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/).

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/).

Marapi in Sumatra, Indonesia, is a massive stratovolcano that rises 2 km above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera and trending ENE-WSW, with volcanism migrating to the west. Since the end of the 18th century, more than 50 eruptions, typically characterized by small-to-moderate explosive activity, have been recorded. The previous eruption consisted of two explosions during April-May 2018, which caused ashfall to the SE (BGVN 43:06). This report covers a new eruption during January-March 2023, which included explosive events and ash emissions, as reported by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and MAGMA Indonesia.

According to a press release issued by PVMBG and MAGMA Indonesia on 26 December, primary volcanic activity at Marapi consisted of white gas-and-steam puffs that rose 500-100 m above the summit during April-December 2022. On 25 December 2022 there was an increase in the number of deep volcanic earthquakes and summit inflation. White gas-and-steam emissions rose 80-158 m above the summit on 5 January. An explosive eruption began at 0611 on 7 January 2023, which generated white gas-and-steam emissions and gray ash emissions mixed with ejecta that rose 300 m above the summit and drifted SE (figure 10). According to ground observations, white-to-gray ash clouds during 0944-1034 rose 200-250 m above the summit and drifted SE and around 1451 emissions rose 200 m above the summit. Seismic signals indicated that eruptive events also occurred at 1135, 1144, 1230, 1715, and 1821, but no ash emissions were visually observed. On 8 January white-and-gray emissions rose 150-250 m above the summit that drifted E and SE. Seismic signals indicated eruptive events at 0447, 1038, and 1145, but again no ash emissions were visually observed on 8 January. White-to-gray ash plumes continued to be observed on clear weather days during 9-15, 18-21, 25, and 29-30 January, rising 100-1,000 m above the summit and drifted generally NE, SE, N, and E, based on ground observations (figure 11).

Figure 10. Webcam image of the start of the explosive eruption at Marapi at 0651 on 7 January 2023. White gas-and-steam emissions are visible to the left and gray ash emissions are visible on the right, drifting SE. Distinct ejecta was also visible mixed within the ash cloud. Courtesy of PVMBG and MAGMA Indonesia.

Figure 11. Webcam image showing thick, gray ash emissions rising 500 m above the summit of Marapi and drifting N and NE at 0953 on 11 January 2023. Courtesy of PVMBG and MAGMA Indonesia.

White-and-gray and brown emissions persisted in February, rising 50-500 m above the summit and drifting E, S, SW, N, NE, and W, though weather sometimes prevented clear views of the summit. An eruption at 1827 on 10 February produced a black ash plume that rose 400 m above the summit and drifted NE and E (figure 12). Similar activity was reported on clear weather days, with white gas-and-steam emissions rising 50 m above the summit on 9, 11-12, 20, and 27 March and drifted E, SE, SW, NE, E, and N. On 17 March white-and-gray emissions rose 400 m above the summit and drifted N and E.

Figure 12. Webcam image showing an eruptive event at 1829 on 10 February 2023 with an ash plume rising 400 m above the summit and drifting NE and E. Courtesy of PVMBG and MAGMA Indonesia.

Geologic Background. Gunung Marapi, not to be confused with the better-known Merapi volcano on Java, is Sumatra's most active volcano. This massive complex stratovolcano rises 2,000 m above the Bukittinggi Plain in the Padang Highlands. A broad summit contains multiple partially overlapping summit craters constructed within the small 1.4-km-wide Bancah caldera. The summit craters are located along an ENE-WSW line, with volcanism migrating to the west. More than 50 eruptions, typically consisting of small-to-moderate explosive activity, have been recorded since the end of the 18th century; no lava flows outside the summit craters have been reported in 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, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1).

Kikai, located just S of the Ryukyu islands of Japan, contains a 19-km-wide mostly submarine caldera. The island of Satsuma Iwo Jima (also known as Satsuma-Iwo Jima and Tokara Iojima) is located at the NW caldera rim, as well as the island’s highest peak, Iodake. Its previous eruption period occurred on 6 October 2020 and was characterized by an explosion and thermal anomalies in the crater (BGVN 45:11). More recent activity has consisted of intermittent thermal activity and gas-and-steam plumes (BGVN 46:06). This report covers similar low-level activity including white gas-and-steam plumes, nighttime incandescence, seismicity, and discolored water during May 2021 through April 2023, using information from the Japan Meteorological Agency (JMA) and various satellite data. During this time, the Alert Level remained at a 2 (on a 5-level scale), according to JMA.

Activity was relatively low throughout the reporting period and has consisted of intermittent white gas-and-steam emissions that rose 200-1,400 m above the Iodake crater and nighttime incandescence was observed at the Iodake crater using a high-sensitivity surveillance camera. Each month, frequent volcanic earthquakes were detected, and sulfur dioxide masses were measured by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Mishima Village, and JMA (table 6).

Table 6. Summary of gas-and-steam plume heights, number of volcanic earthquakes detected, and amount of sulfur dioxide emissions in tons per day (t/d). Courtesy of JMA monthly reports.

Sentinel-2 satellite images show weak thermal anomalies at the Iodake crater on clear weather days, accompanied by white gas-and-steam emissions and occasional discolored water (figure 24). On 17 January 2022 JMA conducted an aerial overflight in cooperation with the Japan Maritime Self-Defense Force’s 1st Air Group, which confirmed a white gas-and-steam plume rising from the Iodake crater (figure 25). They also observed plumes from fumaroles rising from around the crater and on the E, SW, and N slopes. In addition, discolored water was reported near the coast around Iodake, which JMA stated was likely related to volcanic activity (figure 25). Similarly, an overflight taken on 11 January 2023 showed white gas-and-steam emissions rising from the Iodake crater, as well as discolored water that spread E from the coast around the island. On 14 February 2023 white fumaroles and discolored water were also captured during an overflight (figure 26).

Figure 24. Sentinel-2 satellite images of Satsuma Iwo Jima (Kikai) showing sets of visual (true color) and infrared (bands 12, 11, 8a) views on 7 December 2021 (top), 23 October 2022 (middle), and 11 January 2023 (bottom). Courtesy of Copernicus Browser.

Figure 25. Aerial image of Satsuma Iwo Jima (Kikai) showing a white gas-and-steam plume rising above the Iodake crater at 1119 on 17 January 2022. There was also green-yellow discolored water surrounding the coast of Mt. Iodake. Courtesy of JMSDF via JMA.

Figure 26. Aerial image of Satsuma Iwo Jima (Kikai) showing white gas-and-steam plumes rising above the Iodake crater on 14 February 2023. Green-yellow discolored water surrounded Mt. Iodake. Courtesy of JCG.

Geologic Background. Multiple eruption centers have exhibited recent activity at Kikai, a mostly submerged, 19-km-wide caldera near the northern end of the Ryukyu Islands south of Kyushu. It was the source of one of the world's largest Holocene eruptions about 6,300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries. Post-caldera eruptions formed Iodake (or Iwo-dake) lava dome and Inamuradake scoria cone, as well as submarine lava domes. Recorded eruptions have occurred at or near Satsuma-Iojima (also known as Tokara-Iojima), a small 3 x 6 km island forming part of the NW caldera rim. Showa-Iojima lava dome (also known as Iojima-Shinto), a small island 2 km E of Satsuma-Iojima, was formed during submarine eruptions in 1934 and 1935. Mild-to-moderate explosive eruptions have occurred during the past few decades from Iodake, a rhyolitic lava dome at the eastern end of Satsuma-Iojima.

Information Contacts: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/kaiikiDB/kaiyo30-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

The current eruption at Lewotolok, in Indonesian’s Lesser Sunda Islands, began in late November 2020 and has included Strombolian explosions, occasional ash plumes, incandescent ejecta, intermittent thermal anomalies, and persistent white and white-and-gray emissions (BGVN 47:10). Similar activity continued during October 2022-April 2023, as described in this report based on information provided by Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and satellite data.

During most days in October 2022 white and white-gray emissions rose as high as 200-600 m above the summit. Webcam images often showed incandescence above the crater rim. At 0351 on 14 October, an explosion produced a dense ash plume that rose about 1.2 km above the summit and drifted SW (figure 43). After this event, activity subsided and remained low through the rest of the year, but with almost daily white emissions.

Figure 43. Webcam image of Lewotolok on 14 October 2022 showing a dense ash plume and incandescence above the crater. Courtesy of MAGMA Indonesia.

After more than two months of relative quiet, PVMBG reported that explosions at 0747 on 14 January 2023 and at 2055 on 16 January produced white-and-gray ash plumes that rose around 400 m above the summit and drifted E and SE (figure 44). During the latter half of January through April, almost daily white or white-and-gray emissions were observed rising 25-800 m above the summit, and nighttime webcam images often showed incandescent material being ejected above the summit crater. Strombolian activity was visible in webcam images at 2140 on 11 February, 0210 on 18 February, and during 22-28 March. Frequent hotspots were recorded by the MIROVA detection system starting in approximately the second week of March 2023 that progressively increased into April (figure 45).

Figure 44. Webcam image of an explosion at Lewotolok on 14 January 2023 ejecting a small ash plume along with white emissions. Courtesy of MAGMA Indonesia.

Figure 45. MIROVA Log Radiative Power graph of thermal anomalies detected by the VIIRS satellite instrument at Lewotolok’s summit crater for the year beginning 24 July 2022. Clusters of mostly low-power hotspots occurred during August-October 2022, followed by a gap of more than four months before persistent and progressively stronger anomalies began in early March 2023. Courtesy of MIROVA.

Explosions that produced dense ash plumes as high as 750 m above the summit were described in Volcano Observatory Notices for Aviation (VONA) at 0517, 1623, and 2016 on 22 March, at 1744 on 24 March, at 0103 on 26 March, at 0845 and 1604 on 27 March (figure 46), and at 0538 on 28 March. According to the Darwin VAAC, on 6 April another ash plume rose to 1.8 km altitude (about 370 m above the summit) and drifted N.

Figure 46. Webcam image of Lewotolok at 0847 on 27 March 2023 showing a dense ash plume from an explosion along with clouds and white emissions. Courtesy of MAGMA-Indonesia.

Sentinel-2 images over the previous year recorded thermal anomalies as well as the development of a lava flow that descended the NE flank beginning in June 2022 (figure 47). The volcano was often obscured by weather clouds, which also often hampered ground observations. Ash emissions were reported in March 2022 (BGVN 47:10), and clear imagery from 4 March 2022 showed recent lava flows confined to the crater, two thermal anomaly spots in the eastern part of the crater, and mainly white emissions from the SE. Thermal anomalies became stronger and more frequent in mid-May 2022, followed by strong Strombolian activity through June and July (BGVN 47:10); Sentinel-2 images on 2 June 2022 showed active lava flows within the crater and overflowing onto the NE flank. Clear images from 23 April 2023 (figure 47) show the extent of the cooled NE-flank lava flow, more extensive intra-crater flows, and two hotspots in slightly different locations compared to the previous March.

Figure 47. Sentinel-2 satellite images of Lewotolok showing sets of visual (true color) and infrared (bands 12, 11, 8a) views on 4 March 2022, 2 June 2022, and 23 April 2023. Courtesy of Copernicus Browser.

Geologic Background. The Lewotolok (or Lewotolo) stratovolcano occupies the eastern end of an elongated peninsula extending north into the Flores Sea, connected to Lembata (formerly Lomblen) Island by a narrow isthmus. It is symmetrical when viewed from the north and east. A small cone with a 130-m-wide crater constructed at the SE side of a larger crater forms the volcano's high point. Many lava flows have reached the coastline. Eruptions recorded since 1660 have consisted of explosive activity from the summit crater.

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

Barren Island is part of a N-S-trending volcanic arc extending between Sumatra and Burma (Myanmar). The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic flow and surge deposits. Eruptions dating back to 1787, have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western coast. Previous activity was detected during mid-May 2022, consisting of intermittent thermal activity. This report covers June 2022 through March 2023, which included strong thermal activity beginning in late December 2022, based on various satellite data.

Activity was relatively quiet during June through late December 2022 and mostly consisted of low-power thermal anomalies, based on the MIROVA (Middle InfraRed Observation of Volcanic Activity) graph. During late December, a spike in both power and frequency of thermal anomalies was detected (figure 58). There was another pulse in thermal activity in mid-March, which consisted of more frequent and relatively strong anomalies.

Figure 58. Occasional thermal anomalies were detected during June through late December 2022 at Barren Island, but by late December through early January 2023, there was a marked increase in thermal activity, both in power and frequency, according to this MIROVA graph (Log Radiative Power). After this spike in activity, anomalies occurred at a more frequent rate. In late March, another pulse in activity was detected, although the power was not as strong as that initial spike during December-January. Courtesy of MIROVA.

The Suomi NPP/VIIRS sensor data showed five thermal alerts on 29 December 2022. The number of alerts increased to 19 on 30 December. According to the Darwin VAAC, ash plumes identified in satellite images captured at 2340 on 30 December and at 0050 on 31 December rose to 1.5 km altitude and drifted SW. The ash emissions dissipated by 0940. On 31 December, a large thermal anomaly was detected; based on a Sentinel-2 infrared satellite image, the anomaly was relatively strong and extended to the N (figure 59).

Figure 59. Thermal anomalies of varying intensities were visible in the crater of Barren Island on 31 December 2022 (top left), 15 January 2023 (top right), 24 February 2023 (bottom left), and 31 March 2023 (bottom right), as seen in these Sentinel-2 infrared satellite images. The anomalies on 31 December and 31 March were notably strong and extended to the N and N-S, respectively. Images using “Atmospheric penetration” rendering (bands 12, 11, 8a). Courtesy of Sentinel Hub Playground.

Thermal activity continued during January through March. Sentinel-2 infrared satellite data showed some thermal anomalies of varying intensity on clear weather days on 5, 10, 15, 20, and 30 January 2023, 9, 14, 19, and 24 February 2023, and 21, 26, and 31 March (figure 59). According to Suomi NPP/VIIRS sensor data, a total of 30 thermal anomalies were detected over 18 days on 2-3, 7, 9-14, 16-17, 20, 23, 25, and 28-31 January. The sensor data showed a total of six hotspots detected over six days on 1, 4-5, and 10-12 February. During March, a total of 33 hotspots were visible over 11 days on 20-31 March. Four MODVOLC thermal alerts were issued on 25, 27, and 29 March.

Geologic Background. Barren Island, a possession of India in the Andaman Sea about 135 km NE of Port Blair in the Andaman Islands, is the only historically active volcano along the N-S volcanic arc extending between Sumatra and Burma (Myanmar). It is the emergent summit of a volcano that rises from a depth of about 2250 m. The small, uninhabited 3-km-wide island contains a roughly 2-km-wide caldera with walls 250-350 m high. The caldera, which is open to the sea on the west, was created during a major explosive eruption in the late Pleistocene that produced pyroclastic-flow and -surge deposits. Historical eruptions have changed the morphology of the pyroclastic cone in the center of the caldera, and lava flows that fill much of the caldera floor have reached the sea along the western 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/).

During the interval 20 February-2 April no eruptions occurred. One minor eruption did take place at 1530 on 3 April, when the volcano sent an ash plume to 1.4 km height. After that Sakura-jima exhibited a low level of activity (no earthquake swarms) through early May. . . .

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

Information Contacts: JMA.

The active Crater C continued to emit gases and lava, and was the site of sporadic Strombolian eruptions. Crater D continued fumarolic degassing. The lava that began to exit from crater C at the end of December remained active along its two NW-flowing channels. These divided at 1,100 m elev, forming two arms that in April extended to 900 m and 750 m elev. The temperature and pH of hot and cold springs around the volcano remained stable during early April sampling.

Compared to previous months, the Strombolian eruptions that had accompanied strong rumbling have diminished in both number and magnitude. On days with good visibility observers on the W flank saw Arenal noiselessly send up small, ash-bearing eruptions that rose to 300 m above the rim of Crater C.

In April, low-frequency seismic events took place 621 times, and harmonic tremor lasted for 57 hours. Relative to previous months, there was a decrease in the duration of tremor with frequencies below 3 Hz.

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

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

Low-level steam and ash eruptions continued from mid-April through mid-May, although poor weather often obscured the volcano. Observers in Adak . . . reported seeing steam, and possibly minor ash, rising a few hundreds of meters above the volcano early during the week of 15-22 April. On 28 April, FWS personnel in Adak observed and photographed a gray ash cloud erupting from the summit crater. The cloud did not rise much above the summit before strong winds carried the plume to the SW. On 5 May, U.S. Navy personnel in Adak observed steam clouds from the summit crater during a short break in the weather. Early in the week of 6-13 May, U.S. Navy and FWS personnel observed steam clouds rising from the summit crater to an estimated altitude of 1,800-2,400 m. They also observed a dark stripe extending down the SE flank from near the summit; a lack of steaming suggested that it was either a small mudflow or ash concentrated by snowmelt.

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

Information Contacts: AVO.

In April . . . lava traveled through tubes and plunged into the ocean. Lava also occasionally escaped from the tubes and formed surface flows. In mid-April, flows escaped lava tubes at three locations: a) along the E side of the Kamoamoa lava delta, b) at 61 m elevation, and c) at 610 m elevation. The latter site fed aa flows that descended to 488 m elevation.

A 2-day pause . . . on 15-16 April [resulted] in stagnant surface flows. During this time the volume of lava entering the ocean backed off to a residual trickle.

For the interval 29 March to 11 April the surface of the active lava pond rose from 90 m to 84 m below the crater rim. Following the 2-day pause, the surface of the active lava pond at the bottom of Pu`u `O`o crater stood at 79 m below the crater rim.

During 1-17 April, tremor . . . often sustained amplitudes 2-3x larger than background. Beginning 18 April, tremor amplitude began to drop to near-background levels, marking the start of irregular banding patterns on the seismic record. The patterns consisted of low-level tremor alternating with 1-2 hour bursts of higher amplitude tremor.

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

Information Contacts: T. Mattox and P. Okubo, HVO.

Seismic stations continued to register both deep and shallow weak earthquakes (average of 6 events/day) and weak volcanic tremor (0-1.5 hours/day) through the end of April. On 29 April there were 11 events/day, but the number of events decreased to 4/day by 5 May. Weak volcanic tremor decreased to 0.1-0.3 hours/day. Seismicity increased during the second week of May when 11-18 earthquakes/day were recorded. As of 18 May, both deep and shallow earthquakes (8-22 events/day) and weak volcanic tremor were continuing beneath the volcano. When the volcano was not obscured by clouds, weak fumarolic activity above the crater was observed in late April and May with a steam plume to <=1 km above the crater.

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

Information Contacts: V. Kirianov, IVGG.

The increased level of seismicity . . . continued to be recorded beneath the volcano (1-2 earthquakes/day) through the end of April. These high-frequency tectonic events (with some unusual characteristics) occurred at 2-12 km depth with a maximum magnitude of 2. This type of event at this depth range has not been identified since the present seismic net on the volcano was installed in 1982. Although the seismic activity continued into early May at a rate of 1-2 earthquakes/day, the lack of additional increase led to the downgrading of the Level of Concern Color Code back to Green.

Seismicity beneath the Koryaksky-Avachinsky area increased above background levels in mid-May to a rate of 6-9 earthquakes/day. During a 2-day period, 18 earthquakes were detected at ~12 km (11 May) and 0-9 km depth (12 May). Seismicity had returned to background levels by 26 May.

Geologic Background. The large symmetrical Koryaksky stratovolcano is the most prominent landmark of the NW-trending Avachinskaya volcano group, which towers above Kamchatka's largest city, Petropavlovsk. Erosion has produced a ribbed surface on the eastern flanks of the 3430-m-high volcano; the youngest lava flows are found on the upper W flank and below SE-flank cinder cones. Extensive Holocene lava fields on the western flank were primarily fed by summit vents; those on the SW flank originated from flank vents. Lahars associated with a period of lava effusion from south- and SW-flank fissure vents about 3900-3500 years ago reached Avacha Bay. Only a few moderate explosive eruptions have occurred during historical time, but no strong explosive eruptions have been documented during the Holocene. Koryaksky's first historical eruption, in 1895, also produced a lava flow.

Information Contacts: V. Kirianov, IVGG.

Press reports have described renewed activity . . . from late March through early May. Activity apparently began again on 19 March after about five months of quiet. At the end of March, thick black ash plumes rose 300 m while "red flames and glowing lava" were observed at night to rise 200 m. Eruption noises like thunder could be heard at Carita Beach . . . . Scientists from GMU stated in the newspaper Suara Pembaruan that their seismograph was not operational at the time of the eruption, but began functioning again on 26 March.

Wimpy Tjetjep, Director of VSI, reported to the same newspaper on 6 April that volcanic materials had been thrown within a 3,000-m-radius from the crater. Ash and tephra emissions were continuing, and 160-170 weak volcanic earthquakes had been recorded in the previous week. Officials at the local observatory reportedly described the activity as "spouts of fire" and thick black ash emissions alternating with explosions. Ejection of blocks as large as 1-m-diameter were also reported. Fishermen at Carita Beach described the April activity as "gigantic fireworks floating in the middle of the sea at night."

The most recent report of activity was on 5 May, when VSI volcanologists at the local observatory told the Antara News Agency that an eruption ejected lava 200 m into the air. They also noted that as many as 222 "eruptions" had been recorded on 3 May. All of the reports emphasized that the island remains off-limits to visitors.

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

Information Contacts: Suara Pembaruan News; Antara News Service; UPI.

"Observations were conducted during 1-17 and 23-27 April. The level of activity was slightly lower than in March. Emissions from Crater 2 consisted mainly of thin-to-moderate white-grey vapour-and-ash clouds. Explosion noises were heard on 4, 5, and 17 April, and weak rumbling noises were heard during 23-27 April. The explosion on 4 April produced a thick ash column that rose a few hundred metres above the crater and resulted in fine ashfall on the NW side of the volcano. Steady weak red glow was observed on 23-24 April. Crater 3 released emissions of thin white vapour with occasional low ash content. Seismic activity was at a low level."

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

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

The following report presents observations made during crater visits in December 1993 and January 1994. There has been very little activity since June 1993 . . . . As of January, major crater features were unchanged.

On 27 December 1993, Franck Pothe observed that the most recent lava flows had remained dark in color since their emission in June, although there was extensive fracturing of the flow surfaces (figure 33). Carbonatitic lava flows within the crater commonly become a whitish color relatively soon after emplacement. Observers in September reported that the lava flows were covered with ash. Neither bombs, scoria, nor ash were visible within the crater, although the outer SW flank retained a thin layer of gray ash. No recent activity was noted in the N half of the crater, but sulfur was visible. Many fractures on the N crater rim contained sulfuric fumaroles. A generally circular depression between the highest spatter cones (T5/T9 and T20) was 10-15 m across and 1.5 m deep. A fault on the S edge of the depression revealed a 1-m-thick section of the crater floor.

Figure 33. Photograph of the summit crater of Ol Doinyo Lengai, 27 December 1993. View is approximately to the N. The tall spatter cones in the upper left (T20) and upper right (T5/T9) rise about 20 m above the surrounding lava flows. June 1993 lava flows are visible at left center with surface fractures; the blocky lava flow F35 is E of T5/T9. Cone T30 is in the left foreground. Courtesy of Franck Pothe.

The crater was visited on 19-21 January 1994 by Kevin Koepenick, Susan Brantley, and Andrew Nyblade (Penn State), Gary Rowe (USGS), and Constantine Moshi (Tanzania Geological Survey). Field work consisted of fumarole sampling, soil gas sampling, and soil flux measurements in the N rim area. Sporadic rumbling, cracking, and groaning noises emanated from the center of the crater, but were particularly evident at large fractures at the S edge of an oval-shaped depression between cones T5/T9 and T20 (figure 34). Rumbling noises were also heard from near T23, which exhibited vigorous high-temperature gas discharge. Gas emissions were also present from T5/T9, T8, T20, and the fractures at the S edge of the depression.

Figure 34. Sketch map of the summit crater of Ol Doinyo Lengai, 21 January 1994, showing spatter cones and lava flows (hatched and F35). Irregular black areas along the crater rim and on the crater floor are fumaroles. Original sketch by Barry Dawson, Harry Pinkerton, and David Pyle; courtesy of Kevin Koepenick.

A temperature survey of all accessible fumaroles was conducted on 19 January. Fractures in the crater walls were lined with native sulfur crystals and weakly emitted gas and steam, with a distinct sulfur smell, at temperatures of 50-80°C. Temperatures of crater-floor fumaroles ranged from 65 to 273°C; most emitted steam without a sulfur smell. The highest temperatures were recorded from cone T8 (248°C) and from a fracture just to the E (273°C). Temperatures of the larger cones were not measured due to the potential hazard.

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

Information Contacts: K. Koepenick, Pennsylvania State Univ; F. Pothe, LAVE.

At about 0600 on 17 May this glacier-capped stratovolcano. . . began a Strombolian-to-subplinian eruption with associated lahars and flooding. The eruption produced a column composed of ash, gases, and steam that reached ~ 4,000-5,000 m above the summit, which itself has an elevation of 3,125 m. Tephra fell over a cigar-shaped zone trending about ESE (figures 3 and 4). In early assessments of the eruption, workers estimated the VEI as about 2 or 3 (moderate to moderately large).

Figure 3. Map showing location of the 17 May Llaima ash-fall deposit. The shaded area includes ash of thickness greater than about 1 mm. Dash-dot lines show province boundaries in Argentina: Mendoza, Neuquén, and Río Negro. Base map provided courtesy of Daniel Delpino and Adriana Bermudez (A.C.V.V.R.).

Figure 4. Annotated sketch map of the area near Llaima, emphasizing aspects of the 17 May eruption adjacent to the volcano. Contours are only drawn for the volcano, not the surrounding topography. Lowest contour shown is at ~ 1,600 m (200 m contour interval). Labelled times indicate arrival of lahars at various points along the rivers. Courtesy of Hugo Moreno.

Lava, lahars, and flooding. Llaima has two historically active craters, one at the summit and the other to the SE. This eruption issued from a fissure that grew in the crater at the summit (figure 4). The 500-m-long, SW-trending fissure produced explosions at 3-second intervals and lava fountains. A lava flow developed. The flow drained across the bottom of a glacier located on Llaima's W flank.

Runoff from melting snow and ice caused lahars to descend the W flank into the Calbuco and Quepe rivers. Arrival times for the lahars are shown for two points on figure 4. At further distances from the volcano flooding occurred.

Plume. The ash plume was initially seen for at least 40 km, reaching Lake Icalma, Chile (figure 4) and Lake Aluminé, Argentina (figure 3). Later, at 0930, one witness at Paso Icalma (figure 4) saw a "big, dark, and dense ball of smoke" rising in the sky.

Ash also fell in the towns of Zapala and Cutral-Co, in Neuquén Province, Argentina, settlements with a combined population of ~ 105,000 (figure 3). It fell between the hours of 1330 and 1430 in Zapala, and beginning at 1530 in Cutral-Co. In these towns, falling ash hampered all normal activities and irritated people's eyes and respiratory systems. In the first stage of the ash advance, the sky changed from open sunlight to a dark ash cloud looming over the horizon. About a half hour later the ash cloud completely engulfed the sky.

Ashfall was weak at a meteorological station ~20 km NNE of the town of Neuquén, a location outside and E of the 1 mm isopach shown on figure 3, and a distance of 300 km from Llaima. The time given for this ash fall was 1600, just half an hour after ash began falling at Cutral-Co. Another report, however, mentioned light ashfall at essentially the same spot (the Neuquén airport) beginning at 1900.

Pilots on at least five commercial flights recognized the plume; available reports for 17 May covered these times: (a) between 0900 and 1000, (b) 1330, and (c) between 1600 and 1700. For these intervals, the available pilot reports describe the plume as follows: (a) fumarolic, (b) a volcanic cloud, and (c) tenuous clouds quickly dispersing during what was described as a lull in the eruption. Besides these reports (provided by Raul Rodano), Daniel Delpino and Adriana Bermudez mentioned one other: It stated that the plume rose 10 km above Llaima.

Using the infrared capability of geostationary satellite Meteosat 3, Raul Rodano imaged the plume for the time 1200, 17 May. He also detected the plume between 1800 and 1900, but after this it became indistinguishable from weather clouds.

Description of the non-proximal ash fall. In Neuquén Province, Argentina, the blanket of ashfall over 1 mm thick was distributed over an area of ~ 37,680 km² (figure 3). In Argentina, the ash within this isopach was mostly 1-2 mm thick but in some areas the accumulations reached 2-5 cm thick. Near-source deposits in Chile may be thicker, but this has yet to be reported.

Table 1 shows grain-size analyses for tephra collected in Zapala and in Paso Icalma (at 1700). In Zapala the tephra was dominantly dark gray to black in color. The ash was examined by A. M. Case and determined to be composed of amber-colored glass shards of presumed mafic composition. Phenocrysts included plagioclase, pyroxene, and iron-titanium oxides.

Table 1. Preliminary grain-size analysis for two localities in Neuquén Province, Argentina. Dashes indicate lack of significant material in stated size range. Exact values for the size ranges require clarification, but diameters in one classification scheme are given in parenthesis. Data provided courtesy of Daniel Delpino and Adriana Bermudez, A.C.V.V.R.

Ongoing monitoring, damage estimate, and history. Gustavo Fuentealba set up some portable seismographs that were to be joined by additional instruments from the national civil defense agency (ONEMI). According to an ONEMI report the eruption was preceded by seismic activity.

Both during and after the eruption, volcanologists communicated closely with their respective civil authorities. In Argentina, routine commercial flights were suspended in order to prevent encounters with the ash plume. In Chile, according to Norman Banks, ten staff members of the Conguillio national park and 22 families in the highest risk zones were evacuated, 7 by helicopter. Another 2,000-3,000 inhabitants of the towns of Melipeuco (15 km SSE) and Cherquenco (18 km W) were preparing for possible evacuation.

According to Norman Banks: "The current eruption has been preceded by several years of unrest, [with Llaima] sending a steam column to 1 km in September of 1992, and provoking two other brief episodes of steam or heightened seismic activity, the latest in December of 1993." He also noted that lahars associated with the current eruption caused an estimated $1.25 million (US) damage to Chilean roads and bridges.

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

Information Contacts: H. Moreno, SERNAGEOMIN, Temuco (also at Univ de Chile); ONEMI, Santiago; G. Fuentealba, Univ de la Frontera, Temuco; N. Banks, American Embassy, Santiago; El Mercurio news articles; R. Rodano, Aerolineas Argentinas, Buenos Aires; D. Delpino and A. Bermudez, A.C.V.V.R., Neuquén, Argentina.

The following report summarizes caldera seismicity and deformation during 1992-93.

Earthquake activity within the caldera gradually increased through 1993, with the level of activity significantly higher than in 1992 (figure 16). More than 30 earthquakes of M >3 occurred within the caldera during 1993; six were associated with a swarm on 27 November. By comparison, only four events in 1992 had M >3. The level of activity in the Sierra Nevada block, S of the caldera, was also higher in 1993, with over 30 M >3 earthquakes, compared to 14 in 1992. During 10-15 August 1993, the Red Slate Mountain swarm was the most intense earthquake sequence of the year in the region. It included the largest event (M 4.5) and the largest number of events (~400 M >1) for any one swarm.

Figure 16. Earthquake epicenters in the Long Valley region during 1992 (top) and 1993 (bottom). Courtesy of the USGS.

An interesting episode of seismic activity in the Long Valley region followed the 28 June 1992 Landers earthquake (M 7.4) in southern California, >400 km S. Local increases in seismicity triggered by the Landers earthquake were also recognized in the Geysers geothermal area and in Yellowstone National Park, over 1,200 km from the epicenter. These were clear examples of seismic "remote-triggering."

A series of >60 long-period earthquakes began beneath the SW flank of Mammoth Mountain during a mid-1989 earthquake swarm, and continued through 1993. These events lacked significant energy at frequencies above 5 Hz, distinguishing them from the much more common volcano-tectonic earthquakes. Reliable hypocenter locations were determined for 30 long-period events in 1992. Focal depths ranged from 10 to 28 km, distinctly deeper than the 2-10 km depth range for local tectonic earthquakes. Most of the long-period events were located between Mammoth Mountain and Red Cones (a pair of Holocene basaltic cinder cones 4 km SW of Mammoth Mountain), although two were located 5-7 km W of Mono Craters and one was within the caldera beneath the W moat. Intervals between events ranged from weeks to months, with individual occurrences typically consisting of several events within a few minutes. Magnitudes were in the 0.7-2.2 range. In 1993, long-period events beneath the SW flank of Mammoth Mountain were detected at depths of 10-25 km. Instruments detected eleven episodes of long-period activity from March through December 1993. Approximately half were solitary events with the rest consisting of bursts of several events over time intervals of 1-2 minutes. All of the long-period events in 1993 were small; the largest had a magnitude of ~1. Similar sporadic long-period events have been recognized beneath Lassen Peak, Medicine Lake caldera, and young volcanic areas in Japan. Evidence suggests that occasional long-period events at mid- to lower-crustal depths are common beneath areas of Holocene volcanism and are not indicative of imminent volcanic activity.

Deformation measurements from late 1989 through 1992, using a two-color geodetic distance-meter (geodimeter), revealed elevated deformation rates across the caldera compared to the modest rate that characterized the last half of the 1980's. Deformation remained steady in 1993 at 2-3 ppm/year. The increased rate of seismicity, however, is still not adequate to account for the deformation that has accumulated since 1990. Geodimeter data and annual GPS surveys of the regional Long Valley-Mono Craters geodetic network indicated that caldera deformation was dominated by inflation of the resurgent dome, driven by a pressure source at a depth of 5-8 km. However, this single source cannot fully account for either the regional deformation pattern or the geodimeter and 1988-92 leveling data. These data sets require a second, relatively deep (>15 km) inflation source somewhere beneath the SW margin of the caldera and Mammoth Mountain, although the location is poorly constrained. Analysis of the combined data sets indicated that the pressure source beneath the dome may have a flattened ellipsoidal shape, and that normal slip on the medial graben faults may have accompanied dome inflation.

Further References. Hill, D.P., Reasenberg, P.A., Michael, A., and 28 others, 1993, Seismicity remotely triggered by the magnitude 7.3 Landers, California, earthquake: Science, v. 260, p. 1617-1623.

Langbein, J., Hill, D.P., Parker, T.N., and Wilkinson, S.K., 1993, An episode of reinflation of the Long Valley caldera, eastern California: 1989-1991: Journal of Geophysical Research, v. 98, p. 15,851-15,870.

Pitt, A.M., and Hill, D.P., 1994, Long period earthquakes in the Long Valley caldera region, eastern California: Geophysical Research Letters (in press).

Geologic Background. The large 17 x 32 km Long Valley caldera east of the central Sierra Nevada Range formed as a result of the voluminous Bishop Tuff eruption about 760,000 years ago. Resurgent doming in the central part of the caldera occurred shortly afterwards, followed by rhyolitic eruptions from the caldera moat and the eruption of rhyodacite from outer ring fracture vents, ending about 50,000 years ago. During early resurgent doming the caldera was filled with a large lake that left strandlines on the caldera walls and the resurgent dome island; the lake eventually drained through the Owens River Gorge. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation, seismicity, and other unrest in recent years. The late-Pleistocene to Holocene Inyo Craters cut the NW topographic rim of the caldera, and along with Mammoth Mountain on the SW topographic rim, are west of the structural caldera and are chemically and tectonically distinct from the Long Valley magmatic system.

Information Contacts: D. Hill, USGS Menlo Park.

"The level of activity remained low during April. Throughout the month both craters released only emissions of weak white vapour. Seismic recording was restricted to the period 4-13 April because of battery problems. The level of seismicity appeared to be slightly higher than in March, but still within the range of normal background levels. No significant changes in ground deformation were indicated by measurements from the water-tube tiltmeters."

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

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

Seismic activity was monitored on 22-24 April using one station located at 4,480 m elevation on the S flank. The station consisted of a 1-Hz vertical component seismometer, operating at 78 dB amplification with a band-pass filter between 0.3 and 30.0 Hz. The station registered five B-type events within a 5.5 hour time period on 23 April, and three events within 15 minutes of each other on 24 April. Coda duration was 10-18 seconds long; frequencies were in the 3-6 Hz range. Maximum peak-to-peak amplitudes measured 1-3 mm.

Observers who ascended to the summit found no fumarolic activity. Crater morphology was unchanged since visits in October 1992 and 1989. Water discharge from the "La virgen" spring at 4,400 m elevation had decreased to 0.5 liters/hour from 10 liters/hour in 1992.

Geologic Background. Pico de Orizaba (Volcán Citlaltépetl), México's highest peak and North America's highest volcano, was formed in three stages beginning during the mid-Pleistocene. Orizaba lies at the southern end of a volcanic chain extending north to Cofre de Perote volcano and towers up to 4400 m above its eastern base. Construction of the initial Torrecillas and Espolón de Oro volcanoes was contemporaneous with growth of Sierra Negra volcano on the SW flank and was followed by edifice collapses that produced voluminous debris avalanches and lahars. The modern volcano was constructed during the late Pleistocene and Holocene of viscous andesitic and dacitic lavas, forming the current steep-sided cone. Repetitive explosive eruptions beginning during the early Holocene accompanied lava dome growth and lava effusion. Historical eruptions have consisted of moderate explosive activity and the effusion of dacitic lava flows. The latest eruption occurred during the 19th century.

Information Contacts: Guillermo González-Pomposo¹ and Carlos Valdés-González, Departamento de Sismología y Volcanología, Instituto de Geofísica, UNAM, Cd. Universitaría, 04510 D.F., México; ¹ Also at Benemérita Univ Autótonoma de Puebla, México.

In the active, northernmost crater at Poás, the water in the lake dropped several meters since January, almost drying towards the end of April. Fumaroles prevail on the lake bottom, chiefly in the central and northern parts of the crater. Escaping gases were audible from Cerro Pelón, 2 km SW. Gas columns rose up to 1 km above the crater floor and were carried by the wind toward the W and SW flanks. Residents of several nearby settlements reported sulfur smells and the quantity of gas released to the atmosphere appeared to have increased. Near the end of April, two small phreatic eruptions took place.

Park Rangers reported that on the night of 25 April Poás vented a gray-to-clear colored emission containing old lake-floor sediments. The emitted sediments were found over a significant area on the volcano, and reached both Cerro Pelón and the hut at the park entrance on the S. On 30 April the volcano again released a similar emission. Tourists who encountered it left the area rapidly. They came in contact with strong sulfurous gases and suffered from coughs and irritated skin and eyes. Repeated degassing has caused new damage to both cultivated and wild vegetation in and outside the park (last month's report only mentioned damage to local vegetation). The apparent increase in sub-aerial (rather than sub-aqueous) degassing has translated to an increase in the acidity on material deposited at a network of collector sites around the volcano.

In April the local seismic station (POA2, located 2.5 km SW of the active crater) registered a total of 6,919 events, almost as many as the previous month, which had the highest total so far this year. The majority of these events were of low frequency, although 30 were of medium frequency. During the last 8 days of the month there arrived consistent, low-frequency (below 2 Hz), background tremor with peak-to-peak amplitudes of 2-4 mm.

The above-described emissions of lake sediment on 25 and 30 April were accompanied by two different sorts of seismic signals. The first emission was associated with inferred near-surface noise; the second, with a 3.5-4.5 Hz signal of 135-seconds duration and 4-6 mm peak-to-peak amplitude.

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

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

Seismicity during March and April consisted primarily of B-type events. During March, 99 B-type events were recorded, an increase from the 62 recorded in both January and February (19:02). On two days (6 and 22 March) as many as 7 B-type events were recorded, and during a 3-day period (26-28 March) 16 events were registered. A- and AB-type events (11 and 6, respectively, during the month) were also recorded during periods of increased B-type activity. The number of B-type events continued to increase in April, reaching a total of 164 during the month. From 16 to 19 April, 33 events were recorded. The highest daily total was on 29 April, when 13 events were registered. Except for two A-type events on 2 April, no A- or AB-type events were detected. The seismic station, part of the Mexican National Seismic Network, is located at 3,900 m elevation on the N flank.

A new series of ultraviolet absorption correlation spectrometry (COSPEC) measurements were made by Univ de Colima scientists on 4-5 May from a Mexican Navy airplane. The measurements were requested by the Secretaria de Gobernacion through the Centro Nacional para la Prevencion de Desastres (CENAPRED). Between 1151 and 1359 on 4 May, the plume was traversed 20 times at an altitude of 3,900-4,000 m in partially cloudy to overcast conditions. Another 11 traverses were made at 3,950 m altitude between 0936 and 1148 the next day. The weather was again partially cloudy to overcast, and rainfall was detected upon returning. The aircraft's global positioning system (GPS) computed the wind speed independently for each traverse. These measurements were each used to make individual SO₂ flux calculations, removing the need to calculate an SO₂ estimate based on an average wind speed. This procedure is advantageous when the wind speed varies significantly. A statistical analysis of the time series was also performed.

The SO₂ flux on 4 May ranged from 485 to 1,462 metric tons/day (t/d), with a standard deviation of 232 t/d and an average value of 900 t/d. This result is close to the value of 1,200 ± 400 t/d measured on 1 February (19:01). SO₂ values dropped the next day to a range of 386-684, with a standard deviation of 89 and an average of 502 t/d. The 5 May results are not thought to be representative of the actual emissions because weather conditions were much more humid, resulting in a more effective gas-to-particle conversion that produced more H₂SO₄ aerosol droplets. An adequate baseline reference value for SO₂ output has not yet been established, so any interpretations are preliminary and should be made with caution. A regular monthly schedule for additional COSPEC measurements is currently planned.

On 30 January 1994, Delgado, Siebe, and Tobias Fischer (Arizona State Univ) installed sampling boxes near the crater rim to monitor gas emissions. These boxes hold an open container of 500 ml 25% KOH solution to absorb acidic gases and allow measurement of variations in S/Cl/CO₂ ratios (Noguchi and Kamiya, 1963). Since then, visits to the crater have taken place every three weeks. The following is an account of observations made during those visits through mid-May.

Intense fumarolic activity has been observed in the crater, in and around the inner dome, and from the crater walls. Sulfur deposits around the fumarolic vent glowed red during the day. Intense reddish glow at night was more common, although dense emissions often hampered observations. Hissing noises from several vents were due to powerful gas emissions that rose up to 1,000 m before being blown downwind. Most of the fumarolic column was bent by the wind to the E, NE, and SE during January-April. A tall, long plume from the volcano could be observed at distances of 40-60 km. The most dense fumarolic emissions came from the crater floor. Several small fumarolic vents associated with fractures in the crater walls and near the rim produced gas emissions, these were smelled by mountain climbers visiting the summit, and they contaminated the surrounding ice cap with sulfur. Most of the fumaroles were on the inner part of the crater rim, but some diffuse vents were observed on the outer SE and E flanks as low as ~5,000 m elevation.

During the 19 February crater visit, the small milky-green lake nested in the central dome had a temperature of 65°C and a pH of 1.5 ± 0.5. This crater lake description contrasts with observations from 1986 in color (very clear with a greenish tint), temperature (29°C), and pH (6.5) (11:01). Temperatures at fumarolic vents, measured using a thermocouple, ranged from 250 to 380°C. Fumaroles surrounded by the intensely glowing red sulfur deposits were inaccessible. The measured fumaroles were the same as those with temperatures of 97-99°C in 1986. Since late March-early April, additional water introduced to the system by precipitation has increased the vapor phase, causing the plumes to look more dense and whitish when emitted, but more diffuse when dispersed out of the crater. If no wind was present at 5,000-6,000 m altitude to disperse the plume, there was the appearance of a dense, dirty cloud near the volcano.

Increased fumarolic activity has caused alarm in surrounding towns and villages. Concern about future eruptions has prompted many people to climb the summit for observation of the activity. This has led to a proliferation of reports by untrained people describing "molten lava" (overheated sulfur) and "phreatic eruptions" inside the crater (reported as unconfirmed in 18:11). On 29 April a commercial airline pilot reported an "ash cloud" at ~5,800 m altitude 35 km SE of México City. No ash cloud was seen the following day during a routine visit to refill the KOH solution in the boxes. Careful observations inside the crater and on the ice around the crater did not reveal any recent ash emissions. The pilot very likely observed a fumarolic cloud, but the report of ash caused alarm among air-traffic authorities in México City.

Popocatépetl rises to 5,420 m above the Mexico and Puebla valleys, basins with >20 million inhabitants. The last significant eruptive period at this stratovolcano was in 1920-22, with fumarolic activity through 1927; minor ash explosions were reported in 1933, 1943, and 1947.

Following destruction of an ancestral volcano by a Bezymianny-type eruption during which a debris avalanche formed a 6.5 x 11 km caldera, the modern cone was constructed in two stages. The El Fraile volcano, formed prior to 10,000 years BP, was partly destroyed by later explosive activity. The current summit of Popocatépetl was formed to the south of El Fraile by repeated lava effusions until about 1,200 years BP. About 25-30 eruptions have occurred in the last 600 years, most of them apparently consisting of weak summit explosions.

Reference. Noguchi, K., and Kamiya, H., 1963, Prediction of volcanic eruption by measuring the chemical composition and amounts of gases: Bulletin of Volcanology, v. 26, p. 367-378.

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

Information Contacts: Guillermo González-Pomposo and Carlos Valdés-González, Instituto de Geofísica, UNAM; Ignacio Galindo, Arturo Gonzalez, Juan Carlos Gavilanes and Carlos Navarro, CUICT Univ de Colima; Hugo Delgado and Claus Siebe, Instituto de Geofísica, UNAM, Circuito Exterior.

"There was a further decline in the level of seismicity in April. A total of 397 caldera earthquakes were recorded, down from 458 in March and 580 in February. Of this total, 181 earthquakes occurred during four small swarms that took place on the 13th (33), 17th (73), 19th (38), and 25th (37). Thirty-eight of the caldera earthquakes were located. About half of them were scattered evenly in the N part of the caldera seismic zone. The rest were located in the W part of the seismic zone near Vulcan. Routine levelling . . . on 2 May showed that uplift of ~25 mm had taken place at the S end of Matupit Island since the previous survey on 16 March."

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

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

During April, the local seismic station received only 13 low-frequency events. In contrast, there were 283 low-frequency events during the previous month, the most so far this year. Neither the increase nor the decrease in seismicity were associated with any other observed changes.

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

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

Weak shallow seismicity that accompanies growth of the crater dome and associated small explosions decreased in the second half of April from an average of 21 events/day to 4 events/day. This seismic activity continued through mid-May at a rate of 2-8 weak earthquakes/day. Average duration of volcanic tremor decreased from 4 hours/day in mid-April to less than 1 hour/day in early May. Fumarolic activity was observed on 25 April, and in May consisted of steam-and-gas plumes rising to 300-1,000 m above the extrusive dome. Seismic data indicated a small explosion on 8 May when the volcano was obscured by clouds.

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: V. Kirianov, IVGG.

"There was an increase in the level of activity in April. At the beginning of the month, emissions consisted of moderate white vapour. However, these emissions changed through the month to strong thick white vapour and there were occasional reports of grey and blue emissions. On 19 April, very strong, thick, dark grey emissions were reported. Very fine ashfall was reported on the NW side of the volcano on 28 April, and steady weak red glow was seen on the 30th.

"Seismic monitoring was affected by telemetry problems from mid-March until 12 April. When the system was restored, daily earthquake totals were ~400-500. On 18 April there was a sharp increase to ~540 earthquakes/day. The daily totals then increased steadily through the remainder of the month to ~630 at month's end. Earthquake amplitudes showed a progressive increase after 12 April."

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

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

At Unzen the output of lava remained about the same as in previous months, yet the edifice first inflated then deflated, the direction of dome growth reversed, and the paths of pyroclastic flows changed. Recently, seismicity remained high while the number of seismically detected pyroclastic flows remained unusually low (figure 69).

Figure 69. (Top) Plot of earthquakes at Unzen versus time; arrows indicate first appearance of lobes 1-12 of the summit dome complex. (Bottom) Plot of pyroclastic flows versus time. Time interval covered is January 1989 through 15 May 1994. Courtesy of JMA.

Dome growth shifted direction between late-April, when the dome continued to expand northward, and early-May, when it began to expand southward. The late-April growth involved both the upper surface of the dome (the carapace) and the adjacent talus; they both moved nearly horizontally toward the NNW at about 1-2 m/day. The later, southerly motion also reached a similar speed.

Crest-line measurements determined by theodolite from the UWS documented deformation on the W side of the dome (figure 70). These three sequential profiles of the lava dome illustrate the subhorizontal, westward movement of the W dome margin.

Figure 70. Profile of the Unzen lava dome determined by theodolite measurements from about 2 km SSW of the dome. Courtesy of JMA.

In mid-May a split in the dome top formed an E-W trending graben: 200 m long, 50 m wide, and 20 m deep. The graben severed and offset the dome; the dome's S part moved S while the N part subsided. Rockfalls frequently bounded down the dome's upper slopes, chiefly from the unstable, advancing face of the dome. During the shift in growth direction there was neither high seismicity nor a break in the rate of eruption (which remained below ~50,000 m³/day).

A spiny cone sprouted in February 1994. It underwent rapid growth, reaching the top of the talus pile in early April (19:2-3). It subsequently began a day-by-day collapse.

EDM measurements by the GSJ recorded Unzen's inflation and deflation. On the S flank no clear pattern emerged. In contrast, the distance between sites on the dome floor (100-150 m NW of talus) and Unzen's NW flank indicated that the volcano inflated until 29 April, when it suddenly began deflating. Deflation continued until at least mid-May. This inflation followed by deflation took place a few days prior to the shift in the growth direction of the dome, suggesting a possible relationship.

During mid- to late-April pyroclastic flows were similar in number to March. Attributed to partial collapse of the N part of the dome, the flows frequently descended the N flank.

On 26 April several of these N-directed pyroclastic flows traveled a horizontal distance of 2.1 km from their source. For the distal 0.8 km of their path, these flows were guided by two valleys with widths of tens of meters. Observers inspected the resultant deposits the next day from a helicopter. The deposits contained large blocks only as far as the heads of the small valleys; these block-rich facies were associated with a zone of toppled trees that extended ahead of the deposits. Above the valleys, a seared zone extended 100-200 m beyond the deposits. In the valleys, the seared zone only continued several tens of meters beyond the deposits. The portions of the flows guided by valleys are thought to have behaved as pyroclastic surges, conveyed in a manner similar to that seen along the Mizunashi and Nakao rivers in June and July 1993. Since early February, N-directed pyroclastic flows have had an estimated maximum total volume of 1.2 million m³.

Although pyroclastic flows traveling N were also common in late-April, beginning in the evening of 2 May they ceased and flows began to travel SE instead. One such SE-directed flow, on 3 May, followed the Akamatsu Valley. Later, in mid-May, SW-directed pyroclastic flows became common. Rockfalls occurred in Unzen's southern quadrant but they failed to develop into pyroclastic flows because the prehistoric lava dome broke their descent. On 12 May pyroclastic flows cascaded for ~1 km, moving down the length of a large gully S of the volcano. They came to a halt at the lower end of the gully, having retraced a route not used since 20 January.

In April, a high number of microearthquakes occurred beneath the lava dome. A large number of earthquakes also took place in March (5,110) and April (4,606) (figure 69). Over 3,000 people have remained evacuated since the eruption and associated pyroclastic flows began 1991, an interval of more than 1,000 days.

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

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

The eruptive activity . . . continued through mid-May with steam plumes and lava emissions. Residents of Perryville . . . were able to see the top of Veniaminof on 21 April; a small steam plume was noted. During the week of 22-29 April, pilots and ground observers in Perryville reported occasional vigorous steam plumes emanating from near the intracaldera cinder cone. Perryville residents heard loud rumbling noises on 30 April and saw extensive glow over Veniaminof on the next evening (1 May). Large steam columns were reported by pilots and ground observers during that same period. Poor weather obscured the summit in late April and early May.

On 9 May 1994, AVO observers flew over the active cone for about 15-20 minutes around noon, and made 6-8 passes over the E flank of the active intracaldera cone. The summit of the main intracaldera cone was generally shrouded in steam and clouds (at about 2,400 m above sea level) but was obviously degassing vigorously, similar to 1983-84. Photos taken on 3 August 1993 indicated that the eruption began in the summit area.

The eruption has formed an oval-shaped NE-SW lava field about 1,000 m (N-S) x 800 m (E-W) on the SE flank of the main intracaldera cone between ~1,925 and 1,770 m elevation (preliminary measurements are based on guesses as to scale, similarity to 1983-84 features, etc.). The lava is assumed to be similar in composition to material erupted during 1983-84 and earlier eruptions, which was basaltic andesite with about 54% SiO₂. Assuming an area of 0.8-1.0 km² and an average thickness of 20 m, the volume of the lava field is 16-20 x 10⁶ m³. The volume of magma erupted in 1983-84 was ~40 x 10⁶ m³.

The new lava is black with some snow cover along the W margin and a few scattered snow fields low on the S and SE flanks. The lava field is composed of block lavas with well-developed levees and channels; most of the lavas appear to have moved in a general N to S direction. The N and upslope end of the lava field was apparently the source area for most, if not all, of the flows. A sharp-featured cone centered about 100 m from the edge of the ice rises 100-150 m above the surrounding ice-field and dominates this part of the field. Photographs indicate that this feature is the N half of a cinder cone; the S half apparently either collapsed or was blown away, resulting in an E-W trench and debris pile at the base of the cone. This cone, referred to hereafter as "Half Cone," was probably the locus of early activity. Except for part of its steep S slope, however, it was snow-covered, indicating a lack of recent activity. Fumes rose from the cone but no point-source fumaroles were apparent.

Three low mounds at the SW base of Half Cone were arranged in a dog-legged N-S direction with the middle mound slightly W of the others. Covered with yellow-green alteration encrustations typical of vent areas, the mounds were connected by a low fissure-like ridge covered with similar material. These mounds, and possibly the connecting ridge, have been the source of many lava flows. An incandescent bright orange-red fan-shaped lava flow was being extruded from the central mound and moving E into what looked like the E-W trench at the base of Half Cone. This W to E direction is in contrast to the general N to S movement of earlier flows. The incandescent part of the flow was an estimated 80 m long and the flow was perhaps 10 m wide at its point of issue. A line of blue gases (SO₂?) extended for perhaps another 100 m to the E. Eruptive products appeared to be entirely lava; no fresh ash was seen on the surrounding ice field.

The ice pit melted by the lava field had vertical, jagged walls 30-50 m high, exposing numerous black ash layers from earlier eruptions. Fractures in the ice-field are sparsely developed parallel to the W (up-slope) wall and more extensively developed parallel to the E wall (extending perhaps 100-200 m from the wall). A series of crescent-shaped fractures in the ice field, which extended 500 m from the S end of the lava field, suggested that lava had reached the caldera floor and caused sub-glacial melting, local ice cap uplift, and subsequent collapse similar to the 1983-84 eruption. This collapse area partially overlapped the E end of the 1983-84 ice pit. No lake separated the lava flow from the ice, and there was no sign of the tunnels or ductile deformation in the ice so visible in 1983-84. Assuming a 1 km² area and a 30-50 m thickness, 30-50 x 10⁶ m³ of ice (<1% of the ice in the caldera) have been melted during the present eruption. An estimated 150 x 10⁶ m³ of ice melted during the 1983-84 eruption.

Gas and steam was seen rising sparsely and discontinuously over the surface of the lava field. A vigorous fumarolic plume rose from the margin of the lava field E of the active flow. The second most active fumarole issued from the edge of the lava field N of Half Cone, and a third strong fumarole rose from the SE margin of the lava field. All three plumes were white in color and issued from the ice field at the lava-field contact. Whether each one represented an active lava flow or some combination of cooling lava, water, and ice field was unknown.

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

Information Contacts: AVO.