Ambrym contains a 12-km-wide caldera and is part of the New Hebrides Arc, located in the Vanuatu archipelago. The two currently active craters within the caldera are Benbow and Marum, both of which have produced lava lakes, explosions, lava flows, and gas-and-ash emissions. The previous eruption occurred during late January 2022 and was characterized by ash plumes, sulfur dioxide plumes, and crater incandescence (BGVN 47:05). This report covers a new, short eruption during January 2024, which consisted of a lava effusion and an explosion. Information comes from the Vanuatu Meteorology and Geohazards Department (VMGD) and satellite data.

VMGD reported that at 2217 on 13 January an eruption began at Benbow Crater, based on webcam and seismic data. The eruption was characterized by a loud explosion, intense crater incandescence (figure 55), and gas-and-steam emissions. As a result, the Volcano Alert Level (VAL) was raised from 1 to 3 (on a scale of 0-5). A lava flow was reported in Benbow Crater, which lasted for four days. Satellite data showed that 1,116 tons of sulfur dioxide per day (t/d) were emitted on 14 January (figure 56). During the morning of 15 January, ground reports noted loud explosions and minor earthquakes. The sulfur dioxide flux on 15 January was 764 t/d. During 15-17 January activity decreased according to webcam images, seismic data, and field observations. No sulfur dioxide emissions were reported after 15 January. Gas-and-ash emissions also decreased, although they continued to be observed through 31 January, and crater incandescence was less intense (figure 57). The VAL was lowered to 2 on 17 January.

Figure 55. Webcam image showing strong nighttime incandescence coming from Benbow Crater at Ambrym at 2030 on 14 January 2024. Courtesy of VMGD.

Figure 56. A sulfur dioxide plume with a volume of 1,116 t/d was detected on 14 January 2024 drifting W from Ambrym. Courtesy of MOUNTS via VMGD.

Figure 57. Thermal activity was visible in a clear infrared (bands B12, B11, B4) satellite image at Benbow Crater on 23 January 2024. Courtesy of Copernicus Browser.

Geologic Background. Ambrym is a large basaltic volcano with a 12-km-wide caldera formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have been frequently reported since 1774, though mostly limited to extra-caldera eruptions that would have affected local populations. Since 1950 observations of eruptive activity from cones within the caldera or from flank vents have occurred almost yearly.

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

Popocatépetl, located 70 km SE of Mexica City, Mexico, contains a 400 x 600 m-wide summit crater. Records of activity date back to the 14th century; three 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. The current eruption period began in January 2005, characterized by numerous episodes of lava dome growth and destruction within the summit crater. Recent activity has been characterized by daily gas-and-ash emissions, ashfall, and explosions (BGVN 48:09). This report covers similar activity during August through November 2023, according to daily reports from Mexico's Centro Nacional de Prevención de Desastres (CENAPRED) and various satellite data.

Daily gas-and-steam emissions, containing some amount of ash, continued during August through November 2023. CENAPRED reported the number of low-intensity gas-and-ash emissions or “exhalations” and the minutes of tremor, which sometimes included harmonic tremor in their daily reports (figure 220). A total of 21 volcano-tectonic (VT) tremors were detected throughout the reporting period. The average number of exhalations was 117 per day, with a maximum number of 640 on 25 September. Frequent sulfur dioxide plumes that exceeded two Dobson Units (DU) and drifted in multiple directions were visible in satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 221).

Figure 220. Graphs showing the number of daily “exhalations” (in blue, top), and the number of minutes of tremor (in gold, bottom) at Popocatépetl each day during August through November 2023. The maximum number of daily exhalations was 640 on 25 September 2023; the maximum duration of 1,323 minutes of tremor was detected on 14 November 2023. Data from CENAPRED daily reports.

Figure 221. Strong sulfur dioxide plumes were detected at Popocatépetl and drifted in different directions on 26 August 2023 (top left), 5 September 2023 (top right), 9 October 2023 (bottom left), and 21 November 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during August was relatively low and mainly consisted of occasional explosions, ash emissions, and light ashfall. There were 30 explosions (25 minor explosions and four moderate explosions), and nine VT-type events detected. An average number of 60 exhalations occurred each day, which mostly consisted of water vapor, volcanic gases, and a small amount of ash. On 2 August the National Center for Communications and Civil Protection Operations (CENACOM) reported light ashfall in Ocuituco (22 km SW), Yecapixtla (31 km SW), Cuautla (43 km SW), and Villa de Ayala (47 km SW). On 7 August light ashfall was observed in Atlautla (16 km W). A minor explosion at 0305 on 11 August was accompanied by crater incandescence. Explosions at 0618 on 13 August produced a gas-and-ash plume that rose above the summit, and at 0736 another explosion produced a puff of gas-and-ash (figure 222). Two minor explosions were detected at 0223 and 0230 on 16 August that generated eruptive columns with low ash content rising 800 m and 700 m above the crater, respectively. On 24 August an eruptive event lasted 185 minutes and consisted of light ash emissions that did not exceed 300 m above the crater. According to the Washington VAAC, ash plumes identified in daily satellite images rose to 4.6-7.6 km altitude and drifted in multiple directions, the highest of which occurred on 29 August.

Figure 222. Webcam image of an ash plume rising above Popocatépetl at 0738 on 13 August 2023. Courtesy of CENAPRED daily report.

There was an average of 156 exhalations each day during September, a monthly total of seven VT-type events, and 29 explosions, 14 of which were minor and nine of which were moderate. A gas-and-ash plume rose to 2 km above the summit and drifted WSW at 1216 on 1 September. CENACOM reported at 1510 observations of ashfall in Ozumba (18 km W), Atlautla, Tepetlixpa (20 km W), and Ecatzingo (15 km SW), as well as in Morelos in Cuernavaca (65 km WSW), Temixco (67 km WSW), Huitzilac (67 km W), Tepoztlán (49 km W), and Jiutepec (59 km SW). The next day, gas-and-ash plumes rose to 2 km above the summit (figure 223). At 1100 ashfall was reported in Amecameca (15 km NW), Ayapango (24 km WNW), Ozumba, Juchitepec, Tenango del Aire (29 km WNW), Atlautla, and Tlalmanalco (27 km NW). A gas-and-ash plume rose to 1 km above the summit and drifted WNW at 1810. During 5-6, 8-9, 12, 14, 19, and 24-25 September ashfall was reported in Amecameca, Atlautla, Ozumba, Tenango del Aire, Tepetlixpa, Juchitepec, Cuernavaca, Ayala, Valle de Chalco (44 km NW), Ixtapaluca (42 km NW), La Paz (50 km NW), Chimalhuacán, Ecatepec, Nezahualcóyotl (60 km NW), Xochimilco (53 km SE), Huayapan, Tetela del Volcano (20 km SW), Yautepec (50 km WSW), Cuautla (43 km SW), Yecapixtla (30 km SW) and possibly Tlaltizapán (65 km SW), Tlaquiltenango, and Tepalcingo. According to the Washington VAAC, ash plumes identified in daily satellite images rose to 5.8-9.1 km altitude and drifted in multiple directions, the highest of which was identified during 1-2 August.

Figure 223. Webcam image of a strong ash plume rising 2 km above Popocatépetl around 0342 on 2 September 2023. Courtesy of CENAPRED daily report.

Activity during October and November was relatively low. An average of 179 exhalations consisting of gas-and-steam and ash emissions were reported during October and 73 during November. Only one VT-type event and two explosions were detected during October and four VT-type events and one explosion during November. A satellite image from 0101 on 14 October showed ash fanning out to the NW at 6.7 km altitude and an image from 0717 showed a continuously emitted ash plume drifting WNW and NW at the same altitude. Ash emissions at 1831 on 14 October were ongoing and visible in webcam images slowly drifting W at an altitude of 6.4 km altitude (figure 224). On 24 October a tremor sequence began at 0310 that generated a gas-and-ash plume that rose 800 m above the summit and drifted W. Another tremor sequence occurred during 1305-1900 on 25 October that consisted of continuous ash emissions. Ash plumes identified in daily satellite images rose to 5.5-8.5 km altitude and drifted in different directions during October, according to the Washington VAAC. The highest ash plume was detected on 23 October. During 10-13 November ash plumes rose to 6.7 km altitude and drifted N, NNW, NE, and NW. On 13 November a M 1.5 VT-type event was detected at 0339 and light ashfall was reported in Amecameca, Cocotitlán (34 km NW), and Tenango del Aire, and Ocuituco. On 14 November ash plumes rose to 6 km altitude and drifted N, NE, and SE and light ashfall was reported in Cuernavaca (64 km W). The Washington VAAC reported frequent ash plumes that rose to 5.8-7.9 km altitude and drifted in several directions; the highest ash plume was recorded on 28 November.

Figure 224. A strong ash plume rising above Popocatépetl at 0553 on 14 October 2023. Image has been color corrected. Courtesy of CENAPRED daily report.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed frequent low-to-moderate thermal anomalies during the reporting period (figure 225). The intensity of the anomalies was lower compared to previous months. According to data from MODVOLC thermal alerts, a total of ten hotspots were detected at the summit crater on 2 August and 2, 4, 9, 19, and 26 September. Thermal activity in the summit crater was visible in infrared satellite data and was sometimes accompanied by ash plumes, as shown on 17 November (figure 226).

Figure 225. Frequent low-to-moderate power thermal anomalies were detected at Popocatépetl during July through November 2023. During October through November the intensity of the anomalies was lower compared to previous months. Courtesy of MIROVA.

Figure 226. Infrared (bands B12, B11, B4) satellite images show a persistent, yet variably strong, thermal anomaly (bright yellow-orange) in the summit crater of Popocatépetl on 9 August 2023 (top left), 19 August 2023 (top right), 28 October 2023 (bottom left), and 17 November 2023 (bottom right). A strong ash plume drifted S on 17 November. Courtesy of Copernicus Browser.

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: Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665. Coyoacan, México D.F. 04360, México (URL: http://www.cenapred.unam.mx/, Daily Report Archive https://www.gob.mx/cenapred/archivo/articulos); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.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/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Volcán El Reventador, located in Ecuador, is a stratovolcano with a 4-km-wide avalanche scarp open to the E that was formed by edifice collapse. The largest recorded eruption took place in 2002 producing a 17-km-high eruption column, pyroclastic flows that traveled as far as 8 km, and lava flows from summit and flank vents. Recorded eruptions date back to the 16th century and have been characterized by explosive events, lava flows, ash plumes, and lahars. Frequent lahars have built deposits on the scarp slope. The current eruption period began in July 2008 and has recently been characterized daily explosions, gas-and-ash emissions, and block avalanches (BGVN 48:08). This report covers similar activity during August through November 2023 using daily reports from Ecuador's Instituto Geofisico (IG-EPN) and satellite data.

During August through November 2023, IG-EPN reported daily explosions, gas-and-ash plumes that rose as high as 1.3 km above the crater, and frequent crater incandescence, often accompanied by incandescent block avalanches that affected one or multiple flanks. More daily explosions were detected during November, with an average total of 46 per day.

Table 19. Monthly summary of explosions and plume heights recorded at Reventador from August through November 2023. Data could not be collected for 29-30 September 2023 and 6-23 October 2023. Data courtesy of IG-EPN (August-November 2023 daily reports).

Activity during August consisted of 6-75 daily explosions, nighttime crater incandescence, and incandescent avalanches of material. Frequent seismicity was mainly characterized by long-period (LP) events, harmonic tremor (TRARM), tremor-type (TRE), and volcano tectonic (VT)-type events. Daily gas-and-ash emissions rose 200-1,300 m above the summit and drifted W, SW, NW, NE, N, and E, based on webcam and satellite images. The Washington VAAC also reported occasional ash plumes that rose 400-1,600 m above the crater and drifted NW. Avalanches of incandescent material were reported during 1-2, 6-7, 9-14, 16-17, 18-21, and 26-29 August, which traveled 500-900 m below the crater and affected multiple flanks (figure 180). During 24-25 August incandescent material was ejected 300 m above the crater.

Figure 180. Infrared webcam image of incandescent avalanches descending the flanks of Reventador at 2158 (local time) on 21 August 2023. A gas-and-ash plume accompanied this activity more than 700 m above the crater as indicated by the black dotted lines. The white dotted line indicates the direction of the avalanches. The southern flank is located on the left of the photo. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-233, 21 de agosto de 2023).

Gas-and-ash emissions and seismicity characterized by LP, VT, TRARM, and TRE-type events continued during September; data were not available for 29-30 September. Daily gas-and-ash emissions rose 200-1,000 m above the crater and generally drifted W, NW, and SW (figure 181). Near-daily explosions ranged from 16-53 per day, often accompanied by incandescent avalanches, which affected one or multiple flanks and traveled 100-800 m below the crater. During 2-3 September incandescent material was ejected 200 m above the crater and was accompanied by blocks rolling down the flanks. During 16-17 September incandescent material was ejected 100-200 m above the crater and avalanches descended 600 m below the crater. During 21-22 and 24-26 September incandescent material was ejected 100-300 m above the crater. According to the Washington VAAC, ash plumes rose 700 m above the crater and drifted SW, W, and NW on 3, 16, and 20 September, respectfully.

Figure 181. Webcam image of a gas-and-ash plume rising above Reventador on 13 September 2023. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-257, 14 de septiembre de 2023).

During October, daily explosions, gas-and-ash plumes, and crater incandescence continued, with 16-40 explosions recorded each day (figure 182); data was not available for 6-23 October. Seismicity consisted of LP, TRE, and TRARM-type events. Gas-and-ash emissions rose 200-1,000 m above the crater and drifted W, SW, NW, SSW, NNW, and NE. The Washington VAAC reported that ash plumes rose 1-1.3 km above the crater and drifted W, SW, and NW during 1-5 October. During 30 September-1 October incandescent avalanches descended 700 m below the crater. Ejected material rose 200 m above the crater during 2-5 October and was accompanied by avalanches of material that traveled 250-600 m below the crater rim; incandescent avalanches were also reported during 23-29 October.

Figure 182. Photo showing nighttime crater incandescence and an explosion at Reventador on 25 October 2023. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-299, 26 de octubre de 2023).

Daily explosions, LP, TRARM, VT, and TRE-type events, crater incandescence, and avalanches of material continued during November. There were 26-62 daily explosions detected throughout the month. Gas-and-ash emissions rose 300-1,200 m above the crater and drifted in different directions (figure 183). The Washington VAAC reported that ash plumes rose 700-1,620 m above the crater and drifted NW, W, WNW, SW, E, SE, and ESE. Frequent incandescent avalanches descended 500-1,000 m below the crater. Explosions ejected material 100-300 m above the crater during 4-7, 11-12, and 19-23 November.

Figure 183. Webcam image showing an ash plume rising several hundred meters above Reventador on 21 November 2023. Courtesy of IG-EPN (INFORME DIARIO DEL VOLCAN EL REVENTADOR No. 2023-325, 21 de noviembre de 2023).

Satellite data. MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed intermittent thermal anomalies of low-to-moderate power (figure 184). Thermal activity mainly consisted of incandescent avalanches descending the flanks due to the frequently detected explosions. The MODVOLC hotspot system identified a total of ten hotspots on 3 August, 7, 18, 12, 22, and 28 September, and 7, 9, and 19 November.

Figure 184. Intermittent low-to-moderate intensity thermal activity was detected at Reventador during August through November 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Geologic Background. Volcán El Reventador is the most frequently active of a chain of Ecuadorian volcanoes in the Cordillera Real, well east of the principal volcanic axis. The forested, dominantly andesitic stratovolcano has 4-km-wide avalanche scarp open to the E formed by edifice collapse. A young, unvegetated, cone rises from the amphitheater floor to a height comparable to the rim. It has been the source of numerous lava flows as well as explosive eruptions visible from Quito, about 90 km ESE. Frequent lahars in this region of heavy rainfall have left extensive deposits on the scarp slope. The largest recorded eruption took place in 2002, producing a 17-km-high eruption column, pyroclastic flows that traveled up to 8 km, and lava flows from summit and flank vents.

Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional (IG-EPN), Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); 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/).

Erta Ale in Ethiopia has a 50-km-wide edifice that rises more than 600 m from below sea level in the Danakil depression. The summit caldera is 0.7 x 1.6 km and contains at least two pit craters (North and South). Another larger 1.8 x 3.1-km-wide depression is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Lava flows from fissures have traveled into the caldera and locally overflowed the crater rim. The current eruption has been ongoing since 1967, with at least one long-term active lava lake present in the summit caldera. Recent fissure eruptions from 2017 have occurred on the SE flank (BGVN 42:07). Recent activity has been characterized by minor thermal activity at the S crater and an active lava lake at the N crater (BGVN 48:06). This report covers strong lava lake activity primarily at the N pit crater during June through November 2023 using information from satellite infrared data.

Infrared satellite images generally showed an active lava lake as the N pit crater and variable thermal activity at the S pit crater during the reporting period. On 7 June two strong thermal anomalies were detected at the S pit crater and two weaker anomalies were visible at the N pit crater. Those anomalies persisted throughout the month, although the intensity at each declined. On 2 July a possible lava lake was identified at the S pit crater, filling much of the crater. On 7 July both pit craters contained active lava lakes (figure 120). By 12 July the thermal activity decreased; two smaller anomalies were visible through the rest of the month at the S pit crater while the N pit crater showed evidence of cooling.

Figure 120. Infrared satellite images (bands B12, B11, B4) showed strong thermal anomalies at both the N and S pit craters at Erta Ale on 7 July 2023 (top left). On 25 September 2023 (top right) thermal activity intensified at the N pit crater, which overflowed and traveled SE for several hundred meters, as shown on 15 October 2023 (bottom left) and 29 November 2023 (bottom right). Courtesy of Copernicus Browser.

Renewed lava lake activity was identified at the N pit crater, based on a satellite image from 11 August, with two smaller anomalies visible at the S pit crater. By 16 August the lava lake in the N pit had begun to cool and only a small thermal anomaly was identified. Activity restarted on 21 August, filling much of the E and SE part of the N pit crater. The thermal activity at the N pit crater intensified on 31 August, particularly in the NW part of the crater. On 5 September lava filled much of the N pit crater, overflowing to the W and SW. During at least 10-20 September thermal activity at both craters were relatively low.

According to a satellite image on 25 September, strong thermal activity resumed when lava overflowed the N pit crater to the S, SW, and NE (figure 120). A satellite image taken on 5 October showed lava flows from the N had spilled into the S and begun to cool, accompanied by two weak thermal anomalies at the S pit crater. On 15 October lava flows again traveled SE and appeared to originate from the S pit crater (figure 120). Following these events, smaller thermal anomalies were visible on the SE rim of the N pit crater and within the S pit crater.

Lava was visible in the NW part of the N pit crater according to a satellite image taken on 4 November. By 9 November the intensity had decreased, and the lava appeared to cool through the rest of the month; young lava flows were visible along the W side of the S pit crater on 24 and 29 November. Lava flows occurred at the N pit crater trending NE-SW and along the E side on 29 November (figure 120).

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) thermal detection system recorded consistent activity during the first half of 2023 (figure 121). Beginning in June 2023, thermal activity increased and remained variable in intensity through the end of the year indicating the presence of an active lava lake and lava flows. The MODVOLC thermal detection system registered a total of 63 anomalies during 7, 8, and 23 July, 10 and 18 August, 3, 5, 16, 23, 24, and 25 September, 15 and 20 October, and 21, 24, 26, 28, and 30 November. Some of these stronger thermal anomalies were also detected in Sentinel-2 infrared satellite images that showed an active lava lake at the N pit crater and subsequent lava overflows from both pit craters (figure 120).

Figure 121. Graph of Landsat 8 and 9 OLI (red dots) and MODIS (blue bars) thermal anomalies at Erta Ale during 2022-2023. Thermal activity was relatively consistent during much of this time and during June through November activity became more variable due to lava flows and a strong active lava lake. Courtesy of MIROVA.

Geologic Background. The Erta Ale basaltic shield volcano in Ethiopia has a 50-km-wide edifice that rises more than 600 m from below sea level in the Danakil depression. The volcano includes a 0.7 x 1.6 km summit crater hosting steep-sided pit craters. Another larger 1.8 x 3.1 km wide depression elongated parallel to the trend of the Erta Ale range is located SE of the summit and is bounded by curvilinear fault scarps on the SE side. Basaltic lava flows from these fissures have poured into the caldera and locally overflowed its rim. The summit caldera usually also holds at least one long-term lava lake that has been active since at least 1967, and possibly since 1906. Recent fissure eruptions have occurred on the N flank.

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

Ubinas, located in Peru, has had 24 eruptions since 1550, which more recently have been characterized by explosions, ash plumes, and lahars (BGVN 45:03). This report covers a new eruption during June through December 2023 based on reports from Instituto Geofisico del Peru (IGP), Instituto Geológico Minero y Metalúrgico (INGEMMET), and satellite data.

IGP reported that seismic unrest began on 17 May, followed by an increase in seismicity during the second half of the month. There were 168 volcano-tectonic (VT) earthquakes detected, which are associated with rock fracturing processes, and 171 long-period (LP) earthquakes recorded during 16-24 May, which are associated with the movement of volcanic fluid.

Seismicity and fumarolic activity at the crater level continued to increase during June. During 1-18 June there was an average of 631 VT-type earthquakes and 829 LP earthquakes recorded. Webcams showed gas-and-steam emissions rising 500 m above the summit and drifting SE. In addition, the maximum value of emitted sulfur dioxide during this period was 337 tons/day. During 19-22 June an average of 315 VT-type events and 281 LP-type events and tremor were reported. On 20 June the Gobierno Regional de Moquegua raised the Volcano Alert Level (VAL) to Yellow (the second level on a four-color scale), based on recommendations from IGP. Webcam images showed ash emissions rising 1 km above the summit and drifting E at 0011 on 22 June, which IGP reported marked the start of a new eruption. Sporadic and diffuse gas-and-ash emissions continued to rise 800-1,500 m above the summit through the rest of the month and drifted mainly E, N, NW, W, SW, and NE. During 23-25 June there was an average of 402 VT-type earthquakes and 865 LP-type events detected. During 26-28 June the earthquakes associated with ash emissions, which have been observed since 22 June, decreased, indicating the end of the phreatic phase of the eruption, according to IGP. A thermal anomaly was detected in the crater for the first time on 26 June and was periodically visible through 4 July (figure 61). During 29-30 June there was an average of 173 VT-type earthquakes and 351 LP-type events recorded, and sulfur dioxide values ranged between 600 t/d and 1,150 t/d. During this same time, seismicity significantly increased, with 173 VT-type earthquakes, 351 LP-type events, and harmonic tremor which signified rising magma. The Gobierno Regional de Moquegua raised the Alert Level to Orange (the third level on a four-color scale) on 30 June based on the recommendation from IGP and INGEMMET.

Figure 61. A strong thermal anomaly (bright yellow-orange) at Ubinas was visible in an infrared (bands B12, B11, B4) satellite image on 28 June 2023 (left). Natural color images showed an ash plume rising above the summit on 3 July 2023 (middle) and 12 August 2023 (right). Courtesy of Copernicus Browser.

Activity during July consisted of continued seismicity and gas-and-ash emissions. Gas-and-ash emissions rose as high as 5 km above the summit and drifted as far as 40 km in different directions during 1, 4-6, 16, 20-23, 26, and 29 July, based on webcam and satellite images. During 1-2 July an average of 72 VT-type earthquakes and 114 LP-type events were detected. In addition, during that time, ashfall was reported in Ubinas (6.5 km SSE) and Querapi (4.5 km SE). During 2-3 July INGEMMET reported gas-and-ash plumes rose 400 m above the summit and drifted SW, causing ashfall in downwind areas as far as 5 km. During 3-4 July there was an average of 69 VT-type earthquakes and 96 LP-type events reported. On 4 July starting around 0316 there were 16 seismic signals associated with explosive activity and ash emissions detected (figure 62). According to INGEMMET an explosion ejected ballistics and generated a gas-and-ash plume that rose 5.5 km above the summit and drifted SW and S. Ashfall was recorded in Querapi, Ubinas, Sacohaya (7 km SSE), Anascapa (11 km SE), San Miguel (10 km SE), Tonohaya (7 km SSE), Huatahua, Huarina, Escacha (9 km SE), and Matalaque (17 km SSE), and was most significant within 5 km of the volcano. IGP noted that ash fell within a radius of 20 km and deposits were 1 mm thick in towns in the district of Ubinas.

Figure 62. Webcam image showing an ash plume rising 2.5 km above the summit of Ubinas on 4 July 2023. Courtesy of INGEMMET.

During 5-9 July an average of 67 VT-type events and 47 LP-type events were reported. A period of continuous gas-and-ash emissions occurred on 5 July, with plumes drifting more than 10 km SE and E. A total of 11 seismic signals associated with explosions also detected on 6, 16, 17, and 22 July. On 6 July explosions recorded at 0747 and 2330 produced gas-and-ash plumes that rose as high as 3.5 km above the summit and drifted as far as 30 km NW, NE, SE, and S. According to the Washington VAAC the explosion at 0747 produced a gas-and-ash plume that rose to 9.1 km altitude and drifted SW, which gradually dissipated, while a lower-altitude plume rose to 7.6 km altitude and drifted NE. Gobierno Regional de Moquegua declared a state of emergency for districts in the Moquegua region, along with Coalaque Chojata, Icuña, Lloque, Matalaque, Ubinas, and Yunga of the General Sánchez Cerro province, to be in effect for 60 days. On 7 July an ash plume rose to 7.3 km altitude and drifted E at 0320. At 0900 and 1520 gas-and-steam plumes with diffuse ash rose to 6.7 km altitude and drifted SE. Small ash emissions were visible in satellite and webcam images at 0920 and 1520 on 8 July and rose as high as 6.4 km altitude and drifted SE. During 10-16 July there was an average of 80 VT-type earthquakes and 93 LP-type events reported. INGEMMET reported that during 9-11 July sulfur dioxide emissions were low and remained around 300 t/d.

During 17-23 July an average of 46 VT-type events and 122 LP-type events were detected. On 20 July at 0530 an explosion generated an ash plume that rose 3-4.5 km above the crater and drifted 65 km toward Arequipa. An explosion on 21 July at 0922 produced a gas-and-ash plume that rose 5 km above the summit (figure 63). Ashfall was reported in Querapi, Ubinas, Tonohaya, Anascapa, Sacohaya, San Miguel, Escacha, Huatagua (14 km SE), Huarina, Escacha (9 km SE), Matalaque, Logén, Santa Lucía de Salinas, and Salinas de Moche. An explosion on 22 July at 1323 generated an ash plume that rose 5.5 km above the summit and drifted NE, E, and SE. During 24-30 July there were five volcanic explosions detected and an average of 60 VT-type events and 117 LP-type events. An explosion on 29 July at 0957 produced an ash plume that rose 2.5 km above the summit and drifted as far as 40 km NE, E, and SE. As a result, significant ashfall was reported in Ubinas and Matalaque.

Figure 63. Webcam image of Ubinas showing an ash plume rising as high as 5 km above the summit at 0930 on 21 July 2023. Courtesy of INGEMMET.

During August, explosions, gas-and-ash emissions, and seismic earthquakes persisted. During 31 July to 6 August there was an average of 115 VT-type events and 124 LP-type events reported. Gas-and-ash emissions were observed during 1, 6, 10, 13-14, 17-18, 21, and 23 August and they drifted as far as 20 km in different directions; on 14 and 18 August continuous ash emissions extended as far as 40 km S, SE, and NE. An explosion was detected at 2110 on 1 August, which generated a gas-and-ash plume that rose 5.4 km above the summit and drifted SE and E. The explosion ejected blocks and incandescent material as far as 3 km from the crater onto the SW, S, and SE flanks. Ashfall was reported in Ubinas and Chojata (19 km ESE). Gas-and-ash emissions rose as high as 2 km above the summit and drifted in different directions through 5 August, sometimes causing ashfall within a 15-km-radius. An explosion at 0009 on 6 August ejected blocks and produced a gas-and-ash plume that rose 1.4 km above the summit and drifted SE and E, which caused ashfall in Ubinas and Chojata and other areas within a 30-km radius. During 7-13 August there was an average of 102 VT-type events and 60 LP-type events detected. INGEMMET reported that sulfur dioxide emissions were low on 7 August and averaged 400 t/d.

One volcanic explosion that was recorded on 10 August, producing gas-and-ash emissions that rose 2.4 km above the summit and drifted as far as 25 km SE and E. Ashfall was observed in Ubinas, Matalaque, and Chojata. During 10-11 and 13-14 August sulfur dioxide values increased slightly to moderate levels of 2,400-3,700 t/d. The average number of VT-type events was 104 and the number of LP-type events was 71 during 14-21 August. Two explosions were detected at 0141 and 0918 on 21 August, which produced gas-and-ash emissions that rose 3.5 km above the summit and drifted 50 km N, NE, W, and NW (figure 64). The explosion at 0918 generated an ash plume that caused ashfall in different areas of San Juan de Tarucani. During 22-27 August the average number of VT-type events was 229 and the average number of LP-type events was 54. An explosion was reported at 1757 on 25 August, which generated a gas-and-ash plume that rose 4.2 km above the summit and drifted in multiple directions as far as 25 km. During 28 August through 3 September gas-and-ash emissions rose 600 m above the summit and drifted as far as 5 km E and SE. During this time, there was an average of 78 VT-type events and 42 LP-type events.

Figure 64. Webcam image showing an ash plume rising 3 km above the summit of Ubinas on 21 August 2023 at 0932. Courtesy of INGEMMET.

Gas-and-steam emissions rose 600-2,600 m above the summit and drifted as far as 15 km in multiple directions during September. During 4-10 and 11-17 September there was an average of 183 VT-type events and 27 LP-type events, and 114 VT-type events and 86 LP-type events occurred, respectively. On 14 September an explosion at 1049 generated a gas-and-ash plume that rose 2.6 km above the summit and drifted as far as 15 km E, NE, SE, and S (figure 65). During 14-16 September an average of three hours of seismic tremor related to ash emissions was recorded each day. During 18-24 September the average number of VT-type events was 187 and the average number of LP-type events was 45. During 25 September and 1 October, there was an average number of 129 VT-type events and 52 LP-type events detected.

Figure 65. Webcam image showing an ash plume rising 2.6 km above the summit of Ubinas on 14 September 2023. Courtesy of INGEMMET.

Relatively low activity was reported during October; during 2-9 October there was an average number of 155 VT-type events and 27 LP-type events recorded. On 1 October at 1656 seismic signals associated with ash emissions were recorded for an hour and thirty minutes; the ash plumes rose as high as 1 km above the summit and drifted more than 10 km E, S, and SW. On 4 October IGP reported that an ash plume drifted more than 15 km SW and S. Sulfur dioxide emissions were 1,250 t/d on that day. On 7 October a gas-and-ash plume rose 1.9 km above the summit and drifted NE, E, and SE. On 4 October the amount of sulfur dioxide emissions was 1,250 t/d. During 10-15 October there was an average number of 225 VT-type events and 34 LP-type events recorded. On 11 October at 1555 a single seismic signal associated with an ash pulse was recorded; the gas-and-ash emissions rose 700 m above the summit and drifted SW and W. There was an average of 204 VT-type events and 25 LP-type events detected during 16-22 October and 175 VT-type events and 17 LP-type events during 23-29 October. On 27 October at 0043 a gas-and-ash emission rose 500 m above the summit and drifted SE and E. A minor thermal anomaly was visible on the crater floor. During 30 October to 5 November there was an average of 95 VT-type events and 24 LP-type events detected.

Activity remained relatively low during November and December and consisted mainly of gas-and-steam emissions and seismicity. Gas-and-steam emissions rose 900-1,100 m above the summit and drifted mainly E, SE, N, and NE. IGP detected an average of 166 VT-type events and 38 LP-type events during 6-15 November, 151 VT-type events and 17 LP-type events during 16-30 November, 143 VT-type events and 23 LP-type events during 1-15 December, and 129 VT-type events and 21 LP-type events during 16-31 December. No explosions or ash emissions were recorded during November. The VAL was lowered to Yellow (the second level on a four-color scale) during the first week of November. According to the Washington VAAC an ash emission was identified in a satellite image at 0040 on 11 December that rose to 5.5 km altitude and drifted NW. Webcam images at 0620 and 1220 showed continuous gas-and-steam emissions possibly containing some ash rising as high as 7 km altitude. Webcam images during 10-31 December showed continuous gas-and-ash emissions that rose as high as 2.5 km above the summit and drifted up to 5 km NW, W, and SW. On 12 December continuous ash emissions drifted more than 10 km N and NW.

Geologic Background. The truncated appearance of Ubinas, Perú's most active volcano, is a result of a 1.4-km-wide crater at the summit. It is the northernmost of three young volcanoes located along a regional structural lineament about 50 km behind the main volcanic front. The growth and destruction of Ubinas I was followed by construction of Ubinas II beginning in the mid-Pleistocene. The upper slopes of the andesitic-to-rhyolitic Ubinas II stratovolcano are composed primarily of andesitic and trachyandesitic lava flows and steepen to nearly 45°. The steep-walled, 150-m-deep summit crater contains an ash cone with a 500-m-wide funnel-shaped vent that is 200 m deep. Debris-avalanche deposits from the collapse of the SE flank about 3,700 years ago extend 10 km from the volcano. Widespread Plinian pumice-fall deposits include one from about 1,000 years ago. Holocene lava flows are visible on the flanks, but activity documented since the 16th century has consisted of intermittent minor-to-moderate explosive eruptions.

Information Contacts: Instituto Geofisico del Peru (IGP), Calle Badajoz N° 169 Urb. Mayorazgo IV Etapa, Ate, Lima 15012, Perú (URL: https://www.gob.pe/igp); Observatorio Volcanologico del INGEMMET (Instituto Geológical Minero y Metalúrgico), Barrio Magisterial Nro. 2 B-16 Umacollo - Yanahuara Arequipa, Peru (URL: http://ovi.ingemmet.gob.pe); Gobierno Regional Moquegua, Sede Principal De Moquegua, R377+5RR, Los Chirimoyos, Moquegua 18001, Peru (URL: https://www.gob.pe/regionmoquegua); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kanaga lies 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. Most of its previous recorded eruptions are poorly documented, although they date back to 1763. Fumarolic activity at Kanaga 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. Its most recent eruption occurred in February 2012, which consisted of numerous small earthquakes, a possible weak ash cloud, and gas-and-steam emissions (BGVN 38:03). This report covers a new eruption during December 2023, based on information from the Alaska Volcano Observatory (AVO).

A small explosion was detected in local infrasound and seismic data at 2231 on 18 December, followed by elevated seismicity. No ash emissions were visible in partly cloudy satellite images. On 19 December the Volcano Alert Level (VAL) was raised to Advisory (the second level on a four-level scale) and the Aviation Color Code (ACC) was raised to Yellow (the second color on a four-color scale). The rate of seismicity significantly declined after the 18th, although it remained elevated through 30 December. Small, daily earthquakes occurred during 19-28 December. Satellite observations following the event showed a debris flow extending 1.5 km down the NW flank. Possible minor gas-and-steam emissions were visible in a webcam image on 20 December. Weakly elevated surface temperatures were identified in satellite data during 23-26 December. A series of cracks extending from the inner crater to the upper SE flank and debris deposits on the upper flanks were observed in satellite images on 27 December. AVO reported that these were likely formed during the 18 December event. Local webcam and seismic data were temporarily offline due to a power failure during 4-28 January.

On 28 January connection to the seismic stations and webcams was restored and webcam images showed gas-and-steam emissions at the summit. Occasional earthquakes were also detected each day. A period of weak seismic tremor was observed on 31 January. During February, the number of earthquakes declined. On 27 February AVO lowered the VAL to Normal (the lowest level on a four-level scale) and the ACC to Green (the lowest color on a four-color scale) due to decreased levels of seismicity and no new surface changes or elevated temperatures based on satellite and webcam data.

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

Klyuchevskoy, located on the Kamchatka Peninsula, has produced frequent moderate-volume explosive and effusive eruptions and more than 100 flank eruptions have occurred during the past 3,000 years. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. Eruptions over the past 400 years have primarily originated from the summit crater, although numerous major explosive and effusive eruptions have also occurred from flank craters. The previous eruption ended in November 2022 and consisted of Strombolian activity (BGVN 47:12). This report covers a new eruption during June through December 2023, characterized by Strombolian explosions, lava flows, and ash plumes. Information primarily comes from weekly and daily reports from the Kamchatkan Volcanic Eruption Response Team (KVERT) and various satellite data.

KVERT reported that a Strombolian eruption began at 2323 on 22 June. A thermal anomaly was visible in satellite data starting on 22 June (figure 75). As a result, the Aviation Color Code (ACC) was raised to Yellow (the second lowest level on a four-color scale). During 4-6 and 13 July small ash clouds were occasionally observed over the crater. On 19 July a new lava flow began to effuse along the Apakhonchich drainage on the SE flank, which continued through 19 August. Lava fountaining was reported on 21 July in addition to the active lava flow, which continued through 23 August and during 27-30 August. During 22-23 and 27-30 August the lava flow was active along the Apakhonchich drainage on the SE flank.

Figure 75. Photo of Strombolian activity at the summit crater of Klyuchevskoy on 5 July 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk via Volkstat.

Similar activity was observed during September. Lava fountaining resumed on 2 September and continued through 31 October. In addition, on 2 September a lava flow began to effuse along the Kozyrevsky drainage on the SW flank. During 3-5 September resuspended ash plumes rose to 3-3.5 km altitude and extended as far as 170 km E by 1940 on 4 September. The ACC was raised to Orange (the third level on a four-color scale) at 1240 on 4 September. The ACC was briefly lowered back to Yellow at 1954 that same day before returning to Orange during 1532-1808 on 5 September due to resuspended ash plumes that rose to 3 km altitude and drifted 120 km E at 1500. KVERT reported that Strombolian activity continued, feeding the lava flows advancing down the Apakhonchichsky and Kozyrevsky drainages through most of the month. During 25 September through 16 October the lava flow was only active in the Apakhonchichisky drainage (figure 76). During 9-12 September resuspended ash plumes rose to 1.5-4 km altitude and extended 550 km E and SE. On 22 September resuspended ash plumes rose to 2-2.5 km altitude and drifted 50-90 km E, which prompted KVERT to raise the ACC to Orange; the ACC was lowered back to Yellow on 24 September. On 29 September phreatic explosions generated ash plumes that rose to 5.2-5.3 km altitude.

Figure 76. Photo of Strombolian explosions at the summit of Klyuchevskoy accompanied by ash plumes and a lava flow descending the Apakhonchichsky on the SE flank on 28 September 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk, IVS FEB RAS, KVERT.

Activity during October continued with lava fountains, lava flows, and ash plumes. Strombolian activity with lava fountains continued at the crater and active lava flows alternately descended the Apakhonchichisky and Kozyrevsky drainages on the SE and S flanks (figure 77). During 11-12 October gas-and-steam plumes containing some ash rose to 5.5-6 km altitude and extended as far as 65 km NE and SE. The ACC was raised to Orange on 11 October. According to observers at the Kamchatka Volcanological Station, lava effusion was almost continuous, and incandescent material was ejected as high as 300 m above the crater rim. On 13 October at 1420 an ash plume rose to 5-5.5 km altitude and drifted 90-100 km SE. During 14-16 October gas-and-steam plumes containing some ash rose to 4-6 km altitude and drifted 40-145 km ESE and E. On 16 October lava on the SE flank melted the snow and ice, causing phreatic explosions and large collapses of material from the margins of the flow. At 1500 an ash plume rose to 6.5-7 km altitude and drifted 70 km ENE. On 17 October an ash plume was reported extending 360 km NE. Gray-red ashfall was observed in Klyuchi at 0700; this ash was resuspended from older material.

Figure 77. Photo of Strombolian activity at the summit crater of Klyuchevskoy on 23 October 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk, IVS FEB RAS, KVERT.

During 22-31 October phreatic explosions generated ash plumes mainly containing ash from collapses of previously deposited pyroclastic material that rose to 7 km altitude and extended as far as 280 km NE, E, SW, and S on 23 and 29 October the ash plumes rose to 8 km altitude. Ash plumes during 27-29 October rose to 8 km altitude and drifted as far as 300 km SE, ESE, and E. Lava fountains rose up to 500 m above the crater during 27-31 October. Scientists from the Kamchatka Volcanological Station visited the volcano on 28 October and reported that the cinder cone at the summit had grown. They also observed advancing lava on the E flank that extended about 2 km from the summit to 2,700 m elevation, incandescent ejecta 500 m above the crater, and avalanches in the Apakhonchichsky drainage. On 31 October activity intensified, and lava flows were reported moving in the Kretovsky, Kozyrevsky, and Apakhonchichisky drainages on the NW, SW, and SE flanks. At 0930 an ash plume rose to 7 km altitude and at first drifted 169 km SW and then 646 km SE. KVERT reported ash plumes rose to 14 km altitude and extended as far as 1,500 km SSE. The ACC was raised to Red (the highest level on a four-color scale). During 31 October to 1 November ash plumes rose as high as 14 km altitude and drifted as far as 2,255 km ESE.

Activity on 1 November intensified. The lava fountains rose as high as 1 km above the summit (figure 78) and fed the lava flows that were active on the Kretovsky, Kozyrevsky, and Apakhonchichsky drainages on the NW, SW, and SE flanks. Ash plumes rose to 10-14 km altitude and drifted as far as 1,500 km SSE (figure 79). According to the Kamchatka Volcanological Station, observers reported pyroclastic flows descending the flanks. Lahars descended the Studenoy River, blocking the Kozyrevsky-Petropavlovsk federal highway and descended the Krutenkaya River, blocking the road E of Klyuchi. According to news articles the ash plumes caused some flight cancellations and disruptions in the Aleutians, British Columbia (Canada), and along flight paths connecting the Unites States to Japan and South Korea. Ash plumes containing old ash from collapses in the Apakhonchichsky drainage due to phreatic explosions rose to 9.5-9.8 km altitude and drifted 192 km SW at 1400 and to 8.7 km altitude and drifted 192 km SW at 1710 on 1 November.

Figure 78. Photo of the Strombolian activity at Klyuchevskoy accompanied by strong ash plumes taken on 1 November 2023. Photo has been color corrected. Courtesy of Yu Demyanchuk via Volkstat.

Figure 79. Webcam image of an explosive eruption at Klyuchevskoy accompanied by strong ash plumes on 1 November 2023. Courtesy of KB GS RAS, KVERT.

On 2 November ash plumes rose to 6-14 km altitude; the ash plume that rose to 14 km altitude decreased to 6.5 km altitude and drifted NNE by 2000 and continued to drift more than 3,000 km ESE and E. The ACC was lowered to Orange. On 3 November ash plumes rose to 5-8.2 km altitude and drifted 72-538 km ENE, NNE, and ESE; at 0850 an ash plume rose to 6-6.5 km altitude and drifted more than 3,000 km ESE throughout the day. During 4-6 and 8-10 November resuspended ash plumes associated with collapses of old pyroclastic material from the sides of the Apakhonchichsky drainage due to phreatic explosions rose to 4.5-5.5 km altitude and extended 114-258 km NE, ENE, and E. KVERT reported that the eruption stopped on 5 November and the lava flows had begun to cool. Resuspended ash plumes rose to 5-6 km altitude and drifted 60 km E at 0820 on 13 November and to 5 km and 4.5 km altitude at 1110 and 1430 and drifted 140 km E and 150 km ESE, respectively. On 15 November the ACC was lowered to Green.

Activity was relatively low during most of December. On 27 December Strombolian activity resumed based on a thermal anomaly visible in satellite data. On 30 December an ash plume rose to 6 km altitude and extended 195 km NW. The ACC was raised to Orange. On 31 December video and satellite data showed explosions that generated ash plumes that rose to 5-6.5 km altitude and drifted 50-230 km WNW and NW. Though a thermal anomaly persisted through 1 January 2024, no explosions were detected, so the ACC was lowered to Yellow.

Satellite data. Thermal activity was strong throughout the reporting period due to frequent lava fountaining and lava flows. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed strong activity during the entire reporting period, resulting from lava fountaining and lava flows (figure 80). According to data from MODVOLC thermal alerts, a total of 336 hotspots were detected in June (3), July (30), August (11), September (52), October (217), and November (23). Thermal activity was also visible in infrared satellite images, often showing a strong thermal anomaly at the summit crater and a lava flow affecting primarily the SE and SW flanks (figure 81).

Figure 80. Strong thermal activity was detected at Klyuchevskoy during the end of June through early November 2023, according to this MIROVA graph (Log Radiative Power). High levels of activity coincided with lava flows on the SE and SW flanks and Strombolian activity. Courtesy of MIROVA.

Figure 81. Infrared (bands B12, B11, B4) satellite images show a strong thermal anomaly (bright yellow-orange) in the summit crater of Klyuchevskoy, which over time became a lava flow that primarily affected the SE and SW flanks. Lava flows shown here occurred on 31 July 2023 (top right), 27 August 2023 (left middle), 29 September 2023 (right middle), 24 October 2023 (bottom left), and 29 October 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank 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/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Mount Agung, located on the E end of the island of Bali, Indonesia, rises above the SE rim of the Batur caldera. The summit area extends 1.5 km E-W, with the highest point on the W and a steep-walled 800-m-wide crater on the E. Recorded eruptions date back to the early 19th century. A large and deadly explosive and effusive eruption occurred during 1963-64, which was characterized by voluminous ashfall, pyroclastic flows, and lahars that caused extensive damage and many fatalities. More recent activity was documented during November 2017-June 2019 that consisted of multiple explosions, significant ash plumes, lava flows at the summit crater, and incandescent ejecta. This report covers activity reported during April-May 2022 and December 2022 based on data from the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during 2022 was relatively low and mainly consisted of a few ash plumes during April-May and December. An ash plume on 3 April rising to 3.7 km altitude (700 m above the summit) and drifting N was reported in a Darwin VAAC notice based on a ground report, with ash seen in HIMAWARI-8 visible imagery. Another ash plume was reported at 1120 on 27 May that rose to 5.5 km altitude (2.5 m above the summit); the plume was not visible in satellite or webcam images due to weather clouds. An eruption was reported based on seismic data at 0840 on 13 December, with an estimated plume altitude of 3.7 km; however, no ash was seen using satellite imagery in clear conditions before weather clouds obscured the summit.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE rim of the Batur caldera, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

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/).

Saunders is one of eleven islands that comprise the South Sandwich Islands in the South Atlantic. The active Mount Michael volcano has been in almost continuous eruption since November 2014 (BGVN 48:02). Recent activity has resulted in intermittent thermal anomalies and gas-and-steam emissions (BGVN 47:03, 48:02). Visits are infrequent due to its remote location, and cloud cover often prevents satellite observations. Satellite thermal imagery and visual observation of incandescence during a research expedition in 2019 (BGVN 28:02 and 44:08) and a finding confirmed by a National Geographic Society research team that summited Michael in November 2022 reported the presence of a lava lake.

Although nearly constant cloud cover during February 2023 through January 2024 greatly limited satellite observations, thermal anomalies from the lava lake in the summit crater were detected on clear days, especially around 20-23 August 2023. Anomalies similar to previous years (eg. BGVN 48:02) were seen in both MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS instruments and in Sentinel 2 infrared imagery. The only notable sulfur dioxide plume detected near Saunders was on 25 September 2023, with the TROPOMI instrument aboard the Sentinel-5P satellite.

Geologic Background. Saunders Island consists of a large central volcanic edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weather conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

Information Contacts: 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/); 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 (URL: https://dataspace.copernicus.eu/browser).

Tengger Caldera, located at the N end of a volcanic massif in Indonesia’s East Java, consists of five overlapping stratovolcanoes. The youngest and only active cone in the 16-km-wide caldera is Bromo, which typically produces gas-and-steam plumes, occasional ash plumes and explosions, and weak thermal signals (BGVN 44:05, 47:01). This report covers activity during January 2022-December 2023, consisting of mostly white gas-and-steam emissions and persistent weak thermal anomalies. Information was provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and satellite imagery. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay at least 1 km from the crater.

Activity was generally low during the reporting period, similar to that in 2021. According to almost daily images from MAGMA Indonesia (a platform developed by PVMBG), white emissions and plumes rose from 50 to 900 m above the main crater during this period (figure 24). During several days in March and June 2022, white plumes reached heights of 1-1.2 km above the crater.

Figure 24. Webcam image showing a gas-and-steam plume from the Bromo cone in the Tengger Caldera on 2 April 2023. Courtesy of MAGMA Indonesia.

After an increase in activity at 2114 on 3 February 2023, a PVMBG team that was sent to observe white emissions rising as high as 300 m during 9-12 February and heard rumbling noises. A sulfur dioxide odor was also strong near the crater and measurements indicated that levels were above the healthy (non-hazardous) threshold of 5 parts per million; differential optical absorption spectroscopy (DOAS) measurements indicated an average flux of 190 metric tons per day on 11 February. Incandescence originating from a large fumarole in the NNW part of the crater was visible at night. The team observed that vegetation on the E caldera wall was yellow and withered. The seismic network recorded continuous tremor and deep and shallow volcanic earthquakes.

According to a PVMBG press release, activity increased on 13 December 2023 with white, gray, and brown emissions rising as high as 900 m above Bromo’s crater rim and drifting in multiple directions (figure 25). The report noted that tremor was continuous and was accompanied in December by three volcanic earthquakes. Deformation data indicated inflation in December. There was no observable difference in the persistent thermal anomaly in the crater between 11 and 16 December 2023.

Figure 25. Webcam image showing a dark plume that rose 900 m above the summit of the Bromo cone in the Tengger Caldera on 13 December 2023. Courtesy of MAGMA Indonesia.

All clear views of the Bromo crater throughout this time, using Sentinel-2 infrared satellite images, showed a weak persistent thermal anomaly; none of the anomalies were strong enough to cause MODVOLC Thermal Alerts. A fire in the SE part of the caldera in early September 2023 resulted in a brief period of strong thermal anomalies.

Geologic Background. The 16-km-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru volcano. The massive volcanic complex dates back to about 820,000 years ago and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The Ngadisari caldera at the NE end of the complex formed about 150,000 years ago and is now drained through the Sapikerep valley. The most recent of the calderas is the 9 x 10 km wide Sandsea caldera at the SW end of the complex, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, one of Java's most active and most frequently visited volcanoes.

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

Shishaldin is located on the eastern half of Unimak Island, one of the Aleutian Islands. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. The previous eruption ended in May 2020 and was characterized by intermittent thermal activity, increased seismicity and surface temperatures, ash plumes, and ash deposits (BGVN 45:06). This report covers a new eruption during July through November 2023, which consisted of significant explosions, ash plumes, ashfall, and lava fountaining. Information comes from daily, weekly, and special reports from the Alaska Volcano Observatory (AVO) and various satellite data. AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

AVO reported that intermittent tremor and low-frequency earthquakes had gradually become more regular and consistent during 10-13 July. Strongly elevated surface temperatures at the summit were identified in satellite images during 10-13 July. On 11 July AVO raised the Aviation Color Code (ACC) to Yellow (the second color on a four-color scale) and Volcano Alert Level (VAL) to Advisory (the second level on a four-level scale) at 1439. Later in the day on 11 July summit crater incandescence was observed in webcam images. Observations of the summit suggested that lava was likely present at the crater, which prompted AVO to raise the ACC to Orange (the second highest color on a four-color scale) and the VAL to Watch (the second highest level on a four-level scale). The US Coast Guard conducted an overflight on 12 July and confirmed that lava was erupting from the summit. That same day, sulfur dioxide emissions were detected in satellite images.

A significant explosion began at 0109 on 14 July that produced an ash plume that rose to 9-12 km altitude and drifted S over the Pacific Ocean (figure 43). Webcam images and photos taken around 0700 from a ship SW off Unimak Island showed small lahar deposits, which were the result of the interaction of hot pyroclastic material and snow and ice on the flanks. There was also ashfall on the SW and N flanks. A smaller explosion at 0710 generated an ash plume that rose to 4.5 km altitude. Webcam images and pilot reports showed continued low-level ash emissions during the morning, rising to less than 4.6 km altitude; those emissions included a small ash plume near the summit around 1030 resulting from a small explosion.

Figure 43. Photo of a strong ash plume that rose to 9-12 km altitude on the morning of 14 July 2023. Lahar deposits were visible on the SW flank (white arrows). Photo has been color corrected. Courtesy of Christopher Waythomas, AVO.

Seismic tremor amplitude began increasing at around 1700 on 15 July; strongly elevated surface temperatures were also reported. An ash plume rose to 4.6 km altitude and drifted SSE at 2100, based on a satellite image. A continuous ash plume during 2150 through 2330 rose to 5 km altitude and extended 125 km S. At 2357 AVO raised the ACC to Red (the highest color on a four-color scale) and the VAL to Warning (the highest level on a four-level scale), noting that seismicity remained elevated for more than six hours and explosion signals were frequently detected by regional infrasound (pressure sensor) networks. Explosions generated an ash plume that rose to 4.9 km altitude and drifted as far as 500 km SE. Activity throughout the night declined and by 0735 the ACC was lowered to Orange and the VAL to Watch. High-resolution satellite images taken on 16 July showed pyroclastic deposits extending as far as 3 km from the vent; these deposits generated lahars that extended further down the drainages on the flanks. Ash deposits were mainly observed on the SSE flank and extended to the shore of Unimak Island. During 16-17 July lava continued to erupt at the summit, which caused strongly elevated surface temperatures that were visible in satellite imagery.

Lava effusion increased at 0100 on 18 July, as noted in elevated surface temperatures identified in satellite data, increasing seismic tremor, and activity detected on regional infrasound arrays. A significant ash plume at 0700 rose to 7 km altitude and continued until 0830, eventually reaching 9.1 km altitude and drifting SSE (figure 44). As a result, the ACC was raised to Red and the VAL to Warning. By 0930 the main plume detached, but residual low-level ash emissions continued for several hours, remaining below 3 km altitude and drifting S. The eruption gradually declined and by 1208 the ACC was lowered to Orange and the VAL was lowered to Watch. High-resolution satellite images showed ash deposits on the SW flank and pyroclastic deposits on the N, E, and S flanks, extending as far as 3 km from the vent; lahars triggered by the eruption extended farther down the flanks (figure 45). Lava continued to erupt from the summit crater on 19 July.

Figure 44. Photo of an ash-rich plume rising above Shishaldin to 9.1 km altitude on 18 July 2023 that drifted SE. View is from the N of the volcano and Isanotski volcano is visible on the left-hand side of the image. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Figure 45. Near-infrared false-color satellite image of Shishaldin taken on 18 July 2023 showing ash deposits on the N, E, and S flanks extending as far as 3 km from the vent due to recent eruption events. Courtesy of Matthew Loewen, AVO.

Elevated surface temperatures were detected in satellite images during 19-25 July, despite occasional weather cloud cover, which was consistent with increased lava effusion. During 22-23 July satellite observations acquired after the eruption from 18 July showed pyroclastic flow and lahar deposits extending as far as 3 km down the N, NW, and NE flanks and as far as 1.5 km down the S and SE flanks. Ash deposits covered the SW and NE flanks. No lava flows were observed outside the crater. On 22 July a sulfur dioxide plume was detected in satellite data midday that had an estimated mass of 10 kt. In a special notice issued at 1653 on 22 July AVO noted that eruptive activity had intensified over the previous six hours, which was characterized by an hours-long steady increase in seismic tremor, intermittent infrasound signals consistent with small explosions, and an increase in surface temperatures that were visible in satellite data. Pilots first reported low-level ash plumes at around 1900. At 2320 an ash plume had risen to 9 km altitude based on additional pilot reports and satellite images. The ACC was increased to Red and the VAL to Warning at 2343. Satellite images indicated growth of a significantly higher ash plume that rose to 11 km altitude continued until 0030 and drifted NE. During the early morning hours of 23 July ash plumes had declined to 4.6 k altitude. Seismic tremor peaked at 0030 on 23 July and began to rapidly decline at 0109; active ash emissions were no longer visible in satellite data by 0130. The ACC was lowered to Orange and the VAL to Watch at 0418; bursts of increased seismicity were recorded throughout the morning, but seismicity generally remained at low levels. Elevated surface temperatures were visible in satellite data until about 0600. On 24 July pilots reported seeing vigorous gas-and-steam plumes rising to about 3 km altitude; the plumes may have contained minor amounts of ash.

During 24-25 July low level seismicity and volcanic tremor were detected at low levels following the previous explosion on 23 July. Strongly elevated surface temperatures were observed at the summit crater in satellite data. Around 2200 on 25 July seismicity began to increase, followed by infrasound signals of explosions after 0200 on 26 July. An ash plume rose to 3 km altitude at 0500 and drifted ENE, along with an associated sulfur dioxide plume that drifted NE and had an estimated mass of 22 kt. Diffuse ash emissions were visible in satellite data and rose to 6.1-7.6 km altitude and extended 125 km from the volcano starting around 1130. These ash events were preceded by about seven hours of seismic tremor, infrasound detections of explosions, and five hours of increased surface temperatures visible in satellite data. Activity began to decline around 1327, which included low-frequency earthquakes and decreased volcanic tremor, and infrasound data no longer detected significant explosions. Surface temperatures remained elevated through the end of the month.

Seismicity, volcanic tremor, and ash emissions remained at low levels during early August. Satellite images on 1 August showed that some slumping had occurred on the E crater wall due to the recent explosive activity. Elevated surface temperatures continued, which was consistent with cooling lava. On 2 August small explosive events were detected, consistent with low-level Strombolian activity. Some episodes of volcanic tremor were reported, which reflected low-level ash emissions. Those ash emissions rose to less than 3 km altitude and drifted as far as 92.6 km N. Pilots that were located N of the volcano observed an ash plume that rose to 2.7 km altitude. Seismicity began to increase in intensity around 0900 on 3 August. Seismicity continued to increase throughout the day and through the night with strongly elevated surface temperatures, which suggested that lava was active at the surface.

An ash cloud that rose to 7.6-7.9 km altitude and drifted 60-75 km NE was visible in a satellite image at 0520 on 4 August. Pilots saw and reported the plume at 0836 (figure 46). By 0900 the plume had risen to 9.1 km altitude and extended over 100 km NE. AVO raised the ACC to Red and the VAL to Warning as a result. Seismic tremor levels peaked at 1400 and then sharply declined at 1500 to slightly elevated levels; the plume was sustained during the period of high tremor and drifted N and NE. The ACC was lowered to Orange and the VAL to Watch at 2055. During 5-14 August seismicity remained low and surface temperatures were elevated based on satellite data due to cooling lava. On 9 August a small lava flow was observed that extended from the crater rim to the upper NE flank. It had advanced to 55 m in length and appeared in satellite imagery on 11 August. Occasional gas-and-steam plumes were noted in webcam images. At 1827 AVO noted that seismic tremor had steadily increased during the afternoon and erupting lava was visible at the summit in satellite images.

Figure 46. Photo showing an ash plume rising above Shishaldin during the morning of 4 August 2023 taken by a passing aircraft. The view is from the N showing a higher gas-rich plume and a lower gray ash-rich plume and dark tephra deposits on the volcano’s flank. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Strong explosion signals were detected at 0200 on 15 August. An ash cloud that was visible in satellite data extended 100 km NE and may have risen as high as 11 km altitude around 0240. By 0335 satellite images showed the ash cloud rising to 7.6 km altitude and drifting NE. Significant seismicity and explosions were detected by the local AVO seismic and infrasound networks, and volcanic lightning was detected by the World Wide Lightning Location Network (WWLLN). A sulfur dioxide plume associated with the eruption drifted over the S Bering Sea and parts of Alaska and western Canada. Seismicity was significantly elevated during the eruption but had declined by 1322. A pilot reported that ash emissions continued, rising as high as 4.9 km altitude. Elevated surface temperatures detected in satellite data were caused by hot, eruptive material (pyroclastic debris and lava) that accumulated around the summit. Eruptive activity declined by 16 August and the associated sulfur dioxide plume had mostly dissipated; remnants continued to be identified in satellite images at least through 18 August. Surface temperatures remained elevated based on satellite images, indicating hot material on the upper parts of the volcano. Small explosions were detected in infrasound data on the morning of 19 August and were consistent with pilot reports of small, short-lived ash plumes that rose to about 4.3 km altitude. Low-level explosive activity was reported during 20-24 August, according to seismic and infrasound data, and weather clouds sometimes prevented views. Elevated surface temperatures were observed in satellite images, which indicated continued hot material on the upper parts of the volcano.

Seismic tremor began to increase at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with erupting lava. Small explosions were recorded in infrasound data. The ACC was raised to Red and the VAL to Warning at 1204 after a pilot reported an ash plume that rose to 9.1 km altitude. Seismicity peaked at 1630 and began to rapidly decline at around 1730. Ash plumes rose as high as 10 km altitude and drifted as far as 400 km NE. By 2020 the ash plumes had declined to 6.4 km altitude and continued to drift NE. Ash emissions were visible in satellite data until 0000 on 26 August and seismicity was at low levels. AVO lowered the ACC to Orange and the VAL to Watch at 0030. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures were still visible in satellite imagery through the rest of the month. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to 4.5 km altitude and rapidly dissipated. The mass flow was likely caused by the collapse of spatter that accumulated on the summit crater rim.

Similar variable explosive activity was reported in September, although weather observations sometimes prevented observations. A moderate resolution satellite image from the afternoon of 1 September showed gas-and-steam emissions filling the summit crater and obscuring views of the vent. In addition, hot deposits from the previous 25-26 August explosive event were visible on the NE flank near the summit, based on a 1 September satellite image. On 2 and 4 September seismic and infrasound data showed signals of small, repetitive explosions. Variable gas-and-steam emissions from the summit were visible but there was no evidence of ash. Possible summit crater incandescence was visible in nighttime webcam images during 3-4 September.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated at around 0830. A pilot reported an ash plume that rose to 7.6 km altitude at 0842 and continued to rise as high as possibly 9.7 km altitude and drifted SSE based on satellite images (figure 47). The ACC was raised to Red and the VAL to Warning at 0900. In addition to strong tremor and sustained explosions, the eruption produced volcanic lightning that was detected by the WWLLN. Around 1100 seismicity decreased and satellite data confirmed that the altitude of the ash emissions had declined to 7.6 km altitude. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. The ACC was lowered to Orange and the VAL to Watch at 1440. Satellite images showed extensive pyroclastic debris flows on most of the flanks that extended 1.2-3.3 km from the crater rim.

Figure 47. Webcam image taken from the S of Shishaldin showing a vertical ash plume on 5 September 2023. Courtesy of AVO.

During 6-13 September elevated surface temperatures continued to be observed in satellite data, seismicity remained elevated with weak but steady tremor, and small, low-frequency earthquakes and small explosions were reported, except on 12 September. On 6 September a low-level ash plume rose to 1.5-1.8 km altitude and drifted SSE. Occasional small and diffuse gas-and-steam emissions at the summit were visible in webcam images. Around 1800 on 13 September seismic tremor amplitudes began to increase, and small explosions were detected in seismic and infrasound data. Incandescent lava at the summit was seen in a webcam image taken at 0134 on 14 September during a period of elevated tremor. No ash emissions were reported during the period of elevated seismicity. Lava fountaining began around 0200, based on webcam images. Satellite-based radar observations showed that the lava fountaining activity led to the growth of a cone in the summit crater, which refilled most of the crater. By 0730 seismicity significantly declined and remained at low levels.

Seismic tremor began to increase around 0900 on 15 September and rapidly intensified. An explosive eruption began at around 1710, which prompted AVO to raise the ACC to Red and the VAL to Warning. Within about 30 minutes ash plumes drifted E below a weather cloud at 8.2 km altitude. The National Weather Service estimated that an ash-rich plume rose as high as 12.8 km altitude and produced volcanic lightning. The upper part of the ash plume detached from the vent around 1830 and drifted E, and was observed over the Gulf of Alaska. Around the same time, seismicity dramatically decreased. Trace ashfall was reported in the community of False Pass (38 km ENE) between 1800-2030 and also in King Cove and nearby marine waters. Activity declined at around 1830 although seismicity remained elevated, ash emissions, and ashfall continued until 2100. Lightning was again detected beginning around 1930, which suggested that ash emissions continued. Ongoing explosions were detected in infrasound data, at a lower level than during the most energetic phase of this event. Lightning was last detected at 2048. By 2124 the intensity of the eruption had decreased, and ash emissions were likely rising to less than 6.7 km altitude. Seismicity returned to pre-eruption levels. On 16 September the ACC was lowered to Orange and the VAL to Watch at 1244; the sulfur dioxide plume that was emitted from the previous eruption event was still visible over the northern Pacific Ocean. Elevated surface temperatures, gas-and-steam emissions from the vent, and new, small lahars were reported on the upper flanks based on satellite and webcam images. Minor deposits were reported on the flanks which were likely the result of collapse of previously accumulated lava near the summit crater.

Elevated seismicity with tremor, small earthquakes, and elevated surface temperatures were detected during 17-23 September. Minor gas-and-steam emissions were visible in webcam images. On 20 September small volcanic debris flows were reported on the upper flanks. On 21 September a small ash deposit was observed on the upper flanks extending to the NE based on webcam images. Seismic tremor increased significantly during 22-23 September. Regional infrasound sensors suggested that low-level eruptive activity was occurring within the summit crater by around 1800 on 23 September. Even though seismicity was at high levels, strongly elevated surface temperatures indicating lava at the surface were absent and no ash emissions were detected; weather clouds at 0.6-4.6 km altitude obscured views. At 0025 on 24 September AVO noted that seismicity continued at high levels and nearly continuous small infrasound signals began, likely from low-level eruptive activity. Strongly elevated surface temperatures were identified in satellite images by 0900 and persisted throughout the day; the higher temperatures along with infrasound and seismic data were consistent with lava erupting at the summit. Around 1700 similarly elevated surface temperatures were detected from the summit in satellite data, which suggested that more vigorous lava fountaining had started. Starting around 1800 low-level ash emissions rose to altitudes less than 4.6 km altitude and quickly dissipated.

Beginning at midnight on 25 September, a series of seismic signals consistent with volcanic flows were recorded on the N side of the volcano. A change in seismicity and infrasound signals occurred around 0535 and at 0540 a significant ash cloud formed and quickly reached 14 km altitude and drifted E along the Alaska Peninsula. The cloud generated at least 150 lightning strokes with thunder that could be heard by people in False Pass. Seismicity rapidly declined to near background levels around 0600. AVO increased the ACC to Red and the VAL to Warning at 0602. The ash cloud detached from the volcano at around 0700, rose to 11.6 km altitude, and drifted ESE. Trace to minor amounts of ashfall were reported by the communities of False Pass, King Cove, Cold Bay, and Sand Point around 0700. Ash emissions continued at lower altitudes of 6-7.6 km altitude at 0820. Small explosions at the vent area continued to be detected in infrasound data and likely represented low-level eruptive activity near the vent. Due to the significant decrease in seismicity and ash emissions the ACC was lowered to Orange and the VAL to Watch at 1234. Radar data showed significant collapses of the crater that occurred on 25 September. Satellite data also showed significant hot, degassing pyroclastic and lahar deposits on all flanks, including more extensive flows on the ENE and WSW sections below two new collapse scarps. Following the significant activity during 24-25 September, only low-level activity was observed. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease through the end of the month, though tremor and small earthquakes were still reported. No explosive activity was detected in infrasound data through 2 October. Gas-and-steam emissions rose to 3.7 km altitude, as reported by pilots and seen in satellite images. Satellite data from 26 September showed that significant collapses had occurred at the summit crater and hot, steaming deposits from pyroclastic flows and lahars were present on all the flanks, particularly to the ENE and WSW. A small ash cloud was visible in webcam images on 27 September, likely from a collapse at the summit cone. High elevated surface temperatures were observed in satellite imagery during 27-28 September, which were likely the result of hot deposits on the flanks erupted on 25 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September.

During October, explosion events continued between periods of low activity. Seismicity significantly increased starting at around 2100 on 2 October; around the same time satellite images showed an increase in surface temperatures consistent with lava fountaining. Small, hot avalanches of rock and lava descended an unspecified flank. In addition, a distinct increase in infrasound, seismicity, and lightning detections was followed by an ash plume that rose to 12.2 km altitude and drifted S and E at 0520 on 3 October, based on satellite images. Nighttime webcam images showed incandescence due to lava fountaining at the summit and pyroclastic flows descending the NE flank. AVO reported that a notable explosive eruption started at 0547 and lasted until 0900 on 3 October, which prompted a rise in the ACC to Red and the VAL to Warning. Subsequent ash plumes rose to 6-7.6 km altitude by 0931. At 1036 the ACC was lowered back to Orange and the VAL to Watch since both seismic and infrasound data quieted substantially and were slightly above background levels. Gas-and-steam emissions were observed at the summit, based on webcam images. Trace amounts of ashfall were observed in Cold Bay. Resuspended ash was present at several kilometers altitude near the volcano. During the afternoon, low-level ash plumes were visible at the flanks, which appeared to be largely generated by rock avalanches off the summit crater following the explosive activity. These ash plumes rose to 3 km altitude and drifted W. Trace amounts of ashfall were reported by observers in Cold Bay and Unalaska and flights to these communities were disrupted by the ash cloud. Satellite images taken after the eruption showed evidence of pyroclastic flows and lahar deposits in drainages 2 km down the SW flank and about 3.2 km down the NE flank, and continued erosion of the crater rim. Small explosion craters at the end of the pyroclastic flows on the NE flank were noted for the first time, which may have resulted from gas-and-steam explosions when hot deposits interact with underlying ice.

During 4 October seismicity, including frequent small earthquakes, remained elevated, but was gradually declining. Ash plumes were produced for over eight hours until around 1400 that rose to below 3.7 km altitude. These ash plumes were primarily generated off the sides of the volcano where hot rock avalanches from the crater rim had entered drainages to the SW and NE. Two explosion craters were observed at the base of the NE deposits about 3.2 km from the crater rim. Webcam images showed the explosion craters were a source of persistent ash emissions; occasional collapse events also generated ash. Seismicity remained elevated with sulfur dioxide emissions that had a daily average of more than 1,000 tons per day, and frequent small earthquakes through the end of the month. Frequent elevated surface temperatures were identified in satellite images and gas-and-steam plumes were observed in webcam images, although weather conditions occasionally prevented clear views of the summit. Emissions were robust during 14-16 October and were likely generated by the interaction of hot material and snow and ice. During the afternoon of 21 October a strong gas-and-steam plume rose to 3-4.6 km altitude and extended 40 km WSW, based on satellite images and reports from pilots. On 31 October the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Activity in November was characterized by elevated seismicity with ongoing seismic tremor and small, low-frequency earthquakes, elevated surface temperatures, and gas-and-steam emissions. There was an increase in seismic and infrasound tremor amplitudes starting at 1940 on 2 November. As a result, the ACC was again raised to Orange and the VAL was increased to Watch, although ash was not identified in satellite data. An ash cloud rose to 6.1 km altitude and drifted W according to satellite data at 2000. By 0831 on 3 November ash emissions were no longer visible in satellite images. On 6 and 9 November air pressure sensors detected signals consistent with small explosions. Small explosions were detected in infrasound data consistent with weak Strombolian activity on 19 and 21 November. Seismicity started to decrease on 21 November. On 25 November gas-and-steam emissions were emitted from the vent as well as from a scarp on the NE side of the volcano near the summit. A gas-and-steam plume extended about 50 km SSE and was observed in satellite and webcam images on 26 November. On 28 November small explosions were observed in seismic and local infrasound data and gas-and-steam emissions were visible from the summit and from the upper NE collapse scarp based on webcam images. Possible small explosions were observed in infrasound data on 30 November. Weakly elevated surface temperatures and a persistent gas-and-steam plume from the summit and collapse scarps on the upper flanks. A passing aircraft reported the gas-and-steam plume rose to 3-3.4 km altitude on 30 November, but no significant ash emissions were detected.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed a strong pulse of thermal activity beginning in July 2023 that continued through November 2023 (figure 48). This strong activity was due to Strombolian explosions and lava fountaining events at the summit crater. According to data from MODVOLC thermal alerts, a total of 101 hotspots were detected near the summit crater in July (11-14, 16-19, 23-24 and 26), August (4, 25-26, and 29), September (5, 12, and 17), and October (3, 4, and 8). Infrared satellite data showed large lava flows descending primarily the northern and SE flanks during the reporting period (figure 49). Sulfur dioxide plumes often exceeded two Dobson Units (DUs) and drifted in different directions throughout the reporting period, based on satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 50).

Figure 48. Graph of Landsat 8 and 9 OLI thermal data from 1 June 2024 showing a strong surge in thermal activity during July through November 2023. During mid-October, the intensity of the hotspots gradually declined. Courtesy of MIROVA.

Figure 49. Infrared (bands B12, B11, B4) satellite images show several strong lava flows (bright yellow-orange) affecting the northern and SE flanks of Shishaldin on 18 July 2023 (top left), 4 June 2023 (top right), 26 September 2023 (bottom left), and 3 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 50. Strong sulfur dioxide plumes were detected at Shishaldin and drifted in different directions on 15 August 2023 (top left), 5 September 2023 (top right), 25 September 2023 (bottom left), and 6 October 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

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

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ioto (Iwo-jima), located about 1,200 km S of Tokyo, lies within a 9-km-wide submarine caldera along the Izu-Bonin-Mariana volcanic arc. Previous eruptions date back to 1889 and have consisted of dominantly phreatic explosions, pumice deposits during 2001, and discolored water. A submarine eruption during July through December 2022 was characterized by discolored water, pumice deposits, and gas emissions (BGVN 48:01). This report covers a new eruption during October through December 2023, which consisted of explosions, black ejecta, discolored water, and floating pumice, based on information from the Japan Meteorological Association (JMA), the Japan Coast Guard (JCG), and satellite data.

JMA reported that an eruption had been occurring offshore of Okinahama on the SE side of the island since 21 October, which was characterized by volcanic tremor, according to the Japan Maritime Self-Defense Force (JMSDF) Iwo Jima Air Base (figure 22). According to an 18 October satellite image a plume of discolored water at the site of this new eruption extended NE (figure 23). During an overflight conducted on 30 October, a vent was identified about 1 km off the coast of Okinahama. Observers recorded explosions every few minutes that ejected dark material about 20 m above the ocean and as high as 150 m. Ejecta from the vent formed a black-colored island about 100 m in diameter, according to observations conducted from the air by the Earthquake Research Institute of the University of Tokyo in cooperation with the Mainichi newspaper (figure 24). Occasionally, large boulders measuring more than several meters in size were also ejected. Observations from the Advanced Land Observing Satellite Daichi-2 and Sentinel-2 satellite images also confirmed the formation of this island (figure 23). Brown discolored water and floating pumice were present surrounding the island.

Figure 22. Map of Ioto showing the locations of recorded eruptions from 1889 through December 2023. The most recent eruption occurred during October through December 2023 and is highlighted in red just off the SE coast of the island and E of the 2001 eruption site. A single eruption highlighted in green was detected just off the NE coast of the island on 18 November 2023. From Ukawa et al. (2002), modified by JMA.

Figure 23. Satellite images showing the formation of the new island formation (white arrow) off the SE (Okinahama) coast of Ioto on 18 October 2023 (top left), 27 November 2023 (top right), 2 December 2023 (bottom left), and 12 December 2023 (bottom right). Discolored water was visible surrounding the new island. By December, much of the island had been eroded. Courtesy of Copernicus Browser.

Figure 24. Photo showing an eruption off the SE (Okinahama) coast of Ioto around 1230 on 30 October 2023. A column of water containing black ejecta is shown, which forms a new island. Occasionally, huge boulders more than several meters in size were ejected with the jet. Dark brown discolored water surrounded the new island. Photo has been color corrected and was taken from the S by the Earthquake Research Institute, University of Tokyo in cooperation of Mainichi newspaper. Courtesy of JMA.

The eruption continued during November. During an overflight on 3 November observers photographed the island and noted that material was ejected 169 m high, according to a news source. Explosions gradually became shorter, and, by the 3rd, they occurred every few seconds; dark and incandescent material were ejected about 800 m above the vent. On 4 November eruptions were accompanied by explosive sounds. Floating, brown-colored pumice was present in the water surrounding the island. There was a brief increase in the number of volcanic earthquakes during 8-14 November and 24-25 November. The eruption temporarily paused during 9-11 November and by 12 November eruptions resumed to the W of the island. On 10 November dark brown-to-dark yellow-green discolored water and a small amount of black floating material was observed (figure 25). A small eruption was reported on 18 November off the NE coast of the island, accompanied by white gas-and-steam plumes (figure 23). Another pause was recorded during 17-19 November, which then resumed on 20 November and continued erupting intermittently. According to a field survey conducted by the National Institute for Disaster Prevention Science and Technology on 19 November, a 30-m diameter crater was visible on the NE coast where landslides, hot water, and gray volcanic ash containing clay have occurred and been distributed previously. Erupted blocks about 10 cm in diameter were distributed about 90-120 m from the crater. JCG made observations during an overflight on 23 November and reported a phreatomagmatic eruption. Explosions at the main vent generated dark gas-and-ash plumes that rose to 200 m altitude and ejected large blocks that landed on the island and in the ocean (figure 26). Discolored water also surrounded the island. The size of the new island had grown to 450 m N-S x 200 m E-W by 23 November, according to JCG.

Figure 25. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 10 November showing discolored water and a small amount of black floating material were visible surrounding the island. Photo has been color corrected. Photographed by JCG courtesy of JMA.

Figure 26. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 23 November showing a phreatomagmatic eruption that ejected intermittent pulses of ash and dark material that rose to 200 m altitude. Photo has been color corrected. Photographed by JCG courtesy of JMA.

The eruption continued through 11 December, followed by a brief pause in activity, which then resumed on 31 December, according to JMA. Intermittent explosions produced 100-m-high black plumes at intervals of several minutes to 30 minutes during 1-10 December. Overflights were conducted on 4 and 15 December and reported that the water surrounding the new island was discolored to dark brown-to-dark yellow-green (figure 27). No floating material was reported during this time. In comparison to the observations made on 23 November, the new land had extended N and part of it had eroded away. In addition, analysis by the Geospatial Information Authority of Japan using SAR data from Daichi-2 also confirmed that the area of the new island continued to decrease between 4 and 15 December. Ejected material combined with wave erosion transformed the island into a “J” shape, 500-m-long and with the curved part about 200 m offshore of Ioto. The island was covered with brown ash and blocks, and the surrounding water was discolored to greenish-brown and contained an area of floating pumice. JCG reported from an overflight on 4 December that volcanic ash-like material found around the S vent on the NE part of the island was newly deposited since 10 November (figure 28). By 15 December the N part of the “J” shaped island had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands (figure 27).

Figure 27. Photos of the new island formed off the SE (Okinahama) coast of Ioto on 4 December 2023 (left) and 15 December 2023 (right). No gas-and-ash emissions or lava flows were observed on the new land. Additionally, dark brown-to-dark yellow-green discolored water was observed surrounding the new land. During 4 and 15 December, the island had eroded to where the N part of the “J” shape had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands. Courtesy of Copernicus Browser.

Figure 28. Photo of new volcanic ash-deposits (yellow dashed lines) near the S vent on the NE coast of Ioto taken by JCG on 4 December 2023. White gas-and-steam emissions were also visible (white arrow). Photo has been color corrected. Courtesy of JMA.

References. Ukawa, M., Fujita, E., Kobayashi, T., 2002, Recent volcanic activity of Iwo Jima and the 2001 eruption, Monthly Chikyu, Extra No. 39, 157-164.

Geologic Background. Ioto, in the Volcano Islands of Japan, lies within a 9-km-wide submarine caldera. The volcano is also known as Ogasawara-Iojima to distinguish it from several other "Sulfur Island" volcanoes in Japan. The triangular, low-elevation, 8-km-long island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks that are more alkalic than those of other volcanoes in this arc. The island has undergone uplift for at least the past 700 years, accompanying resurgent doming of the caldera; a shoreline landed upon by Captain Cook's surveying crew in 1779 is now 40 m above sea level. The Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama. Numerous recorded phreatic eruptions, many from vents on the W and NW sides of the island, have accompanied the uplift.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/GIJUTSUKOKUSAI/kaiikiDB/kaiyo22-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Asahi, 5-3-2, Tsukiji, Chuo Ward, Tokyo, 104-8011, Japan (URL: https://www.asahi.com/ajw/articles/15048458).

[The following was originally included within the Masaya report, not as a stand-alone report about Apoyo.]

July 2000 seismicity near Masaya and Laguna de Apoyo. During July 2000 there were over 300 earthquakes near Laguna de Apoyo (Apoyo volcano) and Masaya. The earthquakes, determined to be of tectonic rather than volcanic origin, caused surficial damage at both volcanoes.

At 1329 on 6 July a small M 2 earthquake occurred near the N rim of Laguna de Apoyo that was followed at 1330 by a M 5.4 earthquake (figure 1). It was located ~32 km SE of Managua, at 11.96°N, 86.02°E, with a focal depth less than 1 km (figure 2). The earthquake was felt in most of Nicaragua and was most strongly felt in the cities of Managua (Modified Mercalli V-VI) and Masaya (VI), and in the region near Laguna de Apoyo (maximum intensity of VII or VIII). The earthquake caused numerous landslides down the volcano's crater walls and surface faulting was observed. In towns located in the epicentral zone, trees and electric lines fell and many houses were partially or totally destroyed. About 70 people were injured and four children were killed by collapsing walls or roofs of homes. At Masaya volcano, ~8 km from the epicenter, there were minor collapses of Santiago crater's walls. No change in degassing was observed at the volcano.

Figure 1. Seismogram showing the M 2 and M 5.4 earthquakes near the Masaya volcano station on 6 July 2000. Courtesy of INETER.

Figure 2. Epicenters near Masaya for the M 5.4 earthquake on 6 July, and the M 4.8 earthquake on 25 July 2000 (stars). The aftershocks from these earthquakes are also shown (small circles). Courtesy of INETER.

Immediately after the earthquake there were many smaller, shallow earthquakes in a zone that includes the area between Masaya, Laguna de Apoyo, and W of Granada (figure 2). In the epicentral zone property was destroyed, cracks opened in the ground, landslides occurred, and trees fell. Several landslides occurred at the edges and steep walls of Laguna de Apoyo. A large number of earthquakes continued until 10 July (figure 3 and table 1). The number of earthquakes then diminished until 1554 on 25 July when a M 4.8 earthquake took place, initiating a series of smaller earthquakes that lasted until about 27 July.

Figure 3. Graph showing the number of earthquakes in the Masaya region between 4 and 30 July 2000. Courtesy of INETER.

Table 1. A summary of earthquakes in vicinity of Masaya and Laguna de Apoyo in early July 2000. Courtesy of INETER.

The July earthquakes were the most destructive seismic events since the 1972 Managua earthquake. The epicentral zone of the July 2000 earthquakes correlates with the same active zones of past earthquakes, which are caused by fault movement between the Cocos and Caribbean plates.

Geologic Background. The scenic 7-km-wide, lake-filled Apoyo caldera is a large silicic volcanic center immediately SE of Masaya caldera. The surface of Laguna de Apoyo lies only 78 m above sea level; the steep caldera walls rise about 100 m to the eastern rim and up to 500 m to the western rim. An early shield volcano constructed of basaltic-to-andesitic lava flows and small rhyodacitic lava domes collapsed following two major dacitic explosive eruptions. The caldera-forming eruptions have been radiocarbon dated between about 21,000-25,000 years before present. Post-caldera ring-fracture eruptions of uncertain age produced lava flows below the scalloped caldera rim. The slightly arcuate, N-S-trending La Joya fracture system that cuts the eastern flank of the caldera only 2 km east of the caldera rim is a younger regional fissure system structurally unrelated to Apoyo caldera.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/).

During January to July 2000 Arenal's outbursts generally remained low but included frequent pyroclastic explosions, gas emissions, and avalanches. In late August explosions spawned a pyroclastic flow that injured three people several kilometers from the crater; two later died. Three days later a small airplane crashed into the volcano.

In January through parts of June crater C continued its usual activities, consisting of a constant gas emission, sporadic Strombolian eruptions, and occasional incandescent avalanches. Crater D exhibited fumarolic activity. The lava continued to flow variously toward the NNE, E, SE flanks. The NE and SE flank were continually affected by acid rain and pyroclastic material that contributed to the destruction of the vegetation on these flanks, resulting in major erosion that created small avalanches on the rivers Calle de Arenas, Manolo, Guillermina, and Agua Caliente.

The EDM network (established along the subradial lines) continued to show an average annual contraction of 7-10 ppm. The dry inclinometers ("dry tilt") showed variations in the radial component, deflation at ~5 µrad per year.

During the last half of April and throughout May eruptive activity increased, but few ash columns rose to ~500 m over crater C. The columns of ash were carried by the predominating winds toward the NW and SE flank causing both acid rain and ash fall. In May a narrow channel of lava began to flow toward the NNE flank. It later widened into a fan burning vegetation on the N and NE flanks.

From April to May the seismometer detected an increase in both number of eruptions and the hours of tremor. On 16 May two MR 3 earthquakes were recorded and located on the flanks of the volcano at 2 and 5 km from the summit. These earthquakes were reported to be felt in La Fortuna, 6.5 km NE of the volcano.

Eruptive activity remained low in June; few eruption clouds rose more than 500 m over crater C. In July crater C continued with the emission of gases, lava flows, sporadic Strombolian eruptions, and occasional pyroclastic flows. The eruptive activity increased in July with respect to June, although the number of eruptions, their intensity, and the quantity of pyroclastic material ejected remained low.

In August Arenal became more active and underwent a series of explosions. One began at 0945 on 23 August; mutiple pyroclastic flows came down the volcano's NE side (figure 89) as a series of pulses. Pulses occurred at 0955, 0956, and 0958. The most important pulse occurred at 1001 and continued for six minutes. Two more pulses followed at 1008 and 1012. For the next two hours activity returned to normal, but at 1323 a new series of explosions began. At 1336 a pyroclastic flow began and lasted for ten minutes. Various pulses descended the NNE flank. Normal low-level activity resumed 19 hours after the afternoon explosions.

Figure 89. A map of Arenal and vicinity showing the distribution of deposits from the 23 August pyroclastic flows (N-directed swath of dark-gray color). The light gray shows the lava field formed by past eruptions. Courtesy of Rafael Barquero (OSIVAM).

News reports. One of the pyroclastic flows on 23 August engulfed a Costa Rican tour guide and two tourists from the United States. OVSICORI-UNA stated that the victims were burnt by the front of the flow ~2.3 km from the crater. According to a local volcanologist, the flow was traveling at 80 km/hour at that point.

The three victims were sent to San José to be treated for their burns and injuries. On the night of 23 August the tour guide died in the hospital. An 8-year-old girl from Massachusetts died on 6 September as a result of her burns.

The National Emergency Commission (NEC) ordered evacuations of the tourist centers of Los Lagos, the Tabacón hot springs and resort, Hotel Montaña del Fuego, Arenal Lodge, and other areas. The NEC and Red Cross workers evacuated 600 tourists and residents and closed the route around the volcano to Tilarán. On 24 August the volcano returned to its normal behavior. The 23 August explosive eruptions were believed to be the strongest since the deadly 1968 eruption.

On 26 August a ten-passenger airplane crashed into the NE flank ~200 m below the summit. All of the occupants died. The cause of the crash is unknown at this point and no further details are available.

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: Observatorio Vulcanologico y Sismologico de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica; Oficina de Sismología y Vulcanología del Arenal y Miravalles (OSIVAM), Instituto Costarricense de Electricidad (ICE), Apartado 10032-1000, San José, Costa Rica; The Tico Times (URL: http://www.ticotimes.net/); La Nacion (URL: http://www.nacion.co.cr/).

During 14 February to 4 March 2000 an eruption occurred at Piton de la Fournaise that was briefly mentioned in a previous report (BGVN 25:01) and is discussed here in more detail. After 4 March through May, there was no volcanic activity and seismicity was low with 1-2 events per month. On 23 June volcanism recommenced with an eruption that lasted more than a month.

Eruption of 14 February 2000. Three and a half months after its previous eruption (BGVN 24:09), Piton de la Fournaise erupted on 14 February. Throughout January, seismicity was well above normal levels until the beginning of February when a relative lull in seismicity lasted for two weeks (figure 50). At 2314 on 13 February a seismic crisis began that lasted 64 minutes. A total of 261 earthquakes occurred with magnitudes up to 1.9. The deepest events were localized at sea level, just below Dolomieu summit crater (figure 51).

Figure 50. Seismic events at Piton de la Fournaise during December 1999- February 2000 shown as a series of five day averages. Heightened activity occurred through January, and a relative lull in activity occurred two weeks prior to the eruption on 14 February. Seismic information was not available for the beginning of the eruption (February 14-24). Courtesy of OVPDLF.

Figure 51. Map of the N flank of Piton de la Fournaise showing the lava flows from the 14 February 2000 eruption (black), fissure vents (white lines within the flow), and the major features associated with the flow. Note Dolomieu summit crater at lower edge of the map. Courtesy of OVPDLF.

On 13 February, three minutes after the beginning of the seismic crisis, the first significant variations in deformation were recorded at 2317 and 2320, on radial and tangential components, respectively, by the "Dolomieu Sud" tiltmeter station. After initial deformation was observed, tiltmeter and extensometer stations at "Soufriere," "Bory," "Tunnel Catherine," and "Flanc Est" (figure 52) registered variations, with up to 270 µrad recorded for the "Soufriere tiltmeter" radial component. The intrusion of magma caused inflation under the summit crater. The inflation center started S of Dolomieu summit crater, migrated below Dolomieu, and then traveled to the N flank of the volcano where several vents opened (figure 53). At 0018 on 14 February, tremors registered at all of the seismic stations marking the beginning of the eruption.

Figure 52. Map showing the location of radon, deformation, magnetic, and seismic stations on Piton de la Fournaise in February 2000. Courtesy of OVPDLF.

Figure 53. During the 14 February 2000 eruption at Piton de la Fournaise the center of inflation migrated. The incenter of inflation was calculated on 5-minute intervals and plotted on this sketch map. The center of inflation was estimted based on the shift of deformation vectors over time. Courtesy of OVPDLF.

Inclement weather produced by cylone Eline passing 200 km N of Reunion inhibited visual observations for several days. After that, scientists found that several en echelon fissures were localized on the N flank starting at 2,490 m elevation (white lines within black lava flows, figure 53). An aa flow inundated the "Puy Mi-Côte" crater, passed to the W and E of the crater, and continued in the direction of "Piton Partage." Both vents were inactive at the time of observation. Eruptive activity was concentrated on a vent 300 m E of Puy Mi-Côte, where stable 20- to 30-m-high fountains were observed from a new crater, whose rim grew to 20 m high at that time. A second, much smaller crater was active about 100 m above the main crater. A large aa lava flow and meter-sized blocks descended in the direction of "Piton Kapor" (site of the 1998 eruption), then joined the first lava flow and followed the "rempart Fouqué" to the E. This lava flow terminated about 4 km away at 1,950 m altitude near "Nez Coupé de Saint Rose." Beginning on 24 February a large number of small pahoehoe lava flows were observed. For several hours on 4 March a large number of gas-piston events were observed and then at 1800 tremor stopped, marking the end of the eruption.

Retrospective analysis revealed that the initial aa lava flow represented most of the erupted material. The lava was particularly irregular with scoria that ranged in size from tens of centimeters to meter-sized blocks. Pahoehoe flows from the 24 February phase of the eruption partly covered the aa lava that was emitted earlier. The entire lava flow covered an area of about 1.3 x 10⁶ m² and comprised a total volume of about 4 x 10⁶ m³ of aphyric basalt. The main new crater was called "Piton Célimène" (figure 53).

Eruption of 23 June 2000. Beginning in June, long-term deformation was observed at several stations near the volcano. Since the beginning of the month up to 0.1 mm of inflation took place at the "Soufrière" extensometer (figure 52). Starting on 12 June clear inflation of up to 70 µrad was observed at the "Dolomieu Sud" tiltmeter. After 20 June inflation of up to 20 µrad was observed at the "Château Fort" tiltmeter. The Château Fort extensometer showed variations in opening, shear, and vertical movement components.

Seismicity increased during 9-14 June with twelve deep earthquakes ~6 km below the W flank. During 15-21 June seismicity drastically increased with 2, 2, 4, 10, 29, 69, and 101 earthquakes recorded on successive days (figure 54). All of these earthquakes occurred below Dolomieu summit crater, with focal depths between sea level and 1 km above sea level. They had magnitudes up to 1.8 that increased with the number of earthquakes recorded. During the same time period, five deep earthquakes also occurred.

Figure 54. The number of daily seismic events recorded at two seismic stations at Piton de la Fournaise during 1 June through 6 July 2000. Courtesy of OVPDLF.

During 0600-0640 on 22 June, following 50 seismic events, there was a small seismic crisis that consisted of 36 low-energy seismic events. For 36 hours after the seismic crisis only very low-energy earthquakes occurred. At 1650 on 23 June another seismic crisis took place (figure 54). It consisted of about 300 earthquakes, including some greater than M 2 and possibly as high as M 2.5. Some of the earthquakes were recorded at the seismic station in Cilaos, more than 30 km from the volcano.

During the seismic crisis one shallow earthquake was centered under the E flank of the volcano. Around this time the observatory's tiltmeter network showed uplift of the central part of the volcano to over 200 µrad. The inferred effect of an intrusion was first localized under the summit region, then shifted to the SE. At 1800 eruption tremor began, and tremor localization suggested the eruption site was on the SE flank between "Signal de l' Enclose" and "Château Fort" craters between 1.9 and 2.2 km elevation. Figure 55 shows these named locations and the actual fissure vent and extent of lava flows.

Figure 55. Map and image composite of the 23 June 2000 lava flows on the E flank of Piton de la Fournaise. Courtesy of OVPDLF.

According to the observatory staff, the 23 June eruption began with the formation of a short-lived, 500-m-long, SE-trending fissure on the SE flank at an elevation of ~2,100 m (figure 55). A second, 200-m-long, ESE trending vent also formed on the SE flank at ~1,800 m. About eight lava fountains initially rose up to 50 m above the second vent. In addition, a 300-m-long aa lava flow traveled down the "Grandes Pentes" to an elevation of 580 m. About two days after the eruption began, the intensity of the lava fountains decreased, and the crater rim reached a height of 10-15 m.

Within 24 hours after the onset of the eruption, tremor rapidly decreased to less than 10% of the initial value. Unlike typical eruptions at Piton de la Fournaise, seismicity under the central crater continued for the first five days of the eruption. During 24-28 June there were 26, 22, 17, 17 and six seismic events, respectively, up to M 2.5. Similar seismic events occurred during eruptions in 1986, 1988, and 1998; in two cases they preceded the formation of new vents. However, no new vents formed during 24-28 June. After 29 June no seismic events were recorded, and starting on 27 June there was an increase in tremors that remained around initial levels and lasted three weeks. Throughout most of the eruption there was a lava lake in the eruption crater and several meter-sized lava flows emerged at its base reaching up to 300-400 m below the crater. Lava samples were collected during the eruption, and a lava temperature of 1,160°C was measured several times using a thermocouple.

On 30 July the eruption stopped after 37 days of activity. The initial flow was entirely aa lava, while the later outspreading lava flows were aa and pahoehoe lava. The entire lava flow covered an area of ~3 x 10² m² and comprised a total volume of ~1 x 10⁷ m³. The final crater was 26 m high and was named "Piton Pârvédi."

Geologic Background. Piton de la Fournaise is a massive basaltic shield volcano on the French island of Réunion in the western Indian Ocean. Much of its more than 530,000-year history overlapped with eruptions of the deeply dissected Piton des Neiges shield volcano to the NW. Three scarps formed at about 250,000, 65,000, and less than 5,000 years ago by progressive eastward slumping, leaving caldera-sized embayments open to the E and SE. Numerous pyroclastic cones are present on the floor of the scarps and their outer flanks. Most recorded eruptions have originated from the summit and flanks of Dolomieu, a 400-m-high lava shield that has grown within the youngest scarp, which is about 9 km wide and about 13 km from the western wall to the ocean on the E side. More than 150 eruptions, most of which have produced fluid basaltic lava flows, have occurred since the 17th century. Only six eruptions, in 1708, 1774, 1776, 1800, 1977, and 1986, have originated from fissures outside the scarps.

Information Contacts: Thomas Staudacher, Nicolas Villeneuve, Jean Louis Cheminée, Kei Aki, Jean Battaglia, Philippe Catherine, Valérie Ferrazzini, and Philippe Kowalski, Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, Institut National des Sciences de l'Univers, 14 RN3 - Km 27, 97418 La Plaine des Cafres, Réunion, France (URL: http://www.ipgp.fr/fr/ovpf/observatoire-volcanologique-piton-de-fournaise).

This report covers April through June 2000. Activity remained at a low level in April. From visual observation reports received only up to 9 April, Crater 2 periodically gently released moderate to thick ash clouds. However, on 5 and 9 April, the ash clouds were released more forcefully and with rumbling sounds. These ash clouds rose 1-2 km above the summit before being blown SE. Crater 3 released light white vapor throughout the month.

Visual observations were next reported after 16 June. Crater 2 produced thick, white ash clouds in moderate volume. On 23 and 24 June, these clouds were accompanied by blue vapor. On 16 and 18 June, rumbling noises were heard. Crater 3 was inactive in June with the exception of a weak trail of thin white vapor escaping on 16 June.

The seismograph remained non-operational throughout the entire reporting period.

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, D. Lolok, K. Mulina, and F. Taranu, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

This report covers April-June 2000. Inflation that began in January 2000 (BGVN 25:03) peaked in early April. By mid-April the water-tube tiltmeter 4 km SW of the summit detected a 2.5 µrad decrease in tilt. By the end of April the tilt had recovered 1.5 µrad. Emissions from both the summit craters, Main and Southern, consisted of gentle releases of light to moderate volumes of white vapor. Seismicity remained low with the number of events ranging from 500 to 1,200 events a day. Seismic amplitude measurements were steady at background levels.

During May, Manam continued to produce varying amounts of white vapor from both craters. Rabaul Volcanic Observatory (RVO) characterized the seismicity as normal. Tiltmeter readings showed no particular trend.

Throughout June, Main Crater released light to moderate volumes of white vapor. However, during 3-4 June, Southern crater increased in activity.

At 1235 on 3 June, an explosive eruption produced thick, dark ash clouds and produced fine-ash and scoria deposits at Yassa village, W of the summit. The ash clouds reached an altitude of 1-1.2 km. The initial explosion was followed by light to moderate release of ash. At 0004 on 4 June, booming sounds lasting 1-2 minutes were accompanied by the ejection of glowing lava fragments. These fragments fell in the SW valley and had free fall times (FFT) of 5-10 s. Some weak to low fluctuating night time glows were visible during the intervals between lava fragment ejections. Prior to and after the events of 3-4 June, Southern crater produced light amounts of white vapor.

Although there were no water-tiltmeter readings after 19 June, the values taken 4 km S of the crater showed an inflation of 10 µrad from 1-19 June. Since December 1999, there has been an overall inflation of 16 µrad. There were no seismic readings during 1-10 June. Low-level seismicity the remainder of the month had counts ranging from 600-1,360 a day. Seismic amplitude measurements were relatively steady at normal background levels.

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, D. Lolok, K. Mulina, and F. Taranu, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Since the last report on Masaya, of continued degassing and marked gravity decreases (BGVN 24:04), there have been sporadic reports about its activity, which are summarized below prior to discussion of a nearby M 5.4 earthquake on 6 July 2000.

Reports of ash-and-steam emissions. Between November 1999 and January 2000 there were several reports from the Washington VAAC of ash-and-steam emissions from Masaya. On 22 November 1999 the VAAC reported that GOES-8 imagery suggested that Masaya may have awakened. Satellite imagery showed activity at or very near Masaya, including a plume of ash or "smoke" moving to the WSW, and a hotspot that was visible for over two hours. At about 1600 the imagery suggested that an explosion may have occurred and by 1615 the resultant plume was at ~800 m (near Masaya's summit), and had been blown WSW.

On 22 December 1999 the Washington VAAC issued an ash advisory stating that a continuous low-level plume was being emitted from Masaya. Volcanic activity was confirmed by INETER who noted that seismic activity was consistent with ash emissions. The cloud was ~2 km in altitude and was blown to the WSW.

On 18 January 2000 the VAAC reported that GOES-8 imagery through 0845 detected a low-level thin ash plume from Masaya's summit. The plume reached an altitude of ~900 m, was blown to the SW, and rapidly dissipated.

Seismic activity during April 1999-March 2000. Seismic activity at the volcano remained low with eight microearthquakes registered for the month. The RSAM (seismic tremor) stayed at ~30 units. During the first two weeks of April the RSAM signal was not obtained due to technical problems in the seismic power station. On 23 April two explosions were detected by RSAM, which were confirmed by observers at the Masaya Volcano National Park. In that case, RSAM began to show a small increase until 0800, and an hour later the two explosions occurred.

May 1999: The number of microearthquakes was 21 for the month. The RSAM stayed at ~24 units. June: The number of microearthquakes was 18 for the month. The RSAM stayed at ~24 units. August: The number of microearthquakes was 47 for the month. The RSAM remained at ~40 units. Constant gas emissions occurred. September: The number of microearthquakes was 87 for the month. The RSAM stayed constant at ~40 units. Constant gas emissions occurred. October: The number of microearthquakes was 22 for the month. The RSAM stayed constant at ~20 units. Constant gas emissions occurred. November: There were 49 microearthquakes for the month. The RSAM stayed constant. Constant gas emissions occurred. December: Twenty one earthquakes were registered for the month. The RSAM stayed constant.

January 2000: Eleven earthquakes were registered for the month. The RSAM stayed constant. At 1145 on 6 January an explosion occurred in Santiago crater. February: Six microearthquakes and the RSAM remained constant. March: There were three microearthquakes for the month. The RSAM was at a similar level as the previous month.

July 2000 seismicity near Masaya and Laguna de Apoyo. During July 2000 there were over 300 earthquakes near Laguna de Apoyo (Apoyo volcano) and Masaya. The earthquakes, determined to be of tectonic rather than volcanic origin, caused surficial damage at both volcanoes.

At 1329 on 6 July a small M 2 earthquake occurred near the N rim of Laguna de Apoyo that was followed at 1330 by a M 5.4 earthquake (figure 10). It was located ~32 km SE of Managua, at 11.96°N, 86.02°E, with a focal depth less than 1 km (figure 11). The earthquake was felt in most of Nicaragua and was most strongly felt in the cities of Managua (Modified Mercalli V-VI) and Masaya (VI), and in the region near Laguna de Apoyo (maximum intensity of VII or VIII). The earthquake caused numerous landslides down the volcano's crater walls and surface faulting was observed. In towns located in the epicentral zone, trees and electric lines fell and many houses were partially or totally destroyed. About 70 people were injured and four children were killed by collapsing walls or roofs of homes. At Masaya volcano, ~8 km from the epicenter, there were minor collapses of Santiago crater's walls. No change in degassing was observed at the volcano.

Figure 10. Seismogram showing the M 2 and M 5.4 earthquakes near the Masaya volcano station on 6 July 2000. Courtesy of INETER.

Figure 11. Epicenters near Masaya for the M 5.4 earthquake on 6 July, and the M 4.8 earthquake on 25 July 2000 (stars). The aftershocks from these earthquakes are also shown (small circles). Courtesy of INETER.

Immediately after the earthquake there were many smaller, shallow earthquakes in a zone that includes the area between Masaya, Laguna de Apoyo, and W of Granada (figure 11). In the epicentral zone property was destroyed, cracks opened in the ground, landslides occurred, and trees fell. Several landslides occurred at the edges and steep walls of Laguna de Apoyo. A large number of earthquakes continued until 10 July (figure 12 and table 2). The number of earthquakes then diminished until 1554 on 25 July when a M 4.8 earthquake took place, initiating a series of smaller earthquakes that lasted until about 27 July.

Figure 12. Graph showing the number of earthquakes in the Masaya region between 4 and 30 July 2000. Courtesy of INETER.

Table 2. A summary of earthquakes in vicinity of Masaya and Laguna de Apoyo in early July 2000. Courtesy of INETER.

The July earthquakes were the most destructive seismic events since the 1972 Managua earthquake. The epicentral zone of the July 2000 earthquakes correlates with the same active zones of past earthquakes, which are caused by fault movement between the Cocos and Caribbean plates.

Geologic Background. Masaya volcano in Nicaragua has erupted frequently since the time of the Spanish Conquistadors, when an active lava lake prompted attempts to extract the volcano's molten "gold" until it was found to be basalt rock upon cooling. It lies within the massive Pleistocene Las Sierras caldera and is itself a broad, 6 x 11 km basaltic caldera with steep-sided walls up to 300 m high. The caldera is filled on its NW end by more than a dozen vents that erupted along a circular, 4-km-diameter fracture system. The Nindirí and Masaya cones, the source of observed eruptions, were constructed at the southern end of the fracture system and contain multiple summit craters, including the currently active Santiago crater. A major basaltic Plinian tephra erupted from Masaya about 6,500 years ago. Recent lava flows cover much of the caldera floor and there is a lake at the far eastern end. A lava flow from the 1670 eruption overtopped the north caldera rim. Periods of long-term vigorous gas emission at roughly quarter-century intervals have caused health hazards and crop damage.

Information Contacts: Wilfried Strauch and Virginia Tenorio, Dirección General de Geofísica, Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado 1761, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Washington VAAC, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/).

This report covers the period 8 July-31 August 2000, an interval marked by strong outbursts, spectacular plumes, pyroclastic flows, ashfalls, and a remarkable series of concentric crater collapses that followed the initial crater collapse on 8 July 2000 (figures 6 and 7). Striking ash-column photos, some marked with azimuthal angles and calculated plume heights, appear on Japanese-language websites (see below).

Figure 6. An oblique aerial view of Miyake-jima's pre-eruption summit; the sketched-in curve indicates the area of the collapse on 8 July 2000. That area is sub-circular in plan view (figure 7) and has a diameter of ~ 0.9 km. View is looking NNE. Courtesy of Tokyo Metropolitan Islands Promotion Corporation.

Figure 7. Map of Miyake-jima's active summit crater documenting the crater's expansion during July and August 2000. The margins were drawn from aerial photos taken on the specified dates. The progression was thought to be closely linked with summit deflation; this deflation had been detected since the end of June and accelerated on 8 July. Large dots indicate the locations of a series of small migrating vents seen in the crater during 10-26 August. From the website of K. F. Fujita.

Continuous deflation at the summit had been recorded since the end of June. However, on 8 July the deflation accelerated. Following 4 days of earthquake swarms under the summit, at 1841 on 8 July, a small, phreatic explosion sent a cloud to 800 m above the summit (BGVN 25:07). This explosion lasted several minutes. At the same time, a large pit crater formed with a diameter of ~800-1,000 m and a depth of 100-200 m. A small amount of ash was ejected but was not comparable to the volume of the depression. Red ash and cinder deposits from this eruption were estimated to amount to less than 1 x 10⁶ m³. The volume of collapse was estimated at 50 x 10⁶ m³. No scoriae or any other juvenile material was found. The rapid deflation is thought to have formed as the result of "drain-back" of magma that had intruded near the surface. This appears to have been the catalyst for the explosion.

After the 8 July explosion, tiltmeters recorded periods of sudden inflation. Inflations were preceded and accompanied by long-period earthquakes located less than 2 km below the surface. The intervals of inflation and earthquakes were followed by continued steady deflation. This cycle repeated itself approximately every 12 hours from the 8 July eruption to 23 July.

Following a series of foreshocks, at 1601 on 1 July a Mb 6.1 earthquake struck near Kozu-shima Island, NW of Miyake-jima. This was followed on 14 July by a M 5.3 earthquake off the coast of Miyake-jima. At about 0400, shortly after the earthquake, a phreatic eruption occurred. Thick layers of ash were deposited on the N and E parts of the islands. This eruption continued until about 1300 on 15 July. Photographs taken by Asahi News Network (ANN) on the afternoon of 14 July showed that the 8 July crater had expanded to a diameter of 1,000 m and a depth of 400 m. Observers looking at the bottom of the 8 July crater saw small phreatic explosions yielding plumes with convoluted and scrolled shapes (reminiscent of cock's tails); these originated from a new pit crater that was ~100 m in diameter. The volume of ash from this eruption was estimated to be less than 10 x 10⁶ m³. The volume of collapse was estimated at 200 x 10⁶ m³.

Measurements in early August showed that the collapsed crater had enlarged to a diameter of 1.4 km and a depth of 450 m. According to The Japan Times, an eruption on 10 August produced a plume that rose 3 km above the summit and deposited ash over the NE section of the island. Yukio Hayakawa reported that small pyroclastic flows accompanied this event. After 10 August, phreatic explosions occurred intermittently. Figure 7 shows the progressive expansion of the crater associated with the deflation. GPS measurements made at four stations around the summit indicated continued summit deflation, including during the explosion on 18 August.

At 1700 on 18 August, a large phreatic eruption occurred. This was the largest eruption since activity began on 26 June 2000. Yukio Hayakawa reported small pyroclastic flows. According to articles by the Associated Press and Reuters, white clouds rising to 8 km above the summit were encountered by a commercial airline pilot who was in route from Guam to Narita airport in Tokyo. The plane, which was flying over the island of Miyake shortly after the eruption, later landed safely at Narita. Aviation contacts later revealed that while in flight a commercial airliner encountered airborne ash and underwent a dual-engine flame-out, but managed to land safely. The airliner sustained ~$4 million (US dollars) in damage.

Ash fall was reported to be heaviest on the western part of the island, but ash in the NW sector accumulated up to 15 cm thick as far as 3 km from the crater (figure 8). Ballistics, which included basaltic bombs, were ejected at the end of the eruption and were deposited in a uniform, radial pattern around the crater (figure 9). On the W slope of the volcano, 2-m-diameter ballistics destroyed roofs of cowsheds and formed craters in the meadows. To the SE, there were reports of broken car windows and cinders 5 cm in diameter at the airport. It is uncertain whether these ballistics were juvenile material.

Figure 8. Isopach map of ash-fall deposits from Miyake-jima's eruption on 18 August 2000. Courtesy of Joint University Research Group, Geological Survey of Japan.

Figure 9. Isopleth map of ballistics from Miyake-jima's eruption on 18 August 2000. Courtesy of Joint University Research Group, Geological Survey of Japan.

Although several lower plume-height observations and estimates were made, for example by aviators, one based on a photograph of the actively rising ash column indicated that the 18 August plume rose to at least 15 km. Laser radar (lidar) provided additional constraints on the height of airborne volcanic aerosols at distance from the volcano, detecting them on 23 August at 16 and 17.5 km altitude. More details follow.

For the 18 August eruption, lidar data collected by Takashi Shibata established these values at Nagoya, Japan (35°N, 137°E, on S Honshu Island, 290 km SE of the volcano) around 2100 on 23 August: backscatter ratio at 532 nm, 1.1; depolarization ratio at 532 nm, 5%; plume height, 16 km; and plume width, 100 km.

On 23 August the lidar instrument run by Motowo Fujiwara and Kouichi Shiraishi in Fukuoka (33.5°N, 130.4°E, on NW Kyushu Island, 850 km W of the volcano) detected a thin aerosol layer. Their measurement took place over an interval that began at 0013 and extended over the next hour and a half. They detected relatively strong scattering in the lower stratosphere and found these values: peak backscatter ratio at 532 nm, 1.20-1.25; depolarization ratio at 532 nm, 8-15%; layer height, 17.5 km; and layer width, 1 km. The cited height corresponds to the peak (strongest effect) of the layer; this altitude was ~1.7 km above the tropopause observed by Fukuoka Meteorological Observatory at 2100 on 22 August. Fujiwara and Shiraishi suggested aerosols might have come from Miyake-jima, specifically its eruption at 1702 on 18 August. The Meteorological Observatory reported that during the period from 18-21 August the wind direction around the layer height (17-18 km) changed from ENE to SSE (i.e., basically easterly) and its speed changed from 3 to 7 m/s. These easterly winds further suggested that the lidar-detected aerosol layer originated from a Miyake-jima eruption.

Observations made on 20 August by Osamu Oshima of the University of Tokyo revealed 3 small cones with open pits inside the summit crater, multiple mudflows from the crater pits onto the crater floor, and step faults that crossed new ash layers. He interpreted the step faults to indicate continued subsistence of the crater floor.

The Tokyo VAAC reported three small eruptions at Miyake-jima on 28 August. The eruption clouds reached respective heights of about 5.8, 3.8, and 5 km. On 29 August at 0430, Miyake-jima erupted vigorously again; according to the Eruption Committee this was the second-largest outburst of the recent eruptive episode (the most vigorous being the 18 August eruption). There were two pyroclastic flows, one to the NE that extended 5 km to the sea, and one to the SW that extended for 3 km. The pyroclastic flows contained large amounts of HCl, unlike those of 18 August. The eruption was theorized to be the result of either the collapse of an unstable hydrothermal system or contact between magma and meteoric water inside the volcano. Photos of the pyroclastic flows appeared on the internet (see references).

According to an article by the Associated Press and the Japanese news agency Asahi Shimbun, on 29 August all students, teachers, and school officials on Miyake-jima were evacuated to Tokyo, and all remaining residents of the island were ordered to evacuate. Residents who had not yet left the island as of 31 August were being housed in shelters due to the threat of mudslides produced by thick ash and rain.

Geologic Background. The circular, 8-km-wide island of Miyakejima forms a low-angle stratovolcano that rises about 1,100 m from the sea floor in the northern Izu Islands about 200 km SSW of Tokyo. The basaltic volcano is truncated by small summit calderas, one of which, 3.5 km wide, was formed during a major eruption about 2,500 years ago. Numerous craters and vents, including maars near the coast and radially oriented fissure vents, are present on the flanks. Frequent eruptions have been recorded since 1085 CE at vents ranging from the summit to below sea level, causing much damage on this small populated island. After a three-century-long hiatus ending in 1469 CE, activity has been dominated by flank fissure eruptions sometimes accompanied by minor summit eruptions. A 1.6-km-wide summit crater was slowly formed by subsidence during an eruption in 2000.

Information Contacts: Miyake-jima Meterological Observatory and Volcanological Division; Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); Akihiko Tomiya, Geological Survey of Japan, 1-1-3 Higashi, Ibaraki, Tsukuba 305, Japan (URL: https://www.gsj.jp/); Setsuya Nakada, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Takashi Shibata, STEL, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; Yukio Hayakawa, Faculty of Education, Gunma University, Aramaki, Maebashi 371, Japan (URL: http://www.hayakawayukio.jp/); Motowo Fujiwara and Kouichi Shiraishi, Department of Earth System Science, Fukuoka University, 8-19-1 Nanakuma, Jonann-ku, Fukuoka 814-0180, Japan; U.S. Geological Survey, Reston, VA, USA (URL: http://www.usgs.gov); The Japan Times, 5-4, Shibaura 4-chome, Minato-ku, Tokyo 108-0023 (URL: http://www.japantimes.co.jp/); Asahi Shimbun (URL: http://www.asahi.com/english/english.html); Associated Press; Reuters.

An explosion at Semeru on 27 July 2000 took the lives of two dedicated Volcanological Survey of Indonesia (VSI) staff members, Wildan and Mukti. Asep Wildan was born in Bandung and a graduate of the physics department at the Institute of Technology Bandung. He worked with VSI since 1993, most recently as a geophysicist in VSI's Eastern Java section where he investigated volcano seismology at Semeru and other volcanoes in East Java and Bali. He is survived by his wife and young daughter.

Mukti was born in the city of Banyuwangi on the eastern tip of Java. A high-school graduate, he served with VSI since 1990 in the capacity of volcano observer and was posted at Semeru. He is survived by his mother. Efforts are underway to work with VSI to provide economic assistance for the families of Wildan and Mukti.

Asep Wildan and Mukti made important contributions to VSI's volcano research and monitoring programs, and both had, in the past, generously provided vital assistance to international researchers working at Semeru. They will be greatly missed by their many Indonesian and international friends and colleagues.

Geologic Background. Obituary notices for volcanologists are sometimes written when scientists are killed during an eruption or have had a special relationship with the Global Volcanism Program.

Information Contacts:

This report covers the period form 15 June to 22 August 2000. The highest ash column in this period rose to over 5 km above the summit.

Throughout most of the reporting period, activity remained stable with periodic exhalations of small amplitude and duration. However, two small mudflows were reported: one on 23 June and the other on 24 June. According to CENAPRED, the mudflow on 24 June did not reach any human settlements. No information was available concerning the 23 June mudflow.

On 3 July, two small exhalations generated ash clouds that reached 1 and 2.5 km above the summit and ash fell over the volcano's SW sector. On 4 July, ash from a small exhalation fell in Tetela, a town ~15 km SW of the crater. On 14 July, the volcano erupted and produced an ash cloud that reached 1.6 km in height. According to the Associated Press (AP), the ash from this eruption was blown N and did not significantly impact any populated regions surrounding the volcano.

On 4 August, two closely spaced explosive eruptions occurred. The first at 1251, a moderately large exhalation, lasted 2 minutes. The second one occurred at 1255 and lasted 1.5 minutes. The resulting ash cloud rose to greater than 5 km above the volcano. Ash reportedly fell in nearby communities (Atlautla, San Juan Tehuixtitlan, San Pedro Nexapa, Amecameca, and Tenango).

At 0910 on 10 August, Popocatépetl erupted again. Ash reached to 3.5 km above the volcano. The ash clouds traveled to the W. A second eruption was visible in GOES 8 imagery. It was expected that nearby Mexican states would be coated with a thin layer of ash. At 19:15 on 23 August, a moderate exhalation produced ashfall in the nearby communities of San Pedro Nexapa and Amecameca (~12 km NW and ~16 km NW of the summit, respectively). Throughout the rest of the reporting period there were exhalations of low intensity and short duration that mainly involved gas with small amounts of ash.

Several volcano-tectonic earthquakes, ranging in magnitude from 1.7 to 2.3, occurred during the month of July. The first of these was on 2 July. It was followed by earthquakes on 6, 8, 9, 11, 15, and 23 of July. Three volcano-tectonic earthquakes occurred on 20 July, all under M 2.5. On 1 August, three more tectonic earthquakes were recorded, M 1.9 - 2.7. Other earthquakes occurred on 5 and 10 August; both were less than M 2.

Popocatépetl's volcanic hazard level remained at yellow. CENAPRED recommended that all visitors remain 7 km or more from the crater.

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: The National Center of the Prevention of Disasters (CENAPRED) (URL: https://www.gob.mx/cenapred/); Discovery.com (URL: http://www.discovery.com); Washington VAAC (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Volcano World (URL: http://volcano.oregonstate.edu).

This report covers the period April-June 2000. During mid-April, the inflationary trend that began in February 2000 tapered off (BGVN 25:03). However, the realtime GPS system, along with electronic and water tilt data, continued to indicate a long-term inflation trend.

Emissions from the 1941 vent were characterized by thin, white vapor throughout the months of April and May. The 1995 vent was free of vapor emissions except for gentle puffs of grey ash-clouds on 5, 14-16, and 28-30 April, and 5 and 30 May. During April, these ash clouds rose several hundred meters above the summit before being blown to the W, NW, and SW. Towards the end of May, the general haze produced began to contain a weak ash component and there was a strong smell of SO₂.

In April, a single high-frequency earthquake was recorded and located NE of the caldera wall. Low-frequency earthquakes continued to occur throughout April and were related to the eruptive activity associated with Tavurvur (figure 35). The number of these earthquakes fluctuated within background levels. There was a significant decrease in the number of trigger counts from 78 in February and 90 in March to 28 in April. The number rose again in May to 64. However, it should be noted that these trigger counts include only events that trigger two or more stations. The count that includes non-triggered events (seismic events that do not trigger more than one station) is much higher. On 15 and 30 April, bands of sub-continuous, 2-3 hour long, non-harmonic tremor were recorded.

Figure 35. Map of Rabaul caldera showing locations of volcanic vents, selected towns, and features (modified from Almond and McKee, 1982).

For most of May, seismic activity was low. The exception was a ~M 4.8 earthquake that occurred at 1649 on 10 May and was centered 30 km NE of Rabaul. This produced several aftershocks; a total of 95 high-frequency triggered events were recorded on this date. Because of the proximity of these events to the established 'NE earthquake zone,' which is associated with ongoing eruptive activity, there was an expectation that higher levels of summit activity would occur at Tavurvur.

In June, 13 high-frequency events were recorded. Most originated NE of the Rabaul caldera. The S-P interval for these events was 1-4 seconds. Earthquakes occurring in this region have apparently been associated with the ongoing eruptive activity that began on 28 November 1995. A total of 185 low-frequency triggered events were recorded in June. Most of these events were related to explosions during two episodes of ashfall, one on 5 June and the other on 28 June. In addition, quasi-monochromatic volcanic tremor with durations ranging from a few minutes to a few hours were recorded during these periods. An increase in low- frequency non-triggered events was noted before each of the two episodes.

The 5 and 28 June episodes were characterized by moderate ashfall that emanated from Tavurvur. The first episode began on 5 June with a Vulcanian eruption that deposited lithic blocks beyond the crater rim. Through 8 June there was moderate-to-heavy ashfall. On 6 June at 1150 a loud explosion occurred at the 1941 vent. This was followed by increased explosive activity until the afternoon of 7 June when explosions occurred at 30-minute intervals. The explosion clouds contained moderate amounts of ash and rose to about 1.0-1.5 km above the summit. These ash clouds were blown such that they deposited ash towards the N, NE, and NW where Rabaul Town is located. By 8 June, the explosions had subsided to occasional emissions of light-to-moderate white vapor. For the following two weeks, the areas to the N, NE, and NW were continuosly blanketed in a thin fog of white vapor from Tavurvur.

At 0527(?) on 28 June, another explosion from the 1941 vent triggered the second period of light-to-moderate ashfall. The explosion was followed immediately by a dark grey ash cloud that rose to 1.5 km above the summit before being blown to the N and NW. Over the next two days, further ash clouds were produced that attained heights of several hundred meters. Discrete explosions, occurring at long intervals, marked the end of this period of activity. The last explosion occurred on 30 June.

Beginning in early May, electronic and wet-tilt measurements showed a downward tilt with a total deflation of ~9.0 µrad throughout May and June. However, an inflation of 4.0 µrad was recorded before the activity of 5-8 June and 5.5 µrad was recorded before the 27-30 June activity.

The low-lying Rabaul caldera forms a sheltered harbor once utilized by New Britain's largest city Rabaul prior to the 1994 eruption, which forced the abandonment of the city. Tavurvur and Vulcan are two eruption centers within the Rabaul caldera complex. These volcanoes have had virtually simultaneous eruptions in 1878, 1937, and 1994.

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: Ima Itikarai, David Lolok, Herman Patia, and Steve Saunders, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Semeru has been undergoing nearly constant eruptive activity since 1967. Volcanological Survey of Indonesia (VSI) reports through mid-September 1999 (BGVN 24:09) and earlier described seismicity (including seismically detected pyroclastic flows) and ongoing eruptive outbursts. Accessible Darwin VAAC reports since 3 June 1998 help to characterize the long-term eruptive patterns (table 3). VSI reports are not available for September 1999 through January 2000.

Table 3. A summary of aviation reports (Volcanic Ash Advisories) describing Semeru's plumes during 3 June 1998-21 August 2000. The first two columns describe the time and date when a report was issued. Time entries with commas signify that multiple reports were generated with similar comments. Where available, the time of the observations appear with the comment. Dash marks indicate lack of mention in report. Note that for plume heights, Semeru's summit lies at 3,676 m above sea level. Information sources include air reports (for example, routed via airlines, AIREPS), pilot reports (PIREPS), Notice to Airmen (NOTAM), satellite data, and reports from ground observations. Source data was provided by the Darwin VAAC.

Activity during February-July 2000. Explosive activity during February 2000 included ash emissions, numerous rockfalls, and a few deep A-type earthquakes (table 4). Plumes of thick white ash were seen to rise up to 400 m above the summit on many occasions. Persistent haze or cloudy weather prevented direct observation throughout most of the month. At night during the week of 8-14 February observers noted a 60-m-high flame. Generally, explosions and rockfalls dominated recorded seismicity.

Table 4. Summary of seismicity at Semeru, 31 January-29 August 2000. * Six days of data, through 15 July. Courtesy of VSI.

Explosions and lava avalanches continued in March. Clouds and haze often obscured the volcano, but sometimes thick white emissions appeared above the summit to a maximum height of 500 m. Visual activity and seismicity appeared to increase in late March-early April.

During 4-10 April explosions and lava avalanches were still continuing and became stronger. Seismicity also increased significantly; tremor earthquakes took place 56 times, with maximum amplitudes of 3-15 mm. One pyroclastic flow traveled 1,500 m down the Besuk Kembar river. Many observations in clear conditions showed that the ash cloud was thick and white, rising 400-600 m above the summit. Emissions continued the following week, and explosions increased. "Red flames" sometimes appeared at the summit during night observations. Similar activity continued throughout April. The number of pyroclastic flows increased in late April, and continued at a typical rate of 2-7 per week for the next few months (table 4). On 30 April at 0743, from a location 15 km NNW of Semeru, a pyroclastic flow was observed travelling 800 m down the SSW flank.

Ashfall occurred at the Semeru Volcano Observatory during the week of 2-8 May, when five pyroclastic flows were recorded. Seismicity decreased again, but "red flame" was still seen at night and plumes rose as high as 600 m through 23 May.

Explosive activity was continuing in the second half of June; observers noted white-gray plumes ~600 m above the summit. Pyroclastic flows that reached maximum distances of ~2.5-3 km were reported on 1-2, 4, 10, and 15 July.

Observations on 2 May 2000. John Seach and Geoff Mackley made observations during a 3-hour summit stay on 2 May 2000. During the climb from Ranu Pani village in the N, ash deposits were observed to cover vegetation at a distance of 10 km from the volcano. The bottom third of the cone was vegetated, and zones of mass-wasting had sliced away 20- m-wide sections of forest. The top two-thirds of the cone consisted of ash, cinders, and blocks up to 1.5 m in diameter. There were areas of deep erosion and the risk of rockfalls posed a hazard to climbers.

The summit area (Mahameru) lay covered by ash and baseball-sized blocks with a density of 50/m². A 20-m-wide, 60-m-deep, W-sloping valley separated Mahameru from the active Jonggring Seloko crater, but they are joined by a ridge. The highest N rim of the crater was approximately 30 m below the summit peak. A 2-m-diameter block was located 15 m below the summit on the wall of the valley.

Between 0725 and 1010, 13 eruptive events were observed. During this interval the N rim of Jonggring could not be approached because of the intermittent rain of blocks falling outside the crater and into the valley 50 m from the crater. Two vents produced short-lived Vulcanian eruptions with variable timing and size. Eruptions commenced with degassing, explosions, or the sound of breaking rock, followed by falling bombs and brown ash emission. The explosions were relatively quiet and not accompanied by groundshaking. Brown ash clouds rose to 600 m above the vent and drifted SE. The plume detached from the summit before the next eruption began. Steam emission occurred between eruptions.

Observations on 14 July 2000. Volcanologists on an International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) field trip in east Java observed eruptions of Semeru from an observation point on the N rim of the Sand Sea caldera at Bromo (figure 10). Eruption plumes became visible just before sunrise. Gray ash-and-steam plumes rose a few hundred meters and drifted out over the ocean. Multiple plumes from earlier eruptions were visible downwind. Eruptions lasted up to 2 minutes, and occurred at intervals of between 5 and 30 minutes during the approximately 2 hours of observations. One explosion event was quickly followed by another explosion, apparently from a second location within the crater. Plumes were frequently seen during the next two days from other points around the volcano.

Figure 10. Photograph taken just after sunrise on 14 July 2000 showing an ash eruption from Semeru (upper right) and a steam plume rising from Bromo (lower left). The cone in the lower right is Batok, another young cone within the Sand Sea caldera of the Bromo-Tengger volcanic complex. Note the extensive ash cover on the upper part of Semeru. View is towards the S. Courtesy of Ed Venzke, Smithsonian Institution.

Explosion on 27 July 2000. At approximately 0706 on the morning of 27 July an explosion resulted in two deaths and injuries to five other volcanologists near the NE rim of the active summit crater Jonggring Seloko (see map in BGVN 17:10). The group consisted of a five-member Semeru evaluation team of the Volcanological Survey of Indonesia (VSI), four local porters, and foreign scientists who had attended the IAVCEI conference in Bali the previous week. The fatalities and injuries were caused by impacts and burns from ballistic clasts. These originated from the second of two closely spaced explosions from separate vents that ejected material out to a few hundred meters. Both fatalities were VSI staff members: Asep Wildan was the team leader, and Mukti was a volcano observer from the Semeru Volcano Observatory. Those injured included Suparno, a VSI volcano observer from the Semeru Volcano Observatory, Amit Mushkin from the Hebrew University in Israel, Mike Ramsey from the University of Pittsburgh, and Lee Siebert and Paul Kimberly from the Smithsonian Institution. Kimberly sustained the most serious injuries among the five survivors, including a broken hand, broken arm, and 3rd-degree burns. Following surgeries in Singapore and burn treatments in the United States, Kimberly was released from the hospital in early September.

Continuing activity through August. Visual observations were hindered by bad weather the first week of August. Activity generally decreased through 22 August. White to light-brown ash clouds rising to about 600 m in height were frequently seen during this period. Seismicity increased again in late August, and on 25 and 27 August three pyroclastic flows were recorded. Thin white-gray ash plumes rose ~600 m.

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

Information Contacts: Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); John Seach, P.O. Box 16, Chatsworth Island, NSW 2469, Australia; Ed Venzke, Global Volcanism Program, Smithsonian Institution, Washington DC 20560-0119, USA.

During January-July 2000 Tungurahua volcano experienced continuous but relatively mild activity with occasional lava fountaining. There were periods (hours to days) of relative calm during June and July.

The volcano continues to generate a variety of seismic events, most events being the long-period (LP) type. Two episodes of volcano-tectonic (VT) events were observed; one between late January and early March, and one less intense event between early May and mid-June. Epicenters for these events were across the top of the volcano's cone with focal depths at 3-13 km. Hybrid events, whose waveforms consist of a short, higher-frequency onset followed by lower-frequency, larger-amplitude signals, were most abundant in January and February (~50 events/week), partially coinciding with the greater VT activity. Subsequently these events diminished to 1-2 events/week, except for a brief swarm in early April.

Events of classical LP waveform were frequent, varying from ~400 events/week in January, ~600 in February, ~400 in March, ~600 in April, ~500 in May, and ~400 in June. A sharp increase to ~950 events/week was observed in July. Some of the LP events (3.7-4.0 Hz) were located tentatively at depths of 7-10 km below the crater. However, the great majority of LP events (1.5-3.3 Hz) were 3-7 km deep. They were often associated with explosion clouds or forceful emissions of ash-and-steam within 1-3 seconds of the seismic onset, suggesting a high-level origin.

Explosions, recognized principally by their impulsive onset, were more frequent during January and February (~80-90 events/week), but in subsequent months dropped to ~20-30 events/week, with many accompanied by a sonic boom. Reduced displacement values for the explosions typically were 5-10 cm², and occasionally 12-18 cm².

Low-frequency tremor with spectral frequencies between 0.5-1.6 Hz, but monochromatic at times, were observed in April and May, but only sporadically in June and July. During the period from the 2nd week of April through the 2nd week of May, the low-frequency episode coincided with lava fountaining in the summit crater. The fountains, comprised of the continuous ejection of incandescent material 100-500 m into the air, lasted hours; sustained roaring and surf-like noises heard 12 km away.

The constant glow of incandescent material in the crater, which was observed frequently in late 1999, was seen only occasionally during August, possibly due to unfavorable weather conditions. Better viewing conditions in late June and July confirmed that incandescent lava still remained in the crater or immediately below it.

The emissions have consisted of a permanent, grayish-white to light-gray column of steam with varying amounts of fine-grained ash that commonly rise less than 1 km above the crater. Explosions or strong emissions have consisted of blocks being thrown hundreds of meters into the air and by the formation of Vulcanian-like eruption clouds that are medium-to-dark gray in color and sometimes with a mushroom shape. The clouds have reached as high as 5 km above the summit. Primarily, easterly winds have carried the very fine ash to the W and WSW, but occasionally anywhere in the azimuthal arc between NW and SW. Both national and international flights reported the ash plume. The ash deposits were several centimeters thick on the lower W flank of the cone, but only several millimeters in the agriculturally important lands farther W.

Ballistic blocks were vesicular, black, glassy andesite containing phenocrysts of olivine, plagioclase, augite, and hypersthene, in a glassy matrix with 10-20% microlites. More recent samples had fewer olivines and larger augites. Chemical analyses of these blocks as well as collected ash gave the following typical values: SiO₂ ~58.5%, K₂O ~1.72%, MgO ~3.9%, Ni ~33 ppm, and Cr ~65 ppm.

COSPEC monitoring since November was hindered by heavy cloud cover. Following the consistently high SO₂ flux values of 6,000-8,000 metric tons/day (t/d) during September-October 1999, values decreased to an average of 3,000-4,000 t/d in November-December 1999. Values then rose to ~8,000 t/d in January and subsequently dropped to an average of ~1,000-2,000 t/d in June and July 2000. An exception to this trend was an increase to ~4,000 t/d observed in April-May, 2000, which coincided with the lava fountaining episode. In general, higher SO₂ values seem to be associated with greater tremor activity.

Monthly water analyses of hot springs at both the N and S bases of the edifice have not shown any variation in temperature, pH, conductivity, nor in the concentrations of SO₄, Cl^-, Na+, CO₃^--, Ca⁺⁺, Mg⁺⁺, and K⁺, since chemical monitoring began in 1992 and since the activity on Tungurahua began in July 1999.

Lahars coincided with the rainy season and became frequent in October and November 1999; they rapidly cut the main highway at every stream crossing along the western half of the cone (the area of greatest ash fall). Occasional rains from December to June generated flows of debris. The main highway to Baños and to the Amazon Basin was frequently blocked for hours due to lahar deposits.

In general, the activity appeared to be subsiding. However, during the 1916-18 eruptive period the volcano experienced 1.5 years of little activity between major eruptions. An orange alert is still in effect. In the past, Tungurahua typically generated both Merapi- and St. Vincent-like nuées ardentes. The W sector of Baños (17,000 inhabitants) lies at the mouth of a canyon that starts near the summit of the volcano, 9 km away and 3,000 m above the town.

Following the evacuation of Baños on 17 October 1999, the town remained abandoned until late December (BGVN 25:01). As of August 2000, about 80% of the population had returned and tourism has re-established itself.

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

Information Contacts: Geophysical Institute (Instituto Geofísico), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador.

This report covers the period from April to June 2000. There were no unusual reports from Ulawun in April. Throughout May, moderate to thick white vapor was emitted. Emissions in June consisted of thin white vapor. However, on 5 and 7 June, the emissions were thick white vapor. Seismic activity for June was at a moderate level.

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, D. Lolok, K. Mulina, and F. Taranu, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

This report covers June and July 2000. On 18 April 2000, the Institute of Geological and Nuclear Sciences (IGNS) increased the alert level from 1 to 2 (level 5 being the most severe) following minor eruptive activity that began on 7 March 2000 and included elevated seismicity and higher than normal SO₂ gas flux (BGVN 25:03).

The IGNS reported that for the week ending 16 June 2000, the active MH vent continued to emit an ash plume. This plume sometimes extended as far as 60 km downwind and deposited ash as far as 15 km away. Up to several centimeters of ash were deposited on White Island. Until 16 June, seismic activity was significantly less than in May.

Field observations on 12 July indicated little change in activity since April. Furthermore, no direct relationship between seismic activity during this time and the eruptive activity could be determined. The ash continued to be vented to an altitude of 800-1,000 m. By 19 July, strong NE winds had periodically blown the ash plume towards the mainland, resulting in minor ash deposition there. Ashfall at Turango airport led to landing and departure restrictions. Air traffic was also disrupted around the Bay of Plenty.

On 22 July IGNS staff noticed an increase in activity compared to previous observations. A yellowish-brown gas and an ash plume extending to a height of 1500 m were blown to the E and SE. This continued to disrupt air traffic and deposit ash on the mainland. In fact, the IGNS staff were unable to land due to ash accumulation at the landing site. However, they noted that yellowish-brown ash now covered the island with thicknesses ranging from several mm to several cm. They saw no evidence of ballistic bombs or evidence that the eruptive style had changed from the previous months. However, they did note that the height of the MH vent had decreased from its previous location above the acid lake to a height level with the lake.

On Thursday 27 July between 1700 and 2200, a period of strong seismic activity was recorded. Visual and satellite observations were not possible due to poor weather conditions. A tour operator arriving at the island the morning of 28 July, confirmed that there had been an eruption. IGNS staff arrived 29 July and discovered that a large explosive eruption formed a new crater 120 x 150 m wide in the site formerly occupied by a warm acidic lake in the 1978-90 Crater Complex. The eruption deposited as much as 30 cm of ash and pyroclastic material, including juvenile pumice blocks, over the eastern part of the island. This was the largest eruption at White Island in about 20 years; deposits from this eruption were found in areas frequently visited by tourists. The IGNS advised all visitors that similar eruptions pose serious risks to anyone on the island.

Observations on 31 July found the MH vent, which had enlarged to ~50 m, spewing a dark ash cloud while a reddish-brown ash cloud rose from the new 27 July vent. The plumes combined and rose as high as 1-1.2 km above the vents. After this event, activity returned to the level typical since April: minor eruptions that produced plumes of gas, steam, and volcanic ash.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

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