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/).

This report discusses Pavlof's behavior during May 2014 through 26 December 2014, a time period with two clear eruptive intervals that included lava fountaining, spatter, fragmental (agglutinate-rich, clastogenic) lava flows, lahars, pyroclastic flows, and diverse plumes. On 30 May 2014, an eruption began that continued intermittently through the first week of June. A thermal image taken from a satellite on 24 June 2014 showed warm areas ~5 km down the N flank interpreted as the signature of an earlier, still-warm lava flow. (This flow was perhaps similar to (fountain- and spatter-fed, fragmental, agglutinate-rich, clastogenic) lava flows and possible associated lahars seen during 2013; Waythomas and others, 2014; Wolf and Sumner, 2000.) Another eruption took placed during 12-16 November 2014. Besides the previously mentioned characteristics, common observations during eruptions included strombolian emissions, multiple-kilometer-long zones of incandescent lava, plumes ranging from those dominated by steam and gas to others that were rich in ash. Diagnostics from distant instruments included acoustical signals of eruption received with infrasonics and lightning from inferred ash plumes detected with a lightning detection array.

Background. In BGVN 38:05 we reported on the then most recent eruption at Pavlof, which occurred during May-June 2013. Waythomas and others (2014) summarized Pavlof's eruptive behavior during 2013. This is relevant, in part, because similar ice-spatter interactions also prevailed during 2014. "The 2013 eruption of Pavlof Volcano, Alaska began on13 May and ended 49 days later on 1 July. The eruption was characterized by persistent lava fountaining from a vent just north of the summit, intermittent strombolian explosions, and ash, gas, and aerosol plumes that reached as high as 8 km above sea level and on several occasions extended as much as 500 km downwind of the volcano. During the first several days of the eruption, accumulations of spatter near the vent periodically collapsed to form small pyroclastic avalanches that eroded and melted snow and ice to form lahars on the lower north flank of the volcano. Continued lava fountaining led to the production of clastogenic lava flows that extended to the base of the volcano, about 3–4 km beyond the vent. The generation of fountain-fed lava flows was a dominant process during the 2013 eruption; however, episodic collapse of spatter accumulations and formation of hot spatter-rich granular avalanches was a more efficient process for melting snow and ice and initiating lahars. The lahars and ash plumes generated during the eruption did not pose any serious hazards for the area. However, numerous local airline flights were cancelled or rerouted, and trace amounts of ash fall occurred at all of the local communities surrounding the volcano, including Cold Bay, Nelson Lagoon, Sand Point, and King Cove."

The reports by the AVO also announced Volcano Alert Levels and Aviation Color Codes. The four Alert Levels apply to conditions in vicinity to the volcano (of greatest concern to residents). The Levels consist of Normal, typical background or noneruptive state; Advisory, exhibiting signs of unrest or possible renewed increase; Watch, exhibiting escalating or heightened unrest; and Warning, hazardous eruption is eminent or underway. The respective Color Codes address risks to aircraft from ash plumes. The Codes consist, in increasing order of concern, Green, Yellow, Orange, and Red.

Pavlof is monitored by satellite imagery, observers, several in-situ and remote instruments, and by a Federal Aviation Administration (FAA) web camera. Figure 9 shows Pavlof as seen from the FAA web camera, which resides in Cold Bay. The photo shows conditions on a clear day when the volcano was quiet. The camera produces still images sometimes used to convey the volcano's behavior ('FAA supplementary weather products').

Figure 9. A NE view that features snow- and ice-clad Pavlof as seen from the FAA web camera in Cold Bay (Alaska) on a clear day, date unknown. MSL stands for elevation (in this case with respect to MSL, mean sea level, here expressed in feet, 1 foot = 0.305 m). SM stands for statue miles, used to describe the distance from the camera to a building and to Pavlof (~56 km away; 1 SM = 1.61 km). Courtesy of FAA (US Federal Aviation Administration).

Eruption of 30 May to 4 June 2014. The AVO weekly report issued on 6 June 2014 summarized conditions during the 30 May-4 June eruption period as follows: "Pavlof Volcano is experiencing a typical Strombolian eruption, characterized by lava fountaining, minor explosions, and the accumulation of spatter on the upper north flank of the volcano. Accumulations of spatter have occasionally built up and collapsed, forming hot, ashy, particle-rich flows that generate high-rising steam plumes on the lower north flank of the volcano. As these flows interact with ice and snow on the volcano, they produce meltwater and steam plumes. Spatter-fed lava flows also are likely forming".

According to AVO's 6 June 2014 weekly summary, Pavlof began erupting on 30 May 2014. On the morning of 31 May 2014 elevated surface temperatures were detected at the summit of Pavlof, suggesting a low-level eruption with extruding lava. Campers near the volcano confirmed this detection, and noted lava flows originating from a vent on the NE flank. As those lava flows interacted with glacier ice, low-altitude ash clouds and plumes were created. The plumes were detected in satellite imagery, as well as by pilots and with the Cold Bay FAA web camera.

On the evening of 31 May 2014, small explosion signals were detected by a distant infrasound sensor. The eruption continued, followed by incandescence. The FAA web camera in Cold Bay detected weak incandescence glowing at the summit on the evenings of 31 May and 1 June. Clouds obscured views of the volcano by web camera although no ash clouds were detected in satellite imagery. Weak seismic activity was detected on the Pavlof network of seismometers near the volcano. An increase of seismic tremor occurred 2 June at 1500, decreasing around 2300 that evening (Alaska Standard Time = UTC - 9 hours; during May-June, Daylight Saving Time = UTC - 8 hours). The Aviation Color Code and Alert Levels on 31 May were Orange and Watch respectively.

On 2 June 2014, AVO reported a plume discharged almost continuously from the vent rising to an altitude of 6.7 km and extending over 75 km E, as seen in figure 10. The AVO daily report for this eruption stated "Hazardous conditions exist on the north flank and north side drainages heading on the volcano due to continued pyroclastic and lahar activity. Ash in the vicinity of the volcano remains a hazard to local air traffic" (figure 10).

Figure 10. Visible image of a Pavlof plume acquired by the MODIS instrument on the Terra satellite on 2 June 2014 (2145 UTC 2 June, which corresponds to local Daylight Saving Time and date of 1345 on 2 June). The plume extended ~75 km E of Pavlof. Courtesy of NASA and AVO/USGS.

The AVO photo archive for 2 June contained over 40 photos with captions. Some were taken from Cold Bay and others from at sea and aircraft, documenting eruptive activity that day. Chris Waythomas (AVO) noted incandescence associated with lava fountaining and low-level ash and steam plume on images caught by the FAA camera. Several photos by Rachael Kremer were captioned by AVO scientists. The caption of one image (ID #591161 written by Game McGimsey, AVO/USGS) not only described incandescence from lava fountaining at the summit vent, it also stated the presence of "spatter-fed lava flowing down the N flank." Further, "ash and steam clouds rising from lower on the north flank were likely generated by pyroclastic flows intermixing with glacier ice."

AVO daily reports issued on 2 and 3 June 2014 described a vigorous continuing eruption. Late on the 2nd, tremor increased again. During the night included observers noted intense lava fountaining and a spatter fed lava flow down the N flank. By the morning of the 3rd, and ash and steam plumes rose up to 7.3 km altitude. The AVO report issued at 1233 on the 3rd noted a wind shift and wind at the time of that report carrying the main plume SSW. Lower winds (below ~3 km altitude) carried a plume that may have contained trace ash to the WSW.

The AVO report issued at 1754 on the 3rd made these statements: "Although the eruption of Pavlof continues, seismic tremor has deceased over the past 12 hours and has remained relatively steady throughout the day at a much lower level than that of yesterday. Recent satellite data and web camera views of the eruption plume indicate that there are now two distinct parts of the plume. The part of the plume that reaches high above the volcano appears to be mainly steam and gas with minor ash present, extending south of the volcano. Additionally, pyroclastic flow activity on the north flank is producing diffuse ash emissions that result in areas of hazy air, with variable concentrations of ash below [~3 km]. Low-level winds are likely to disperse this ash to the west-southwest with no more than trace amounts accumulating. There are no reports of ash falling in nearby communities." The Aviation Color Code was reduced from Red to Orange and the Alert Level to Watch. Ash remained a hazard to local air traffic.

Similar conditions prevailed on 4 June, with plumes containing minor ash but rich in sulfur dioxide extending 30 to 100 km downwind over Cold Bay. Although incandescence was visible in early morning web cam images, seismicity had remained stable for the past 24 hours. Incandescence from lava fountaining was visible in webcam images on 4 June. According to a news article, flights in and out of Cold Bay and Unalaska were canceled on 4 June, affecting about 200 people. At 0205 and 0245 on 5 June 2014, seismic data indicated two distinct explosions. AVO inferred these represented the collapse of spatter built up around the vent, with a possible explosive component. A similar third, less energetic, event occurred at 0844. The explosions generated lightning, which was detected by the World Wide Lightning Location Network (WWLLN, a collaboration of over 50 universities) (Morton, 2014). AVO inferred that hot debris moved down the N flank, resulting in localized low-level clouds of fine ash. There was no ash above the meteorological clouds whose tops reached 8.8 km in height. As of 6 June 2014, elevated surface temperatures persisted but cited that on this morning they had observed greatly diminished ash and lava emissions. Steam or ash plumes were absent in satellite images since 4 June. A weekly summary issued on 6th noted plumes during the eruption that started on the evening of 30 May 2014 had reached about 9.1 km in altitude. Seismic data indicated lahars occurred intermittently.

Comparative quiet. During 7-23 June 2014, Pavlof was comparatively quiet. Although extreme temperatures associated with fountaining were not seen, a thermal image of Pavlof on 24 June 2014 suggested broad areas of warm temperatures from what AVO interpreted as a recent lava flow (figure 11). According to the scientist who prepared the image, David Schneider, "Composite satellite image of Pavlof Volcano showing the extent of the lava flows on the northeast flank. The base image was collected by the Worldview-2 satellite on May 9, 2014 (prior to the onset of eruptive activity) and is overlain (in color) with a Landsat-8 thermal infrared image collected early in the morning on June 24, 2014. The thermal infrared sensor measured the heat given off by the still-warm lava flow. The length of the longest branch of the lava flow is about 5 km (3 miles). Note that the lava flow appears to have traveled under the ice on the upper flank of the volcano."

Figure 11. A thermal image from Landsat 8, with areas of increased infrared radiation, acquired in the early morning of 24 June 2014 showing the path of lava flows down the slopes. For scale, the longest arm of the flow was about 5 km. The lava flow traveled under the ice in an area of the upper flank. For more details, see text. Courtesy of AVO. Caption details and image preparation by D. Schneider (AVO/USGS).

An AVO Notification issued on the 25th indicated that AVO had observed no evidence of ash emission from the volcano since early June. Clear web camera and satellite images of the volcano over the past several days showed no evidence of continued lava fountaining. The Aviation Color Code was reduced to Yellow and the Volcano Alert Level was reduced to Advisory. AVO further added that small discrete seismic events continued. They suggested that the signals may have been related to several processes including, (1) degassing of unerupted magma within the volcano's conduit and (2) periodic collapse of ejecta and other debris down the steep flanks of the volcano. The latter, appears consistent with the lava flow seen on figure 11.

On 30 July 2014 the Color Code was lowered to Green and the Volcano Alert Level to Normal. Since mid-June, levels of unrest had gradually declined. Rockfalls and small avalanches of debris still occurred sporadically on the NNW flank of volcano. The next eruptive event did not occur until 12 November.

Eruption of 12-16 November 2014. As previously mentioned, an eruption occurred during 12-16 November 2014. On 12 November 2014, AVO reported a ground observer in Cold Bay sighted ash emissions from Pavlof rising to an altitude of 2.7 km, signifying a new eruption. Minor ash emissions were visible in the Cold Bay web camera beginning around 1650 Alaska Standard Time (AKST) on 12 November. AVO raised the Aviation Color Code and Volcano Alert Level at 1957 on 12 November. Tremor remained elevated on the 12th, 13th, and 14th, with lava fountaining and ash emissions. On 14 November satellite imagery revealed a narrow ash plume extending ~200 km at 4.8 km altitude.

On 15 November 2014, AVO reported the eruption of had intensified. Thus, the Aviation Color Code was raised to Red and the Volcano Alert Level to Warning. Behavior was characterized by explosive eruptions, lava fountaining from a vent just N of the summit, and flows of rock debris and ash descending the N flank of the volcano. Ash emissions were observed from the ground and in satellite images. The intensity of seismic tremor had increased significantly, and satellite data indicated the ash cloud top at 7.6 km altitude extending 200 km NW from the vent. Figure 12 shows a Landsat 8 image captured on the 15th. The top of an ash plume in the image had reached an altitude of ~9 km. Another satellite image taken the same day showed ash plume above cloud cover and extending ~300 km NW from the volcano.

Figure 12. On 15 November 2015, Pavlof was lofting ash plumes to an altitude of 9 kilometers as shown in the natural-color image, acquired by the Operational Land Imager (OLI) on the Landsat 8 satellite. Pavlof's volcanic plume rises well above the cloud deck. NASA Earth Observatory image by Jesse Allen, using Landsat 8 data from the U.S. Geological Survey. Original image by David Schneider.

Although as mentioned above, on 15 November 2014, the ash plume reached more than 9 km, tremor had abruptly decreased at about 1900 that day. This was accompanied by a large decrease in ash emissions, and the next day no evidence of an ash plume at the volcano was reported. On the 16th, the Aviation Color Code decreased to Orange and the Volcano Alert Level to Watch. During 17-18 November seismicity remained low; surface temperatures on the upper NW flank were elevated. The AVO weekly report issued on 21 November 2014 described the week's activity as still remaining low. Intermittent tremor was detected, and satellite images still showed lava flow on the volcano's NW flank. At that stage it reached ~7 km from the summit. On 25 November 2014, AVO further lowered the Aviation Color Code/Volcano Alert Level to Yellow/Advisory, citing continued low seismicity and lack of any observations to suggest ongoing lava fountaining or ash emission. According to the last AVO weekly report issued on 26 December 2014, the status of Pavlof remained unchanged. Seismicity at Pavlof continued slightly above background levels. Weather conditions continued to be cloudy during the week and no activity was observed in satellite or web camera views of the volcano.

References. Demas, A., (3 June) 2014, Volcano Warning Alert Issued for Alaska's Pavlof Volcano, U.S. Geological Survey [accessed August 2014] (URL: http://www.usgs.gov/blogs/features/usgs_top_story/volcano-warning-alert-issued-for-alaskas-pavlof-volcano/ ). [accessed August 2014]

Morton, M, (6 April) 2014, Volcanic Lightning Generated in a Bottle, Earth Magazine (URL: http://www.earthmagazine.org/article/volcanic-lightning-generated-bottle)

Schwaiger, H.F., Denlinger, R.P., and Mastin, L.G., April 2012, Ash3d: A finite-volume, conservative numerical model for ash transport and tephra deposition. Journal of Geophysical Research, v. 117, Issue B4, 20 p.[accessed August 2014] (URL: http://onlinelibrary.wiley.com/doi/10.1029/2011JB008968/pdf).

Schwartz, D., (11 August) 2013, Ash3D is Federal Answer to Ash Cloud Response, Peninsula Clarion [accessed August 2014] (URL: http://peninsulaclarion.com/news/2013-08-10).

Waythomas, C. F., Haney, M. M., Fee, D., Schneider, D. J., and Wech, A., 2014, The 2013 eruption of Pavlof Volcano, Alaska: a spatter eruption at an ice-and snow-clad volcano. Bulletin of Volcanology, 76(10), pp. 1-12.

Wolff, J. A., & Sumner, J. M. (2000). Lava fountains and their products. Encyclopedia of volcanoes, H Sigurdsson, B Houghton, S McNutt, H Rymer, J Stix (Eds.); pp. 321-329.

Geologic Background. The most active volcano of the Aleutian arc, Pavlof is a Holocene stratovolcano that was constructed along a line of vents extending NE from the Emmons Lake caldera. Pavlof and Pavlof Sister to the NE form a dramatic pair of symmetrical, glacier-covered stratovolcanoes that overlook Pavlof and Volcano bays. Little Pavlof is a smaller cone on the SW flank of Pavlof volcano, near the rim of Emmons Lake caldera. Unlike Pavlof Sister, eruptions have frequently been reported from Pavlof, typically Strombolian to Vulcanian explosive eruptions from the summit vents and occasional lava flows. The active vents lie near the summit on the north and east sides. The largest recorded eruption took place in 1911, at the end of a 5-year-long eruptive episode, when a fissure opened on the N flank, ejecting large blocks and issuing lava flows.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320,Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys,794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.dggs.alaska.gov/); Christopher Waythomas, Game McGimsey, and Cheryl Cameron, AVO; Rachel Kremer (affiliation unknown); Federal Aviation Administration (FAA), 800 Independence Ave, SW, Washington, DC 20591, USA (URL: http://www.faa.gov/); and National Aeronautics and Space Administration (NASA) (URL: http://modis.gsfc.nasa.gov/).

Tangkubanparahu (Tankuban Parahu) erupted multiple times during the interval of reporting from February 2013 through December 2014. The eruptions were from Ratu crater and of quite small size (highest reported plumes only rose to 100 m tall). The vent grew in size as a result of these eruptions, reaching in early March 2013 a diameter of 20 m. The small eruptions contained minor ash but did not emit a dome or lava flows and accordingly did not lead to thermal anomalies detected via the MODVOLC satellite-based infrared detection system (and this is the case going back to at least the year 2010).

In past reports during the past few decades, Tangkubanparahu has largely been quiet but with occasional tremor and volcanic earthquakes (eg., late August-October 2002, 12-19 April 2005, and August-September 2012; BGVN 27:09, 28:08, 30:12, and 37:11). The location of the volcano in Java is shown in figure 1 of BGVN 37:11.

According to the Center of Volcanology and Geological Hazard Mitigation (CVGHM, also known as Pusat Vulkanologi dan Mitigasi Bencana Geologi, PVMBG), tremor increased on 21 February 2013 and diffuse ash emissions rose from Ratu Crater. Based on the seismicity, visual observations, and temperature increases of the land around the crater, CVGHM raised the Alert Level to 2 (on a scale of 1-4) and visitors were reminded not to approach the crater within a radius of 1.5 km.

CVGHM reported that phreatic eruptions from Tangkubanparahu's Ratu Crater occurred on 28 February and during 4-6 March 2013, and generated ash plumes that rose up to 100 m above the crater.

A news report (kompas.com) quoted CVGHM as stating that the March explosion was much stronger than the one on 21 February 2013. The news report said that the 6 March eruption lasted for ~8 minutes. The Jakarta Post also said that the 6 March eruption lasted ~8 minutes and ejected ash about 30 m above Ratu Crater. The Jakarta Post reported that on 18 March, CVGHM lowered the Alert Level to 1 (normal) because of a significant decrease in the tremor frequency. The article also quoted CVGHM as stating that deformation, using a Global Positioning System (GPS) and Electronic Distance Measurement (EDM), found at one or more stations a decline in relative elevation from 6.84 cm to a few millimeters by 18 March. Deflation was again detected from 24 February through early March 2013, but was stable during 7-14 March 2013.

According to CVGHM, sulfur dioxide emissions increased to 5.3 metric tons per day (t/d) on 24 February 2013, decreased through 3 March 2013 to 2.1 t/d, and then increased again during 5-9 March 2013 to 4.9 t/d. CVGHM speculated that the increase was due to an enlargement of the eruptive vent, which had grown to a diameter of 20 m.

Gas emissions decreased abruptly on 10 March 2013 to 2.1 t/d and emission sounds stopped. On 4 March 2013, a new solfatara vent opened, but SO₂ levels could not be measured on that day because of weather conditions.

On 5 October 2013, a phreatic eruption occurred, causing CVHGM to raise the Alert Level to 2. Figure 2 is an image of Ratu Crater.

Figure 2. Photo of Tangkubanparahu's Ratu crater taken (or posted?) in June 2014. Ratu crater is the currently active crater and one of two large craters on the volcano; it is about 1 km in diameter and has a depth of about 400 m. CVGHM reporting notes that, overall, the volcano hosts 9 craters. Image courtesy of Marietha S as posted on Tripadvisor.com.

CVGHM reported that during November-December 2014 white plumes rose up to 50 m above Ratu Crater. Deformation occurred and seismicity increased. On 31 December the Alert level rose to 2 (on a scale of 1-4), cautioning people to remain at least 1.5 km from the crater.

Seismicity. The CVGHM report discussing late 2014 features a plot of seismic data during December 2012 through December 2014, which the authors termed significant, the chief observation prompting a rise in alert level (to II).

Tremor was most prominent beginning mid-2013 to early March 2014. Both low-frequency and hybrid earthquakes were nearly absent except during a short sequence in late 2014 (each with over 100 earthquakes; see table below). Type-B earthquakes were common at levels from a few to ten events per 20-day interval, and like the low-frequency and hybrid earthquakes, peaked in latest December 2014 (~50 type-B events). Type-A earthquakes showed little or no tendency to cluster and remained below 5 events per 20 day interval and on many days they were absent.

Table 3 indicates the types and frequencies of seismic activity at Tangkubanparahu during selected, mostly active periods during 2013. Shallow volcanic earthquakes predominated during many of these periods. The number of tremor was high during the first week of March 2013, but significantly declined thereafter. The 25 September 2013-5 October 2013 period contained somewhat elevated seismicity, yet apparently lacked significant eruptive activity. Note the emergence of 513 low-frequency earthquakes during 1-31 December 2014 (lower right). That data is in the same year-end report (issued in early 2014 and written in Indonesian) and is also noteworthy in terms of the plot of distance (EDM) data to various reflectors around the crater during the entire year of 2013.

Table 3. A compilation of earthquake counts and tremor durations recorded at Tangkubanparahu for selected periods during 2012-2014. Definitions: -- signifies no data (presumably no episodes); VA, volcanic type-A earthquake; VB, type B (shallow volcanic earthquake); TJ, deep tectonic earthquake; BQ, an earthquake indicative of emissions; and TL, local tectonic earthquake. Courtesy of CVGHM.

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

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM) (URL: http:proxy.vsi.esdm.go.id/index.php); kompas.com (URL: kompas.com); and The Jakarta Post (URL: http://www.thejakartapost.com/).

Tofua is a remote volcano in Tonga that is not monitored. The primary sources of information about the volcano's activity are from infrequent field visits, ash advisories from the Wellington Volcanic Ash Advisory Centre, and MODIS thermal sensors aboard the Aqua and Terra satellites.

No ash advisories from the Wellington Volcanic Ash Advisory Centre were issued during the reporting period, 28 September 2013-30 June 2015. Since the last report through 27 September 2013 (BGVN 38:07), five thermal alerts were recorded through 30 June 2015 (table 3). Two of those alerts, on 14 and 23 September 2014, were located outside and NW of the caldera rim and therefore were probably not associated with volcanic activity. No thermal alerts were issued between 18 October 2014 and 30 June 2015.

Table 3. Thermal alerts between 28 September 2013 and 30 June 2015. Thermal alerts are derived from data collected by the MODIS thermal sensors aboard the Aqua and Terra satellites and processed by the Hawaii Institute of Geophysics and Planetology using the MODVOLC algorithm. Courtesy of Hawaii Institute of Geophysics and Planetology.

Several articles on Tofua's volcanic geology and geochemistry published in the past few years have come to our attention (Caulfield, 2011, 2012, 2015). Caulfield and others (2011, 2012) include helpful aerial and cross-section sketches of the volcano's various geologic features.

References: Caulfield, J. T., Cronin, S.J., Turner, S.P., & Cooper, L.B., 2011, Mafic Plinian volcanism and ignimbrite emplacement at Tofua volcano, Tonga, Bull. Volcanology, v. 73, pp.1259–1277.

Caulfield, J. T., Turner, S. P., Smith, I. E. M., Cooper, L. B., & Jenner, G. A., 2012, Magma evolution in the primitive, intra-oceanic Tonga arc: petrogenesis of basaltic andesites at Tofua volcano. Journal of Petrology, v. 53(6), pp. 1197-1230.

Caulfield, J. T., Blichert-Toft, J., Albarède, F., & Turner, S. P., 2015, Corrigendum to 'Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Petrogenesis of Basaltic Andesites at Tofua Volcano'and 'Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Rapid Petrogenesis of Dacites at Fonualei Volcano, Journal of Petrology, v. 56(3), pp. 641-644.

Geologic Background. The low, forested Tofua Island in the central part of the Tonga Islands group is the emergent summit of a large stratovolcano that was seen in eruption by Captain Cook in 1774. The summit contains a 5-km-wide caldera whose walls drop steeply about 500 m. Three post-caldera cones were constructed at the northern end of a cold fresh-water caldera lake, whose surface lies only 30 m above sea level. The easternmost cone has three craters and produced young basaltic-andesite lava flows, some of which traveled into the caldera lake. The largest and northernmost of the cones, Lofia, has a steep-sided crater that is 70 m wide and 120 m deep and has been the source of historical eruptions, first reported in the 18th century. The fumarolically active crater of Lofia has a flat floor formed by a ponded lava flow.

Information Contacts: 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/).

This report primarily summarizes activity during January 2013 through mid-December 2014 (although a plot of SO₂ flux during 1 October 2008-30 November 2013 is also presented). That activity included frequent gas emissions, occasional increases in seismicity, intermittent gas explosions that generated ash plumes and ashfall, and strong gas explosions on 21 May 2013 and 29-31 October 2014. Material here are primarily extracted from a 2013 annual report and the suite of 2014 monthly reports, all prepared by the Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Recent Bulletin reports (BGVN 37:06 and 38:02) indicated that the number of volcanic earthquakes and degassing events at Turrialba's W crater during 2012 were lower than those in 2010 and 2011. The three main fumaroles present in the W crater were as follows: Boca 2010 on the W wall, Boca 2011 on the N wall, and Boca 2012 on the E wall.

Gas data, 2008-early 2013. Ultraviolet spectral analysis can yield estimates of volcanogenic SO₂. The methods to assess and express volcanogenic SO₂ vary, with some methods looking at the atmospheric column (total column mass) and others the flux of the gas close to the volcano (mass per unit time, for example, metric tons per day).

The Ozone Monitoring Instrument (OMI) travels in space onboard NASA's Aura satellite and yields estimate of the column SO₂ mass. For Turrialba during the 2008-2013 period OMI determined SO₂ mass burdens generally below 1,500 metric tons and in a few cases to higher values including two cases in the range 2,500-4,000 metric tons (figure 36).

Figure 36. OMI satellite retrievals for SO2 masses in the atmospheric column during 1 October 2008-1 November 2013. Headers are in Spanish (unchanged from original source): Y-axis is SO2 mass in thousands of metric tons, X-axis is date (dd/mm/yyyy). Note the use of commas on the X-axis scale in the place of decimal points (0,5 = 0.5). Graphic is directly from the 2013 annual OVSICORI-UNA report (p. 6).

During 1 April 2013 to 27 November 2013, the ground-based differential optical absorption spectroscopy (DOAS) stations near Turrialbal recorded fluxes generally between 500-1,000 metric tons/day. Based on the DOAS observations, OVSICORI-UNA plotted the CO₂ / SO₂ molar ratio. After an explosion on 21 May 2013, the observatory found this ratio generally increased progressively in available data during the nearly six months that followed (figure 37). In a similar manner, the H₂S / SO₂ molar ratio also showed a tendancy towards progressive increase in available data (figure 38).

Figure 37. CO2/SO2 molar ratios at the Boca 2010 vent from DOAS measurements at Turrialba in the interval from 1 April 2013 to 27 November 2013. DOAS stands for Differential Optical Absorption Spectroscopy, measurements made by stations at the volcano. Spanish labels (left to right): ash emission, increase in seismicity, decrease in seismicity. Courtesy of OVSICORI-UNA.

Figure 38. H2S/SO2 ratios between 1 April 2013 and late November 2013 at Turrialba's Boca 2010, as measured by DOAS stations. Spanish-language labels correspond to triangles on the X-axis stating, from left to right: "ash emission," "increase in seismicity," and "decrease in seismicity." Note error bar (incertidumbre) at upper left. Courtesy of OVSICORI-UNA.

2013 events and monitoring. According to OVSICORI-UNA, the year 2013 began with low seismic activity (shallow hybrid earthquakes) and weak gas emissions similar to those in 2012. In March and April 2013, volcano-tectonic earthquakes originating more than 5 km below the summit began to occur, along with the first tornillo earthquakes of the year. (Tornillo-type earthquakes are long period with wave forms that, at or near the start, contain higher amplitude signals that gradually decrease with time. Their shape on seismograms resembles a woodscrew.) The number of volcanic earthquakes increased from 10/day on 18 April to more than 500/day on 13 July. This high level persisted until the end of August 2013.

On 20 May 2013, increased gas emissions produced a sky-blue plume visible from nearby areas. At 0452 on 21 May, the number of hybrid earthquakes became numerous. Continuous harmonic tremor increased at 0720. At 0830 and after 1100, explosions from both Boca 2010 and Boca 2012 vents generated ash plumes that rose more than 500 m (figure 39). Ashfall was reported in nearby communities to the N, W, and WSW. At noon on 21 May 2013, ash emissions ceased and seismicity decreased. Seismic activity declined sharply after the 21 May explosions, as did the CO₂ /SO₂ ratio, as measured in situ by a portable Multigas station. As previously noted (figures 37 and 38), for plotted measurements, the CO₂/SO₂ and H₂S/SO₂ ratios tended to progressively rise during the months that followed.

Figure 39. Gas explosions on 21 May 2013 at Boca 2010 and Boca 2012 on Turriabla's W crater. Photo taken by the webcam OVSICORI-UNA-A. Courtesy of OVSICORI-UNA.

OVSICORI-UNA reported that a pilot flying past Turrialba about 40 km away observed a blackish plume on 29 May 2013. Officials from the Parque Nacional Volcán Turrialba observed a gas plume that was slightly darker than usual between 0730 and 0745; however, seismic records showed no abnormal activity at those times or seismic data signifying the discharge of a plume during the previous 48 hours. In addition, web camera images lacked evidence of ash emissions since 23 May. Gas plumes with temperatures more than 750°C were emitted from the two vents. The plume from Boca 2010 was whiter than the plume emitted from Boca 2012.

On 4 June 2013, light ashfall was reported in Pacayas (about 13 km W) and San Pablo in Oreamuno de Cartago (25 km SW). An observer in the previously closed National Park engulfing Turrialba noted that gas emissions that day were slightly stronger and more grayish than usual.

According to OVSICORI-UNA, seismic activity increased significantly again on 13 July 2013 with low-frequency signals (figure 40). On that day, the number of seismic events increased to more than 500/day. Seismicity remained at this level until late August when it decreased. During this period the gas temperature from Boca 2012 decreased from ~800°C to ~600°C. During 18-19 July, low-frequency tremor was detected. No morphological changes at the surface were observed.

Figure 40. As recorded at Turrialba between January-November 2013, the number of volcanic earthquakes (y-axis on plot at left) and the number of very long period (VLP) earthquakes (y-axis on plot at right). Courtesy of OVSICORI-UNA.

Volcanic earthquakes with very long periods ceased in November 2013. Tornillos also became less frequent.

2014. The 29 October magmatic eruption discussed below culminated years of high gas emissions at Turrialba. The eruption was sudden and impulsive, termed an explosion by OVSICORI-UNA, but was led by ongoing ash-bearing emission and a clear multihour escalation in tremor. No human injuries were reported. Costa Rica has bolstered its hazard infrastructure in recent years. According to GFDRR (2012) the legislation called the "Emergencies and Risk Prevention Law (No. 8488) requires Government agencies and municipalities to allocate resources for disaster risk reduction activities in their programs and budgets. Presidential Decree (No.36721-MP-PLAN) enhanced the risk management competencies of the CNE [the National Risk Prevention and Emergencies Management Commission] and provides a model to assess vulnerability (compulsory in governmental planning processes)."

During January-September 2014, the number of volcanic earthquakes often remained relatively low (under 100, figure 41, left plot). Occasionally the number approached 200. The low seismicity was broadly similar to that in the last half of 2013; the majority of earthquakes were of low magnitude, including those of tornillo, volcanic-tectonic, and hybrid affinities. During January-September 2014, volcano-tectonic (VT) seismicity was generally stable (at 3 or fewer events per day)(figure 41, right plot).

Figure 41. The number of daily seismic events at Turrialba during 1 January 2014-30 September 2014. Courtesy of OVSICORI-UNA.

On 28 July 2014, a swarm of small, low-amplitude, short-duration, and high-frequency events lasted two hours. OVSICORI-UNA attributed the swarm to movement of fluids through cracks.

Conde and others (2014a) published an article about volcanic SO₂ and CO₂ fluxes at Turrialba during early 2013. They discussed SO₂ and CO₂ measurement methodologies used at Turrialba and Telica. OVSICORI-UNA reports during January-March 2014 noted the development of significantly more accurate, continuous ground-based SO₂ monitoring. In addition, OVSICORI-UNA acquired and used an additional instrument, a Flyspec (a mini-spectrometer to measure SO₂ levels). According to the OVSICORI-UNA September 2014 monthly report, SO₂ fluxes in 2014 through September ranged from 400 to 1,500 metric tons/day, well below the maximum ~3,500 t/d they recorded during several days in June-August 2009 (Conde and others, 2014b).

In addition, reported CO₂/SO₂ ratios were ~8 in May, 2-4 in June, and ~2.5 in July 2014. H₂S/SO₂ molar ratios were ~1.2 in May and 0.2-0.7 in June 2014. Several authors in the two cited articles by Conde and others are affiliated with the NOVAC project (Network for Observation of Volcanic and Atmospheric Change). According to its website, the main objective of NOVAC is to establish a network for the measurements of volcanic gas and aerosol emissions--in particular SO₂ and BrO--and to use the data from this network for risk assessment and volcanological research, both locally and on a regional and global scale. OVSICORI-UNA is part of the NOVAC consortium.

The temperatures at the W crater vents during January-July 2014 were about 600°C or lower, similar to the values of the previous six months as measured 15-20 m from the vents. In August and September, temperatures rose slightly to ~650°C; the composition of the gases were stable and interpreted as primarily magmatic.

Deformation in the August and September 2014 OVSICORI-UNA reports was determined by using interferometric synthetic aperture radar (InSAR), Global Position System (GPS), and electronic distance meter (EDM) surveys. According to the August 2014 report, the InSAR and EDM measurements showed, in the 2013-2014 time interval, a relative contraction of several centimeters around the E and W craters. The September 2014 OVSICORI-UNA reported that a GPS survey on a 4-point transect from the base of the volcano to the summit yielded preliminary results indicating that one of the stations (VTQU, on the S flank) had sunk 2-3 cm/year since 2011. The September 2014 report did not report deformation at other stations.

According to OVSICORI-UNA, seismic activity, which had been low earlier in the year, began to increase in late September 2014. In mid-October instruments recorded a three-day swarm of volcano-tectonic earthquakes. The largest event, M 2.8, occurred at 2035 on 16 October at a depth of 5 km beneath the active crater. SO₂ flux remained low to moderate ranging between 400 and 1,500 metric tons per day during through October 2014. Magmatic influenced degassing intensified during 28-29 October; the SO₂ flux was ~2,000 t/d, higher than the 1,300 t/d average measured in September 2014 and the highest to date during 2014. (Recalling the previously mentioned interval 1 April-27 November 2013, the recorded fluxes also stood lower, generally in the range 500-1,000 tons/day).

The 30 October report by OVSICORI-UNA, which contains informative graphics omitted here, including photos of the plume, tephra deposited on a car, seismic instrument records and spectral information, a helicorder record for a 24-hour interval bracketing the explosion). OVSICORI-UNA described the eruption on the 29th as a moderate eruption of ash between 2310 and 2335 (25 munutes).

According to that report, tremor began at 0600 on the 29th and continued unbroken into at least early the next day. The tremor and the associated RSAM escalation was sufficiently ominous as to lead OVSICORI to notify locals of the situation (including the CNE, the National Park, as well as a nearby lodge. The same OVSICORI-UNA report added that at unstated time during this episode the lodge's chief Tony Lachner noted the plume was darker than usual, contained a yellowish tinge, and was judged to contain ash. At 1700, OVSICORI-UNA again informed local authorities on the situation. The tremor had increased in amplitude and continuity (duration) during the afternoon. Tremor became strongest around 2310-2320 on the 29th coincident with the strong explosion then. The same report noted that OVSICORI-UNA had alerted aviation authorities of the explosion around midnight.

The explosion, heard by local residents, also left a clear record on instruments in the region including those at Poas and Irazu. The explosion ended what started as an initially small eruption from the West Crater that lasted about 25 minutes. The explosion was heard by nearby villagers. An ash cloud rose to an altitude of 5.8 km and drifted WSW. Ash fell on numerous nearby communities, including parts of the capital of San José (whose outskirts are ~30 km W) and Heredia (centered less than 40 km WNW of the volcano). In more detail, settlements noted by OVSICORI-UNA included San Gerardo de Irazú, San Ramón de Tres Ríos, Coronado, Moravia, Curridabat, Desamparados, Aserrí, Escazú, Santa Ana, Belén, Guácima de Alajuela, Río Segundo de Alajuela, San Pedro Montes de Oca, Guadalupe, areas of Heredia, and the capital of San José (population ~350,000, with central downtown located~ 70 km SW of Turrialbla).

The explosion on the 29th destroyed the wall between the West and Central craters, depositing material around the Central Crater and partially burying it. According to a news report (Agence France-Presse), Turrialba National Park remained closed, and eleven people from Santa Cruz de Turrialba were evacuated to shelters. Some schools were also temporarily closed, affecting over 300 area students. OVSICORI-UNA literature (including the 30 October report discussed above) noted that magma had not previously reached the surface at Turrialba since an eruption in 1866 (~150 years ago).

The magmatic eruption continued during 30-31 October (figure 42) with growing magmatic components seen in samples. Analyses of tephra showed that the proportion of juvenile material increased during 30 and 31 October, respectively, rising from the range of 3-5% by volume to the range of 7-10% by volume. A 30 October OVSICORI-UNA report noted that the ash dispersion modeling assumed a plume height of 1.5 km, consistent with a photo they showed (time unstated), which showed much of the plume remaining comparatively low in the area of view near the volcano. According to the Washington Volcanic Ash Advisory Center (VAAC), the 30-31 October eruption produced a continuous emission of gas and light ash with an occasional burst of heavier ash, generally moving W and SW.

Figure 42. A photo of emissions at Turrialba's West Crater on 31 October 2014. The photo was taken from the tourist vista point at Turrialba Volcano National Park. The image shows two distinct plumes adjacent each other, a dark ash-bearing plume and a white plume rich in condensed steam. The plumes rose ~1 km above the vent. Courtesy of Raúl Mora (National Seismological Network, RSN, and University of Costa Rica).

In their 7 November 2014 report, OVSICORI-UNA discussed how named staff collected and ran tests on leachate acidity for material deposited in the explosions during 29-31 October. Leachate reached pH 3.3 (highly acidic). In contrast, ash erupted during 4-5 January 2010 yielded leachate with pH 6.7-7.1 (near neutral). The 2014 report cautioned that such values were of considerable concern to human health, to environmental impacts (native vegetation, aquatic species, etc.), to cultivated plants, and to the well being of livestock and farm animals. The authors attributed the low pH values to the magmatic nature of the eruption and to absorption of those gases on the ash particle surfaces.

An explosion at 0520 on 1 November 2014 generated an ash plume that drifted toward the E and N parts of the Central Valley. A 3 November report stated that during the previous 24 hours seismicity had decreased significantly and no explosions had been detected; seismicity remained elevated. An phone and online (Facebook) public survey allowed residents to record if they had observed ashfall in their localities during the eruptive interval. Responses depicted a W-directed dispersal pattern that covered much of the urban area around San Jose.

OVSICORI-UNA reported a seismic signal indicating a strong emission lasting 50 minutes that started at 2320 on 6 November. The same 7 November report noted that in broad terms seismicity had decreased overall during the previous few days.

An ash-bearing explosion from Turrialba started at 1926 on 13 November and lasted about 10 minutes. Another explosion occurred at 1342 on 14 November and lasted about 15 minutes, although the strongest part was 7-minutes long. The OVSICORI-UNA report issued at 1635 on the 14th emphasized the associated explosive signal of these two emissions in terms of seismicity, for example, noting the dominant frequencies for the respective events were centered at 6.8 and 4.0 Hz. The report also said that of National Park officials reported ashfall at the top of Irazú. Volcanologists observed the 14 November explosion and collected samples at Hacienda La Central, 3 km SE of West Crater.

According to news reports (The Tico Times and crhoy.com), OVSICORI-UNA reported a strong gas emission on 13 November, accompanied by a massive outpouring of ash. A pilot reported ash plume S of the volcano at an altitude of 3.7-4.3 km.

According to OVSICORI-UNA, a strong Strombolian explosion occurred at 2128 on 8 December 2014, considered by them as one of the large explosions in the series that started with the magmatic eruption on 29 October 2014. The explosion lasted about ten minutes and had no precursory activity. The main pulse of ash emissions took place in under 100 seconds. Ashfall, 1 cm thick, and ballistics up to ~5 kg were deposited as far as 300 m W. Ashfall of 0.01 to 2 cm thickness was reported in the Central Valley and in towns to the W and SW, with 23 reports from citizens consistent with ash at distances of 45-80 km from the source. The report also noted constant inflation at Turriabla, ~10-15 mm annually, since the year 2010.

Citizen input to acquire ash thickness data. The Turriabla reporting took advantage of an OVSICORI questionare (Encuesta Alcance de Cenizas, V. Turrialba) to engage citizen observations on ash deposition. The online questionnaire (find link in "Information Contacts" section below) features a scalable map that features a positionable icon to show the location of ash-thickness observation. This position then automatically computes the resulting coordinates (latitude and longitude). The questionare includes several other questions relating to thickness, date and time of observation, rainfall, and weather conditions (which can perturb the original thickness). Entering contact information is optional.

References. Conde V., Robidoux, P., Avard, G., Galle, B. Aiuppa, A.,? Muñoz, A., and Giudice, G., 2014a, Measurements of volcanic SO₂ and CO₂ fluxes by combined DOAS, Multi.GAS and FTIR observations: a case study from Turrialba and Telica volcanoes, Int J Earth Sci (Geol Rundsch), 103, pp. 2335-2347, Springer-Verlag, Berlin Heidelberg. (Also Errata November 2014, 103 (8), p 2349.)

Conde, V., Bredemeyer, S., Duarte, E., Pacheco, J., Miranda, S., Galle, B., and Hansteen, T., 2014b, SO₂ degassing from Turrialba Volcano linked to seismic signatures during the period 2008–2012, International Journal of Earth Sciences (Geol Rundsch) 103, pp. 1983–1998, Springer-Verlag, Berlin Heidelberg.

GFDRR, 2012, Costa Rica Country Update – GFDRR, October 2012; Global Facility for Disaster Reduction and Recovery (GFDRR). (URL: http://www.gfdrr.org/sites/gfdrr.org/files/COSTA_RICA.pdf) (Accessed 12 July 2015).

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

Information Contacts: Observatorio Vulcanologico y Sismologico de Costa Rica-Universidad Nacional (OVSICORI-UNA) (URL: http://www.ovsicori.una.ac.cr/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Network for Observation of Volcanic and Atmospheric Change (NOVAC) (URL: http://www.novac-project.eu/); The Tico Times (URL: http://www.ticotimes.net/); Agence France-Presse (URL: http://www.afp.com/); The Costa Rica Star (URL: http://news.co.cr/); and crhoy.com (URL: http://www.crhoy.com/).