The lava lake in the summit crater of Erebus has been active since at least 1972. Located in Antarctica overlooking the McMurdo Station on Ross Island, it is the southernmost active volcano on the planet. Because of the remote location, activity is primarily monitored by satellites. This report covers activity during 2023.

The number of thermal alerts recorded by the Hawai'i Institute of Geophysics and Planetology’s MODVOLC Thermal Alerts System increased considerably in 2023 compared to the years 2020-2022 (table 9). In contrast to previous years, the MODIS instruments aboard the Aqua and Terra satellites captured data from Erebus every month during 2023. Consistent with previous years, the lowest number of anomalous pixels were recorded in January, November, and December.

Table 9. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2023. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the HIGP – MODVOLC Thermal Alerts System.

Sentinel-2 infrared images showed one or two prominent heat sources within the summit crater, accompanied by adjacent smaller sources, similar to recent years (see BGVN 46:01, 47:02, and 48:01). A unique image was obtained on 25 November 2023 by the OLI-2 (Operational Land Imager-2) on Landsat 9, showing the upper part of the volcano surrounded by clouds (figure 32).

Figure 32. Satellite view of Erebus with the summit and upper flanks visible above the surrounding weather clouds on 25 November 2023. Landsat 9 OLI-2 (Operational Land Imager-2) image with visible and infrared bands. Thermal anomalies are present in the summit crater. The edifice is visible from about 2,000 m elevation to the summit around 3,800 m. The summit crater is ~500 m in diameter, surrounded by a zone of darker snow-free deposits; the larger circular summit area is ~4.5 km diameter. NASA Earth Observatory image by Lauren Dauphin, using Landsat data from the U.S. Geological Survey.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: https://earthobservatory.nasa.gov/images/152134/erebus-breaks-through).

Rincón de la Vieja is a volcanic complex in Costa Rica with a hot convecting acid lake that exhibits frequent weak phreatic explosions, gas-and-steam emissions, and occasional elevated sulfur dioxide levels (BGVN 45:10, 46:03, 46:11). The current eruption period began June 2021. This report covers activity during July-December 2023 and is based on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Numerous weak phreatic explosions continued during July-December 2023, along with gas-and-steam emissions and plumes that rose as high as 3 km above the crater rim. Many weekly OVSICORI-UNA bulletins included the previous week's number of explosions and emissions (table 9). For many explosions, the time of explosion was given (table 10). Frequent seismic activity (long-period earthquakes, volcano-tectonic earthquakes, and tremor) accompanied the phreatic activity.

Table 9. Number of reported weekly phreatic explosions and gas-and-steam emissions at Rincón de la Vieja, July-December 2023. Counts are reported for the week before the Weekly Bulletin date; not all reports included these data. Courtesy of OVSICORI-UNA.

Table 10. Summary of activity at Rincón de la Vieja during July-December 2023. Weak phreatic explosions and gas emissions are noted where the time of explosion was indicated in the weekly or daily bulletins. Height of plumes or emissions are distance above the crater rim. Courtesy of OVSICORI-UNA.

According to OVSICORI-UNA, during July-October the average weekly sulfur dioxide (SO₂) flux ranged from 68 to 240 tonnes/day. However, in mid-November the flux increased to as high as 334 tonnes/day, the highest value measured in recent years. The high SO₂ flux in mid-November was also detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 43).

Figure 43. Sulfur dioxide (SO2) maps from Rincón de la Vieja recorded by the TROPOMI instrument aboard the Sentinel-5P satellite on 16 November (left) and 20 November (right) 2023. Mass estimates are consistent with measurements by OVSICORI-UNA near ground level. Some of the plume on 20 November may be from other volcanoes (triangle symbols) in Costa Rica and Nicaragua. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

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

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); 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/).

Bezymianny, located on Russia’s Kamchatka Peninsula, has had eruptions since 1955 characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. Activity during November 2022-April 2023 included gas-and-steam emissions, lava dome collapses generating avalanches, and persistent thermal activity. Similar eruptive activity continued from May through October 2023, described here based on information from weekly and daily reports of the Kamchatka Volcano Eruptions Response Team (KVERT), notices from Tokyo VAAC (Volcanic Ash Advisory Center), and from satellite data.

Overall activity decreased after the strong period of activity in late March through April 2023, which included ash explosions during 29 March and 7-8 April 2023 that sent plumes as high as 10-12 km altitude, along with dome growth and lava flows (BGVN 48:05). This reduced activity can be seen in the MIROVA thermal detection system graph (figure 56), which was consistent with data from the MODVOLC thermal detection system and with Sentinel-2 satellite images that showed persistent hotspots in the summit crater when conditions allowed observations. A renewed period of strong activity began in mid-October 2023.

Figure 56. The MIROVA (Log Radiative Power) thermal data for Bezymianny during 20 November 2022 through October 2023 shows heightened activity in the first half of April and second half of October 2023, with lower levels of thermal anomalies in between those times. Courtesy of MIROVA.

Activity increased significantly on 17 October 2023 when large collapses began during 0700-0830 on the E flanks of the lava dome and continued to after 0930 the next day (figure 57). Ash plumes rose to an altitude of 4.5-5 km, extending 220 km NNE by 18 October. A large explosion at 1630 on 18 October produced an ash plume that rose to an altitude of 11 km (8 km above the summit) and drifted NNE and then NW, extending 900 km NW within two days at an altitude of 8 km. Minor ashfall was noted in Kozyrevsk (45 km WNW). At 0820 on 20 October an ash plume was identified in satellite images drifting 100 km ENE at altitudes of 4-4.5 km.

Figure 57. Sentinel-2 satellite images of Bezymianny from 1159 on 17 October 2023 (2359 on 16 October UTC) showing a snow-free S and SE flank along with thermal anomalies in the crater and down the SE flank. Left image is in false color (bands 8, 4, 3); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Lava flows and hot avalanches from the dome down the SE flank continued over the next few days, including 23 October when clear conditions allowed good observations (figures 58 and 59). A large thermal anomaly was observed over the volcano through 24 October, and in the summit crater on 30 October (figure 60). Strong fumarolic activity continued, with numerous avalanches and occasional incandescence. By the last week of October, volcanic activity had decreased to a level consistent with that earlier in the reporting period.

Figure 58. Daytime photo of Bezymianny under clear conditions on 23 October 2023 showing a lava flow and avalanches descending the SE flank, incandescence from the summit crater, and a small ash plume. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 59. Night photo of Bezymianny under cloudy conditions on 23 October 2023 showing an incandescent lava flow and avalanches descending the SE flank. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 60. Sentinel-2 satellite images of Bezymianny from 1159 on 30 October 2023 (2359 on 29 October UTC) showing a plume drifting SE and thermal anomalies in the summit crater and down multiple flanks. Left image is in true color (bands 4, 3, 2); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Aviation warnings were frequently updated during 17-20 October. KVERT issued a Volcano Observatory Notice for Aviation (VONA) on 17 October at 1419 and 1727 (0219 and 0527 UTC) raising the Aviation Color Code (ACC) from Yellow to Orange (second highest level). The next day, KVERT issued a VONA at 1705 (0505 UTC) raising the ACC to Red (highest level) but lowered it back to Orange at 2117 (0917 UTC). After another decrease to Yellow and back to Orange, the ACC was reduced to Yellow on 20 October at 1204 (0004 UTC). In addition, the Tokyo VAAC issued a series of Volcanic Ash Advisories beginning on 16 October and continuing through 30 October.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).chr

Kīlauea is the southeastern-most volcano in Hawaii and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has recently been characterized by lava effusions, spatter, and sulfur dioxide emissions in the active Halema’uma’u lava lake (BGVN 47:08). Lava effusions, some spatter, and sulfur dioxide emissions have continued during this reporting period of July through December 2022 using daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Summary of activity during July-December 2022. Low-level effusions have continued at the western vent of the Halema’uma’u crater during July through early December 2022. Occasional weak ooze-outs (also called lava break outs) would occur along the margins of the crater floor. The overall level of the active lava lake throughout the reporting period gradually increased due to infilling, however it stagnated in mid-September (table 13). During September through November, activity began to decline, though lava effusions persisted at the western vent. By 9 December, the active part of the lava lake had completely crusted over, and incandescence was no longer visible.

Table 13. Summary of measurements taken during overflights at Kīlauea that show a gradual increase in the active lava lake level and the volume of lava effused since 29 September 2021. Lower activity was reported during September-October. Data collected during July-December 2022. Courtesy of HVO.

Activity during July 2022. Lava effusions were reported from the western vent in the Halema’uma’u crater, along with occasional weak ooze-outs along the margins of the crater floor. The height of the lava lake was variable due to deflation-inflation tilt events; for example, the lake level dropped approximately 3-4 m during a summit deflation-inflation event reported on 1 July. Webcam images taken during the night of 6-12 July showed intermittent low-level spattering at the western vent that rose less than 10 m above the vent (figure 519). Measurements made during an overflight on 7 July indicated that the crater floor was infilled about 130 m and that 95 million cubic meters of lava had been effused since 29 September 2021. A single, relatively small lava ooze-out was active to the S of the lava lake. Around midnight on 8 July there were two brief periods of lava overflow onto the lake margins. On 9 July lava ooze-outs were reported near the SE and NE edges of the crater floor and during 10-11 July they occurred near the E, NE, and NW edges. On 16 July crater incandescence was reported, though the ooze-outs and spattering were not visible. On 18 July overnight webcam images showed incandescence in the western vent complex and two ooze-outs were reported around 0000 and 0200 on 19 July. By 0900 there were active ooze-outs along the SW edge of the crater floor. Measurements made from an overflight on 19 July indicated that the crater floor was infilled about 133 m and 98 million cubic meters of lava had erupted since 29 September 2021 (figure 520). On 20 July around 1600 active ooze-outs were visible along the N edge of the crater, which continued through the next day. Extensive ooze-outs occurred along the W margin during 24 July until 1900; on 26 July minor ooze-outs were noted along the N margin. Minor spattering was visible on 29 July along the E margin of the lake. The sulfur dioxide emission rates ranged 650-2,800 tons per day (t/d), the higher of which was measured on 8 July (figure 519).

Figure 519. Minor spattering rising less than 10 m was visible at the E end of the lava lake within Halema‘uma‘u, at the summit of Kīlauea on 8 July 2022. Sulfur dioxide is visible rising from the lake surface (bluish-colored fume). A sulfur dioxide emission rate of approximately 2,800 t/d was measured on 8 July. Courtesy of K. Mulliken, USGS.

Figure 520. A helicopter overflight on 19 July 2022 allowed for aerial visible and thermal imagery to be taken of the Halema’uma’u crater at Kīlauea’s summit crater. The active part of the lava lake is confined to the western part of the crater. The scale of the thermal map ranges from blue to red, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

Activity during August 2022. The eruption continued in the Halema’uma’u crater at the western vent. According to HVO the lava in the active lake remained at the level of the bounding levees. Occasional minor ooze-outs were observed along the margins of the crater floor. Strong nighttime crater incandescence was visible after midnight on 6 August over the western vent cone. During 6-7 August scattered small lava lobes were active along the crater floor and incandescence persisted above the western vent through 9 August. During 7-9 August HVO reported a single lava effusion source was active along the NW margin of the crater floor. Measurements from an overflight on 4 August indicated that the crater floor was infilled about 136 m total and that 102 million cubic meters of lava had been erupted since the start of the eruption. Lava breakouts were reported along the N, NE, E, S, and W margins of the crater during 10-16 August. Another overflight survey conducted on 16 August indicated that the crater floor infilled about 137 m and 104 million cubic meters of lava had been erupted since September 2021. Measured sulfur dioxide emissions rates ranged 1,150-2,450 t/d, the higher of which occurred on 8 August.

Activity during September 2022. During September, lava effusion continued from the western vent into the active lava lake and onto the crater floor. Intermittent minor ooze-outs were reported through the month. A small ooze-out was visible on the W crater floor margin at 0220 on 2 September, which showed decreasing surface activity throughout the day, but remained active through 3 September. On 3 September around 1900 a lava outbreak occurred along the NW margin of the crater floor but had stopped by the evening of 4 September. Field crews monitoring the summit lava lake on 9 September observed spattering on the NE margin of the lake that rose no higher than 10 m, before falling back onto the lava lake crust (figure 521). Overflight measurements on 12 September indicated that the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had been erupted since September 2021. Extensive breakouts in the W and N part of the crater floor were reported at 1600 on 20 September and continued into 26 September. The active part of the lava lake dropped by 10 m while other parts of the crater floor dropped by several meters. Summit tiltmeters recorded a summit seismic swarm of more than 80 earthquakes during 1500-1800 on 21 September, which occurred about 1.5 km below Halema’uma’u; a majority of these were less than Mw 2. By 22 September the active part of the lava lake was infilled about 2 m. On 23 September the western vent areas exhibited several small spatter cones with incandescent openings, along with weak, sporadic spattering (figure 522). The sulfur dioxide emission rate ranged from 930 t/d to 2,000 t/d, the higher of which was measured on 6 September.

Figure 521. Photo of spattering occurring at Kīlauea's Halema’uma’u crater during the morning of 9 September 2022 on the NE margin of the active lava lake. The spatter material rose 10 m into the air before being deposited back on the lava lake crust. Courtesy of C. Parcheta, USGS.

Figure 522.The active western vent area at Kīlauea's Halema’uma’u crater consisted of several small spatter cones with incandescent openings and weak, sporadic spattering. Courtesy of M. Patrick, USGS.

Activity during October 2022. Activity during October declined slightly compared to previous months, though lava effusions persisted from the western vent into the active lava lake and onto the crater floor during October (figure 523). Slight variations in the lava lake were noted throughout the month. HVO reported that around 0600 on 3 October the level of the lava lake has lowered slightly. Overflight measurements taken on 5 October indicated that the crater floor was infilled a total of about 143 m and that 111 million cubic meters of lava had been effused since September 2021. During 6-7 October the lake gradually rose 0.5 m. Sulfur dioxide measurements made on 22 October had an emission rate of 700 t/d. Another overflight taken on 28 October showed that there was little to no change in the elevation of the crater floor: the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had erupted since the start of the eruption.

Figure 523. Photo of the Halema’uma’u crater at Kīlauea looking east from the crater rim showing the active lava lake, with active lava ponds to the SE (top) and west (bottom middle) taken on 5 October 2022. The western vent complex is visible through the gas at the bottom center of the photo. Courtesy of N. Deligne, USGS.

Activity during November 2022. Activity remained low during November, though HVO reported that lava from the western vent continued to effuse into the active lava lake and onto the crater floor throughout the month. The rate of sulfur dioxide emissions during November ranged from 300-600 t/d, the higher amount of which occurred on 9 November.

Activity during December 2022. Similar low activity was reported during December, with lava effusing from the western vent into the active lava lake and onto the crater floor. During 4-5 December the active part of the lava lake was slightly variable in elevation and fluctuated within 1 m. On 9 December HVO reported that lava was no longer erupting from the western vent in the Halema’uma’u crater and that sulfur dioxide emissions had returned to near pre-eruption background levels; during 10-11 December, the lava lake had completely crusted over, and no incandescence was visible (figure 524). Time lapse camera images covering the 4-10 December showed that the crater floor showed weak deflation and no inflation. Some passive events of crustal overturning were reported during 14-15 December, which brought fresh incandescent lava to the lake surface. The sulfur dioxide emission rate was approximately 200 t/d on 14 December. A smaller overturn event on 17 December and another that occurred around 0000 and into the morning of 20 December were also detected. A small seismic swarm was later detected on 30 December.

Figure 524. Photo of Halema’uma’u crater at Kīlauea showing a mostly solidified lake surface during the early morning of 10 December 2022. Courtesy of J. Bard, USGS.

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

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Nyamulagira (also known as Nyamuragira) is a shield volcano in the Democratic Republic of Congo with the summit truncated by a small 2 x 2.3 km caldera with walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from numerous flank fissures and cinder cones. The current eruption period began in April 2018 and has more recently been characterized by summit crater lava flows and thermal activity (BGVN 48:05). This report describes lava flows and variable thermal activity during May through October 2023, based on information from the Observatoire Volcanologique de Goma (OVG) and various satellite data.

Lava lake activity continued during May. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system recorded moderate-to-strong thermal activity throughout the reporting period; activity was more intense during May and October and relatively weaker from June through September (figure 95). The MODVOLC thermal algorithm, detected a total of 209 thermal alerts. There were 143 hotspots detected during May, eight during June, nine during September, and 49 during October. This activity was also reflected in infrared satellite images, where a lava flow was visible in the NW part of the crater on 7 May and strong activity was seen in the center of the crater on 4 October (figure 96). Another infrared satellite image taken on 12 May showed still active lava flows along the NW margin of the crater. According to OVG lava effusions were active during 7-29 May and moved to the N and NW parts of the crater beginning on 9 May. Strong summit crater incandescence was visible from Goma (27 km S) during the nights of 17, 19, and 20 May (figure 97). On 17 May there was an increase in eruptive activity, which peaked at 0100 on 20 May. Notable sulfur dioxide plumes drifted NW and W during 19-20 May (figure 98). Drone footage acquired in partnership with the USGS (United States Geological Survey) on 20 May captured images of narrow lava flows that traveled about 100 m down the W flank (figure 99). Data from the Rumangabo seismic station indicated a decreasing trend in activity during 17-21 May. Although weather clouds prevented clear views of the summit, a strong thermal signature on the NW flank was visible in an infrared satellite image on 22 May, based on an infrared satellite image. On 28 May the lava flows on the upper W flank began to cool and solidify. By 29 May seismicity returned to levels similar to those recorded before the 17 May increase. Lava effusion continued but was confined to the summit crater; periodic crater incandescence was observed.

Figure 95. Moderate-to-strong thermal anomalies were detected at Nyamulagira during May through October 2023, as shown on this MIROVA graph (Log Radiative Power). During late May, the intensity of the anomalies gradually decreased and remained at relatively lower levels during mid-June through mid-September. During mid-September, the power of the anomalies gradually increased again. The stronger activity is reflective of active lava effusions. Courtesy of MIROVA.

Figure 96. Infrared (bands B12, B11, B4) satellite images showing a constant thermal anomaly of variable intensities in the summit crater of Nyamulagira on 7 May 2023 (top left), 21 June 2023 (top right), 21 July 2023 (bottom left), and 4 October 2023 (bottom right). Although much of the crater was obscured by weather clouds on 7 May, a possible lava flow was visible in the NW part of the crater. Courtesy of Copernicus Browser.

Figure 97. Photo of intense nighttime crater incandescence at Nyamulagira as seen from Goma (27 km S) on the evening of 19 May 2023. Courtesy of Charles Balagizi, OVG.

Figure 98. Two strong sulfur dioxide plumes were detected at Nyamulagira and drifted W on 19 (left) and 20 (right) May 2023. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 99. A map (top) showing the active vents (yellow pins) and direction of active lava flows (W) at Nyamulagira at Virunga National Park on 20 May 2023. Drone footage (bottom) also shows the fresh lava flows traveling downslope to the W on 20 May 2023. Courtesy of USGS via OVG.

Low-level activity was noted during June through October. On 1 June OVG reported that seismicity remained at lower levels and that crater incandescence had been absent for three days, though infrared satellite imagery showed continued lava effusion in the summit crater. The lava flows on the flanks covered an estimated 0.6 km². Satellite imagery continued to show thermal activity confined to the lava lake through October (figure 96), although no lava flows or significant sulfur dioxide emissions were reported.

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

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); 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/); Charles Balagizi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

The remote volcano of Bagana is located in central Bougainville Island, Papua New Guinea. Recorded eruptions date back to 1842 and activity has consisted of effusive activity that has built a small lava dome in the summit crater and occasional explosions that produced pyroclastic flows. The most recent eruption has been ongoing since February 2000 and has produced occasional explosions, ash plumes, and lava flows. More recently, activity has been characterized by ongoing effusive activity and ash emissions (BGVN 48:04). This report updates activity from April through September 2023 that has consisted of explosions, ash plumes, ashfall, and lava flows, using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

An explosive eruption was reported on 7 July that generated a large gas-and-ash plume to high altitudes and caused significant ashfall in local communities; the eruption plume had reached upper tropospheric (16-18 km altitude) altitudes by 2200, according to satellite images. Sulfur dioxide plumes were detected in satellite images on 8 July and indicated that the plume was likely a mixture of gas, ice, and ash. A report issued by the Autonomous Bougainville Government (ABG) (Torokina District, Education Section) on 10 July noted that significant ash began falling during 2000-2100 on 7 July and covered most areas in the Vuakovi, Gotana (9 km SW), Koromaketo, Laruma (25 km W) and Atsilima (27 km NW) villages. Pyroclastic flows also occurred, according to ground-based reports; small deposits confined to one drainage were inspected by RVO during an overflight on 17 July and were confirmed to be from the 7 July event. Ashfall continued until 10 July and covered vegetation, which destroyed bushes and gardens and contaminated rivers and streams.

RVO reported another eruption on 14 July. The Darwin VAAC stated that an explosive event started around 0830 on 15 July and produced an ash plume that rose to 16.5 km altitude by 1000 and drifted N, according to satellite images. The plume continued to drift N and remained visible through 1900, and by 2150 it had dissipated.

Ashfall likely from both the 7 and 15 July events impacted about 8,111 people in Torokina (20 km SW), including Tsito/Vuakovi, Gotana, Koromaketo, Kenaia, Longkogari, Kenbaki, Piva (13 km SW), and Atsinima, and in the Tsitovi district, according to ABG. Significant ashfall was also reported in Ruruvu (22 km N) in the Wakunai District of Central Bougainville, though the thickness of these deposits could not be confirmed. An evacuation was called for the villages in Wakunai, where heavy ashfall had contaminated water sources; the communities of Ruruvu, Togarau, Kakarapaia, Karauturi, Atao, and Kuritaturi were asked to evacuate to a disaster center at the Wakunai District Station, and communities in Torokina were asked to evacuate to the Piva District station. According to a news article, more than 7,000 people needed temporary accommodations, with about 1,000 people in evacuation shelters. Ashfall had deposited over a broad area, contaminating water supplies, affecting crops, and collapsing some roofs and houses in rural areas. Schools were temporarily shut down. Intermittent ash emissions continued through the end of July and drifted NNW, NW, and SW. Fine ashfall was reported on the coast of Torokina, and ash plumes also drifted toward Laruma and Atsilima.

A small explosive eruption occurred at 2130 on 28 July that ejected material from the crater vents, according to reports from Torokina, in addition to a lava flow that contained two lobes. A second explosion was detected at 2157. Incandescence from the lava flow was visible from Piva as it descended the W flank around 2000 on 29 July (figure 47). The Darwin VAAC reported that a strong thermal anomaly was visible in satellite images during 30-31 July and that ash emissions rose to 2.4 km altitude and drifted WSW on 30 July. A ground report from RVO described localized emissions at 0900 on 31 July.

Figure 47. Infrared (bands B12, B11, B4) satellite images showed weak thermal anomalies at the summit crater of Bagana on 12 April 2023 (top left), 27 May 2023 (top right), 31 July 2023 (bottom left), and 19 September 2023 (bottom right). A strong thermal anomaly was detected through weather clouds on 31 July and extended W from the summit crater. Courtesy of Copernicus Browser.

The Darwin VAAC reported that ash plumes were identified in satellite imagery at 0800 and 1220 on 12 August and rose to 2.1 km and 3 km altitude and drifted NW and W, respectively. A news report stated that aid was sent to more than 6,300 people that were adversely affected by the eruption. Photos taken during 17-19 August showed ash emissions rising no higher than 1 km above the summit and drifting SE. A small explosion generated an ash plume during the morning of 19 August. Deposits from small pyroclastic flows were also captured in the photos. Satellite images captured lava flows and pyroclastic flow deposits. Two temporary seismic stations were installed near Bagana on 17 August at distances of 7 km WSW (Vakovi station) and 11 km SW (Kepox station). The Kepox station immediately started to record continuous, low-frequency background seismicity.

Satellite data. Little to no thermal activity was detected during April through mid-July 2023; only one anomaly was recorded during early April and one during early June, according to MIROVA (Middle InfraRed Observation of Volcanic Activity) data (figure 48). Thermal activity increased in both power and frequency during mid-July through September, although there were still some short gaps in detected activity. MODVOLC also detected increased thermal activity during August; thermal hotspots were detected a total of five times on 19, 20, and 27 August. Weak thermal anomalies were also captured in infrared satellite images on clear weather days throughout the reporting period on 7, 12, and 17 April, 27 May, 1, 6, 16, and 31 July, and 19 September (figure 48); a strong thermal anomaly was visible on 31 July. Distinct sulfur dioxide plumes that drifted generally NW were intermittently captured by the TROPOMI instrument on the Sentinel-5P satellite and sometimes exceeded two Dobson Units (DUs) (figure 49).

Figure 48. Low thermal activity was detected at Bagana during April through mid-July 2023, as shown on this MIROVA graph. In mid-July, activity began to increase in both frequency and power, which continued through September. There were still some pauses in activity during late July, early August, and late September, but a cluster of thermal activity was detected during late August. Courtesy of MIROVA.

Figure 49. Distinct sulfur dioxide plumes rising from Bagana on 15 July 2023 (top left), 16 July 2023 (top right), 17 July 2023 (bottom left), and 17 August 2023 (bottom right). These plumes all generally drifted NW; a particularly notable plume exceeded 2 Dobson Units (DUs) on 15 July. Data is from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.0

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); 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/); Autonomous Bougainville Government, P.O Box 322, Buka, AROB, PNG (URL: https://abg.gov.pg/); Andrew Tupper (Twitter: @andrewcraigtupp); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn); Radio NZ (URL: https://www.rnz.co.nz/news/pacific/494464/more-than-7-000-people-in-bougainville-need-temporary-accommodation-after-eruption); USAID, 1300 Pennsylvania Ave, NW, Washington DC 20004, USA (URL: https://www.usaid.gov/pacific-islands/press-releases/aug-08-2023-united-states-provides-immediate-emergency-assistance-support-communities-affected-mount-bagana-volcanic-eruptions).

Mayon is located in the Philippines and has steep upper slopes capped by a small summit crater. Historical eruptions date back to 1616 CE that have been characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during June through October 2022 and consisted of lava dome growth and gas-and-steam emissions (BGVN 47:12). A new eruption was reported during late April 2023 and has included lava flows, pyroclastic density currents, ash emissions, and seismicity. This report covers activity during April through September 2023 based on daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

During April through September 2023, PHIVOLCS reported near-daily rockfall events, frequent volcanic earthquakes, and sulfur dioxide measurements. Gas-and-steam emissions rose 100-900 m above the crater and drifted in different directions. Nighttime crater incandescence was often visible during clear weather and was accompanied by incandescent avalanches of material. Activity notably increased during June when lava flows were reported on the S, SE, and E flanks (figure 52). The MIROVA graph (Middle InfraRed Observation of Volcanic Activity) showed strong thermal activity coincident with these lava flows, which remained active through September (figure 53). According to the MODVOLC thermal algorithm, a total of 110 thermal alerts were detected during the reporting period: 17 during June, 40 during July, 27 during August, and 26 during September. During early June, pyroclastic density currents (PDCs) started to occur more frequently.

Figure 52. Infrared (bands B12, B11, B4) satellite images show strong lava flows descending the S, SE, and E flanks of Mayon on 13 June 2023 (top left), 23 June 2023 (top right), 8 July 2023 (bottom left), and 7 August 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 53. Strong thermal activity was detected at Mayon during early June through September, according to this MIROVA graph (Log Radiative Power) due to the presence of active lava flows on the SE, S, and E flanks. Courtesy of MIROVA.

Low activity was reported during much of April and May; gas-and-steam emissions rose 100-900 m above the crater and generally drifted in different directions. A total of 52 rockfall events and 18 volcanic earthquakes were detected during April and 147 rockfall events and 13 volcanic events during May. Sulfur dioxide flux measurements ranged between 400-576 tons per day (t/d) during April, the latter of which was measured on 29 April and between 162-343 t/d during May, the latter of which was measured on 13 May.

Activity during June increased, characterized by lava flows, pyroclastic density currents (PDCs), crater incandescence and incandescent rockfall events, gas-and-steam emissions, and continued seismicity. Weather clouds often prevented clear views of the summit, but during clear days, moderate gas-and-steam emissions rose 100-2,500 m above the crater and drifted in multiple directions. A total of 6,237 rockfall events and 288 volcanic earthquakes were detected. The rockfall events often deposited material on the S and SE flanks within 700-1,500 m of the summit crater and ash from the events drifted SW, S, SE, NE, and E. Sulfur dioxide emissions ranged between 149-1,205 t/d, the latter of which was measured on 10 June. Short-term observations from EDM and electronic tiltmeter monitoring indicated that the upper slopes were inflating since February 2023. Longer-term ground deformation parameters based on EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano remained inflated, especially on the NW and SE flanks. At 1000 on 5 June the Volcano Alert Level (VAL) was raised to 2 (on a 0-5 scale). PHIVOLCS noted that although low-level volcanic earthquakes, ground deformation, and volcanic gas emissions indicated unrest, the steep increase in rockfall frequency may indicate increased dome activity.

A total of 151 dome-collapse PDCs occurred during 8-9 and 11-30 June, traveled 500-2,000 m, and deposited material on the S flank within 2 km of the summit crater. During 8-9 June the VAL was raised to 3. At approximately 1947 on 11 June lava flow activity was reported; two lobes traveled within 500 m from the crater and deposited material on the S (Mi-isi), SE (Bonga), and E (Basud) flanks. Weak seismicity accompanied the lava flow and slight inflation on the upper flanks. This lava flow remained active through 30 June, moving down the S and SE flank as far as 2.5 km and 1.8 km, respectively and depositing material up to 3.3 km from the crater. During 15-16 June traces of ashfall from the PDCs were reported in Sitio Buga, Nabonton, City of Ligao and Purok, and San Francisco, Municipality of Guinobatan. During 28-29 June there were two PDCs generated by the collapse of the lava flow front, which generated a light-brown ash plume 1 km high. Satellite monitors detected significant concentrations of sulfur dioxide beginning on 29 June. On 30 June PDCs primarily affected the Basud Gully on the E flank, the largest of which occurred at 1301 and lasted eight minutes, based on the seismic record. Four PDCs generated between 1800 and 2000 that lasted approximately four minutes each traveled 3-4 km on the E flank and generated an ash plume that rose 1 km above the crater and drifted N and NW. Ashfall was recorded in Tabaco City.

Similar strong activity continued during July; slow lava effusion remained active on the S and SE flanks and traveled as far as 2.8 km and 2.8 km, respectively and material was deposited as far as 4 km from the crater. There was a total of 6,983 rockfall events and 189 PDCs that affected the S, SE, and E flanks. The volcano network detected a total of 2,124 volcanic earthquakes. Continuous gas-and-steam emissions rose 200-2,000 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 792-4,113 t/d, the latter of which was measured on 28 July. During 2-4 July three PDCs were generated from the collapse of the lava flow and resulting light brown plumes rose 200-300 m above the crater. Continuous tremor pulses were reported beginning at 1547 on 3 July through 7 July at 1200, at 2300 on 8 July and going through 0300 on 10 July, and at 2300 on 16 July, as recorded by the seismic network. During 6-9 July there were 10 lava flow-collapse-related PDCs that generated light brown plumes 300-500 m above the crater. During 10-11 July light ashfall was reported in some areas of Mabinit, Legazpi City, Budiao and Salvacion, Daraga, and Camalig, Albay. By 18 July the lava flow advanced 600 m on the E flank as well.

During 1733 on 18 July and 0434 on 19 July PHIVOLCS reported 30 “ashing” events, which are degassing events accompanied by audible thunder-like sounds and entrained ash at the crater, which produced short, dark plumes that drifted SW. These events each lasted 20-40 seconds, and plume heights ranged from 150-300 m above the crater, as recorded by seismic, infrasound, visual, and thermal monitors. Three more ashing events occurred during 19-20 July. Short-term observations from electronic tilt and GPS monitoring indicate deflation on the E lower flanks in early July and inflation on the NW middle flanks during the third week of July. Longer-term ground deformation parameters from EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano was still generally inflated relative to baseline levels. A short-lived lava pulse lasted 28 seconds at 1956 on 21 July, which was accompanied by seismic and infrasound signals. By 22 July, the only lava flow that remained active was on the SE flank, and continued to extend 3.4 km, while those on the S and E flanks weakened markedly. One ashing event was detected during 30-31 July, whereas there were 57 detected during 31 July-1 August; according to PHIVOLCS beginning at approximately 1800 on 31 July eruptive activity was dominated by phases of intermittent ashing, as well as increased in the apparent rates of lava effusion from the summit crater. The ashing phases consisted of discrete events recorded as low-frequency volcanic earthquakes (LFVQ) typically 30 seconds in duration, based on seismic and infrasound signals. Gray ash plume rose 100 m above the crater and generally drifted NE. Shortly after these ashing events began, new lava began to effuse rapidly from the crater, feeding the established flowed on the SE, E, and E flanks and generating frequent rockfall events.

Intensified unrest persisted during August. There was a total of 4,141 rockfall events, 2,881 volcanic earthquakes, which included volcanic tremor events, 32 ashing events, and 101 PDCs detected throughout the month. On clear weather days, gas-and-steam emissions rose 300-1,500 m above the crater and drifted in different directions (figure 54). Sulfur dioxide emissions averaged 735-4,756 t/d, the higher value of which was measured on 16 August. During 1-2 August the rate of lava effusion decreased, but continued to feed the flows on the SE, S, and E flanks, maintaining their advances to 3.4 km, 2.8 km, and 1.1 km from the crater, respectively (figure 55). Rockfall and PDCs generated by collapses at the lava flow margins and from the summit dome deposited material within 4 km of the crater. During 3-4 August there were 10 tremor events detected that lasted 1-4 minutes. Short-lived lava pulse lasted 35 seconds and was accompanied by seismic and infrasound signals at 0442 on 6 August. Seven collapses were recorded at the front of the lava flow during 12-14 August.

Figure 54. Photo of Mayon showing a white gas-and-steam plume rising 800-1,500 m above the crater at 0645 on 25 August. Courtesy of William Rogers.

Figure 55. Photo of Mayon facing N showing incandescent lava flows and summit crater incandescence taken at 1830 on 25 August 2023. Courtesy of William Rogers.

During September, similar activity of slow lava effusion, PDCs, gas-and-steam emissions, and seismicity continued. There was a total of 4,452 rockfall events, 329 volcanic earthquakes, which included volcanic tremor events, two ashing events, and 85 PDCs recorded throughout the month. On clear weather days, gas-and-steam emissions rose 100-1,500 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 609-2,252 t/d, the higher average of which was measured on 6 September. Slow lava effusion continued advancing on the SE, S, and E flanks, maintaining lengths of 3.4 km, 2.8 km, and 1.1 km, respectively. Rockfall and PDC events generated by collapses along the lava flow margins and at the summit dome deposited material within 4 km of the crater.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); 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/); William Rogers, Legazpi City, Albay Province, Philippines.

Nishinoshima, located about 1,000 km S of Tokyo, is a small island in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent submarine peaks to the S, W, and NE. Eruptions date back to 1973 and the current eruption period began in October 2022. Recent activity has consisted of small ash plumes and fumarolic activity (BGVN 48:07). This report covers activity during May through August 2023, using information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports and satellite data.

Activity during May through June was relatively low. The Japan Coast Guard (JCG) did overflights on 14 and 22 June and reported white gas-and-steam emissions rising 600 m and 1,200 m from the central crater of the pyroclastic cone, respectively (figure 125). In addition, multiple white gas-and-steam emissions rose from the inner rim of the W side of the crater and from the SE flank of the pyroclastic cone. Discolored brown-to-green water was observed around almost the entire perimeter of the island; on 22 June light green discolored water was observed off the S coast of the island.

Figure 125. A white gas-and-steam plume rising 600 m above the crater of Nishinoshima at 1404 on 14 June 2023 (left) and 1,200 m above the crater at 1249 on 22 June 2023 (right). Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, June, 2023).

Observations from the Himawari meteorological satellite confirmed an eruption on 9 and 10 July. An eruption plume rose 1.6 km above the crater and drifted N around 1300 on 9 July. Satellite images acquired at 1420 and 2020 on 9 July and at 0220 on 10 July showed continuing emissions that rose 1.3-1.6 km above the crater and drifted NE and N. The Tokyo VAAC reported that an ash plume seen by a pilot and identified in a satellite image at 0630 on 21 July rose to 3 km altitude and drifted S.

Aerial observations conducted by JCG on 8 August showed a white-and-gray plume rising from the central crater of the pyroclastic cone, and multiple white gas-and-steam emissions were rising from the inner edge of the western crater and along the NW-SE flanks of the island (figure 126). Brown-to-green discolored water was also noted around the perimeter of the island.

Figure 126. Aerial photo of Nishinoshima showing a white-and-gray plume rising from the central crater taken at 1350 on 8 August 2023.

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity), showing an increase in both frequency and power beginning in July (figure 127). This increase in activity coincides with eruptive activity on 9 and 10 July, characterized by eruption plumes. According to the MODVOLC thermal alert algorithm, one thermal hotspot was recorded on 20 July. Weak thermal anomalies were also detected in infrared satellite imagery, accompanied by strong gas-and-steam plumes (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Nishinoshima during May through August 2023, showing an increase in both frequency and power in July, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 128. Infrared (bands B12, B11, B4) satellite images showing a small thermal anomaly at the crater of Nishinoshima on 30 June 2023 (top left), 3 July 2023 (top right), 7 August 2023 (bottom left), and 27 August 2023 (bottom right). Strong gas-and-steam plumes accompanied this activity, extending NW, NE, and SW. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

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); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of Strombolian eruptions and ash plumes (BGVN 48:07). This report describes lower levels of activity consisting of ash and white gas-and-steam plumes during May through August 2023, based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, and satellite data.

Activity was relatively low during May and June. Daily white gas-and-steam emissions rose 25-200 m above the crater and drifted in different directions. Five ash plumes were detected at 0519 on 10 May, 1241 on 11 May, 0920 on 12 May, 2320 on 12 May, and at 0710 on 13 May, and rose 1-2.5 km above the crater and drifted SW. A webcam image taken on 12 May showed ejection of incandescent material above the vent. A total of nine ash plumes were detected during 6-11 June: at 1434 and 00220 on 6 and 7 June the ash plumes rose 500 m above the crater and drifted NW, at 1537 on 8 June the ash plume rose 1 km above the crater and drifted SW, at 0746 and at 0846 on 9 June the ash plumes rose 800 m and 3 km above the crater and drifted SW, respectively, at 0423, 1431, and 1750 on 10 June the ash plumes rose 2 km, 1.5 km, and 3.5 km above the crater and drifted NW, respectively, and at 0030 on 11 June an ash plume rose 2 km above the crater and drifted NW. Webcam images taken on 10 and 11 June at 0455 and 0102, respectively, showed incandescent material ejected above the vent. On 19 June an ash plume at 0822 rose 1.5 km above the crater and drifted SE.

Similar low activity of white gas-and-steam emissions and few ash plumes were reported during July and August. Daily white gas-and-steam emissions rose 25-300 m above the crater and drifted in multiple directions. Three ash plumes were reported at 0843, 0851, and 0852 on 20 July that rose 500-2,000 m above the crater and drifted NW.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during May through August 2023 (figure 140). Although activity was often obscured by weather clouds, a thermal anomaly was visible in an infrared satellite image of the crater on 12 May, accompanied by an eruption plume that drifted SW (figure 141).

Figure 140. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during May through August 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 141. A single thermal anomaly (bright yellow-orange) was visible at Krakatau in this infrared (bands B12, B11, B4) satellite image taken on 12 May 2023. An eruption plume accompanied the thermal anomaly and drifted SW. Courtesy of Copernicus Browser.

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

Information Contacts: 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); 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/).

Villarrica, in central Chile, consists of a 2-km-wide caldera that formed about 3,500 years ago and is located at the base of the presently active cone at the NW margin of a 6-km-wide caldera. Historical eruptions eruptions date back to 1558 and have been characterized by mild-to-moderate explosive activity with occasional lava effusions. The current eruption period began in December 2014 and has recently consisted of nighttime crater incandescence, ash emissions, and seismicity (BGVN 48:04). This report covers activity during April through September 2023 and describes occasional Strombolian activity, gas-and-ash emissions, and nighttime crater incandescence. Information for this report primarily comes from the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN) and satellite data.

Seismicity during April consisted of long period (LP) events and tremor (TRE); a total of 9,413 LP-type events and 759 TR-type events were detected throughout the month. Nighttime crater incandescence persisted and was visible in the degassing column. Sulfur dioxide data was obtained using Differential Absorption Optical Spectroscopy Equipment (DOAS) that showed an average value of 1,450 ± 198 tons per day (t/d) during 1-15 April and 1,129 ± 201 t/d during 16-30 April, with a maximum daily value of 2,784 t/d on 9 April. Gas-and-steam emissions of variable intensities rose above the active crater as high as 1.3 km above the crater on 13 April. Strombolian explosions were not observed and there was a slight decrease in the lava lake level.

There were 14,123 LP-type events and 727 TR-type events detected during May. According to sulfur dioxide measurements taken with DOAS equipment, the active crater emitted an average value of 1,826 ± 482 t/d during 1-15 May and 912 ± 41 t/d during 16-30 May, with a daily maximum value of 5,155 t/d on 13 May. Surveillance cameras showed continuous white gas-and-steam emissions that rose as high as 430 m above the crater on 27 May. Nighttime incandescence illuminated the gas column less than 300 m above the crater rim was and no pyroclastic emissions were reported. A landslide was identified on 13 May on the E flank of the volcano 50 m from the crater rim and extending 300 m away; SERNAGEOMIN noted that this event may have occurred on 12 May. During the morning of 27 and 28 May minor Strombolian explosions characterized by incandescent ejecta were recorded at the crater rim; the last reported Strombolian explosions had occurred at the end of March.

Seismic activity during June consisted of five volcano-tectonic (VT)-type events, 21,606 LP-type events, and 2,085 TR-type events. The average value of sulfur dioxide flux obtained by DOAS equipment was 1,420 ± 217 t/d during 1-15 June and 2,562 ± 804 t/d, with a maximum daily value of 4,810 t/d on 17 June. White gas-and-steam emissions rose less than 480 m above the crater; frequent nighttime crater incandescence was reflected in the degassing plume. On 12 June an emission rose 100 m above the crater and drifted NNW. On 15 June one or several emissions resulted in ashfall to the NE as far as 5.5 km from the crater, based on a Skysat satellite image. Several Strombolian explosions occurred within the crater; activity on 15 June was higher energy and ejected blocks 200-300 m on the NE slope. Surveillance cameras showed white gas-and-steam emissions rising 480 m above the crater on 16 June. On 19 and 24 June low-intensity Strombolian activity was observed, ejecting material as far as 200 m from the center of the crater to the E.

During July, seismicity included 29,319 LP-type events, 3,736 TR-type events, and two VT-type events. DOAS equipment recorded two days of sulfur dioxide emissions of 4,220 t/d and 1,009 t/d on 1 and 13 July, respectively. Constant nighttime incandescence was also recorded and was particularly noticeable when accompanied by eruptive columns on 12 and 16 July. Minor explosive events were detected in the crater. According to Skysat satellite images taken on 12, 13, and 16 July, ashfall deposits were identified 155 m S of the crater. According to POVI, incandescence was visible from two vents on the crater floor around 0336 on 12 July. Gas-and-ash emissions rose as high as 1.2 km above the crater on 13 July and drifted E and NW. A series of gas-and-steam pulses containing some ash deposited material on the upper E flank around 1551 on 13 July. During 16-31 July, average sulfur dioxide emissions of 1,679 ± 406 t/d were recorded, with a maximum daily value of 2,343 t/d on 28 July. Fine ash emissions were also reported on 16, 17, and 23 July.

Seismicity persisted during August, characterized by 27,011 LP-type events, 3,323 TR-type events, and three VT-type events. The average value of sulfur dioxide measurements taken during 1-15 August was 1,642 ± 270 t/d and 2,207 ± 4,549 t/d during 16-31 August, with a maximum daily value of 3,294 t/d on 27 August. Nighttime crater incandescence remained visible in degassing columns. White gas-and-steam emissions rose 480 m above the crater on 6 August. According to a Skysat satellite image from 6 August, ash accumulation was observed proximal to the crater and was mainly distributed toward the E slope. White gas-and-steam emissions rose 320 m above the crater on 26 August. Nighttime incandescence and Strombolian activity that generated ash emissions were reported on 27 August.

Seismicity during September was characterized by five VT-type events, 12,057 LP-type events, and 2,058 TR-type events. Nighttime incandescence persisted. On 2 September an ash emission rose 180 m above the crater and drifted SE at 1643 (figure 125) and a white gas-and-steam plume rose 320 m above the crater. According to the Buenos Aires VAAC, periods of continuous gas-and-ash emissions were visible in webcam images from 1830 on 2 September to 0110 on 3 September. Strombolian activity was observed on 2 September and during the early morning of 3 September, the latter event of which generated an ash emission that rose 60 m above the crater and drifted 100 m from the center of the crater to the NE and SW. Ashfall was reported to the SE and S as far as 750 m from the crater. The lava lake was active during 3-4 September and lava fountaining was visible for the first time since 26 March 2023, according to POVI. Fountains captured in webcam images at 2133 on 3 September and at 0054 on 4 September rose as high as 60 m above the crater rim and ejected material onto the upper W flank. Sulfur dioxide flux of 1,730 t/d and 1,281 t/d was measured on 3 and 4 September, respectively, according to data obtained by DOAS equipment.

Figure 125. Webcam image of a gray ash emission rising above Villarrica on 2 September 2023 at 1643 (local time) that rose 180 m above the crater and drifted SE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 02 de septiembre de 2023, 17:05 Hora local).

Strong Strombolian activity and larger gas-and-ash plumes were reported during 18-20 September. On 18 September activity was also associated with energetic LP-type events and notable sulfur dioxide fluxes (as high as 4,277 t/d). On 19 September Strombolian activity and incandescence were observed. On 20 September at 0914 ash emissions rose 50 m above the crater and drifted SSE, accompanied by Strombolian activity that ejected material less than 100 m SSE, causing fall deposits on that respective flank. SERNAGEOMIN reported that a Planet Scope satellite image taken on 20 September showed the lava lake in the crater, measuring 32 m x 35 m and an area of 0.001 km². Several ash emissions were recorded at 0841, 0910, 1251, 1306, 1312, 1315, and 1324 on 23 September and rose less than 150 m above the crater. The sulfur dioxide flux value was 698 t/d on 23 September and 1,097 t/d on 24 September. On 24 September the Volcanic Alert Level (VAL) was raised to Orange (the third level on a four-color scale). SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

During 24-25 September there was an increase in seismic energy (observed at TR-events) and acoustic signals, characterized by 1 VT-type event, 213 LP-type events, and 124 TR-type events. Mainly white gas-and-steam emissions, in addition to occasional fine ash emissions were recorded. During the early morning of 25 September Strombolian explosions were reported and ejected material 250 m in all directions, though dominantly toward the NW. On 25 September the average value of sulfur dioxide flux was 760 t/d. Seismicity during 25-30 September consisted of five VT-type events, 1,937 LP-type events, and 456 TR-type events.

During 25-29 September moderate Strombolian activity was observed and ejected material as far as the crater rim. In addition, ash pulses lasting roughly 50 minutes were observed around 0700 and dispersed ENE. During 26-27 September a TR episode lasted 6.5 hours and was accompanied by discrete acoustic signals. Satellite images from 26 September showed a spatter cone on the crater floor with one vent that measured 10 x 14 m and a smaller vent about 35 m NE of the cone. SERNAGEOMIN reported an abundant number of bomb-sized blocks up to 150 m from the crater, as well as impact marks on the snow, which indicated explosive activity. A low-altitude ash emission was observed drifting NW around 1140 on 28 September, based on webcam images. Between 0620 and 0850 on 29 September an ash emission rose 60 m above the crater and drifted NW. During an overflight taken around 1000 on 29 September scientists observed molten material in the vent, a large accumulation of pyroclasts inside the crater, and energetic degassing, some of which contained a small amount of ash. Block-sized pyroclasts were deposited on the internal walls and near the crater, and a distal ash deposit was also visible. The average sulfur dioxide flux measured on 28 September was 344 t/d. Satellite images taken on 29 September ashfall was deposited roughly 3 km WNW from the crater and nighttime crater incandescence remained visible. The average sulfur dioxide flux value from 29 September was 199 t/d. On 30 September at 0740 a pulsating ash emission rose 1.1 km above the crater and drifted NNW (figure 126). Deposits on the S flank extended as far as 4.5 km from the crater rim, based on satellite images from 30 September.

Figure 126. Webcam image of a gray ash plume rising 1.1 km above the crater of Villarrica at 0740 (local time) on 30 September 2023. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 30 de septiembre de 2023, 09:30 Hora local).

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) showed intermittent thermal activity during April through September, with slightly stronger activity detected during late September (figure 127). Small clusters of thermal activity were detected during mid-June, early July, early August, and late September. According to the MODVOLC thermal alert system, a total of four thermal hotspots were detected on 7 July and 3 and 23 September. This activity was also intermittently captured in infrared satellite imagery on clear weather days (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Villarrica during April through September 2023, according to this MIROVA graph (Log Radiative Power). Activity was relatively low during April through mid-June. Small clusters of activity occurred during mid-June, early July, early August, and late September. Courtesy of MIROVA.

Figure 128. Consistent bright thermal anomalies (bright yellow-orange) were visible at the summit crater of Villarrica in infrared (bands B12, B11, B4) satellite images, as shown on 17 June 2023 (top left), 17 July 2023 (top right), 6 August 2023 (bottom left), and 20 September 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Av. Beauchef 1671, Santiago, Chile (URL: https://web.senapred.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Merapi, located just north of the major city of Yogyakarta in central Java, Indonesia, has had activity within the last 20 years characterized by pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome. The current eruption period began in late December 2020 and has more recently consisted of ash plumes, intermittent incandescent avalanches of material, and pyroclastic flows (BGVN 48:04). This report covers activity during April through September 2023, based on information from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data.

Activity during April through September 2023 primarily consisted of incandescent avalanches of material that mainly affected the SW and W flanks and traveled as far as 2.3 km from the summit (table 25) and white gas-and-steam emissions that rose 10-1,000 m above the crater.

Table 25. Monthly summary of avalanches and avalanche distances recorded at Merapi during April through September 2023. The number of reported avalanches does not include instances where possible avalanches were heard but could not be visually confirmed as a result of inclement weather. Data courtesy of BPPTKG (April-September 2023 daily reports).

BPPTKG reported that during April and May white gas-and-steam emissions rose 10-750 m above the crater, incandescent avalanches descended 500-2,000 m on the SW and W flanks (figure 135). Cloudy weather often prevented clear views of the summit, and sometimes avalanches could not be confirmed. According to a webcam image, a pyroclastic flow was visible on 17 April at 0531. During the week of 28 April and 4 May a pyroclastic flow was reported on the SW flank, traveling up to 2.5 km. According to a drone overflight taken on 17 May the SW lava dome volume was an estimated 2,372,800 cubic meters and the dome in the main crater was an estimated 2,337,300 cubic meters.

Figure 135. Photo showing an incandescent avalanche affecting the flank of Merapi on 8 April 2023. Courtesy of Øystein Lund Andersen.

During June and July similar activity persisted with white gas-and-steam emissions rising 10-350 m above the crater and frequent incandescent avalanches that traveled 300-2,000 m down the SW, W, and S flanks (figure 136). Based on an analysis of aerial photos taken on 24 June the volume of the SW lava dome was approximately 2.5 million cubic meters. A pyroclastic flow was observed on 5 July that traveled 2.7 km on the SW flank. According to the Darwin VAAC multiple minor ash plumes were identified in satellite images on 19 July that rose to 3.7 km altitude and drifted S and SW. During 22, 25, and 26 July a total of 17 avalanches descended as far as 1.8 km on the S flank.

Figure 136. Photo showing an incandescent avalanche descending the flank of Merapi on 23 July 2023. Courtesy of Øystein Lund Andersen.

Frequent white gas-and-steam emissions continued during August and September, rising 10-450 m above the crater. Incandescent avalanches mainly affected the SW and W flanks and traveled 400-2,300 m from the vent (figure 137). An aerial survey conducted on 10 August was analyzed and reported that estimates of the SW dome volume was 2,764,300 cubic meters and the dome in the main crater was 2,369,800 cubic meters.

Figure 137. Photo showing a strong incandescent avalanche descending the flank of Merapi on 23 September 2023. Courtesy of Øystein Lund Andersen.

Frequent and moderate-power thermal activity continued throughout the reporting period, according to a MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 138). There was an increase in the number of detected anomalies during mid-May. The MODVOLC thermal algorithm recorded a total of 47 thermal hotspots: six during April, nine during May, eight during June, 15 during July, four during August, and five during September. Some of this activity was captured in infrared satellite imagery on clear weather days, sometimes accompanied by incandescent material on the SW flank (figure 139).

Figure 138. Frequent and moderate-power thermal anomalies were detected at Merapi during April through September 2023, as shown on this MIROVA plot (Log Radiative Power). There was an increase in the number of anomalies recorded during mid-May. Courtesy of MIROVA.

Figure 139. Infrared (bands B12, B11, B4) satellite images showed a consistent thermal anomaly (bright yellow-orange) at the summit crater of Merapi on 8 April 2023 (top left), 18 May 2023 (top right), 17 June 2023 (middle left), 17 July 2023 (middle right), 11 August 2023 (bottom left), and 20 September 2023 (bottom right). Incandescent material was occasionally visible descending the SW flank, as shown in each of these images. Courtesy of Copernicus Browser.

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

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); 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/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Øystein Lund Andersen (URL: https://www.oysteinlundandersen.com/, https://twitter.com/oysteinvolcano).

Ebeko, located on the N end of Paramushir Island in Russia’s Kuril Islands just S of the Kamchatka Peninsula, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Observed eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruptive period began in June 2022, consisting of frequent explosions, ash plumes, and thermal activity (BGVN 47:10, 48:06). This report covers similar activity during June-November 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Moderate explosive activity continued during June-November 2023 (figures 50 and 51). According to visual data from Severo-Kurilsk, explosions sent ash 2-3.5 km above the summit (3-4.5 km altitude) during most days during June through mid-September. Activity after mid-September was slightly weaker, with ash usually reaching less than 2 km above the summit. According to KVERT the volcano in October and November was, with a few exceptions, either quiet or obscured by clouds that prevented satellite observations. KVERT issued Volcano Observatory Notices for Aviation (VONA) on 8 and 12 June, 13 and 22 July, 3 and 21 August, and 31 October warning of potential aviation hazards from ash plumes drifting 3-15 km from the volcano. Based on satellite data, KVERT reported a persistent thermal anomaly whenever weather clouds permitted viewing.

Figure 50. Ash explosion from the active summit crater of Ebeko on 18 July 2023; view is approximately towards the W. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Figure 51. Ash explosion from the active summit crater of Ebeko on 23 July 2023 with lightning visible in the lower part of the plume. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/).

Thermal anomalies were detected by MODIS throughout 2001 and 2002 in zones proximal to the summit of Agung. The first alert occurred on 23 September 2001 when two alert-pixels were detected with a maximum alert ratio of -0.789. Larger anomalies were detected on 12 August 2002 (two alert-pixels with maximum alert ratio of -0.429) and 5 October 2002 (one alert-pixel with alert ratio of -0.536). All the alerts seem to occur outside the summit crater, with the possible exception of 5 October 2002, and are more likely to represent fires than volcanic activity.

No volcanic activity has been reported recently by the Volcanological Survey of Indonesia.

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: Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

Thermal alerts detected by MODIS within the 2001-2002 period occurred only during August-October 2002 (figure 3) in the summit area. The first alert occurred on 13 August 2002 when a single alert-pixel had an alert ratio of -0.542. On 10 October the anomaly consisted of two alert-pixels with a maximum alert ratio of -0.409, and on 21 October the anomaly was characterized by six alert-pixels (clustered SW of the summit) with a maximum alert ratio of -0.571.

Figure 3. MODIS-detected alerts on Arjuno-Welirang during May-December 2002. Thermal alerts collated by Diego Coppola and David Rothery; data courtesy of the Hawaii Institute of Geophysics and Planetology's MODIS Thermal Alert Team.

The Volcanological Survey of Indonesia reported that the volcano was at Status Level I (no activity) in October 2002. No observations were reported, but only distant tectonic earthquakes were detected at the seismograph station.

An explosive eruption took place in the NW part of Gunung Welirang in October 1950, and eruptive activity was reported on the NW flank (Kawah Plupuh) in August 1952. Steam plumes from the summit of Welirang were photographed from space on 13 September 1991 (BGVN 16:08) and in mid-November 1994.

Geologic Background. The Arjuno and Welirang volcanoes anchor the SE and NW ends, respectively, of a 6-km-long line of volcanic cones and craters. The complex overlies most of the Gunung Ringgit edifice, whose summit is about 3 km NE from the main ridge. Pyroclastic cones are located on the north flank of Gunung Welirang and along an E-W line cutting across the southern side of Gunung Arjuno that extends to the lower SE flank. Fumarolic areas with sulfur deposition occur at several locations on Welirang.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

The last reported activity at Dukono consisted of a plume that reached 6 km altitude on 25 September 1995 (BGVN 20:10). Post-May 2000 MODIS data suggested a significant event during 26 August-7 September 2002. During that period, anomalies rose well above alert detection threshold, triggering 10 thermal alerts. All of the alert pixels were located within a 1-km radius.

Geologic Background. The Dukono complex in northern Halmahera is on an edifice with a broad, low profile containing multiple peaks and overlapping craters. Almost continuous explosive eruptions, sometimes accompanied by lava flows, have occurred since 1933. During a major eruption in 1550 CE, a lava flow filled in the strait between Halmahera and the Gunung Mamuya cone, 10 km NE. Malupang Wariang, 1 km SW of the summit crater complex, contains a 700 x 570 m crater that has also had reported eruptions.

Information Contacts: Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

The last reported activity at Ibu included ash emission and mild ash explosions in September 1999. A May 2000 photo showed a lava dome covering the crater floor. MODIS data after May 2000 indicated thermal alerts during 28 May-3 October 2001 (figure 1). The series of alerts was consistent with continued inflation of, or extrusion onto, this dome. Note that the alert was barely above threshold, and it is likely that Ibu was just below detection threshold through 2002. A discussion of the MODIS technique was included in BGVN 28:01.

Figure 1. MODIS thermal alerts on Ibu during 2001. Thermal alerts collated by Diego Coppola and David Rothery; data courtesy of the Hawaii Institute of Geophysics and Planetology's MODIS Thermal Alert Team.

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

Information Contacts: Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

During 9 December 2002-26 January 2003, the Volcanological Survey of Indonesia (VSI) reported that seismicity at Ijen was dominated by shallow volcanic and tectonic earthquakes (table 6). The number of weekly volcanic earthquakes decreased significantly in December compared to July-November 2002 (BGVN 27:08 and 27:11). One deep volcanic earthquake was registered during 13-19 January. Continuous tremor occurred throughout the report period. The Alert Level remained at 2.

Table 6. Seismicity at Ijen during 9 December 2002-26 January 2003. Courtesy VSI.

Thermal anomalies were detected by MODIS throughout 2001 and 2002 adjacent to the Ijen (Kendeng) caldera. The center coordinates of the alert-pixels are widely dispersed, so it seems likely that these represent fires. Alerts occurred in August-September 2001, May 2002, and September-October 2002. The biggest anomaly occurred on 19 October 2002 close to Kawah Ijen, the only currently known locus of activity in the complex. This was characterized by four alert-pixels with a maximum alert ratio of +0.568. This is an extremely high ratio and is comparable to that seen elsewhere during lava effusion. However, VSI confirmed that there was no eruption that day, only a bush fire that also damaged seismic sensors.

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

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

The Philippine Institute for Volcanology and Seismology (PHIVOLCS) reported a sudden increase in steaming activity at Canlaon (also spelled Kanlaon) on 28 June 2002. At about 0436, "dirty white steam" was observed rising up to 200 m above the active crater and drifting SW and SSW. However, there was no corresponding significant earthquake activity; the seismic network detected only two high-frequency volcanic earthquakes in the 24-hour window around the event. A small ash puff on 28 November 2002 at 0721 rose ~100 m above the active crater and drifted SW. The event was recorded as a volcanic tremor at the Cabagnaan and Guintubdan seismic stations. Traces of ash deposits were observed at Cabagnaan Station, located SSW of the active crater. Moderate emission of white to dirty white steam was observed immediately after the ash puff. As of 1100 on 28 November, activity had decreased to only minor white steaming from the summit with a few discrete tremors.

A PHIVOLCS report on 17 March indicated that the hazard status of Canlaon had been raised to Alert Level 1 following an ash emission on that day and one the previous week. At about 0530 on 17 March observatory personnel noted the emission of a grayish volcanic plume. The dirty white steam clouds rose 50 m above the active crater and drifted SW and SSW. No corresponding significant earthquake activity accompanied the event; the seismic network detected only two small low-frequency volcanic earthquakes in the preceding 24 hours. PHIVOLCS interpreted the activity as being hydrothermal in nature at shallow levels in the crater, with no indication of active magma intrusion. Details of the ash emission that occurred "last week" were not provided.

Alert Level 1 signifies that there could be possible ash explosions in the coming days or weeks. For this reason, PHIVOLCS reiterated that the public should avoid entering the 4-km-radius Permanent Danger Zone.

Geologic Background. Kanlaon volcano (also spelled Canlaon) forms the highest point on the Philippine island of Negros. The massive andesitic stratovolcano is covered with fissure-controlled pyroclastic cones and craters, many of which are filled by lakes. The largest debris avalanche known in the Philippines traveled 33 km SW from Kanlaon. The summit contains a 2-km-wide, elongated northern caldera with a crater lake and a smaller but higher active vent, Lugud crater, to the south. Eruptions recorded since 1866 have typically consisted of phreatic explosions of small-to-moderate size that produce minor local ashfall.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs. dost.gov.ph/).

MODIS thermal alerts at Kawi-Butak during 2001 and 2002 occurred only in August and October 2002 mostly to the SE of the summit. These almost certainly represent fires rather than volcanic events. The biggest detected alert occurred on 12 October and was characterized by seven alert-pixels with maximum alert ratio of -0.298. These alert pixels were in a group including the summit and the N flank, and are the best candidate for an eruption, though it is unlikely that an eruption of the kind required to trigger such an alert (a significant lava dome or flow) would have gone unreported. The Volcanological Survey of Indonesia confirmed that there was no eruption at Kawi-Butak on 12 October 2002 and that the thermal alert was indeed caused by a bush fire.

Geologic Background. The broad Kawi-Butak volcanic massif lies immediately E of Kelut volcano and S of Arjuno-Welirang volcano. Gunung Kawi was constructed to the NW of Gunung Butak. No historical eruptions are known from either volcano, but both are primarily of Holocene age.

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

Seismicity at Krakatau was dominated by volcanic and tectonic earthquakes during 30 December 2002-23 March 2003 (table 3). The hazard status remained unchanged at Alert Level 2.

Table 3. Seismicity at Krakatau during 30 December 2002-23 March 2003. Courtesy VSI.

Throughout 2001 and 2002, MODIS thermal alerts for Krakatau occurred only during July-September 2001. The first alert occurred on 31 July when one alert pixel was detected with an alert ratio of -0.793. The anomalies increased during August and on 9 August the anomaly consisted of two alert-pixels with a maximum alert ratio of -0.306. Other major anomalies occurred on 1 September (four alert-pixels with maximum alert ratio of -0.327) and on 17 September (two alert-pixels with maximum alert ratio of -0.284). These anomalies correspond to an increase of activity at Krakatau characterized by ash and bomb emission during August 2001 and an increase in the number of explosion and volcanic earthquakes during the first half of September 2001, reported by the Volcanological Survey of Indonesia (BGVN 26:09 and 27:09). The coordinates of the centers of the alert pixels are tightly grouped around the summit of the main cone. Bearing in mind that each pixel represents radiance from an area of ground more than 1 km across, the alert pixels could represent radiance from the active vent or from hot ejecta close to the vent.

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

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

The summit area was obscured by rain and clouds on many days in January and February. During clear days (4-5, 8-16, 18-21, and 25 January; 1-9 and 13-17 February), Crater 2 released weak to moderate emissions of white and white-gray vapor. Occasional ash-laden gray-brown and forceful dark gray emissions were produced on 10 and 14 January, respectively. The forceful emissions on the 14th were accompanied by low roaring noises. On 18 January a large explosion produced a thick dark ash column that penetrated the atmospheric clouds over the summit area. Occasional white-gray and gray-brown ash-laden emissions were observed on 1-6 February. On 3 and 4 February the same vent forcefully ejected dark gray ash clouds. Night glow was observed at Crater 2 on 14 and 15 January; some of the glow on the 15th changed into weak incandescent lava projections. Variable weak to bright red glow was observed at night on 3-6 and 14 February. On 3 February the glow fluctuated. Low rumbling noises were only heard on 6 February. Crater 3 released thin white vapor gently on 9-10, 12-13, and 19 January, and during 3-4, 6-9, 14, and 16 February. No emissions were observed on other clear days. There was no seismic recording.

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: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

An international team of scientists conducted an interdisciplinary research project at Lascar from 13 October 2002 to15 January 2003. The group of scientists from Argentina, Chile, Italy, Puerto Rico, United Kingdom, and the United States, includes volcanologists who have directly observed the volcano from before the 1993 eruption (BGVN 18:04). During the first part of the project the team took the first ever direct measurements of fumarole temperatures and gas compositions within the crater, which are to be compared with measurements acquired through remote sensing techniques. The combination of direct and ground- and satellite-based measurements at very different spatial scales will hopefully corroborate results from the different techniques. A significant change in crater geometry over the last few years was identified through comparison with work carried out by Gardeweg and others (1993) and Matthews and others (1997).

Visual observations. On 26 October 2002 small explosive eruptive events (reaching heights of 300 m above the crater) were observed at 0905, 0910, and 0915 by both the remote-sensing team 7 km SE of the vent and the direct sampling team on the crater rim (figure 25). Winds from the NW rapidly dispersed the ash cloud. On 27 October at 0845, loud noises were heard, and an ash plume was observed by people 7 km NW of the volcano. At 1340 a much more vigorous explosion produced a plume that rose at least 1,500 m above the vent (figure 26), which was observed by the volcanologists from Pozo Tres, 60 km NW.

Figure 25. Photograph of an ash eruption at Lascar on 26 October 2002 seen from the crater rim. Courtesy of Franco Tassi.

Figure 26. Photograph of an eruption at Lascar on 27 October 2002 seen from "Pozo Tres." Courtesy of J.G. Viramonte.

On 1 November 2002 the direct-measurement team reached the crater for a second time to collect gas samples. Comparison with previous descriptions (Gardeweg and others, 1993; Matthews and others, 1997) and photographs taken by J.G. Viramonte at the beginning of the 1990's indicated that after the 2000 eruption (BGVN 25:06; http://www.unsa.edu.ar/varias/lascar; http://www. conae.gov.ar) several changes in crater morphology and locations of the high-flux fumaroles occurred. The dome had collapsed by several tens of meters, producing a deep, steep, hole ~200 m in diameter and 200 m deep, with a number of large fumaroles around the internal rim and at the base (figure 27). Observations suggest that Lascar is presently at or near the climax of the "dome subsidence phase," as described by Matthews and others (1997). There was no evidence of new dome emplacement after the July 2000 eruption.

Figure 27. Cross-section sketch of the Lascar crater showing fractures, high-temperature fumaroles, and areas of recent ash and bombs. Courtesy of J.G. Viramonte.

Direct techniques. Team members from Universita' degli Studi di Firenze (Italy), Universidad Nacional de Salta (Argentina), and Universidad Catolica del Norte (Chile) took, for the first time, direct temperature measurements of Lascar's fumaroles and collected gas samples using vacuum bottles filled with a 4N NaOH + 0.15N CdOH solution (Montegrossi and others, 2001). Sampled fumaroles were aligned along the upper collapse ring fault in the NW internal flank of the active crater (figure 28). A maximum temperature of 385°C was measured. Preliminary results indicate a very high concentration of acidic gases, with a paucity of water vapor. A more complete analysis, performed by gas chromatography and mass spectrometry, will be done in the Department of Earth Sciences at the Univ. Firenze.

Figure 28. Photograph of the NW side of the Lascar crater, modified to show the collapsed rims and fumarole sampling locations in October 2002. Courtesy of Franco Tassi.

Remote-sensing techniques. Team members from Michigan Technological University (MTU), Cambridge, and Universidad Nacional de Salta (UNSa) provided a suite of state-of-the-art ground-based instruments, including a miniature UV spectrometer that utilizes Differential Optical Absorption Spectroscopy (DOAS), a MICROTOPS II sun-photometer, and a Kestrel 4000 weather station. The instruments will help provide a more complete understanding of S-bearing species, and their fates in a high, dry atmosphere. The mini UV spectrometer provides an open path line-of-site burden of SO₂ through spectral analysis (Galle and others, 2002; Edmonds and others, 2002), which can be used to derive SO₂ emission rates (using the plume's speed and width). The sun-photometer will provide information about the plume's liquid- and solid-phase species, specifically sulfate aerosol. The aerosol's spectral signature can be used to derive the particle size distribution from the spectral optical depth (Watson and Oppenheimer, 2000). The weather station, in conjunction with the other instruments, will elucidate the effects of Lascar's high, dry, and extremely transmissive atmosphere upon SO₂ conversion rates. The team will also derive SO₂ burdens and emission rates using satellite imagery from NASA's ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) sensor.

Lascar provides an opportunity to study the effects of an end-member atmosphere upon volcanic plumes with the aim of better understanding the fates of volcanic species in the high troposphere (and hence the lower stratosphere). The DOAS is an exciting new instrument, first applied to volcanic studies by volcanologists from the Montserrat Volcano Observatory (MVO), Cambridge University (UK), and Chalmer's University of Technology (Sweden) that is now rapidly replacing the older, bulkier, and much more expensive correlation spectrometer (COSPEC). This experiment is a continuation of that work in a new and different environment.

Future work. The Cambridge team planned to begin a new round of remote studies in early 2003, using the DOAS system and sun-photometers, in particular to investigate evolution of the aerosol phase of the plume. The direct gas sampling by the Florence, Salta, and Del Norte team will be repeated, hopefully in 2003. The group, led by the MTU and UNSa contingent, plan to use recently acquired ASTER data to investigate SO₂ emission. Hotspot activity will be studied using ASTER, MODIS, and GOES data. A study of the morphological evolution of the crater is planned for the near future, hopefully incorporating previous investigators' work on cyclic activity at Lascar.

References. Déruelle, B., Medina, E.T., Figueroa, O.A., Maragaño, M.C., and Viramonte, J.G., 1995, The recent eruption of Lascar volcano (Atacama-Chile, April 1993): petrological and volcanological relationships: C.R. Acad. Sci. Paris, 321, série II, p. 377-384.

Déruelle, B., Figueroa, O.A., Medina, E.T., Viramonte, J.G., and Maragaño, M.C., 1996, Petrology of pumices of April 1993 eruption of Lascar (Atacama, Chile): Blackwell Science Ltd, Terra Nova, v. 8, p. 191-199.

Edmonds, M., Herd, R.A., Galle, B., and Oppenheimer, C.M., 2002, Automated, high time resolution measurements of SO₂ flux at Soufriere Hills Volcano, Montserrat: in review.

Galle, B., Oppenheimer, C., Geyer, A., McGonigle, A., Edmonds, M., and Horrocks, L.A., 2002, A miniaturised ultraviolet spectrometer for remote sensing of SO₂ fluxes: a new tool for volcano surveillance: Journal of Volcanology and Geothermal Research, v. 119, p. 241-254.

Gardeweg, M.C., Sparks, S., Matthews, S., Fuentealba, C., Murillo, M., and Espinoza, A., 1993, V informe sobre el comportamiento del volcan Lascar (II región): Enero-Marzo 1993: SERNAGEOMIN, Chile, Marzo 1993.

Gardeweg, M.C., and Medina, E., 1994, La erupción subpliniana del 19-20 de Abril del volcan Lascar N de Chile: Congreso Geológico Chileno, Actas I, p. 299-304.

Matthews, S.J., Gardeweg, M.C., and Sparks, R.S.J., 1997, The 1984 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions: Bulletin of Volcanology, v. 59, p. 72-82.

Montegrossi, G., Tassi, F., Vaselli, O., Buccianti, A., and Garofalo, K., 2001, Sulphur species in volcanic gases: Anal. Chem., v. 73, p. 3,709-3,715.

Viramonte, J.G., Seggiaro, R.E., Becchio, R.A., and Petrinovic, I.A., 1994, Erupción del Volcán Lascar, Chile, Andes Centrales, Abril de 1993: 4ta Reunión Internacional del Volcán de Colima, Colima, México, Actas I, p. 149-151.

Watson, I.M., and Oppenheimer, C., 2000, Particle size distributions of Mt. Etna's aerosol plume constrained by sunphotometry: Journal of Geophysical Research, Atmospheres, v. 105, no. D8, p. 9,823-9,829.

Geologic Background. Láscar is the most active volcano of the northern Chilean Andes. The andesitic-to-dacitic stratovolcano contains six overlapping summit craters. Prominent lava flows descend its NW flanks. An older, higher stratovolcano 5 km E, Volcán Aguas Calientes, displays a well-developed summit crater and a probable Holocene lava flow near its summit (de Silva and Francis, 1991). Láscar consists of two major edifices; activity began at the eastern volcano and then shifted to the western cone. The largest eruption took place about 26,500 years ago, and following the eruption of the Tumbres scoria flow about 9000 years ago, activity shifted back to the eastern edifice, where three overlapping craters were formed. Frequent small-to-moderate explosive eruptions have been recorded since the mid-19th century, along with periodic larger eruptions that produced ashfall hundreds of kilometers away. The largest historical eruption took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit and ashfall in Buenos Aires.

Information Contacts: José G. Viramonte and Mariano Poodts, Instituto GEONORTE, Universidad Nacional de Salta, Buenos Aires 177, Salta 4400, Argentina (URL: http://www.unsa.edu.ar/); Matt Watson and Lizzette Rodríguez, Department of Geology, Michigan Technological University, Houghton, MI 49931, USA (URL: http://www.geo.mtu.edu/volcanoes/); Franco Tassi, Dipartimento di Scienze della Terra, Università degli studi di Firenze, Via La Pira 4, 50121 Firenze, Italy (URL: https://www.dst.unifi.it/); Eduardo Medina, Claudio Martinez, and Felipe Aguilera, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile (URL: http://www.ucn.cl/en/carrera/geology/).

The following are summaries from the U.S. Geological Survey (USGS) of activity at Long Valley during 2001 (Hill, 2001) and 2002 (Hill, 2002). Summaries of activity during 1996, 1997, and 1998 are found in BGVN 22:11-22:12 and 24:06; activities during 1999 through 2000 are found in BGVN 26:07. Figure 25 shows some of the locations mentioned in this report.

Figure 25. Map of the Long Valley caldera area. Courtesy of USGS.

Summary of activity during 2001. Activity levels in Long Valley caldera and vicinity were incrementally lower in 2001 than in 2000, thus continuing the trend of extended quiescence that began toward the end of 1999. Low-level seismic activity within the caldera typically included five or fewer earthquakes per day large enough to be located by the online computer system. Most were smaller than M 2.0, and none were as large as M 3.0; the largest was a M 2.8 earthquake beneath the southern margin of the caldera 800 m N of Convict Lake on 21 May. Seismic activity in the Sierra Nevada S of the caldera continued to be concentrated within the aftershock zone of the 1998-99 sequence of three M 5 earthquakes. The 2001 activity (figure 26) included eight earthquakes of M 3.0 or larger. The largest was the M 3.4 earthquake of 2 December located near the epicenter of the M 5.6 earthquake of 15 May 1999.

Figure 26. Earthquake epicenters in the Long Valley region for the year 2001. Courtesy of USGS.

Mid-crustal long-period (LP) volcanic earthquakes continued to occur at depths of 10-25 km beneath the W flank of Mammoth Mountain (figure 27), although at a much reduced rate compared with the peak in activity in 1997-98. Some 60 LP earthquakes were detected during 2001, with over 15 of these occurring in a cluster on 10 February.

Figure 27. Time history of deep (depth 10-25 km) LP earthquakes in the Long Valley caldera beneath Mammoth Mountain from 1989 through 2002. Vertical bars indicate number of LP earthquakes per week (left axis) and the continuous curve shows the cumulative number of events (right axis). Courtesy of USGS.

Deformation within the caldera was limited to continuing slow subsidence of the resurgent dome at a rate of roughly 1 cm/year. All together, the center of the resurgent dome has lost some 2 cm in elevation since inflation stopped in late 1998, leaving the center of the resurgent dome roughly 75 cm or so higher at the end of 2001 than in the late 1970's. The continuous strain and deformation monitoring networks detected no short-term deformation transients during the year. The same is true for the magnetometer networks.

The diffuse carbon dioxide (CO₂) degassing at the Horseshoe Lake tree-kill area (BGVN 22:11) and other sites around the flanks of Mammoth Mountain has shown no significant change over the past several years. The total CO₂ flux continued to fluctuate ~200 tons per day, with the Horseshoe Lake area contributing roughly 90 tons per day.

The lull in caldera unrest over the past couple of years has provided the Long Valley Observatory (LVO) an opportunity to look back over the wealth of data collected during the previous two decades of activity and to investigate the nature and significance of the processes driving the unrest, toward the goal of assessing future unrest episodes and their significance in terms of potential volcanic hazards. Data from the intense unrest during the 1997-98 episode in the S moat, for example, indicate that fluids (magmatic brine or perhaps magma) played a central role in this activity. This underscores the value of a closely integrating the seismic, deformation, and hydrologic monitoring efforts.

Summary of activity during 2002. Activity in 2002 was dominated by the onset of renewed inflation of the resurgent dome following nearly three years of gradual subsidence. Earthquake activity within the caldera, which remained low through the first half of the year, showed a slight increase through the second half. Of particular note was the response of the caldera to the shear and surface waves generated by the M 7.9 Denali Fault earthquake of 3 November 2002 in the form of a burst of some 60 small earthquakes beneath the S flank of Mammoth Mountain, a coincident strain transient consistent with aseismic slip on a normal fault beneath the E flank of the mountain, and an earthquake swarm the following day in the S moat that included the first M 3.0 earthquake since 1999. This is the third time Long Valley has shown a well-documented response to large, distant earthquakes, the first two being with the M 7.4 Landers earthquake of 28 June 1992 and the M 7.2 Hector Mine earthquake of 16 October 1999. No other significant changes occurred within the caldera during the year. Both the carbon dioxide flux from the flanks of Mammoth Mountain and the rate of deep long-period (LP) volcanic earthquakes beneath Mammoth Mountain showed little change from previous years. The LVO detected no very-long-period (VLP) earthquakes during 2002.

Beginning around the first of the year, both the 2-color EDM and continuous GPS data for the baselines radiating from the CASA monument turned from gradual contraction to renewed extension that persisted through the year at rate of 2.5-3.0 cm/year. This rate is comparable to extension rates that prevailed through the mid-1990's. Cumulative uplift of the center of the resurgent dome associated with this extension has returned to its 1999 value of roughly 80 cm with respect to the late 1970's.

Earthquake activity within the caldera remained low through the first half of the year averaging fewer than five earthquakes per day, most with M 2.0 (figures 27 and 28). The largest event within the caldera during this period was a M 2.8 earthquake on 15 March located in the W lobe of the S moat seismic zone, 1.6 km S of the 203-395 Highway junction. Activity increased slightly in mid-June beginning with a cluster of small earthquakes beneath the W flank of Mammoth Mountain on 26 June that included four events of about M 2. A number of small (M 2) events with the appearance of LP earthquakes occurred at shallow depths (less than 2 km) beneath the southern section of the resurgent dome during the last half of August.

Figure 28. Earthquake epicenters in the Long Valley caldera region for 2002. Courtesy of USGS.

The most notable activity began with a burst of over 60 small earthquakes of M 1 beneath the S flank of Mammoth Mountain as the surface waves generated by the M 7.9 Denali Fault, Alaska, earthquake of 3 November 2002 passed through just 17 minutes after the mainshock rupture. At the same time, the borehole dilatometers detected a 0.1-microstrain strain transient that is consistent with slow (aseismic) slip on a normal fault at a depth of about 7 km beneath the W flank of Mammoth Mountain. As with the caldera activity remotely triggered by the M 7.4 Landers earthquake of 28 June 1992 and the M 7.2 Hector Mine earthquake of 16 October 1999, this strain transient is much larger than can be explained by cumulative slip for the 60 or so earthquakes of M 1 triggered by the Denali Fault earthquake. The following day, 4 November, the largest earthquake swarm in the S moat of the caldera since 1998 developed as a sequence that included six earthquakes of M 2 and one of M 3.0. This S-moat swarm was unusual in that it occurred in a relatively aseismic section of the S moat, focal depths of the swarm earthquakes were unusually shallow (4 km), and the NNW lineations of the swarm epicenters cuts across the prevailing WNW-trend of the usual S-moat swarm activity. The latter was also true for the swarm activity triggered by the M 7.4 Landers earthquake of 1992. This S-moat earthquake swarm was not accompanied by detectable strain changes. Mid-crustal long-period (LP) earthquakes have continued at depths of 10-25 km beneath Mammoth Mountain at a fairly steady rate over the past three years. Occasional bursts of activity included 12-15 events per week.

Diffuse emission of carbon dioxide from the flanks of Mammoth Mountain showed little change from previous years. Emission rates estimated for the Horseshoe Lake tree-kill area continued to fluctuate between 50 and 150 tons of CO₂ per day, with an average flux of 100 tons per day since 1995. The Horseshoe Lake area produced roughly one-third of the total CO₂ flux from the flanks of Mammoth Mountain.

Values for the helium isotope ratio ³He/ 4He from samples taken in early and mid-2002 from the Mammoth Mountain Fumarole (MMF), located at 3,000 m elevation some 300 m E of the Chair 3 ski lift, averaged 5.5, or essentially the same as the 2001 values. These values are significantly higher than the 1999 value of 3.0. The increase with respect to 1999 is consistent with an increase in the magmatic component in the gas emissions from the fumarole. Whether the elevated values for 2001-2002 are related to the very-long-period (VLP) volcanic earthquakes that occurred at a depth of 3 km beneath the summit of Mammoth Mountain in July and August of 2000 remains to be seen.

Seismic activity in the region surrounding Long Valley caldera continued to be dominated by earthquakes in the SSW-trending aftershock zone of the June and July 1998 and the May 1999 earthquakes in the Sierra Nevada S of the caldera. Activity within this aftershock zone included a cluster of earthquakes near the southern end of the zone centered just E of Grinnell Lake that began on 6 June and persisted through the end of the month. Elsewhere, a M 3.7 earthquake on 15 July just 3.2 km NNW of Bishop produced felt shaking throughout the Bishop area. Earthquakes of M 2.9 and 3.5 on 12 December were located beneath the Volcanic Tableland 19 km NNW of Bishop.

An updated revision of the USGS Response Plan for Volcanic Unrest in the Long Valley Caldera - Mono Craters Region, California was released in March 2002 as USGS Bulletin 2185. This bulletin is available in print and in electronic form at ttp://geopubs.wr.usgs.gov/bulletin/b2185/.

References. Hill, D.P., 2001, Long Valley Observatory quarterly report October-December 2001 and annual summary for 2001: Long Valley Observatory, U.S. Geological Survey, Menlo Park, CA (URL: http://lvo.wr.usgs.gov/Annual/lvc_01.html).

Hill, D.P., 2002, Long Valley Observatory quarterly report July-September and October-December 2002 and annual summary for 2002: Long Valley Observatory, U.S. Geological Survey, Menlo Park, CA (URL: http://lvo.wr.usgs.gov/Quarterly/qrt_rpt3-4-02.html).

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

Information Contacts: David Hill, Long Valley Observatory, Volcano Hazards Program, U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025, USA (URL: https://volcanoes.usgs.gov/observatories/calvo/).

The summit area of Manam was obscured by rain and atmospheric clouds on most days during January-March 2003, making it difficult to observe emissions from the two summit craters. When clear, the Main Crater released small-to-moderate volumes of thin white vapor. Southern Crater generally released small-volume white emissions. Seismicity was low. Small low-frequency earthquakes were recorded on most days. Slightly greater numbers of earthquakes occurred on 16, 17, 23, 25, and 27 January. Some volcano-tectonic earthquakes were recorded on 11 (1), 12 (1), and 16 January (3); the events on the 16th were larger than the others. No volcano-tectonic earthquakes were recorded in February, and there was no seismic recording during March.

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: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

Until 11 October 2002, no significant volcanic activity had been reported since eruptions in June and July 2001 (BGVN 26:08). Subsequent deflation, combined with declining seismicity and sulfur dioxide flux, resulted in the Alert Level being lowered to 0 (no eruption is forecast in the foreseeable future, but entry in the 6-km radius Permanent Danger Zone (PDZ) is not advised because phreatic explosions and ash puffs may occur without precursors) in February 2002 (BGVN 27:04).

Mayon remains intermittently active, with tremor episodes, a small ash puff in October 2002, steam emission in January 2003, and an explosion and ash plume in March 2003. Small ash explosions on 5 May and 6 April will be described in the next Bulletin.

Activity during October 2002. At 0635 on 11 October 2002 the volcano produced a small ash puff that reached 500 m above the summit crater. The small ash cloud from this minor explosion quickly diffused and drifted E without noticeable deposits on the slopes. The ash puff followed a series of imperceptible volcanic tremors that began in the early hours of 22 September and occurred sporadically until the last tremor was recorded on 9 October. The 11 October report from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) also noted that slight swelling of the volcano's edifice was detected by an electronic tiltmeter on the S flank. However, the Alert Level remained at 0.

A 30 October notice from PHIVOLCS indicated that the number of volcanic earthquakes, although imperceptible, remained significantly above background levels since the ash emission of 11 October. Another notable observation was the occurrence of small volcanic tremors and consistent inflation detected by electronic tiltmeters, which suggested that magma was intruding into the volcano. Gas output from the summit had increased from recent emission rates of ~950 metric tons per day (t/d) to ~2,200 t/d on 29 October. Because of these consistent increases in monitored parameters, PHIVOLCS raised the Alert Level to 1. Although a major explosive eruption was still considered unlikely at this stage, the persistent unrest over the previous weeks clearly indicates a shift from its former period of repose. Alert Level 1 is meant to call attention to increased volcanic activity specifically an increased likelihood for steam-driven or ash explosions to occur with little or no warning. During the last week of October PHIVOLCS augmented its monitoring network around Mayon with additional personnel and equipment.

Activity during January 2003. A brief period of vigorous steam emission occurred at 1753 on 31 January after an episode of volcanic tremor the previous day. The steam ejection lasted for about a minute and produced a dirty white steam cloud that rose ~500 m above the summit crater. A low-frequency, short duration, harmonic tremor coincided with the steam venting. The sulfur dioxide emission rate increased slightly to 764 t/d on 31 January from the previous reading of 441 t/d taken on 21 January, which followed several episodes of low-frequency volcanic tremor during the previous weeks.

Activity during March 2003. An explosion from the crater at 1819 on 17 March sent ash and steam ~1 km above the summit before it was blown WNW by winds. The explosion was recorded as a high-frequency seismic signal, indicating a sudden release of pressure. No significant seismicity was apparent prior to the event. Measurements of SO₂ flux within the emission plume between 0900 and 1100 earlier that morning averaged ~890 t/d, which is more than the usual 500 t/d typical during periods of repose. Electronic tiltmeters on the N and S flanks indicated slight inflation of the edifice beginning on 13 March. Due to the increased possibility of additional ash ejections, the hazard status was raised to Alert Level 1.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, PHIVOLCS Building, C.P. Garcia Avenue, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs. dost.gov.ph/).

During late July-1 September 2002, the Volcanological Survey of Indonesia (VSI) reported frequent lava avalanches and plumes up to 550 m above the summit of Merapi (BGVN 27:09). No further reports were issued by VSI through at least March 2003.

MODIS thermal alerts during 2001 and 2002 indicated continuous activity through mid-January 2002 (figures 24 and 25). This period was characterized by dome collapse and hot avalanches (BGVN 26:01, 26:07, 26:10, and 27:02). Pyroclastic flows occurred too frequently to correlate them with the MODIS alerts, for which data are collected only about once per day (weather permitting). There were no alerts detected during the rest of 2002 except for late March-late May, which corresponded to a temporary renewal of pyroclastic flows before a quieter second half of the year (BGVN 27:06 and 27:09).

Figure 24. MODIS-detected alerts on Merapi during 2001-2002. Thermal alerts collated by Diego Coppola and David Rothery; data courtesy of the Hawaii Institute of Geophysics and Planetology's MODIS Thermal Alert Team.

Figure 25. Center coordinates of alert-pixels on Merapi, relative to the published summit location. Grid squares are 1 km. Thermal alerts collated by Diego Coppola and David Rothery; data courtesy of the Hawaii Institute of Geophysics and Planetology's MODIS Thermal Alert Team.

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

Information Contacts: Dali Ahmad, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Diego Coppola and David A. Rothery, Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. Thermal alerts courtesy of the HIGP MODIS Thermal Alerts Team (URL: http://modis.higp.hawaii.edu/).

On 3 November 2002, intense degassing caused bubbling activity near the small islet of Lisca Bianca, very close to the island of Panarea (BGVN 27:10). On 13-14 November 2002, observers Orlando Vaselli (University of Florence), Bruno Capaccioni (University of Urbino), and Piermaria Luigi Rossi (University of Bologna) noted 10 points of boiling water when they visited the area to sample gas emissions.

Geochemical monitoring and research is being regularly performed by the Fluid Geochemistry group from the Osservatorio Vesuviano (Istituto Nazionale di Geofisica e Vulcanologia), led by Giovanni Chiodini. Submarine gas emissions were sampled during 29-30 November and 10-17 December 2002, as well as 23-24 January and 9-11 February 2003. Samples obtained during March, April, and May have not yet been analyzed. Chiodini noted that although the intensity of emissions decreased after 5 November 2002 (BGVN 27:10), the gas flux remained much higher than before the November event. That observation, along with chemical variations in gas samples, indicate that the process is ongoing. Research results posted on the Osservatorio Vesuviano website provide additional details, analytical findings, and hypotheses about these phenomena.

Geologic Background. The mostly submerged Panarea volcanic complex lies about midway between Stromboli and Lipari in the eastern part of the Aeolian Islands. Panarea, the smallest island in the Aeolian Archipelago, lies on the western side of a shallow platform whose shelf margin is at about 130 m depth. A series of small islands breach the surface to form the Central Reefs, the rim of a crater 2 km E of Panarea, whose shallow submerged floor contains Roman ruins. The submerged Secca dei Pesci lava dome lies at the SE end of the platform, and the rhyolitic Basiluzzo lava dome rises 165 m above the surface at the NE end, along a ridge trending towards Stromboli volcano. The complex was constructed in two main stages: an initial effusive activity phase that produced lava domes, and an explosive stage. The youngest subaerial airfall-tephra deposits are dated to about 20,000 years ago; a date of less then 10,000 BP on a lava flow is uncertain. Vigorous hydrothermal activity has continued at fumarolic fields at several locations on the submerged platform; submarine hydrothermal explosions have occurred in historical time.

Information Contacts: Giovanni Chiodini, Unità Funzionale di Geochimica dei Fluidi, Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Via Diocleziano, 328-80124 Napoli, Italy (URL: http://www.ov.ingv.it/); Orlando Vaselli, Dipartimento di Scienze della Terra, Universita' degli Studi di Firenze, Via La Pira 4, 50121 Firenze, Italy; Stromboli Online (URL: http://www.stromboli.net/).

Eruptions at Tavurvur continued to occur throughout January-March 2003. The eruptions were characterized by forceful and convoluted, sub-continuous, light to pale gray ash cloud emissions at irregular intervals. The following was provided by the Rabaul Volcano Observatory.

Activity during January 2003. During the first several days of January (except the 4th), activity was similar to late December 2002. The eruptions consisted of sub-continuous ash emissions occurring at intervals ranging from a few minutes to ~10 minutes. Many of the ash emissions were sustained for 1-2 minutes. On the 4th, activity was at a low point, shown by the fewest ash emissions of the month. Between 8 and 17 January, the pattern of eruption changed slightly to a mixture of events. The sub-continuous ash emissions persisted, but forceful emissions began as well, although not in significant numbers. A complete change in the pattern of eruptive activity began on the 18th. The sub-continuous ash emissions reduced significantly and the sharp forceful emissions became more prominent. They occurred at very short intervals of 2-4 minutes. This pattern of activity was maintained until the 26th. A lot of the forceful emissions between 20 and 26 January were accompanied by low roaring noises. Noises were also heard on the 7th. After 26 January, the magnitude of the forceful emissions eroded and activity changed back to sub-continuous ash emissions at slightly longer intervals. This trend of summit activity continued until the end of the month.

Ash plumes from the eruptive activity rose variably in height. Those from the forceful emissions rose to a maximum of about 1,500 m, while ash plumes from the sub-continuous emissions rose to several hundred meters above the summit. Variable winds blew the ash plumes to the E and SE (1-14 and 22-31 January), and N and NW (15-21 January). Rabaul Town and villages that are located N and NW from Tavurvur had fine ashfall between 15 and 21 January. The S and SE drifting ash fell mainly in the sea; however, very fine specks of it fell on Cape Gazelle including the nearby Tokua Airport, ~20 km from Tavurvur.

Seismic activity reflected the summit activity. Both the sharp forceful and the sub-continuous ash emissions generated seismic waves characteristic of their nature. Seismic waves associated with the forceful emissions had greater amplitudes reflecting greater energy. Average duration of this type of event was about 40-50 seconds. On the other hand, events associated with the sub-continuous ash emissions had lower amplitudes, and their duration ranged between one and several minutes. Only one volcano-tectonic earthquake was recorded.

During the month ground-deformation measurements showed deflation. Real-time GPS measurements showed 5-8 mm of deflation. The electronic tiltmeter showed a few microradians of down-tilt towards the perceived uplift center SE of Matupit Island and SW of Tavurvur.

Activity during February 2003. Forceful ash emissions were observed in February, but not as abundantly as in January. In February, ash emissions were slightly more frequent during the first few and last few days of the month. The emissions occurred at intervals of 4 and 10 minutes. The longest duration for an ash emission during these periods was about 4-6 minutes. Between 5 and 24 February activity fluctuated, and ash emissions occurred at intervals of several minutes. The longest duration for an ash emission during this period was about 15 minutes. This does not necessarily imply that the amount or volume of ash contained in the emissions was consistent throughout the entire duration of emission. Rather, there was higher ash content in the initial stages of the emissions, which faded thereafter to white to pale gray emissions with very little ash content.

Plume heights were similar to those in January. During the month ash plumes were blown mainly to the E and SE, and occasionally to the SW. On 3 and 4 February, some ash plumes drifted N and NW, resulting in fine ashfall in Rabaul Town and nearby villages farther downwind.

Seismic activity was dominated by the long-duration, low-amplitude, tremor-type events, associated with the convoluted, sub-continuous ash emissions. The duration of these events ranged between 2 and 19 minutes. Only one high-frequency, volcano-tectonic earthquake was recorded.

Real-time GPS measurements fluctuated in February. During the first half of the month, measurements showed an inflationary trend. This is a rebound from the month-long deflationary trend observed in January. During the second half of February, movements changed to show deflation. The electronic tiltmeter fluctuated showing no obvious trends.

Activity during March 2003. The general level of eruptive activity in March had minor fluctuations but did not deviate much from previous months. Activity during the first two weeks was a continuation of the last few days of February. Thereafter, activity waned slightly, with ash emissions occurring at slightly longer intervals, with the exception of a couple of half-days on 15 and 16 March, when ash emissions were a bit more frequent. At the same time forceful-type emissions began until about the 23rd, when rates of sub-continuous ash emissions picked up again slightly, surpassing the activity for the first two weeks of the month. The slightly increased level continued until the end of the month. A handful of forceful emissions also occurred.

Ash plumes from the March activity rose 500-1,500 m above the summit before they were blown mainly to the SE. Most ash fell immediately downwind near Tavurvur and the deserted Talvat village. Lighter ash particles drifted farther downwind and fell in the sea.

Seismicity reflected the summit activity. It consisted mainly of low-amplitude tremor-type events with durations ranging from a couple of minutes to about eight minutes. These events were associated with sub-continuous convoluted ash emissions. Short duration, higher amplitude events associated with forceful ash emissions were also recorded but were outnumbered by the former event type. Four volcano-tectonic earthquakes were recorded during the month on the 2nd (2) and 3rd (2).

Ground-deformation measurements in March showed a more distinct and consistent sense of surface movement. Both the realtime GPS and electronic tilt measurements showed inflation. The long-term trend between January and March, as per realtime GPS measurements, was characterized by diurnal-type fluctuations of peaks and troughs, the range being about 20 mm between the highest peak and lowest trough. The cumulative movement for the three-month period was deflation of ~8 mm.

A ML 6.8 tectonic earthquake occurred on 11 March. The quake, located about 120 km SE from Rabaul in offshore southern New Island, and was felt strongly at Rabaul with MM VI. It caused minor landslides in parts of the Gazelle Peninsula.

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, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

The main summit crater continued to release variable amounts of thin-to-thick white vapor during January-March 2003, and no activity was observed from the N valley vent that formed in May 2001. Heavy rains during February and especially on the 19th, 21st, 22nd, and 24th, caused debris flows on the NW side of Ulawun. The debris channeled into Namo creek and later swept down to the coast. Along its course it overflowed into Ubili village. Muddy water flowed into six houses built on concrete floors and left a thin sheet of dried mud a few centimeters thick.

The long-term deformation trend based on measurements from an electronic tiltmeter is slow deflation of the summit area. No significant changes were noted in January. In February there was 2 µrad of deflation, and measurements showed a very small amount (~2-3 µrad) of deflation between the beginning of March through the 25th. After that the trend became steady.

Seismic activity had been low through January-February, but an increase was evident starting on 2 March. This was shown by an increase in RSAM values on the same day. The increased activity remained at low to moderate levels between 2 and 12 March. After that, it declined gradually, reaching low levels on the 20th. Due to technical problems with the only seismograph to monitor Ulawun, no analogue waveforms were recorded, making it difficult to ascertain the type of seismicity associated with the increased RSAM values. However, it is assumed that another of the sporadic volcanic tremor episodes recorded since the September 2000 and April 2001 eruptions was the cause.

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: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.

An increase in seismicity since mid-December was a constant trend through February 2003 (BGVN 28:01). During the week of 7 March, discrete seismic events occurred at a rate of about 1-2 events per minute. On 11 March, a 4-hour period of continuous seismic tremor was followed by 17 hours of discrete seismic events and 3-4-minute-long tremor bursts. This culminated with another 4-hour period of continuous tremor on 12 March. Seismic activity later that week was characterized by discrete small-amplitude events occurring every 1-2 minutes. Satellite images collected during clear periods on 4, 6, 7, and 12 March did not reveal any elevated surface temperatures, ash emissions, or ash deposits. Observers in Perryville, 35 km S of Veniaminof, reported no significant plume or other signs of volcanic activity on 12 March. Consistent elevated seismicity, with small-amplitude discrete events every 1-2 minutes continued during the week of 21 March.

Seismicity declined during the last week of March, characterized by very low-amplitude tremors. Satellite images collected during numerous clear periods that week did not reveal any elevated surface temperatures, ash emissions, or ash deposits. There was a dramatic decrease in volcanic activity during the week of 4 April. However, short periods of volcanic tremor and low frequency events were still recorded. This continued into the week of 11 April, prompting the lowering of the level of concern. The Alaska Volcano Observatory (AVO) announced a code color of green, under which the volcano is classified as dormant with normal seismicity and fumarolic activity occurring.

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

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

The eruption that began in August 2002 continued during early 2003 with lava effusion through at least 28 February and vapor emissions. The following is from the Rabaul Volcano Observatory.

Activity during January 2003. No field or aerial observations of the caldera or lava flow were made in January. However, blue vapor was observed throughout January from the NW-most lava-producing vent and other vents along the NW-SE-trending fissure system, suggesting that hot lava was near the surface and presumably still flowing. Besides the blue vapor emissions, variable amounts of white vapor were released. Evidence of dead and dried vegetation downwind of the fissure system indicated that hazardous gases, such as sulfur dioxide, were present in the vapor emissions. The dead vegetation is restricted to an area extending 1-2 km to the S (downwind). This is unlike similar vegetation effects during the SE-wind season, which extended as far as 10 km to the NW from the source of the vapor emissions. Occasional low roaring noises were heard on 9, 21, 22, 25, and 26 January.

Seismic activity was relatively steady with no significant deviation from the background levels determined since the permanent seismic network was established in early October 2002. Earthquakes consisted mainly of volcano-tectonic (VT) events averaging 45 per day, with a low of 18 (recorded on the 20th) and a high of 71 (on the 4th). The events occurred randomly over each day. Low-frequency earthquakes were recorded on some days; a maximum of six events was recorded on the 18th.

Airlink began to use Hoskins airport in the latter half of January after winds began to blow away from the airport. Furthermore, the absence of ash emissions since August and early September 2002 made conditions favorable. The decision to re-use the airport followed information provided by RVO to the Papua New Guinea Civil Aviation Authority and aviation industry.

Activity during February 2003. An aerial inspection on 28 February showed that lava effusion continued from the NW-most vent of the fissure system (figure 19). The lava flow had two lobes. The main lobe was directed initially to the N but later curved to a northeasterly direction, dictated by topographic features of the Witori caldera floor. On 28 February it appeared that horizontal lateral flow of this lobe had stopped after it reached a topographic barrier. As a result, the lava flow began to gain height along its entire northern portion. The height of the flow was estimated to be ~25-33% of the height of the ~240-m-high Witori Caldera wall. The second lobe of the lava flow, which flowed to the S, showed slow progress. Between October 2002 and February 2003 it advanced only a few hundred meters. The thickness of this flow was ~30-40 m. As of 28 February the total volume of erupted lava from this single vent was estimated to be ~0.09-0.12 km³.

Figure 19. Lava flow lobes from the NW-most vent of the fissure system on 28 February 2003. The view is to the N. The white cloud at the top right of the photo is caused by vapor emissions from the line of vents on the fissure system. Photo by Ima Itikarai, RVO.

Emissions of minor to moderate volumes of white vapor continued from all vents along the fissure system. The lower vents to the NW released more vapor than the upper ones to the SE. Small amounts of blue vapor were released from the lava-producing vent. Because the vapor emissions were blown S and SE, vegetation within 2 km downwind turned brown. No ash emissions were produced during the month. Low jet-roaring noises were heard on 4, 9-11, 13, and 21 February. Hoskins airport continued to be used by Airlink in February.

Seismic activity was low during the month. Earthquakes were mainly volcano-tectonic. The daily count was ~30 compared to 45 in January. Most of the earthquakes were very small ones, but moderate-sized events were recorded on 1 (2 events), 10 (2), 12 (1), and 18 February (6). The six earthquakes on the 18th were recorded within a time span of 1.5 hours. A handful of low-frequency earthquakes were also recorded on the 6th (2), 10th (1) and 11th (1).

Activity during March 2003. No field or aerial observations of the lava flow were made in March, so it is uncertain whether lava effusion from the NW-most vent continued. The upper vents continued to release weak emissions of thin white vapor. The lower vents released weak to moderate emissions of white vapor and bluish vapor emissions on 13, 18, 23, and 28-30 March, indicative of hot material. Low roaring noises heard on 13, 16, 18, 23, and 29 March did not accompany explosive activity. No seismic recordings were made in March.

Geologic Background. The active Pago cone has grown within the Witori caldera (5.5 x 7.5 km) on the northern coast of central New Britain contains the active Pago cone. The gently sloping outer caldera flanks consist primarily of dacitic pyroclastic-flow and airfall deposits produced during a series of five major explosive eruptions from about 5,600 to 1,200 years ago, many of which may have been associated with caldera formation. Pago cone may have formed less than 350 years ago; it has grown to a height above the caldera rim, and a series of ten dacitic lava flows from it covers much of the caldera floor. The youngest of these was erupted during 2002-2003 from vents extending from the summit nearly to the NW caldera wall. The Buru caldera cuts the SW flank.

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P.O. Box 386, Rabaul, Papua New Guinea.