued with increased vigor through the end of the month. Some littoral explosions threw incandescent lava as high as 50 m in the air, and detonations could be heard from the highway (>500 m away). Large cracks were observed running parallel to the pre-April shoreline. Surface flows were rare during May. The Pu`u `O`o lava pond was active and its surface was 79-88 m below the crater rim.

"On 3 June a large channelized aa flow broke out of the lava tube at the 125-m elevation and advanced down to the coastal plain. Within a day, all break-outs from this flow were pahoehoe. The flow spread out on the coastal flats and was within 500 m of the shoreline by 6 June. More skylights opened at 150 m elevation."

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: T. Mattox, HVO.

Both craters at Langila continued at a low activity level in May. Emissions from Crater 2 consisted of weak-to-moderate white-gray vapour and ash clouds. Occasional forceful ejections of thick, dark-grey ash columns accompanied by explosion noises were reported on the 2nd, 7th, 9th, 20th, 29th, and 31st. Fine ashfall was reported on the 2nd and 20th on the NW side of the volcano. A steady weak red glow was visible on the 5th. Crater 3 released thin white vapour with very low ash content accompanied by thin blue vapor. Seismic activity was at a low level at the beginning of the month. No seismic recording was achieved after the 3rd because of equipment failure."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: I. Itikarai and C. McKee, RVO.

. . .The eruption produced Strombolian-fed, partially subglacial lava flows. The resulting meltwater caused lahars and chocolate-colored floods (figure 5). On 17 May Llaima also produced a column composed of ash, gas, and steam that reached ~ 4,000-5,000 m above its summit. Tephra fell over a 300-km-long, cigar-shaped zone trending about ESE (figure 6); it fell mainly on 17 May but limited falls also took place on 18 and 19 May.

Figure 5. Annotated sketch map of the area near Llaima on 21 May 1994. Contour intervals are 50 m (but note that in some snow-and ice-covered areas intermediate contours are missing due to data-transmission problems). The map emphasizes lava and subsequent lahars produced when lava melted glacial ice. The extent and path of the subglacial lava flow are incompletely known. Courtesy of H. Moreno.

Figure 6. Isopach and isopleth map of the Llaima tephra falls of 17-19 May 1994. Values given are in units of millimeters with thicknesses shown first, and grain-size diameters in parentheses. Courtesy of H. Moreno.

Observations prior to eruption. Hugo Moreno compiled the following list of pre-eruptive observations. In July 1993 after a long rainstorm, Conguillío lake, located on the NE foot of Llaima (figure 6), rose ~ 10 m above its typical seasonal height. It stayed at this elevated height until at least late-December. The magnitude and duration of the lake level rise were unprecedented since 1957, the year of the last big eruption that brought lava to the surface.

In November 1993, rangers of Conguillío national park reported underground rumbling on the N foot of the volcano (Captrén). A video taken from a small aircraft on 25 December showed that the crater area lacked many visible fumaroles. Specifically, the main crater, which was covered by ice, only hosted a very weak fumarole on its SW side. Llaima typically exhibits more vigorous fumaroles; their absence was an anomaly.

A seismic survey (14-17 February, ~10 km E of Llaima at Verde lake: figure 6) found seismic events had an average frequency of about 1.0 Hz, a typical result for Llaima (e.g., 1.2-1.4 Hz in September 1992, 17:8). During 16-17 February a 2-fold increase in the number of events took place, from 90 to 180 events. The events were interpreted as due to magma degassing. A later seismic survey from the same area, 8-10 March, recorded 150-160 events/day with average frequencies in the range 1.0-2.4 Hz. On 22 March a portable seismic station on the W slope of the volcano (Los Paraguas) recorded events reaching still higher average frequency (1.6-3.0 Hz). The consistent increase of the average frequency since February was interpreted as due to slow ascent of magma along the volcano's main conduit.

H. Moreno and S. Barrientos conducted precise leveling, dry tilt, and electronic distance meter (EDM) measurements during 24 February-1 March on the volcano's E flank. Again, except for weak fumes on the SW rim, no fumaroles were seen coming from the main crater. The S summit area ("Pichillaima," figure 5) displayed many small fumaroles; these have progressively increased since 1984.

17-19 May eruptions. The first report of an eruption came from the Melipeuco Police Station, located ~20 km S of the volcano (figure 6), where at 0500-0600 on 17 May observers saw explosions above the main crater. At about 0600 they watched a dense column of ash, gas, and steam issue from the crater; a strong wind dispersed these products toward the ESE.

Between 0900 and 1000 three Chilean domestic (LAN) flights reported the ash column rising 4-5 km above the summit. Ultimately, the plume disrupted several other commercial flights, especially in Argentina.

Between 1100 and 1530 a Chilean Air Force helicopter carried observers to the erupting snow- and ice-capped stratovolcano's W and N sides. On the SSW side of the main crater the aerial observers saw at least four lava fountains escaping from a fissure. The areas covered by spatter from these fountains are shown on figure 5. The fissure was ~500-m long, trending N10°E; it vented small explosions at intervals of ~3 seconds. Lava fountains reached up to ~ 200 m high and joined a lava flow that ran under the adjacent glacier to the W. Llaima's western glacier is significant. Prior to the eruption it had an area of ~ 17.2 km² and a liquid-phase volume of ~ 367 x 10⁶ m³. Along the fissure the ice underwent rapid, violent melting and vaporization. Many explosions penetrated through the ice.

Aerial observers noted that downslope of the eruption fissure the W glacier discharged steam and explosions. These exhalations indicated that the lava continued some distance beneath the glacial ice, apparently turning toward the W and entering the alpine reaches of either the Lanlan or Calbuco rivers, or both (figure 5). The lava's subglacial path became more apparent later, on 21 May, when the volcano next became visible from the ground. The main crater rim then contained a small notch on its SSW side. The notch held an "ice channel" with a pronounced westerly bend (figure 5). On 21 May, the channel's width varied from about 50 m above the bend to 150 m below it.

On 17 May the invading lava melted sufficient glacial ice so that at about 1200 a lahar was identified moving down the Calbuco river (figure 5). Downstream at a village off the W edge of figure 5 (El Danubio, ~16 km WSW of the summit), the lahar passed at about 1245-0100 carrying trees, sediments, ice blocks, and boulders up to 9 m in diameter. Within a deep gully the lahar reached 35-m wide, 19-m high, and its volume was estimated as 2.5 x 10⁶ m³.

After the lahar reached the Quipe river (~25 km W of Llaima's summit) it advanced as a chocolate-colored flood. At about this point observers in the helicopter flew to the town of Vilcún (43 km W of Llaima's summit), landed on a small bridge, and alerted residents of the advancing floodwaters. The floodwaters arrived at 1515; subsequently the river rose 4.3 m and widened from 32 to 61 m. Estimated water velocity was 13-14 km/hr. During the interval 1630-1700, observers at El Danubio noted the passage of a second flood. In addition to stranding and killing thousands of fish, the lahar and associated flooding nearly covered a cemetery, cut across roads, and destroyed five bridges across the Rio Calbuco; 59 people were rescued from its path.

Observers near the volcano on 17 May saw the ash column blow toward the ESE in the region below about 5 km elevation. During the interval from 0800-1230 ash affected the area immediately adjacent Llaima's E and SE flanks (the Trufultruful river-Verde lake area). During 1000-1330, peaking at 1300-1330, ash fall increased in the area along the ash-distribution axis near the E border of Chile (the Icalma-Cruzaco area). The ash column contained both ash- and water-rich zones.

At 2000 on 18 May, a new, coarser ash fell for several minutes on Cruzaco (~46 km SSE of Llaima). Cruzaco again received ash for the last time on 19 May at 1200; this time it was very fine. Ash samples collected in Cruzaco contained 0.1-4 mm diameter grains of black and reddish-colored scoria with phenocrysts of plagioclase, olivine, and magnetite. Some samples were also taken of water and Coirón grasses that feed livestock, in order to make sulfide, chloride, and other chemical analyses.

Seismic and satellite data. Abnormally high seismicity occurred after the eruption until at least 14 June when monitoring ceased. During this interval, increased seismicity took place on 31 May-1 June, coincident with loud subterranean noises reported from 20 km S at Melipeuco, and summit incandescence seen from 24 km W at Cherquenco.

During the nights of 13-19 June, subterranean rumblings were heard by Pablo Parra of the Hosteria Hue-Telen (Melipeuco) when he was at Verde Lake (figure 6). The rumblings lacked associated smoke-puffs or incandescence. He also reported that although clouds and rain generally shrouded the summit in mid-June, on either 14 or 15 June clear weather revealed a gray-white plume ("normal" for the volcano) changing to a dark-gray plume (distinctly different from "normal"). Parra also noted that Pichillaima exhibited a recent slump on its SE side. He thought the slump was reminiscent of the one seen prior to the explosive 1957 eruption, and he recalled how he and area residents heard similar rumblings for several years prior to that eruption.

Satellite data of Llaima includes GOES-E images collected between 17 and 23 May, excepting 19 May. Steve Matthews, Kath Walley, and Robin Sharphouse have stored the GOES-E images in PDF and TIFF computer format.

The first GOES-E image, at 0230, shortly before the eruption, shows no eruption plume. Plume-like reflectors were observed on the E side of the Chile-Argentina border as follows: (a) on 18 May at 0926, (b) on 20 May at 0926 and 1430, and (c) on 22 May at 1430. On other days cloud cover obscured the area.

The GOES-E image for 18 May contains a small, compact reflector ~100 km E of the volcano. The two 20 May images depict an elongate, plume-like reflector extending from the border directly east of the volcano for ~ 150 km in a SE direction. On the 22 May image a similar feature extends from the border for ~150 km in a NE direction. In all cases these features were more intense than nearby clouds and may represent the ash plume.

Other remarks. The 17 May eruption was ranked by Hugo Moreno as VEI 2 with a strong phreatic component. The exact extent of the subglacial lava flow remains uncertain. The eruption caused no reported casualties.

Geologic Background. Llaima, one of Chile's largest and most active volcanoes, contains two main historically active craters, one at the summit and the other, Pichillaima, to the SE. The massive, dominantly basaltic-to-andesitic, stratovolcano has a volume of 400 km3. A Holocene edifice built primarily of accumulated lava flows was constructed over an 8-km-wide caldera that formed about 13,200 years ago, following the eruption of the 24 km3 Curacautín Ignimbrite. More than 40 scoria cones dot the volcano's flanks. Following the end of an explosive stage about 7200 years ago, construction of the present edifice began, characterized by Strombolian, Hawaiian, and infrequent subplinian eruptions. Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

Information Contacts: H. Moreno¹, G. Fuentealba², M. Murillo², M. Petit-Breuilh², J. Cayupi², and P. Peña², SERNAGEOMIN, Temuco, Chile; A. Rivera, Univ de Chile, Santiago; D. Lescinsky, Arizona State University; S. Mathews, Univ of Bristol, U.K.; K. Walley and R. Sharphouse, Ulverston Victoria High School, U.K.

During May, activity . . . remained low. Crater emissions consisted of thin white vapor released at weak to moderate rates. Throughout the month seismic activity remained at low to moderate inter-eruptive levels. Tilt, measured in the water-tube tiltmeter . . . , remained stable.

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

Information Contacts: I. Itikarai, and C. McKee, RVO.

A fire of unknown origin burned 10 m² of accumulated sulfur deposits in the Soufriere Sulphur Springs area (~700 m SSW of the summit), causing false eruption reports. The alleged eruption was reported by residents to have started on 24 April with the formation of small lava flows. Authorities in the capital of Roseau passed the information to the Seismic Research Unit in Trinidad. A team was sent to investigate the report on 27 April. No local seismic activity was detected at the permanent seismographic station, located 1.5 km away, or by the portable seismometer installed at the site during the visit.

Geologic Background. The Morne Plat Pays volcanic complex occupies the southern tip of the island of Dominica and has been active throughout the Holocene. An arcuate caldera that formed about 39,000 years ago as a result of a major explosive eruption and flank collapse is open to Soufrière Bay on the west. This depression cuts the SW side of Morne Plat Pays stratovolcano and extends to the southern tip of Dominica. At least a dozen small post-caldera lava domes were emplaced within and outside this depression, including one submarine dome south of Scotts Head. The latest dated eruptions occurred from the Morne Patates lava dome about 1270 CE, although younger deposits have not yet been dated. The complex is the site of extensive fumarolic activity, and at least ten swarms of small-magnitude earthquakes, none associated with eruptive activity, have occurred since 1765 at Morne Patates.

Information Contacts: W. Ambeh, L. Lynch, and R. Robertson, UWI.

In May, gases from the shrunken and nearly dry lake, Laguna Caliente, continued to present an environmental problem. Dry weather and persistent eruptive activity led to a decrease in the level of both the lake and surrounding groundwater. The retreat of the lake had reached the point that it appeared nearly dry in March, but fumarolic degassing persisted from a number of locations on the crater floor (figure 48). In the absence of abundant water, volcanic gases vented more directly into the atmosphere, causing fumaroles to degas more vigorously and sometimes even to resemble low-energy explosions.

Figure 48. The active crater at Poás in late May 1994. Original sketch provided by G.J. Soto of ICE.

Volcanic gas concentrations have risen in the area adjacent to the National Park (SE, S, SW, and W of the main crater); residents in its vicinity have reported a "strong sulfur smell." These odors forced the Park to close on 26-27 May and at least once in June. They were particularly strong at dawn, and some emissions had yellow and bluish colors. Acidic rainfall also increased such that economic losses since 1988 were on the order of several million dollars (US). Areas of loss encompassed timber, crops, machinery, grazing land, livestock, habitations, and human health. Health complaints have included nausea and coughing, and irritated throat, eyes, and skin.

In contrast, the fumaroles located on the S part of the crater toward the dome appeared comparatively unchanged. They had stable temperatures (89°C) and continued to emit steam-rich components.

ICE reported that microseismicity at Poás has mainly consisted of low-frequency events located beneath the crater lake. From last January through May 1994 the microseismicity has doubled.

OVSICORI reported that during May, station POA2 (located 2.5 km SW of the active crater) registered a total of 5,228 low-frequency events (figure 49). POA2 registered medium-frequency events (99), and high-frequency events (9). POA2 also registered continuous low-frequency tremor with peak-to-peak amplitude slightly under 3 mm, at times reaching 5 mm. The tremor signal was strong in the frequency range 2.0-3.2 Hz (figure 50). The highest seismicity took place on 25 and 31 May, the lowest, 15 May, a day that still received continuous tremor.

Figure 49. Poás seismicity for January-May 1994. Courtesy of OVSICORI.

Figure 50. Poás tremor beginning at 1343 GMT, 16 May 1994 (top) and spectral analysis of the tremor (bottom). Amplitudes are arbitrary. Courtesy of OVSICORI.

Compared with the month of April, low-frequency seismicity decreased 13%, medium-frequency increased 76%, and the high-frequency remained about the same. In May, the number of hours of tremor increased—coincident with the above mentioned rise in the vigor of fumarolic activity. On 18 May a M 2.5 earthquake took place at a depth of 15 km centered 3.3 km NE of the active crater. During April and May there was no significant deviation in deformation.

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: G. Soto, G. Alvarado, and F. Arias, ICE; H. Flores, UCR; E. Fernández, J. Barquero, V. Barboza, and W. Jiménez, OVSICORI.

Cordón Caulle began generating a series of small to moderate felt earthquakes and discontinuous subterranean noises during the final week of May. The Univ of Chile and the Univ of the Frontera monitored the activity with two seismometers on 28 and 29 May. They detected harmonic tremor and small earthquakes centered N of Puyehue, generally located on Cordón Caulle. Santiago radio reported that four tremors were felt in the area over a 12-hour period on the night of 29 May. The tremors shook with Mercalli-scale intensity IV and V.

The radio report said that the activity had also drawn a team of professionals from the Geosciences Institute of Valdivia Austral Univ to the area. Meanwhile, the police, army officers, civil authorities, and scientists had formed an emergency action committee and established a "White Alert," which signifies the detection of possibly abnormal volcanic activity and mandates that emergency plans be reviewed and updated.

Geologic Background. The Puyehue-Cordón Caulle volcanic complex (PCCVC) is a large NW-SE-trending late-Pleistocene to Holocene basaltic-to-rhyolitic transverse volcanic chain SE of Lago Ranco. The 1799-m-high Pleistocene Cordillera Nevada caldera lies at the NW end, separated from Puyehue stratovolcano at the SE end by the Cordón Caulle fissure complex. The Pleistocene Mencheca volcano with Holocene flank cones lies NE of Puyehue. The basaltic-to-rhyolitic Puyehue volcano is the most geochemically diverse of the PCCVC. The flat-topped, 2236-m-high volcano was constructed above a 5-km-wide caldera and is capped by a 2.4-km-wide Holocene summit caldera. Lava flows and domes of mostly rhyolitic composition are found on the E flank. Historical eruptions originally attributed to Puyehue, including major eruptions in 1921-22 and 1960, are now known to be from the Cordón Caulle rift zone. The Cordón Caulle geothermal area, occupying a 6 x 13 km wide volcano-tectonic depression, is the largest active geothermal area of the southern Andes volcanic zone.

Information Contacts: N. Banks, US Embassy, Santiago.

"During May, 694 earthquakes were detected, compared to 397 in April and 458 in March. Of these, 51 earthquakes were located, 28 with errors <1 km.

"Seismic activity was low until 25 May; it consisted of small swarms and discrete events. On 25 May, Rabaul was subjected to its strongest seismic activity in about a year. Starting at 1043, earthquakes were felt for ~20 minutes. The maximum felt intensity was in the airport region, IV-V on the modified Mercalli scale. Two spatially separated swarms were involved. The first, including an M_L 3.3 earthquake, was located in a linear zone between the airport region and Vulcan. The second swarm, which included an M_L 3.0 earthquake, started ~15 minutes after the first. The second swarm was located just off the E shore of Vulcan and Vulcan Island, near the site of swarm activity in February and April (19:2-3). Both swarms were shallow (< 2 km), consistent with previous activity in these areas. Seismic activity at both centers continued throughout the rest of the day at a declining rate.

"For the rest of the month, seismic activity consisted of small and discrete events, probably located in the same region as the large swarms on the 25th. On the 26th there were two earthquakes just off the SW shore of Matupit Island, at depths around 2.2 km. These locations are not on the ring fault system.

"At 0212 on 26 May, a low-frequency earthquake was recorded on the harbor network. The signal had dominant frequencies around 1 Hz and probably originated near Matupit Island. There may have been as many as 10 similar events in the 24-hour period following the felt earthquakes.

"Routine leveling on 27 May showed that about 35-40 mm of uplift had taken place at the S end of Matupit Island since . . . 2 May. Additional leveling to Vulcan Point on 30 May showed an uplift of ~30 mm since September 1993."

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: I. Itikarai and C. McKee, RVO.

In May a glow was noticed on the crater floor of Barujari cone, which has undergone no significant activity since August 1966. A portable seismograph (PS-2) and telemetry seismograph (Teledyne) were put into operation on 27 May and 9 June, respectively. One volcanic earthquake event/day was recorded on 27, 28, 30, and 31 May. After 4 June, however, volcanic tremor with a maximum amplitude of 35 mm was recorded, presumably associated with the upward movement of magma.

At 0200 on 3 June, Barujari cone began erupting by sending an ash plume 500 m high. One 8 June press report described emission of "smoldering lava" and "thick smoke," as well as ashfall in nearby villages from an ash cloud rising 1,500 m above the summit. Between 3 and 10 June, up to 172 explosions could be heard each day from the Sembalun Lawang volcano observatory (~15 km NE). During this period, seismic data indicated a dramatic increase in the number of explosions per day, from 68 to 18,720 (figure 2). Eruptions were continuous at least through 19 June, with maximum ash plume heights of 2,000 m on 9-11 June (figure 2).

Figure 2. Daily number of explosion earthquakes and height of the ash plume at Rinjani, 3-19 June 1994. Courtesy of VSI.

The ash plume generally drifted SE, depositing up to 30 mm of ash on the island (figure 3). Strombolian eruptions ejected pyroclastic material <2 m in size as high as 600 m above the vent; this material fell in a restricted proximal area around the cone and in the lake. Lava flows began on 8 June and partially covered previous lava flows from Rombongan (in 1944) and Barujari (in 1966) (figure 4).

Figure 3. Distribution of ash from Rinjani eruptions, 3-19 June 1994. Courtesy of VSI.

Figure 4. Segara Anak caldera lake at Rinjani showing lava flows from the Rombongan dome (1944), Barujari cone (1966), and the recent lava flows of June 1994 (slash pattern). Courtesy of VSI.

A series of aircraft warnings based on pilot reports and weather satellite images indicated much larger plumes than suggested by the ground observations. First, an eruption at about 1200 on 7 June produced a long plume that caused a large number of aviation warnings. The plume, located on satellite imagery, extended 120 km S of Rinjani and was beginning to disperse by 1530. A pilot report at 1645 on 7 June indicated a "smoke" plume to 13.5 km altitude moving ESE, but by 2345 the plume was indistinguishable on satellite imagery. The imagery showed a plume around 0633 on 8 June, which extended at least 83 km SE of the volcano. Aircraft were advised to avoid this area to an altitude of 10.5 km (35,000 feet).

Second, at 1645 on 9 June a cloud with volcanic ash was evident on satellite imagery within 56 km of the volcano rising up to an altitude of 4.5 km (15,000 feet). The plume was apparently not elongated on the image but the report stated: "Expect cloud to drift W."

In apparent conflict with ground observations and satellite imagery observed by Australian meteorologists, a GOES satellite image at 1831 on 9 June obtained by Steve Matthews revealed a N-directed plume. This straight, distinct plume originating from Lombok Island trailed N for 800 km over SE Borneo, where it merged with a dense cloud bank. The plume widened from ~50 km across at a point 100 km N of the island to 100 km across where it met the Borneo coast.

Satellite imagery at 0830 on 10 June indicated a cloud with ash from 74 km SE to 56 km NW of the volcano to an altitude of 9 km (30,000 feet) with upper level drift to the S. Between 1700 and 2330, an ash cloud (bounded by the following corner points: 8°S, 116°E; 8°S, 117°E; 10°S, 117°E; and 12°S, 118.5°E) reached a height of almost 10 km (34,000 feet). The tongue of ash cloud previously detected drifting S was no longer evident on satellite imagery by 0600 on 11 June, but at 1940 the ash cloud was detected within an area slightly smaller than the previous day. The plume, as seen on satellite imagery at 0800 on 13 June through about 0500 on 16 June, remained over the vicinity of the island, but it exhibited some streaming to the N. At that time the plume began streaming E before drifting N. Pilot reports indicated a plume to 7.5 km (25,000 feet), with patches to 10.5 km (35,000 feet) and spreading N and NE. On 17 June, islands could be seen through the thin plume on satellite imagery. Enhanced AVHRR imagery indicated the probable presence of ash within the plume through 1300 on 18 June. Pilot reports at ~1200 on 18 June again confirmed an ash "smoke" cloud SW of the volcano for a distance of 80 km and an altitude of 10 km (34,000 feet). The plume was consistently observed on the imagery during the night of 18-19 June, but remained thin.

Geologic Background. Rinjani volcano on the island of Lombok rises to 3726 m, second in height among Indonesian volcanoes only to Sumatra's Kerinci volcano. Rinjani has a steep-sided conical profile when viewed from the east, but the west side of the compound volcano is truncated by the 6 x 8.5 km, oval-shaped Segara Anak (Samalas) caldera. The caldera formed during one of the largest Holocene eruptions globally in 1257 CE, which truncated Samalas stratovolcano. The western half of the caldera contains a 230-m-deep lake whose crescentic form results from growth of the post-caldera cone Barujari at the east end of the caldera. Historical eruptions dating back to 1847 have been restricted to Barujari cone and consist of moderate explosive activity and occasional lava flows that have entered Segara Anak lake.

Information Contacts: W. Tjetjep, VSI; BOM Darwin, Australia; S. Matthews, Univ of Bristol, UK; UPI; ANS.

Heatflow during April remained low (table 4), but evidence of convection (dark slicks from the central vent) on 6 May indicated some recent increase. Lake temperature at 20 m depth continued to decline from 47°C on 18 February to 23.6°C on 6 May. Two bursts of strong tremor, on 5 and 8 May, corresponded to a renewed steady temperature rise to 24.9°C by 11 May. As with the previous heating phase, this activity occurred several weeks after strong low-frequency acoustic signals were recorded.

Table 4. Temperature, outflow measurements, and water analyses from the crater lake of Ruapehu, 18 January 1994 to 27 August 1994. Discharge of "0" indicates a lake level below overflow stage. A dash (--) signifies no measurement. Courtesy of IGNS.

On 18 April the lake was a uniform battleship gray color with no evidence of upwelling, although the N vents were not fully visible from the observation point. No signs of surging were seen around the shoreline or at Outlet. A dark khaki-green slick emanating from the central vent area on 6 May drifted slowly onto the SE shore, but no upwelling was observed. Broken yellow slicks originating from several weak upwelling cells in the N vent area were also present over the N half of the lake. The general color of the lake was the same as in April, and there was no sign of recent activity. Prior to the heating episode in February, the ratio of Mg to Cl in the lake water decreased slightly from 0.042 in late 1993 to 0.038 in January (table 4), due mainly to a decrease in Mg. This ratio had remained stable at least through 18 April.

Inspection of photographs taken during the reported steam eruption on 1 March revealed an apparently passive steam cloud, a common atmospheric effect at the crater lake. The rising cloud was most intense over an area of discolored water, and may have been caused by vigorous convection or a minor phreatic event shortly beforehand. This incident is a reminder that even reports from reliable eyewitnesses should be treated with caution; reports of possible eruptions in February-April should be regarded as unproven.

The only deformation change of possible volcanic significance detected on 6 May was a reversal of the 9 mm contraction of the crater width indicator line recorded between 12 and 28 March. This suggested a return to the mildly inflated level first recorded in January. It is not yet known if the evidence of minor inflation is significant. A leveling survey on 18 April indicated 21 µrad of tilt towards the crater (deflation) at the Dome location over the past year, the largest tilt since 1981. Because this follows a period of slow apparent deflation (0.7 µrad/year), the measurement may not be reliable. Southern benchmarks may have been lowered by downhill creep of a lava slab. However, large systematic apparent tilts of

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. Otway, IGNS Wairakei.

A high-frequency earthquake swarm in mid-March and early April ended nearly two years of low activity. Significant long-period earthquakes began in mid-April. Several swarms on 19, 22, and 23 April culminated in an explosion at 1554 on the 23rd. Seismic activity gradually declined after the explosion. The Emergency Committee of Caldas declared a yellow alert and suspended visitor and tourist passes until the seismicity had decreased to acceptable levels. [INGEOMINAS stated that there was no emission of ash at the time of the 23 April earthquake swarm.]

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: INGEOMINAS, Manizales; U.S. Embassy, Bogota.

A . . . small eruption at Suoh hot-spring field that expelled gas-charged hot mud [followed] a major, destructive earthquake in the same region (19:02). The earthquake, Ms 7.2, took place at 1707 GMT on 15 February, or in terms of local time and date, at 0007 on 16 February.

"We sent our team to investigate the area where the phreatic explosion occurred. The team arrived at Suoh on 19 February, three days after the earthquake. Two new mud explosion pits, 5 m in diameter, were found W of the Suoh depression. Liquifaction was consistently found at fractures associated with the earthquake. The two explosion pits contained boiling water."

Tables 1 and 2 present data on water and gas samples taken from two sites in the Suoh area during the investigation.

Table 1. Water chemical analyses for two sites in the Suoh hot-spring field (sampled 19 February 1994). Courtesy of VSI.

Table 2. Gas chemical analyses for two sites in the Suoh hot-spring field (sampled 19 February 1994). Courtesy of VSI.

Geologic Background. The 8 x 16 km Suoh (or Suwoh) depression appears to have a dominantly tectonic origin, but contains a smaller complex of overlapping calderas oriented NNE-SSW. Historically active maars and silicic domes lie along the margins of the depression, which falls along the Great Sumatran Fault that extends the length of the island. Numerous hot springs occur along faults within the depression, which contains the Pematang Bata fumarole field. Large phreatic explosions (0.2 km2 tephra) occurred at the time of a major tectonic earthquake in 1933. Very minor hydrothermal explosions produced two 5-m-wide craters at the time of a February 1994 earthquake.

Information Contacts: R. Sukhyar, VSI.

Annual fieldwork was carried out on 30 March and 29 April 1994. Maximum fumarole temperatures had fallen to 78°C by the end of April. ... There was insufficient fumarole discharge for adequate sampling, and temperatures and pressures were at the lowest levels ever recorded. Except for minor landslide debris, no significant changes were noted in the Ngāuruhoe crater.

Tilt leveling surveys were carried out at the Tama Lakes (1.7 km SSW) and Mangatepopo (1.8 km NNW) locations on 30 March. Apparent tilt recorded at Tama Lakes during the previous 11 months represented 4 µrad of inflation, but was within the range of random fluctuations recorded since installation in 1978. At Mangatepopo approximately 14 µrad of tilt towards Ngāuruhoe (deflation) was recorded over the same period. This is ~2-3x the past noise level resulting from normal survey errors and seasonal movements. The most likely explanation, based on earlier experiences, is that two benchmarks near a walking trail have settled.

Repairs were made to the three highest crater rim stations on 30 March and two new stations were installed; two old stations are scheduled for removal after the 1995 survey. All six rim sites were surveyed for horizontal deformation on 29 April. Measurements were made by EDM and theodolite from 2 km N on Tongariro volcano. Relative movement vectors for the 1992-94 period at three stations were well within the normal noise range. Instabilities noted at the other sites resulted from various surface movements. Overall, there was no indication of recent volcanic deformation.

Geological mapping of the crater, N flank, and SW flank accomplished during these visits is part of the ongoing mapping project of the Tongariro complex.

Geologic Background. Tongariro is a large volcanic massif, located immediately NE of Ruapehu volcano, that is composed of more than a dozen composite cones constructed over a period of 275,000 years. Vents along a NE-trending zone extending from Saddle Cone (below Ruapehu) to Te Maari crater (including vents at the present-day location of Ngauruhoe) were active during several hundred years around 10,000 years ago, producing the largest known eruptions at the Tongariro complex during the Holocene. North Crater stratovolcano is truncated by a broad, shallow crater filled by a solidified lava lake that is cut on the NW side by a small explosion crater. The youngest cone, Ngauruhoe, is also the highest peak.

Information Contacts: P. Otway, IGNS Wairakei.

The increase in the level of venting activity . . . continued into May. Throughout the month the summit crater emitted moderate to thick white vapor, although there were occasional reports of gray and blue emissions on 17 and 18 May, and towards the end of the month. On 23 May, because of the ash cloud, pilots in the region were notified to "exercise caution and to report any increase in activity including height and movement of the ash cloud." In addition, during most nights in the first three weeks of the month the crater emitted a red glow that remained weak but steady.

May seismic activity underwent a slight progressive decrease: Daily earthquake totals early in the month were in the range 400-600; by month's end they had dropped to 400. Since the end of April earthquake amplitudes also decreased.

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: I. Itikarai, and C. McKee, RVO; BOM, Darwin.

Endogenous dome growth to the W and NW . . . had ceased by the end of April. However, the dome began to grow in a SW direction in mid-May. This SW growth continued through at least mid-June at a rate of 1-2 m/day. EDM measurements taken by the GSJ revealed that a line on the N flank had shortened between January and April, implying that inflation of the entire mountain had ceased by the end of April, but the same line showed elongation in May.

Elevations of lava-dome peaks have steadily increased since the eruption began (figure 71). The highest peak in early June was 250 m above the level of the Jigokuato crater floor. Peaks were commonly formed just above the magma-supply vent during both exogenous and endogenous growth, but no lava extrusion has taken place above 1,420 m elevation.

Figure 71. Elevation of lava-dome peaks at Unzen, 20 May 1991-June 1994. The highest peak as of June 1994 is lobe 12 (L12); base elevation shown (1,250 m) is for the Jigokuato crater. Different lobes are indicated by symbols and lobe numbers. All measurements were made using a theodolite and mirror-less laser distance meter by geologists from the Joint University Research Group. Courtesy of S. Nakada.

A time plot of the eruption rate shows two pulses of magma during the current eruption (figure 72). The first pulse (May 1991-December 1992) was characterized mainly by exogenous growth. The second pulse (December 1992), which started with lobe 9, was dominated by exogenous growth early (first half of the pulse), but then changed to endogenous growth. The volume of magma erupted during the first pulse, 1.3 x 10⁸ m³, is roughly double that erupted during the second pulse (0.6 x 10⁸ m³). Total volume of the lava dome, based on analysis of aerial photos by the GSJ, was 90 x 10⁶ m³ as of 9 April. The lava extrusion rate between 7 February and 9 April was 60,000 m³/day (figure 72). The eruption rate declined in May to3/day as determined by the Joint University Research Group. No fresh lava has been extruded onto the dome surface since February.

Figure 72. Daily eruption rate at Unzen, 20 May 1991-June 1994, showing two distinct pulses of magma-supply. Eruption rates were estimated by the Joint University Research Group (JURG) using photographs from daily helicopter inspections and theodolite surveys. Only aerial photographs were used by the Geographical Survey Institute (GSI), the Public Works Research Institute (PWRI), and the Geological Survey of Japan (GSJ) to calculate the volume change of eruption products. Courtesy of S. Nakada.

Most pyroclastic flows traveled down the SW and SE flanks, only rarely did they descend N of the dome. The longest pyroclastic flow of the month went 2.5 km on 3 May. Pyroclastic flows are detected seismically at a station ~1 km WSW of the dome. Real-time monitoring of both the dome and pyroclastic flows is conducted from the Unzen Weather Station using four visible and thermal infrared video cameras. Microearthquakes beneath the dome averaged >100/day. The total of 3,171 earthquakes in May continues the decline in seismicity . . . .

The Coordination Committee for Prediction of Volcanic Eruption had a meeting on 3 June. A statement issued after the meeting noted that both the lava dome and the entire volcanic edifice were very unstable, and that pyroclastic flows generated by collapse of lava might occur despite the decline in lava extrusion. As of 31 May, 3,307 people remained evacuated.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: S. Nakada, Kyushu Univ; JMA.

Residents in Perryville . . . reported a large steam plume rising from Veniaminof on the afternoon of 20 May. Inclement weather prevented observation of any other activity during the second half of May. Residents of Port Heiden . . . who were able to see the volcano on 2 June reported that no plume was present over the summit caldera. However, they did observe a steam plume on 9 June. AVO received no pilot reports of continuing eruptive activity in early June.

Geologic Background. Veniaminof, on the Alaska Peninsula, is truncated by a steep-walled, 8 x 11 km, glacier-filled caldera that formed around 3,700 years ago. The caldera rim is up to 520 m high on the north, is deeply notched on the west by Cone Glacier, and is covered by an ice sheet on the south. Post-caldera vents are located along a NW-SE zone bisecting the caldera that extends 55 km from near the Bering Sea coast, across the caldera, and down the Pacific flank. Historical eruptions probably all originated from the westernmost and most prominent of two intra-caldera cones, which rises about 300 m above the surrounding icefield. The other cone is larger, and has a summit crater or caldera that may reach 2.5 km in diameter, but is more subdued and barely rises above the glacier surface.

Information Contacts: AVO.