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/).

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).

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/).

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/).

From 8 November through mid-December 2003 the hazard status of Alaid remained at Yellow. Weak seismic activity has remained slightly above background levels since 31 October. The volcano was also obscured by clouds during this period. Continuous spasmodic volcanic tremor was recorded (0.15-3.4 x 10^-6 m/s), and a large number of weak local events were registered, during each week. The report for the week of 12-19 December indicated that seismologists have now decided that the tremor is probably not of volcanic origin, but has probably been a result of strong winds. The Level of Concern Color Code was lowered to Green on 19 December 2003.

Geologic Background. The highest and northernmost volcano of the Kuril Islands, Alaid is a symmetrical stratovolcano when viewed from the north, but has a 1.5-km-wide summit crater that is breached open to the south. This basaltic to basaltic andesite volcano is the northernmost of a chain constructed west of the main Kuril archipelago. Numerous pyroclastic cones are present the lower flanks, particularly on the NW and SE sides, including an offshore cone formed during the 1933-34 eruption. Strong explosive eruptions have occurred from the summit crater beginning in the 18th century. Reports of eruptions in 1770, 1789, 1821, 1829, 1843, 1848, and 1858 were considered incorrect by Gorshkov (1970). Explosive eruptions in 1790 and 1981 were among the largest reported in the Kuril Islands.

Information Contacts: Olga Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Asama has been seismically active since 18 September 2000. Heightened seismicity occurred in June 2002, when the daily number of volcanic earthquakes exceeded 300 (BGVN 27:06). The Asama Volcano Observatory (ERI, University of Tokyo) and Japan Meteorological Agency (JMA) reported a new episode of elevated seismicity during 18-19 September 2002 (BGVN 28:04). According to JMA there were brief ash eruptions on 6 February, 30 March, 7 April, and 18 April 2003 to heights of 200-300 m above the crater with minor ashfall around the summit (BGVN 28:04).

Seismic data and plume observations compiled from JMA reports for September 2000 through April 2003 (table 2) reflect this recent activity. White plumes were reported from the Kama-yama crater during every month in this period, with the addition of grayish white plumes on 6 February, 7 April, and 18 April. These white plumes only rose to 1 km or above in April and May 2001, and June and August 2002. In addition, short isolated episodes of volcanic tremor were recorded in October 2001, February 2003, and March 2003. However, 12 episodes occurred in April 2003, with five on the 29th.

Table 2. Summary of seismicity and plume observations at Asama, January 2000-April 2003. All reported plumes originated from the Kama-yama crater, and were described as either white (W) or grayish white (GW). Data courtesy of JMA.

Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asamayama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has observed activity dating back at least to the 11th century CE. Maekake has had several major Plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/).

This report largely discusses seismicity at Cayambe during January-October 2003 (figure 1). On the whole, the numbers of daily earthquakes remained fairly constant at low to moderate levels (typically fewer than 25 earthquakes per day). On six days the number of daily earthquakes approached 50 or greater (1 January, 15, 28, 29, and 30 March, and 1 April). Epicenters were concentrated on the SW flank, similar to the pattern in December 2002. During the year, residents did not report feeling earthquakes at Cayambe, but did notice sulfurous odors. Although some seismic signals had an uncertain origin, others were interpreted as related to magma movement.

Figure 1. The total number of earthquakes each day at Cayambe between 1 January and 27 October 2003. Courtesy of the Instituto Geofisico-Escuela Politecnica Nacional, Ecuador.

Observers saw no changes at Cayambe during January, although strong sulfurous odors were reported early in the month. Very low seismicity prevailed, with an average of five earthquakes per day during the second week in January. In general, the earthquakes registered since 2 January 2003 were long-period earthquakes; a few hybrid events and fracture events also were recorded. Some small volcano-tectonic (VT) earthquakes registered. On 31 January, following a week of low to moderate seismicity, two larger earthquakes were recorded: the first under the volcano, M 3.9; the second to the E, M 3.5. Neither earthquake was felt by area residents. Associated, small long-period (LP) earthquakes also registered, although the late-January daily totals were still low to modest. Following this activity, seismicity dropped and generally remained low through February and early March.

On 8 March an M 3.6 earthquake triggered about 2 hours of small VT earthquakes beneath Cayambe's S flanks. Seismicity again dropped to low levels until 14 March when there was a small cluster of shallow VT earthquakes lasting about 1 hour. These events were under M 2, and afterwards seismicity dropped to background levels.

The high for the year occurred during 24-30 March when earthquakes peaked at approximately 335 per day. Clusters of events were noted. Those on 27 March included an M 3.9 earthquake, and those on 29 March included two M 3.6 earthquakes. The events were located at 5-6 km depth below the SW flank. This was the same area in which seismic clusters occurred during December 2002. There were 99 earthquakes counted on 31 March. None larger than M 3, they consisted of up to 95 VT and four hybrid events.

In early April seismicity again dropped to low levels, increasing slightly in the second week of the month. The recorded LP and hybrid events registered were thought likely related to fluid movement inside cracks. Small VT earthquakes were recorded on 13 April; otherwise, seismicity remained low through the rest of the month. The smell of sulfur was noticed during a 14-20 April visit, but no other changes were noted. Seismicity remained low throughout May, with only occasional VT, LP, and hybrid events.

Seismicity increased slightly in early June, but still remained close to background levels. A new signal appeared at the Refugio station, characterized by high frequency at the beginning and a very long coda (tail). Thereafter, activity dropped and remained low for the rest of June.

A series of VT earthquakes occurred during the first week of July; the maximum activity was on 1 July, with 16 events. Nevertheless seismicity remained near background levels, and stayed low until early August. A VT earthquake occurred on 4 August beneath the W flank; on 6 August seismicity increased slightly and was characterized by hybrid events. Activity was low through mid-August. A series of VT events related to rock fracturing occurred between 18 and 31 August; the majority occurred on 22 and 25 August, with events reaching M 3.5. These events beneath the SW flank were similar to the spike in activity in March. Activity dropped again during the following month.

For the last week of September, seismicity was still at background levels. A single VT event registered, with a magnitude of 3.2, located beneath the volcano at a depth of 4 km. As with the previous cases, the event was too small to be felt by area residents. Between 13 and 19 October, seismicity generally remained normal, but on 15 October, a small cluster of VT earthquakes occurred S of the crater. On 23 October another small cluster of VT earthquakes (M <3) was recorded beneath the SE flank, in a part of the same zone as the events of the previous week.

Over these weeks the base seismicity level seemed to increase slightly (figure 1). During 3-9 November, about nine earthquakes occurred per day, near the daily average since August 2003 and about twice the daily average of 4/day before that. Seismicity also included small clusters of tectonic earthquakes with magnitudes less than 3. There were reports of a strong scent of sulfur in the vicinity of Picos Jarrína at an elevation of about 5,460 m. The scent was strongest near cracks.

Geologic Background. The massive compound andesitic-dacitic Cayambe stratovolcano is located on the western edge of the Cordillera Real, east of the Inter-Andean Valley. The volcano, whose southern flank is on the equator, is capped by extensive glaciers. The modern Nevado Cayambe, constructed to the east of older Pleistocene volcanic complexes, contains two summit lava domes about 1.5 km apart. Several other lava domes on the upper flanks have been the source of pyroclastic flows that reached the lower flanks. A prominent Holocene pyroclastic cone on the lower E flank, La Virgen, fed thick andesitic lava flows that traveled about 10 km E. Nevado Cayambe has also produced frequent explosive eruptions beginning about 4,000 years ago, with the most recent known activity during 1785-86.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

As previously reported (BGVN 28:08) a new crater formed at the summit following large explosions in July and August 2003. Smaller ash-bearing eruptions continued during September-December 2003.

On 6 September a strong ash emission resulted in an ash cloud that rose to ~6.7 km and drifted N. Ash was not visible on satellite imagery, but a second ash emission on 8 September was visible on the Colima video camera. Into early October, volcanic activity consisted of an average of two explosions per day, producing ash clouds that rose ~2 km above the crater and drifted predominately W. Tropical storm Olaf inundated the Colima area on 7 October, dropping 150 mm of rain in less than 2 hours. The heavy rain mixed with material on the S flank, producing a lahar down the Montegrande ravine. On 9 and 10 October ash clouds were visible on satellite imagery rising to a maximum of ~5 km above the volcano.

On 16 October ash rose to a height of ~6 km; a second plume followed on 18 October, rising to ~7.3 km. Neither plume was visible on satellite imagery. Two small eruptions consisting mainly of steam and some ash on 30 October rose to ~7.3 km altitude and mainly drifted W.

A subtle ash plume, visible in satellite imagery, was emitted on 18 November and rose to ~5.5 km altitude. On 1 and 2 December, ash clouds were visible on satellite imagery at a maximum altitude of ~7 km. As of 12 December, the volcano continued with an average of three explosions a day, usually to 2 to 3 km above the crater. The majority of these explosions have produced ash that drifted toward the ENE. The most significant of these early December explosions occurred early on 11 December, when materials descended the SE, NE, and N flanks, and ashfall was reported in the town of Guzman (25 km NE of Colima volcano).

Geologic Background. The Colima complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide scarp, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent recorded eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Observatorio Vulcanológico de la Universidad de Colima, Colima, Col., 28045, México (URL: https://portal.ucol.mx/cueiv/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

This report contains details of seismicity at Cotopaxi during January through 2 May 2003. The seismicity was generally low (averaging ~20 earthquakes per day), as it has been since 24 November 2001. Despite the low seismicity, during January seismic signals suggestive of emissions registered, although these lacked visual confirmations at the volcano. Moreover, a cluster composed of a variety of kinds of shallow earthquakes took place in mid-March. This was the first such cluster since 19 July 2002.

Activity during January-February 2003. Seismicity was generally low in January 2003 and located earthquakes commonly had focal depths down to 5 km below the summit. During the first week of January one volcano-tectonic (VT) event occurred N of the volcano. Around this time the rate of energy release was very low and no unusual observations were reported. Seismicity decreased after the first week of January, although some long-period (LP) events occurred, including one of high frequency (10 Hz) on 9 January that was followed immediately by another with a slowly decaying coda or tail (a so-called "tornillo" event, with a dominant frequency of 2.7 Hz). Two LP events were located at depths of 1 km. The rate of energy release remained very low, with some peaks on 8 January. Seismicity stayed low through the next week; some hybrid and LP events did occur. Some signals characteristic of emissions were received, although these were not visually confirmed.

During 20-26 January the number of hybrid events increased slightly, to above average. Emission signals were again received, similar to the previous week. No LP earthquakes were recorded this week, but a small group of earthquakes were located at the headwaters of the Pita river. Events such as these were also noted in November 2001. During the last week of January, seismicity remained low, on a par with activity seen since 24 November 2001. However, the low number of events registered or located was partly because arrivals were not clear at many stations.

Seismicity remained low in February, particularly for the first week. During 10-16 February it rose slightly due to larger numbers of hybrid events. No other changes in the volcano were noted. Although the third week of February brought no important variations in seismicity, beginning in late February LP events dominated the record. Still, the number of LP event stayed below the 2002 average.

Activity during March-April 2003. Although low seismicity generally prevailed throughout this interval, there was some variations in the abundance of earthquake types and a mid-March cluster of earthquakes occurred. During early March hybrid earthquakes increased to slightly higher than the 2002 average; in addition another LP-type tornillo was recorded on 6 March. On 7 March LP earthquakes were common.

On 16 March a cluster of hybrid, VT, and LP earthquakes was located 1-3 km below the volcano. Following eight months of low seismicity (averaging ~20 events per day), this was the first seismic swarm registered at Cotopaxi since 19 July 2002. However, the energy released per number of events was similar to earlier activity.

Seismicity increased after 16 March. Clusters similar to that of the 16th continued, but with lower magnitudes. By the beginning of April seismicity decreased to within the base level, although on 4, 7, and 8 April VT events were recorded to the S and SE, approximately 3 km below the summit. No significant changes were noted at the volcano, although the usual smell of sulfur was noted on a visit to the summit. During 14-20 April, the number of LP events decreased from the previous week, but VT events of M 2.5-M 3.4 continued to the N. VT events persisted through the rest of April, particularly in late April, which on 23 April included an M 3.6 event. VT events occurred on the N, NE, and S sides of the volcano up to 15 km from the summit at depths between 3 and 15 km. The VT events were interpreted as related to rock fracturing.

On the morning of 2 May a VT event registered on the S flank, located ~3 km deep. It was M 3.2, moderate for Cotopaxi. Later that day an event registered at the seismic stations at Cotopaxi, Antisana, and Guagua Pichincha. This event had a duration of 180 seconds and was made up of an LP earthquake followed by a tremor-like signal with a duration of 150 seconds that was of low frequency (1.6 Hz).

Geologic Background. The symmetrical, glacier-covered, Cotopaxi stratovolcano is Ecuador's most well-known volcano and one of its most active. The steep-sided cone is capped by nested summit craters, the largest of which is about 550 x 800 m in diameter. Deep valleys scoured by lahars radiate from the summit of the andesitic volcano, and large andesitic lava flows extend to its base. The modern edifice has been constructed since a major collapse sometime prior to about 5,000 years ago. Pyroclastic flows (often confused in historical accounts with lava flows) have accompanied many explosive eruptions, and lahars have frequently devastated adjacent valleys. Strong eruptions took place in 1744, 1768, and 1877. Pyroclastic flows descended all sides of the volcano in 1877, and lahars traveled more than 100 km into the Pacific Ocean and western Amazon basin. Smaller eruptions have been frequent since that time.

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

Ash explosions during 29 September-19 October rose 50-100 m above the crater. Some of the explosions were accompanied by blasting sounds, and ash fell to the E around the Tobelo area. White-gray ash eruptions continued during 27 October-30 November accompanied by booming sounds, a 100-m-high gray ash column, and more ashfall in the Tobelo area. The hazard status remained at Alert Level 2 (on a scale of 1-4) throughout this period.

The monitoring effort by the Volcanological Survey of Indonesia has been affected by the civil unrest on Halmahera. By 19 November the Dukono observatory had been completely destroyed by the recent "riots," with no idea when it might be rebuilt. Officially there is no longer a VSI officer there, but sometimes two staff check on the volcano. On 18 November they called Bandung with news that activity had increased, with larger, more frequent eruptions (every 5-15 minutes) generating higher plumes. Over the few weeks prior to 18 November eruptive activity has been tending towards bigger explosions, sometimes producing pyroclastic falls and lava flows down to the beach.

Ash plumes from Dukono have been identified in satellite imagery by the Darwin Volcanic Ash Advisory Centre almost every week since early June 2003. Plumes were usually reported rising to altitudes of 3-4.5 km and extending downwind 45-75 km from the summit. Longer plumes to distances of 80-130 km were reported after 18 August. Distinct visual plumes reaching distances of over 200 km were seen in November (figure 2), with a maximum of ~300 km during 12-18 November. Aviation notices continued to be issued through December, warning of almost continuous activity and plumes extending ~90 km from the volcano.

Figure 2. Satellite image showing an ash plume extending S and SE from Dukono on 17 November 2003. The volcano is located below the "D" in Dukono, and the approximate extent of the ash cloud has been outlined. Courtesy of Darwin VAAC.

Paul Taylor provided the following account from Baptist missionary Charles W. Cole. He also noted that the Tobelo "o dukono" just means "(the) volcano," but that the word is now used as a proper name for the volcano on Halmahera. On 16 November 2003 Cole wrote: "The situation in Tobelo continues to be on edge as unknown parties continue to explode bombs. After the distribution of the food packets in Ternate, Kenneth and Oky accompanied the Tobelo pastors on a seven hour ride in a small van to Tobelo. When they arrived in Tobelo it looked like it had just snowed. The ground and all the buildings were covered with white ash. This past week it has rumbled and put out large clouds of white ash. Entire villages and neighborhoods have been destroyed (not by ash)."

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: Dali Ahmad, Hetty Triastuty, Nia Haerani, and Suswati, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Charles W. Cole, Jakarta, Indonesia; Paul W. Taylor, Australian Volcanological Investigations, PO Box 291, Pymble, NSW 2073, Australia; Andrew Tupper, Darwin Volcanic Ash Advisory Centre (VAAC), Commonwealth Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

White gas emissions and glow were reported at Karangetang during October 2003. The Volcanological Survey of Indonesia (VSI) reported continuing activity over the period 26 October-30 November, with white gas plumes rising 350-400 m above the S crater rim and 50-150 m above the N crater. On 28 October an ash explosion produced a 2,000-m-high column with ashfall reaching the sea to the E and a lava avalanche toward the Batu Awang area, 750 m from the summit. Except for the week of 17-23 November, local seismicity decreased compared to the first half of October (table 10). The hazard status remained at Alert Level 2 (on a scale of 1-4).

Table 10. Seismicity at Karangetang during 27 October-30 November 2003. One explosion and one avalanche also occurred during the week of 27 October-2 November. Courtesy of VSI.

Geologic Background. Karangetang (Api Siau) volcano lies at the northern end of the island of Siau, about 125 km NNE of the NE-most point of Sulawesi. The stratovolcano contains five summit craters along a N-S line. It is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675 and many additional small eruptions that were not documented (Neumann van Padang, 1951). Twentieth-century eruptions have included frequent explosive activity sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters; collapse of lava flow fronts have produced pyroclastic flows.

Information Contacts: Dali Ahmad, Hetty Triastuty, Nia Haerani, and Suswati, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

From late August through 5 December seismic activity at Karymsky was above background levels (100-230 events per week) and intermittent explosions continued. The Level of Concern Color Code was Yellow through most of September and October, with a week at the higher Orange status during 3-10 October. The color code was raised to Orange again on 31 October and remained at that level through 5 December. Thermal anomalies identified in satellite data were usually 1-4 pixels in size, with a maximum of 6 pixels on 30 August, and 10, 11, 14, and 16 October. However, the weather was frequently cloudy after 12 September, obscuring observations.

Ash explosions rising up to 4.0 and 4.7 km were observed from aircraft on 29 August. About 2 hours of continuous spasmodic tremor (6.0 x 10^-6 m/s) on 30 August, followed by the detection of a thermal anomaly (6 pixels) less than an hour later, may have been caused by a pyroclastic flow.

On 9 and 10 September, continuous high-frequency spasmodic tremor and a series of shallow seismic events indicated possible ash-and-gas explosions to heights of 1.5-2.0 km above the volcano. A gas-and-steam plume extending 100 km E was noted on 9 September. On 14 September an ash-and-gas plume was seen rising 500 m above the crater. On 23 September there was an explosive ash plume up to 5 km altitude according to visual data from the Institute of Volcanology.

The number of shallow seismic events increased during 4-24 October to weekly highs of 350; these events indicated possible ash-and-gas explosions to heights of 1-1.5 km. Ash plumes extending 60 and 30 km SE and NE were observed on 4 and 7 October, respectively. An extensive gas-and-steam plume extending 85 km SE was noted on 10 October. Continuous high-frequency spasmodic tremor detected for almost an hour on 10 October probably indicated pyroclastic flows. Ash plumes extending 45-50 km NW were observed on 16 October. On 31 October, a possible plume extending ~65 km NNE was observed in a satellite image. Gas-and-steam plumes with possible minor ash ~40-60 km long were detected on 20, 21, 24, and 26 November; clouds obscured the volcano on other days.

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: Olga Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Through September and into early October, lava was moving along the E and W sides of the Mother's Day flow. The E-side lava (known as the 9 August breakout) came from the 9 August rootless shield (see figure 2 in BGVN 28:09), itself fed by the main Mother's Day tube from Pu`u `O`o. The W side lava, known as the Kohola arm of the Mother's Day flow, branched off the tube system below the rootless shield. In early October the 9 August breakout stopped, the Kohola died back to a trickle, and the rootless shield gained prominence. By 16 October, however, the shield had partly collapsed, leaving several drained perched ponds behind. Upstream from the shield, many hornitos and small flows formed over the Mother's Day tube.

During 1-7 October, surface lava flows were sometimes visible on Kīlauea's coastal flat and upslope areas. On 2 October lava began to flow W after filling West Gap Pit on the W flank of Pu`u `O`o cone. Fairly vigorous spattering was visible in the pit, but died to only sporadic bursts later in the day. The flow appeared to have stopped by 4 October when no glow was observed coming from the pit. During 8 October-17 November, a few areas of surface lava were visible upslope of Kīlauea's coastal flat. On 5 November, two small breakouts occurred. The freshly escaping lava was seen on the Kohola arm of the Mother's Day flow just below the top of Pulama pali. Observers watching a 30-40-m-diameter crater on the SW side of Pu`u `O`o noted a new lava pond, a new lava flow, and a fuming cone-pit. Visits to active flow fields on 7 November resulted in observations of hornitos, a 200-m-wide rootless shield, and the leading edge of a 45-m-wide flow.

Seismicity at the summit continued at moderate levels, with 1-2 small low-frequency earthquakes per minute occurring at shallow depths beneath the summit caldera during October and November 2003. Volcanic tremor at Pu`u `O`o remained moderate to high, as is the norm. There were some larger earthquakes at depths of a few kilometers.

Also, there were small inflation and deflation events during this period. Tiltmeters on the NW side of Kīlauea's caldera rim (Uwekahuna) and on the NW flank of the active vent along the East rift zone (Pu`u `O`o cone) showed several microradians of radial tilt during 5-11 November, but the patterns were complex and plagued by instrument problems. During 12-17 November, small amounts of inflation and deflation occurred, including inflation on 17 November that started when the surface waves from a M 7.5 earthquake at Rat Island in the Aleutians reached Kīlauea. The inflation was small, ~0.5 rad at Pu`u `O`o tilt station and 0.3 rad at Uwekahuna station. Small amounts of inflation and deflation were recorded through the week of 19-25 November with sharp deflation beginning at both Uwekahuna and Pu`u O`o early on the morning of 25 November.

Moderate, shallow seismicity was recorded beneath the summit, and moderate to high seismicity occurred beneath Pu`u O`o. The seismic record at Kīlauea's summit during 15-16 December was nearly devoid of earthquakes, though the background is steady weak tremor. Tremor at Pu`u `O`o was continuously at a moderate level. Otherwise, seismicity at Kīlauea was at a low level during this period.

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, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/).

Significant activity from Kliuchevskoi continued throughout 1 August to 5 December 2003, so the hazard status remained at Color Code Orange. Activity included ash explosions that generated long plumes, Strombolian activity in the central crater, thermal anomalies seen in satellite imagery, relatively strong shallow seismicity, and continuous spasmodic tremor. Kamchatka Volcanic Eruptions Response Team (KVERT) reports obtained via the Alaska Volcano Observatory (AVO) provided detailed reports of significant daily activity that is summarized below.

Gas-and-steam plumes, sometimes with ash, were frequently seen rising above the crater to heights of less than 1,500 m. However, on some days plumes were seen rising as high as 2,500-3,000 m. Most of the plumes dissipated after reaching distances described as greater than 10 or 20 km downwind. Satellite imagery showed that on 8-9 September ash-and-gas plumes extended 172 km to SW and 153 km to W. Long ash plumes to distances of 18-63 km SE were seen on 4 October. During mid-October (12, 16, 17, and 18) gas-and-steam plumes reached distances of 25-70 km in many directions. On 24 October an airline pilot reported an ash plume at ~6,800 m altitude extending to the NNE. A gas-and-steam plume approximately 50-55 km long extending to the ESE was noted on 10 November, and another with minor ash extended ~40 km E on the 16th.

Strombolian activity at the central crater was detected on 26 August, when volcanic bombs rose up to 200 m above the crater and explosions occurred at intervals of about 5 minutes. More Strombolian activity was seen by observers in Klyuchi and Kozyrevsk on 25 and 30 September, 2-4, 6-8, and 10-11 October, and 9-10, 14-15, 21, 27, and 29 November. Thermal anomalies were detected every week by USA and Russian satellites, sometimes as large as 8-9 pixels.

Recorded earthquakes at 30-km depth usually ranged up to 9/day through early November, with up to 18/day the week of 1-7 August, and 30 on 3 October; magnitudes were 1.6-2.6. Continuous spasmodic tremor had geophone velocities below 8 x 10^-6 m/s until 4 October, when velocities increased into the 8-20 x 10^-6 m/s range. Geophone velocities dropped again to 5-11 x 10^-6 m/s during 22 November-2 December, then rose to 18 x 10^-6 m/s through 5 December. Large shallow seismic events (M 1.7-2.6) were first reported during the week of 11-17 October. Nine such events that week were followed by totals of 4, 22, 48, and 43 per week over the next month. Counts increased to 75 for the week of 15-21 November, 80 during 22-28 November, and 130 for the week ending on 5 December. Large numbers of weak shallow earthquakes (counts not reported) were also recorded every week.

Geologic Background. Klyuchevskoy is the highest and most active volcano on the Kamchatka Peninsula. Since its origin about 6,000 years ago, this symmetrical, basaltic stratovolcano has produced frequent moderate-volume explosive and effusive eruptions without major periods of inactivity. It rises above a saddle NE of Kamen volcano and lies SE of the broad Ushkovsky massif. More than 100 flank eruptions have occurred during approximately the past 3,000 years, with most lateral craters and cones occurring along radial fissures between the unconfined NE-to-SE flanks of the conical volcano between 500 and 3,600 m elevation. Eruptions recorded since the late 17th century have resulted in frequent changes to the morphology of the 700-m-wide summit crater. These eruptions over the past 400 years have originated primarily from the summit crater, but have also included numerous major explosive and effusive eruptions from flank craters.

Information Contacts: Olga Girina, Kamchatka Volcanic Eruptions Response Team (KVERT), a cooperative program of the Institute of Volcanic Geology and Geochemistry, Far East Division, Russian Academy of Sciences, Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia, the Kamchatka Experimental and Methodical Seismological Department (KEMSD), GS RAS (Russia), and the Alaska Volcano Observatory (USA); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), the Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and the Alaska Division of Geological and Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.

Activity at Manam remained low during 10 November-14 December 2003. Occasional emissions of weak to moderate gray-brown ash clouds continued from Main Crater, at a lower level compared to late October-early November. An explosion on 11 November produced an ash plume that rose slowly to ~400 m above the summit crater, causing ashfall to the E. Occasional low rumbling and weak roaring noises were heard on 12 and 28-30 November. No night-time glow was observed during November. A forceful gas emission on 5 December sent an ash column ~500 m above Main Crater, and a steady glow was observed on the night of 10 December. Southern Crater gently released weak thin white vapor gently throughout the period. Small low-frequency volcanic earthquakes continued, with a slight increase in seismicity characterized by sub-continuous volcanic tremors after 1 December.

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.

Monowai is a frequently active submarine volcano that has had volcanic earthquake swarms recorded in May 2002 (BGVN 27:05), November 2002 (BGVN 28:02), and April-May 2003 (BGVN 28:05). Monowai exhibited increased activity for the first 11 months of 2003, during which more than 1,300 events were detected and located by the French Polynesian seismic network via hydro-acoustic waves (also called T-waves) generated by this submarine volcano. Each volcanic eruption, creating explosions and boiling water, generates hydro-acoustic waves that are able to propagate several thousands of kilometers through the ocean in a wave guide (a low velocity zone located at ~1,000 m depth called the SOFAR—Sound Fixing and Ranging—channel). Consequently, a major part of the volcanic activity can be monitored at great distances in the oceans (> 3,000 km) using the T-waves. The amplitudes of T-waves correlate with the strength or intensity of the eruptions.

Activity at Monowai during 2003 has been much greater than in 2002. From April through November 2003, 11 swarms have been detected (figure 14); about one swarm was detected every two weeks except in the months of June and September, when no T-waves were detected from Monowai. A volcanic swarm started suddenly on 10 April 2003 with an average rate of about 50-60 events per day (figure 15). The activity seemed to decrease smoothly in 2003 compared to abrupt halts in activity in 2002.

Figure 14. Amplitudes of T-waves from Monowai recorded at the TVO station on Tahiti during 1 April through early November 2003. Courtesy of Laboratoire de Geophysique, Tahiti.

Figure 15. Number of daily seismic events at Monowai recorded by the Polynesian seismic network, RSP (Réseau Sismique Polynésien) during 5 April through early November 2003. These numbers are probably largely underestimated. Courtesy of Laboratoire de Geophysique, Tahiti.

The observed swarms are composed of a lot of events over at least 30 minutes containing many tens of signals, producing a quasi-continuous and fluctuating noise. However, these types of signals are detected across the entire seismic network. If a particular wave (a maximum amplitude, or the beginning of a wave) visible in all the seismic records is selected, it is possible to locate the source by the inversion of the travel time. Sometimes strong amplitudes are recorded that can be correlated with the strength of the volcanic explosion.

Geologic Background. Monowai, also known as Orion seamount, is a basaltic stratovolcano that rises from a depth of about 1,500 to within 100 m of the ocean surface about halfway between the Kermadec and Tonga island groups, at the southern end of the Tonga Ridge. Small cones occur on the N and W flanks, and an 8.5 x 11 km submarine caldera with a depth of more than 1,500 m lies to the NNE. Numerous eruptions have been identified using submarine acoustic signals since it was first recognized as a volcano in 1977. A shoal that had been reported in 1944 may have been a pumice raft or water disturbance due to degassing. Surface observations have included water discoloration, vigorous gas bubbling, and areas of upwelling water, sometimes accompanied by rumbling noises. It was named for one of the New Zealand Navy bathymetric survey ships that documented its morphology.

Information Contacts: Dominique Reymond and Olivier Hyvernaud, Laboratoire de Geophysique, CEA/DASE/LDG Tahiti, P.O. Box 640, Papeete, French Polynesia.

Seismic activity at On-take has been ongoing in recent years. Data provided by the Japan Meteorological Agency indicates that from January 2000 through April 2003 an average of 140 volcanic earthquakes per month were recorded by the local seismic station. The number usually ranged between 90 and 200 each month. Activity was higher in July 2001 (300 total events, with 65 on the 1st) and December 2002 (206 total events, with 63 on the 4th). No volcanic tremor was registered. White plumes rising no higher than 300 m were observed once in June 2000 and March 2001, and more frequently during November 2001-January 2002. Small white plumes were seen once per month in September-November 2002 and January-March 2003.

Geologic Background. The massive Ontakesan stratovolcano, the second highest volcano in Japan, lies at the southern end of the Northern Japan Alps. Ascending this volcano is one of the major objects of religious pilgrimage in central Japan. It is constructed within a largely buried 4 x 5 km caldera and occupies the southern end of the Norikura volcanic zone, which extends northward to Yakedake volcano. The older volcanic complex consisted of at least four major stratovolcanoes constructed from about 680,000 to about 420,000 years ago, after which Ontakesan was inactive for more than 300,000 years. The broad, elongated summit of the younger edifice is cut by a series of small explosion craters along a NNE-trending line. Several phreatic eruptions post-date the roughly 7300-year-old Akahoya tephra from Kikai caldera. The first historical eruption took place in 1979 from fissures near the summit. A non-eruptive landslide in 1984 produced a debris avalanche and lahar that swept down valleys south and east of the volcano. Very minor phreatic activity caused a dusting of ash near the summit in 1991 and 2007. A significant phreatic explosion in September 2014, when a large number of hikers were at or near the summit, resulted in many fatalities.

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/).

The Centro Nacional de Prevención de Desastres (CENAPRED) provided daily reports for Popocatépetl describing the comparatively quiet interval of 1 August-5 December 2003. When the volcano was visible it typically gave off minor gas plumes characterized by statements such as "low fumarolic activity" and "without important emissions." The hazard status remained at Yellow-Phase II.

A series of aerial photos enabled scientists to view the state of the crater floor on 21 July, 25 August, 17 October, and 6 November 2003. All of these failed to disclose the growth of an external lava dome. In addition, some of the reports suggested that the floor of the inner crater had subsided.

On the vast majority of days during the reporting interval there were fewer than 10 exhalations, and on ~45% of these days, four or fewer exhalations. Although some resulting plumes contained ash, the vast majority of exhalations (which are detected seismically) were described as low intensity. In a few cases, particularly in August and on 1 September, exhalations occurred 20-89 times per day and reached moderate intensity. Daily reports on some of those days cited elevated groundwater levels due to recent snow or rainfall (rather than deeper magmatic processes) as the cause of increased exhalations.

The most exhalations were registered during August 2003, a month when six days had 12 or more exhalations. In contrast, during September-November 2003 there were only four days reported to have had more than 10 exhalations. Exhalations exceeded twenty on 2 August (35), 23 August (60), 28 August (89), and 1 September (43). On days when exhalations exceeded twenty, often (though not always) one or more of the plumes contained small amounts of ash. For example, an ash-bearing plume was noted at 0300 on 2 August. On that day low-amplitude tremor registered for about 1 hour. The 60 exhalations on 23 August were described as small to moderate, generating plumes composed of gas and steam. They were thought to be related to intense rains during the preceding days. The 89 exhalations on 28 August 2003 were similarly described as low to moderate and accompanied by small steam and gas emissions. On 28 August at 1330 an eruption occurred that bore a low density of ash. The plume reached a height of about 1,500 m above the crater; it dispersed towards the W with no reported ashfall. This event was accompanied by episodes of high-frequency and low-amplitude tremor.

Tremor frequently went unreported. When mentioned, CENAPRED said it took place for up to approximately 2 hours per day, but in some cases only several minutes per day. Small (M ~2-3) earthquakes were repeatedly noted during the interval, including a few in the last half of August, several in September, two in October, and seven in November. During 1-5 December one such earthquake occurred. During the August-5 December interval the largest earthquake, M 2.9, took place on 5 November 2003.

Several examples can serve to illustrate the reported data on many of these earthquakes, which occurred in vicinity of the volcano at depths of a few kilometers. On 20 August seismometers recorded an M 2 volcano-tectonic earthquake 1 km N of the summit at 4.8 km depth. At 2312 on 7 September there was a M 2.2 volcano-tectonic earthquake 6.5 km SE of the crater. At 2137 on 8 September a M 2.3 volcano-tectonic earthquake 5 km below the crater registered.

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

Information Contacts: Angel Gómez Vázquez, Alicia Martinez Bringas, Roberto Quass Weppen, Enrique Guevara Ortiz, Gilberto Castela Pescina, and Javier Ortiz Castro, Centro Nacional de Prevención de Desastres (CENAPRED), Av. Delfín Madrigal No.665, Coyoacan, México D.F. 04360, Mexico (URL: https://www.gob.mx/cenapred/); Servando De la Cruz-Reyna and Carlos Valdez Gonzalez, Instituto de Geofísica, UNAM, Cd. Universitaria, Circuito Institutos, Coyoácan, México D.F. 04510, Mexico (URL: http://www.geofisica.unam.mx/).

Emissions of light to pale ash clouds from Tavurvur characterized activity during November 2003. Between 10 and 20 November the ash emissions occurred frequently at irregular intervals. During 20-24 November Tavurvur produced only a handful of emissions at very long intervals, but after 24 November the emissions became frequent. Occasional moderate explosions through 14 December produced thick ash plumes that rose 1-2.5 km above the summit. Incandescent lava fragments from some explosions were visible at night and occasional roaring and rumbling noises were heard. After 16 November winds were consistently from the SE, blowing ash plumes N and NW. Ashfall resulted in downwind areas, including Rabaul Town and villages on Tavui Peninsula, Malagura and Matupit; accumulation was heaviest in the area of Rabaul Town. Fine ashfall also occurred to the W, SW, S, and SE.

Seismicity has been low, with some high-frequency earthquakes from the NE. Ground deformation in November remained low. The real-time GPS and electronic tilt site on Matupit Island, in the center of the caldera, continued to indicate a slow gradual uplift. This uplift is part of the long-term trend reported earlier (BGVN 28:03).

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.

This report contains updates from Reventador for July through November 2003. During this time seismicity varied from generally low to occasionally high. Lahars recurred, as rain and drainage systems continued to move tephra left after the eruptions that began on 3 November 2002 (BGVN 27:11, 28:02, and 28:06). Except for degassing, steam plumes, and the cooling of lava flows, further eruptive behavior (or cessation of activity) was not mentioned.

Activity during July-August 2003. Rainfall at Reventador during 7-13 July caused renewed movement of ash on the volcano's flanks. This led to lahars down the Montana River, and a consequent interruption in highway travel. Tremor associated with degassing was noted, with an increase during the last week of July and early August. Steam plumes were noted on 30 July and 2 August and lahars coincident with tremors were observed on 30 July. Seismicity was moderate, with about five volcano-tectonic (VT) events per day and a total of four tectonic events between 30 July and 3 August. These four events were located between 5.4 and 35 km away. The tectonic earthquake on 3 August registered the highest magnitude, M 3.2.

On 9 August detectors in three locations registered a lahar; this was later confirmed by staff from the Chaco-Santa Rosa station. Seismicity was low between 4 and 10 August, with six local tectonic events. Three of these occurred on 8 August, at varying depths, but all were within 10-12 km of the volcano. The second of these was the largest of any event that week, M 2.9. Seismicity stayed low through the end of the month, with an average of one VT event per day during the last week of August. Small fracture events related to the cooling of lava flows were noted. However, no rains capable of generating lahars were recorded, and there were no reports of steam or gas emissions.

Activity during September-October 2003. Reports were not available for the first three weeks of September and the first two weeks of October. During the week of 22-28 September, two lahar signals were registered, as well as 44 hybrid events, 43 VT events, and seven long-period (LP) events. During the following week, hybrid and LP events dropped to 17 and two events, respectively, but the number of VT events increased considerably, to 78. Lahars lasting ~ 4 hours each were recorded on 30 September and 1 October, following moderate rain on the 30th. During 13-19 October instruments registered 77 VT and 17 LP earthquakes (i.e., averaging 11 VT earthquakes and two LP earthquakes per day). Lahars were reported on 13, 14, and 19 October. During the following week, seismicity stayed at similar levels. Due to intense rains, more lahars were registered, on 20 and 22 October and again on 28 and 29 October. Traffic was again affected as a route had to be closed. Also between 27 October and 2 November, there was a small increase in the number of volcano-tectonic events.

Activity during November 2003. Seismicity remained constant, averaging 8-9 earthquakes each day. Following strong rains on 7 and 9 November, seismometers detected signals attributed to lahars. After the lahar signals had diminished, tremor was again detected. Lahar signals were also recorded on 11, 12, 14, and 26 November.

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

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

Seismic activity recorded at Soputan during November was dominated by avalanche earthquakes (table 5). Frequent ash explosions occurred during July and on 31 August, when a lava flow was also seen (BGVN 28:08). Only a white gas plume reaching heights of 25-50 m was observed during 27 October-30 November. The hazard status of the volcano remained at Alert Level 2 (on a scale of 1-4).

Table 5. Seismicity at Soputan, 27 October-30 November 2003. Courtesy of VSI.

Geologic Background. The Soputan stratovolcano on the southern rim of the Quaternary Tondano caldera on the northern arm of Sulawesi Island is one of Sulawesi's most active volcanoes. The youthful, largely unvegetated volcano is the only active cone in the Sempu-Soputan volcanic complex, which includes the Soputan caldera, Rindengan, and Manimporok (3.5 km ESE). Kawah Masem maar was formed in the W part of the caldera and contains a crater lake; sulfur has been extracted from fumarolic areas in the maar since 1938. Recent eruptions have originated at both the summit crater and Aeseput, a prominent NE-flank vent that formed in 1906 and was the source of intermittent major lava flows until 1924.

Information Contacts: Dali Ahmad, Hetty Triastuty, Nia Haerani, and Suswati, Volcanological Survey of Indonesia (VSI), Jalan Diponegoro No. 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/).

The Instituto Geofísico (IG) provided Tungurahua reports discussing the year 2003. Ash-bearing eruptions sent plumes as high as 9.4 km altitude, with resulting noticeable ashfall 40 km distant. Lahars were common and occasionally incandescent material descended the upper flanks. Activity was low during January and February, and increased slightly in March and again in June. In August activity increased again, and for the rest of the year it generally remained elevated. IG recognized a new phase of eruptive activity beginning 20 August. That phase consisted of long-period earthquakes followed by emissions reaching up to 3 km above the volcano (~8 km altitude).

Activity during January-February 2003. During these months volcanism generally remained low, with occasional emissions of gas and ash that produced low-level plumes. Incandescence was sometimes visible in the crater at night. Seismicity was low and was characterized by sporadic long-period earthquakes and low intensity emissions. Activity increased slightly beginning 12 February with an emission that rose to low levels and drifted W. A moderate explosion on 19 February deposited a small amount of ash on the ENE flanks (Cerro de Ulba and the Ulba valley). Seismicity increased slightly during the eruption, but returned to low levels afterwards. Volcanic and seismic activity remained low through early March with continuing gas and ash emissions.

Activity during March 2003. Activity began to intensify on 5 March when lahars descended the gorges on Tungurahua's NW flank, obstructing the road between the towns of Baños (~8 km N of the summit) and Pelileo (~13 km NNW of the summit). Around 7 March ash rose to ~7 km altitude and drifted SW. No ash was visible on satellite imagery. By 9 March several low-to-moderate explosions had occurred and this activity continued. On 11 March three small-to-moderate explosions deposited ash in the W-flank village of Pillate (8 km from the summit). That day a pilot reported ash to ~8.2 km altitude.

On 16 March a fine layer of ash accumulated in Baños. Sporadic explosions continued for the rest of the month, with one on 19 March that sent incandescent material ~1 km down the flanks. Explosions during this period were accompanied by Strombolian activity, gas-and-ash emissions, and loud roaring. Seismicity was dominated by tremor and long-period earthquakes, with tremor starting to decrease after 13 March.

Activity during April-May 2003. During early April, explosions occasionally occurred at the volcano. A pilot reported seeing ash at a height of around 2.3 km over Tungurahua on 6 April. No ash was detected on satellite imagery, however. Three explosions occurred on 7 April, with the largest plume rising to ~3 km above the volcano. Very little ash was visible in the plume. Activity dropped slightly for a few days, with sporadic explosions, until a large explosion occurred on 10 April, producing a plume with low ash content to ~2 km above the volcano. Volcanic explosions, generally small, continued the following week; minor vapor columns were also noted. Cloud cover obscured the volcano on some days, but an aviation report on 16 April mentioned that IG staff reported an ash cloud rising up to ~7 km altitude (~2 km above the summit). On 17 April two ash columns rose 1.5 and 2 km above the summit and blew SW and W, respectively. The volcano generally appeared relatively placid, but concern about mudflows and sudden increases in eruptive output remained. Limited visibility often prevailed, but it was noted that Tungurahua's behavior alternated between days of tranquility and those with small to moderate explosions. Few earthquakes occurred.

On 1 May an explosion sent ash to 2 km above the summit; incandescent material fell onto the flanks up to 0.8 km from the crater. Based on information from IG, the Washington VAAC reported that a small 6 May explosion yielded a cloud composed mainly of gas, with some ash. The cloud drifted W and seismic activity decreased after the explosion.

Activity during June-July 2003. Volcanic activity increased in early June. On 6 June, strong Strombolian activity hurled incandescent volcanic blocks ~500 m from the summit; plumes of mainly steam rose to around 2 km above the volcano and drifted W. Ash fell in the settlements of Pillate (8 km W of the summit), San Juan (~40 km WSW of the summit), and Riobamba (32 km SW of the summit), with accumulations of less than 1 mm. There were reports of airborne ash interfering with main flight routes across Ecuador. Emissions on 9 June reached 3-6 km above the volcano. On 10 June vibrations from an explosion were felt in Baños, explosions could be heard in towns near the volcano, and ash fell in several villages.

On 15 June incandescent blocks were hurled to ~150 m above the crater and rolled ~1 km down the N flank. During the evening of 17 June, Strombolian activity was visible at the summit, and an explosion on 18 June deposited ash on the settlements of Cusúa (~8 km NW of the summit), Juive (7 km NNW), and Pillate. Gas emissions with small amounts of ash occurred regularly, and on 19 June observers saw ash rise to 3 km above the summit.

During the last week of June, several explosions produced ash clouds; on 25 June ash fell in Pillate and in the town of Mocha (25 km W). Ash was visible on satellite imagery, with the highest-rising ash cloud reaching ~9.4 km altitude on 27 June. Emissions on 29 June deposited ash in Pillate, and in the towns of Cotaló (8 km NW of summit) and Cevallos.

On 1 and 2 July ash plumes rose to ~2 km above the volcano and ash fell in several towns near the volcano. Strombolian activity also occurred, and ash from the eruptions damaged crops and livestock near the volcano. A state of emergency was declared on 3 July, and food rations were distributed to residents of the town of Chimborazo. After 2 July, eruptive vigor remained relatively low through the rest of the month and into August. Reports noted mainly steam and gas emissions and low plumes.

Activity during August 2003. Tungurahua entered a new phase of activity on 20 August. The new phase was characterized by a short sequence of long-period earthquakes followed by gas-and-ash emissions that reached a maximum height of 3 km above the volcano.

A small amount of ash fell in Cusúa on 20 August. During the evening the volcano hurled incandescent blocks ~300 m above the summit and some traveled ~1 km downslope. On 21 August emissions of mostly steam and small amounts of ash rose ~1 km above the volcano and drifted W; ash fell in the Riobamba, Ambato (~33 km NW), and Santa Fé de Galán areas. On 23 August plumes rose to 0.5-2.5 km above the volcano, and ash fell in the town of Guaranda. On 24 August an explosion, heard in the town of Baños, ejected blocks that traveled ~1 km down the volcano's flanks. An emission on 27 August deposited ash in Ambato and caused flight restrictions to and from the airport there. During this week, volcanic block-and-ash emissions continued, with ash plumes rising to heights of ~4 km above the volcano. These drifted primarily W and SW and deposited ash in several towns.

Activity during September-October 2003. Moderate ash emissions and ashfall continued during September and October, accompanied in mid-September by tremor related to gas discharge. Seismicity ranged from moderate levels in September to a series of long-period earthquakes and explosions in early October.

Incandescence was observed in the crater on the evening of 7 September. On 15 September two emissions produced gas-and-ash plumes that reached a maximum height of 2 km above the volcano; ash fell predominately W of the volcano. On 22 September ash clouds reached a height of 3 km above the volcano and drifted W. On 24 September ash emissions produced plumes that drifted NW, depositing small amounts of ash in the towns of Quero (~20 km WNW of the summit), Puela (~8 km SW), Juive, and Cusúa. Volcanic blocks emitted during the eruption rolled ~1 km down the NW flank.

On 1 October gas-and-ash emissions reached a height of ~4 km and drifted NE and NW, depositing ash in San Juan (~40 km WSW), Pillate, and Valle del Patate. On 9 October ash fell on northerly sectors near the volcano, including Runtún (~6 km NNE of the summit), Juive, and Baños. Strombolian activity was seen during the evening of 12 October. Associated gas-and-ash plumes up to 2 km high drifted NNE and ash fell in Ambato. On the night of 18 October incandescent blocks rolled down the crater's W side. Incandescence and Strombolian activity were observed the following night. Activity decreased slightly on 20 October with fewer explosions and no major gas-and-ash eruptions recorded. Ash plumes were frequently visible on satellite imagery during 15-20 October.

Activity during November-December 2003. Tungurahua maintained generally low activity in early November, increasing towards month's end. Following a week of small-to-moderate eruptions of gas and ash, an eruption on 2 November produced a plume that rose to ~3 km above the volcano and drifted W. Over the next few days, occasional ash-poor plumes rose to less than 1 km above the summit; a few ash-bearing emissions did occur, including ashfalls of low intensity on 5, 6, and 7 November to the E. Also on 6 November seismic stations recorded two larger-than-average explosions, one associated with an ash column rising to 2 km. Seismicity returned to low levels, with relatively few earthquakes, but tremor continued.

During 12-18 November, small-to-moderate eruptions of steam, gas, and some ash continued; plumes rose to ~2.5 km above the volcano, but there were no reports of ashfall in nearby towns. Strombolian activity was visible at the crater and avalanches of incandescent volcanic material rolled ~1 km down the volcano's flanks. Activity increased to high levels beginning 19 November; numerous moderate explosions produced plumes that were frequently visible on satellite imagery and rose up to 2 km above the crater. Ash was dispersed SSW and SW on 19 and 20 November and WNW and NW on 23 and 24 November, respectively. Throughout the week Strombolian activity was visible at night.

During 22 November to 1 December, a large number of emissions of gas, steam, and ash occurred, depositing ash to the SW, W, and NW. Plumes were visible on satellite imagery at a maximum of ~7 km altitude.

During 11-16 December, volcanic activity remained relatively high with several explosions producing ash-and-gas plumes to a maximum of 9 km altitude. There were also many long-period earthquakes, occurring with nearly constant gas-and-ash emissions. Explosions on 11 December deposited ash in the towns of Quero, Santa Fe de Galán, and lesser amounts in Bilbao. Ash-and-gas plumes were visible on satellite imagery several times during the week.

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

Information Contacts: Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); El Comercio, Quito, Ecuador (URL: http://www.elcomercio. com/); Agence France-Presse.

The Darwin Volcanic Ash Advisory Centre reported that an ash plume from Ulawun was visible on satellite imagery on 22 September at an altitude of ~3.7 km extending NW. On 5 October a faint ash plume was identified on satellite imagery at ~4.3 km altitude, extending 55 km WSW of the summit. Another ash plume was seen reaching ~75 km WNW of the summit on satellite imagery on 10 October at an altitude around 3 km.

According to the Rabaul Volcano Observatory, the main summit crater at Ulawun released weak to moderate volumes of white-gray vapor emissions over the period 6 November-22 December 2003. The two north valley vents were quiet, with no emissions observed. The seismograph, restored on 31 October 2003; showed seismicity was low throughout this period, with small low frequency volcanic earthquakes and some high frequency volcano-tectonic events. The electronic tiltmeter, restored at the same time; recorded no significant changes.

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; Darwin Volcanic Ash Advisory Centre (VAAC), Commonwealth Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).