Manam is a 10-km-wide island that consists of two active summit craters: the Main summit crater and the South summit crater and is located 13 km off the northern coast of mainland Papua New Guinea. Frequent mild-to-moderate eruptions have been recorded since 1616. The current eruption period began during June 2014 and has more recently been characterized by intermittent ash plumes and thermal activity (BGVN 47:11). This report updates activity that occurred from November 2022 through May 2023 based on information from the Darwin Volcanic Ash Advisory Center (VAAC) and various satellite data.

Ash plumes were reported during November and December 2022 by the Darwin VAAC. On 7 November an ash plume rose to 2.1 km altitude and drifted NE based on satellite images and weather models. On 14 November an ash plume rose to 2.1 km altitude and drifted W based on RVO webcam images. On 20 November ash plumes rose to 1.8 km altitude and drifted NW. On 26 December an ash plume rose to 3 km altitude and drifted S and SSE.

Intermittent sulfur dioxide plumes were detected using the TROPOMI instrument on the Sentinel-5P satellite, some of which exceeded at least two Dobson Units (DU) and drifted in different directions (figure 93). Occasional low-to-moderate power thermal anomalies were recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system; less than five anomalies were recorded each month during November 2022 through May 2023 (figure 94). Two thermal hotspots were detected by the MODVOLC thermal alerts system on 10 December 2022. On clear weather days, thermal activity was also captured in infrared satellite imagery in both the Main and South summit craters, accompanied by gas-and-steam emissions (figure 95).

Figure 93. Distinct sulfur dioxide plumes were captured, rising from Manam based on data from the TROPOMI instrument on the Sentinel-5P satellite on 16 November 2022 (top left), 6 December 2022 (top right), 14 January 2023 (bottom left), and 23 March 2023 (bottom right). Plumes generally drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Figure 94. Occasional low-to-moderate power thermal anomalies were detected at Manam during November 2022 through May 2023, as shown in this MIROVA graph (Log Radiative Power). Only three anomalies were detected during late November, one in early December, two during January 2023, one in late March, four during April, and one during late May. Courtesy of MIROVA.

Figure 95. Infrared (bands B12, B11, B4) satellite images show a consistent thermal anomaly (bright yellow-orange) in both the Main (the northern crater) and South summit craters on 10 November 2022 (top left), 15 December 2022 (top right), 3 February 2023 (bottom left), and 24 April 2023 (bottom right). Gas-and-steam emissions occasionally accompanied the thermal activity. Courtesy of Copernicus Browser.

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: 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/).

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 explosions, ash plumes, and thermal activity (BGVN 47:11). This report covers activity during November 2022 through April 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, the Darwin Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

Activity was relatively low during November and December 2022. Daily white gas-and-steam plumes rose 25-100 m above the summit and drifted in different directions. Gray ash plumes rose 200 m above the summit and drifted NE at 1047 and at 2343 on 11 November. On 14 November at 0933 ash plumes rose 300 m above the summit and drifted E. An ash plume was reported at 0935 on 15 December that rose 100 m above the summit and drifted NE. An eruptive event at 1031 later that day generated an ash plume that rose 700 m above the summit and drifted NE. A gray ash plume at 1910 rose 100 m above the summit and drifted E. Incandescent material was ejected above the vent based on an image taken at 1936.

During January 2023 daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions. Gray-to-brown ash plumes were reported at 1638 on 3 January, at 1410 and 1509 on 4 January, and at 0013 on 5 January that rose 100-750 m above the summit and drifted NE and E; the gray-to-black ash plume at 1509 on 4 January rose as high as 3 km above the summit and drifted E. Gray ash plumes were recorded at 1754, 2241, and 2325 on 11 January and at 0046 on 12 January and rose 200-300 m above the summit and drifted NE. Toward the end of January, PVMBG reported that activity had intensified; Strombolian activity was visible in webcam images taken at 0041, 0043, and 0450 on 23 January. Multiple gray ash plumes throughout the day rose 200-500 m above the summit and drifted E and SE (figure 135). Webcam images showed progressively intensifying Strombolian activity at 1919, 1958, and 2113 on 24 January; a gray ash plume at 1957 rose 300 m above the summit and drifted E (figure 135). Eruptive events at 0231 and 2256 on 25 January and at 0003 on 26 January ejected incandescent material from the vent, based on webcam images. Gray ash plumes observed during 26-27 January rose 300-500 m above the summit and drifted NE, E, and SE.

Figure 135. Webcam images of a strong, gray ash plume (left) and Strombolian activity (right) captured at Krakatau at 0802 on 23 January 2023 (left) and at 2116 on 24 January 2023 (right). Courtesy of PVMBG and MAGMA Indonesia.

Low levels of activity were reported during February and March. Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in different directions. The Darwin VAAC reported that continuous ash emissions rose to 1.5-1.8 km altitude and drifted W and NW during 1240-1300 on 10 March, based on satellite images, weather models, and PVMBG webcams. White-and-gray ash plumes rose 500 m and 300 m above the summit and drifted SW at 1446 and 1846 on 18 March, respectively. An eruptive event was recorded at 2143, though it was not visible due to darkness. Multiple ash plumes were reported during 27-29 March that rose as high as 2.5 km above the summit and drifted NE, W, and SW (figure 136). Webcam images captured incandescent ejecta above the vent at 0415 and around the summit area at 2003 on 28 March and at 0047 above the vent on 29 March.

Figure 136. Webcam image of a strong ash plume rising above Krakatau at 1522 on 28 March 2023. Courtesy of PVMBG and MAGMA Indonesia.

Daily white gas-and-steam plumes rose 25-300 m above the summit and drifted in multiple directions during April and May. White-and-gray and black plumes rose 50-300 m above the summit on 2 and 9 April. On 11 May at 1241 a gray ash plume rose 1-3 km above the summit and drifted SW. On 12 May at 0920 a gray ash plume rose 2.5 km above the summit and drifted SW and at 2320 an ash plume rose 1.5 km above the summit and drifted SW. An accompanying webcam image showed incandescent ejecta. On 13 May at 0710 a gray ash plume rose 2 km above the summit and drifted SW (figure 137).

Figure 137. Webcam image of an ash plume rising 2 km above the summit of Krakatau at 0715 on 13 May 2023. Courtesy of PVMBG and MAGMA Indonesia.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during November 2022 through April 2023 (figure 138). Some of this thermal activity was also visible in infrared satellite imagery at the crater, accompanied by gas-and-steam and ash plumes that drifted in different directions (figure 139).

Figure 138. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during November 2022 through April 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 139. A thermal anomaly (bright yellow-orange) was visible at Krakatau in infrared (bands B12, B11, B4) satellite images on clear weather days during November 2022 through May 2023. Occasional gas-and-steam and ash plumes accompanied the thermal activity, which drifted in different directions. Images were captured on 25 November 2022 (top left), 15 December 2022 (top right), 27 January 2023 (bottom left), and 12 May 2023 (bottom right). 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); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Stromboli, located in Italy, has exhibited nearly constant lava fountains for the past 2,000 years; recorded eruptions date back to 350 BCE. Eruptive activity occurs at the summit from multiple vents, which include a north crater area (N area) and a central-southern crater (CS area) on a terrace known as the ‘terrazza craterica’ at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano-island. Activity typically consists of Strombolian explosions, incandescent ejecta, lava flows, and pyroclastic flows. Thermal and visual monitoring cameras are located on the nearby Pizzo Sopra La Fossa, above the terrazza craterica, and at multiple flank locations. The current eruption period has been ongoing since 1934 and recent activity has consisted of frequent Strombolian explosions and lava flows (BGVN 48:02). This report updates activity during January through April 2023 primarily characterized by Strombolian explosions and lava flows based on reports from Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Frequent explosive activity continued throughout the reporting period, generally in the low-to-medium range, based on the number of hourly explosions in the summit crater (figure 253, table 16). Intermittent thermal activity was recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 254). According to data collected by the MODVOLC thermal algorithm, a total of 9 thermal alerts were detected: one on 2 January 2023, one on 1 February, five on 24 March, and two on 26 March. The stronger pulses of thermal activity likely reflected lava flow events. Infrared satellite imagery captured relatively strong thermal hotspots at the two active summit craters on clear weather days, showing an especially strong event on 8 March (figure 255).

Figure 253. Explosive activity persisted at Stromboli during January through April 2023, with low to medium numbers of daily explosions at the summit crater. The average number of daily explosions (y-axis) during January through April (x-axis) are broken out by area and as a total, with red for the N area, blue for the CS area, and black for the combined total. The data are smoothed as daily (thin lines) and weekly (thick lines) averages. The black squares along the top represent days with no observations due to poor visibility (Visib. Scarsa). The right axis indicates the qualitative activity levels from low (basso) to highest (altissimo) with the green highlighted band indicating the most common level. Courtesy of INGV (Report 17/2023, Stromboli, Bollettino Settimanale, 18/04/2023 - 24/04/2023).

Table 16. Summary of type, frequency, and intensity of explosive activity at Stromboli by month during January-April 2023; information from webcam observations. Courtesy of INGV weekly reports.

Figure 254. Intermittent thermal activity at Stromboli was detected during January through April 2023 and varied in strength, as shown in this MIROVA graph (Log Radiative Power). A pulse of activity was captured during late March. Courtesy of MIROVA.

Figure 255. Infrared (bands B12, B11, B4) satellite images showing persistent thermal anomalies at both summit crater on 1 February 2023 (top left), 23 March 2023 (top right), 8 March 2023 (bottom left), and 27 April 2023. A particularly strong thermal anomaly was visible on 8 March. Courtesy of Copernicus Browser.

Activity during January-February 2023. Strombolian explosions were reported in the N crater area, as well as lava effusion. Explosive activity in the N crater area ejected coarse material (bombs and lapilli). Intense spattering was observed in both the N1 and N2 craters. In the CS crater area, explosions generally ejected fine material (ash), sometimes to heights greater than 250 m. The intensity of the explosions was characterized as low-to-medium in the N crater and medium-to-high in the CS crater. After intense spattering activity from the N crater area, a lava overflow began at 2136 on 2 January that flowed part way down the Sciara del Fuoco, possibly moving down the drainage that formed in October, out of view from webcams. The flow remained active for a couple of hours before stopping and beginning to cool. A second lava flow was reported at 0224 on 4 January that similarly remained active for a few hours before stopping and cooling. Intense spattering was observed on 11 and 13 January from the N1 crater. After intense spattering activity at the N2 crater at 1052 on 17 January another lava flow started to flow into the upper part of the Sciara del Fuoco (figure 256), dividing into two: one that traveled in the direction of the drainage formed in October, and the other one moving parallel to the point of emission. By the afternoon, the rate of the flow began to decrease, and at 1900 it started to cool. A lava flow was reported at 1519 on 24 January following intense spattering in the N2 area, which began to flow into the upper part of the Sciara del Fuoco. By the morning of 25 January, the lava flow had begun to cool. During 27 January the frequency of eruption in the CS crater area increased to 6-7 events/hour compared to the typical 1-7 events/hour; the following two days showed a decrease in frequency to less than 1 event/hour. Starting at 1007 on 30 January a high-energy explosive sequence was produced by vents in the CS crater area. The sequence began with an initial energetic pulse that lasted 45 seconds, ejecting predominantly coarse products 300 m above the crater that fell in an ESE direction. Subsequent and less intense explosions ejected material 100 m above the crater. The total duration of this event lasted approximately two minutes. During 31 January through 6, 13, and 24 February spattering activity was particularly intense for short periods in the N2 crater.

Figure 256. Webcam images of the lava flow development at Stromboli during 17 January 2023 taken by the SCT infrared camera. The lava flow appears light yellow-green in the infrared images. Courtesy of INGV (Report 04/2023, Stromboli, Bollettino Settimanale, 16/01/2023 - 22/01/2023).

An explosive sequence was reported on 16 February that was characterized by a major explosion in the CS crater area (figure 257). The sequence began at 1817 near the S2 crater that ejected material radially. A few seconds later, lava fountains were observed in the central part of the crater. Three explosions of medium intensity (material was ejected less than 150 m high) were recorded at the S2 crater. The first part of this sequence lasted approximately one minute, according to INGV, and material rose 300 m above the crater and then was deposited along the Sciara del Fuoco. The second phase began at 1818 at the S1 crater; it lasted seven seconds and material was ejected 150 m above the crater. Another event 20 seconds later lasted 12 seconds, also ejecting material 150 m above the crater. The sequence ended with at least three explosions of mostly fine material from the S1 crater. The total duration of this sequence was about two minutes.

Figure 257. Webcam images of the explosive sequence at Stromboli on 16 February 2023 taken by the SCT and SCV infrared and visible cameras. The lava appears light yellow-green in the infrared images. Courtesy of INGV (Report 08/2023, Stromboli, Bollettino Settimanale, 13/02/2023 - 19/02/2023).

Short, intense spattering activity was noted above the N1 crater on 27 and 28 February. A lava overflow was first reported at 0657 from the N2 crater on 27 February that flowed into the October 2022 drainage. By 1900 the flow had stopped. A second lava overflow also in the N crater area occurred at 2149, which overlapped the first flow and then stopped by 0150 on 28 February. Material detached from both the lava overflows rolled down the Sciara del Fuoco, some of which was visible in webcam images.

Activity during March-April 2023. Strombolian activity continued with spattering activity and lava overflows in the N crater area during March. Explosive activity at the N crater area varied from low (less than 80 m high) to medium (less than 150 m high) and ejected coarse material, such as bombs and lapilli. Spattering was observed above the N1 crater, while explosive activity at the CS crater area varied from medium to high (greater than 150 m high) and ejected coarse material. Intense spattering activity was observed for short periods on 6 March above the N1 crater. At approximately 0610 a lava overflow was reported around the N2 crater on 8 March, which then flowed into the October 2022 drainage. By 1700 the flow started to cool. A second overflow began at 1712 on 9 March and overlapped the previous flow. It had stopped by 2100. Material from both flows was deposited along the Sciara del Fuoco, though much of the activity was not visible in webcam images. On 11 March a lava overflow was observed at 0215 that overlapped the two previous flows in the October 2022 drainage. By late afternoon on 12 March, it had stopped.

During a field excursion on 16 March, scientists noted that a vent in the central crater area was degassing. Another vent showed occasional Strombolian activity that emitted ash and lapilli. During 1200-1430 low-to-medium intense activity was reported; the N1 crater emitted ash emissions and the N2 crater emitted both ash and coarse material. Some explosions also occurred in the CS crater area that ejected coarse material. The C crater in the CS crater area occasionally showed gas jetting and low intensity explosions on 17 and 22 March; no activity was observed at the S1 crater. Intense, longer periods of spattering were reported in the N1 crater on 19, 24, and 25 March. Around 2242 on 23 March a lava overflow began from the N1 crater that, after about an hour, began moving down the October 2022 drainage and flow along the Sciara del Fuoco (figure 258). Between 0200 and 0400 on 26 March the flow rate increased, which generated avalanches of material from collapses at the advancing flow front. By early afternoon, the flow began to cool. On 25 March at 1548 an explosive sequence began from one of the vents at S2 in the CS crater area (figure 258). Fine ash mixed with coarse material was ejected 300 m above the crater rim and drifted SSE. Some modest explosions around Vent C were detected at 1549 on 25 March, which included an explosion at 1551 that ejected coarse material. The entire explosive sequence lasted approximately three minutes.

Figure 258. Webcam images of the lava overflow in the N1 crater area of Stromboli on 23 March 2023 taken by the SCT infrared camera. The lava appears light yellow-green in the infrared images. The start of the explosive sequence was also captured on 25 March 2023 accompanied by an eruption plume (e) captured by the SCT and SPT infrared webcams. Courtesy of INGV (Report 13/2023, Stromboli, Bollettino Settimanale, 20/03/2023 - 26/03/2023).

During April explosions persisted in both the N and CS crater areas. Fine material was ejected less than 80 m above the N crater rim until 6 April, followed by ejection of coarser material. Fine material was also ejected less than 80 m above the CS crater rim. The C and S2 crater did not show significant eruptive activity. On 7 April an explosive sequence was detected in the CS crater area at 1203 (figure 259). The first explosion lasted approximately 18 seconds and ejected material 400 m above the crater rim, depositing pyroclastic material in the upper part of the Sciara del Fuoco. At 1204 a second, less intense explosion lasted approximately four seconds and deposited pyroclastic products outside the crater area and near Pizzo Sopra La Fossa. A third explosion at 1205 was mainly composed of ash that rose about 150 m above the crater and lasted roughly 20 seconds. A fourth explosion occurred at 1205 about 28 seconds after the third explosion and ejected a mixture of coarse and fine material about 200 m above the crater; the explosion lasted roughly seven seconds. Overall, the entire explosive sequence lasted about two minutes and 20 seconds. After the explosive sequence on 7 April, explosions in both the N and CS crater areas ejected material as high as 150 m above the crater.

Figure 259. Webcam images of the explosive sequence at Stromboli during 1203-1205 (local time) on 7 April 2023 taken by the SCT infrared camera. Strong eruption plumes are visible, accompanied by deposits on the nearby flanks. Courtesy of INGV (Report 15/2023, Stromboli, Bollettino Settimanale, 03/04/2023 - 09/04/2023).

On 21 April research scientists from INGV made field observations in the summit area of Stromboli, and some lapilli samples were collected. In the N crater area near the N1 crater, a small cone was observed with at least two active vents, one of which was characterized by Strombolian explosions. The other vent produced explosions that ejected ash and chunks of cooled lava. At the N2 crater at least one vent was active and frequently emitted ash. In the CS crater area, a small cone contained 2-3 degassing vents and a smaller, possible fissure area also showed signs of degassing close to the Pizzo Sopra La Fossa. In the S part of the crater, three vents were active: a small hornito was characterized by modest and rare explosions, a vent that intermittently produced weak Strombolian explosions, and a vent at the end of the terrace that produced frequent ash emissions. Near the S1 crater there was a hornito that generally emitted weak gas-and-steam emissions, sometimes associated with “gas rings”. On 22 April another field inspection was carried out that reported two large sliding surfaces on the Sciara del Fuoco that showed where blocks frequently descended toward the sea. A thermal anomaly was detected at 0150 on 29 April.

Geologic Background. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy, (URL: http://www.ct.ingv.it/en/); 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/).

Nishinoshima is a small island located about 1,000 km S of Tokyo in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. Eruptions date back to 1973; the most recent eruption period began in October 2022 and was characterized by ash plumes and fumarolic activity (BGVN 47:12). This report describes ash plumes and fumarolic activity during November 2022 through April 2023 based on monthly reports from the Japan Meteorological Agency (JMA) monthly reports and satellite data.

The most recent eruptive activity prior to the reporting internal occurred on 12 October 2022, when an ash plume rose 3.5 km above the crater rim. An aerial observation conducted by the Japan Coast Guard (JCG) on 25 November reported that white fumaroles rose approximately 200 m above the central crater of a pyroclastic cone (figure 119), and multiple plumes were observed on the ESE flank of the cone. Discolored water ranging from reddish-brown to brown and yellowish-green were visible around the perimeter of the island (figure 119). No significant activity was reported in December.

Figure 119. Aerial photo of gas-and-steam plumes rising 200 m above Nishinoshima on 25 November 2022. Reddish brown to brown and yellowish-green discolored water was visible around the perimeter of the island. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, November 2022).

During an overflight conducted by JCG on 25 January 2023 intermittent activity and small, blackish-gray plumes rose 900 m above the central part of the crater were observed (figure 120). The fumarolic zone of the E flank and base of the cone had expanded and emissions had intensified. Dark brown discolored water was visible around the perimeter of the island.

Figure 120. Aerial photo of a black-gray ash plume rising approximately 900 m above the crater rim of Nishinoshima on 25 January 2023. White fumaroles were visible on the E slope of the pyroclastic cone. Dense brown to brown discolored water was observed surrounding the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, January, 2023).

No significant activity was reported during February through March. Ash plumes at 1050 and 1420 on 11 April rose 1.9 km above the crater rim and drifted NW and N. These were the first ash plumes observed since 12 October 2022. On 14 April JCG carried out an overflight and reported that no further eruptive activity was visible, although white gas-and-steam plumes were visible from the central crater and rose 900 m high (figure 121). Brownish and yellow-green discolored water surrounded the island.

Figure 121. Aerial photo of white gas-and-steam plumes rising 900 m above Nishinoshima on 14 April 2023. Brown and yellow-green discolored water is visible around the perimeter of the island. Photo has been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, April, 2023).

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during November 2022 through April 2023 (figure 123). A cluster of six to eight anomalies were detected during November while a smaller number were detected during the following months: two to three during December, one during mid-January 2023, one during February, five during March, and two during April. Thermal activity was also reflected in infrared satellite data at the summit crater, accompanied by occasional gas-and-steam plumes (figure 124).

Figure 123. Intermittent low-to-moderate thermal anomalies were detected at Nishinoshima during November 2022 through April 2023, according to this MIROVA graph (Log Radiative Power). A cluster of anomalies occurred throughout November, while fewer anomalies were detected during the following months. Courtesy of MIROVA.

Figure 124. Infrared (bands B12, B11, B4) satellite images show a small thermal anomaly at the summit crater of Nishinoshima on 9 January 2023 (left) and 8 February 2023 (right). Gas-and-steam plumes accompanied this activity and extended S and SE, respectively. 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); 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/).

Karangetang (also known as Api Siau), at the northern end of the island of Siau, Indonesia, contains five summit craters along a N-S line. More than 40 eruptions have been recorded since 1675; recent eruptions have included frequent explosive activity, sometimes accompanied by pyroclastic flows and lahars. Lava dome growth has occurred in the summit craters and collapses of lava flow fronts have produced pyroclastic flows. The two active summit craters are Kawah Dua (the N crater) and Kawah Utama (the S crater, also referred to as the “Main Crater”). The most recent eruption began in late November 2018 and has more recently consisted of weak thermal activity and gas-and-steam emissions (BGVN 48:01). This report updates activity characterized by lava flows, incandescent avalanches, and ash plumes during January through June 2023 using reports from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM, or the Center of Volcanology and Geological Hazard Mitigation), MAGMA Indonesia, the Darwin VAAC (Volcano Ash Advisory Center), and satellite data.

Activity during January was relatively low and mainly consisted of white gas-and-steam emissions that rose 25-150 m above Main Crater (S crater) and drifted in different directions. Incandescence was visible from the lava dome in Kawah Dua (the N crater). Weather conditions often prevented clear views of the summit. On 18 January the number of seismic signals that indicated avalanches of material began to increase. In addition, there were a total of 71 earthquakes detected during the month.

Activity continued to increase during the first week of February. Material from Main Crater traveled as far as 800 m down the Batuawang (S) and Batang (W) drainages and as far as 1 km W down the Beha (W) drainage on 4 February. On 6 February 43 earthquake events were recorded, and on 7 February, 62 events were recorded. White gas-and-steam emissions rose 25-250 m above both summit craters throughout the month. PVMBG reported an eruption began during the evening of 8 February around 1700. Photos showed incandescent material at Main Crater. Incandescent material had also descended the flank in at least two unconfirmed directions as far as 2 km from Main Crater, accompanied by ash plumes (figure 60). As a result, PVMBG increased the Volcano Alert Level (VAL) to 3 (the second highest level on a 1-4 scale).

Figure 60. Photos of the eruption at Karangetang on 8 February 2023 that consisted of incandescent material descending the flanks (top left), ash plumes (top right and bottom left), and summit crater incandescence (bottom right). Courtesy of IDN Times.

Occasional nighttime webcam images showed three main incandescent lava flows of differing lengths traveling down the S, SW, and W flanks (figure 61). Incandescent rocks were visible on the upper flanks, possibly from ejected or collapsed material from the crater, and incandescence was the most intense at the summit. Based on analyses of satellite imagery and weather models, the Darwin VAAC reported that daily ash plumes during 16-20 February rose to 2.1-3 km altitude and drifted NNE, E, and SE. BNPB reported on 16 February that as many as 77 people were evacuated and relocated to the East Siau Museum. A webcam image taken at 2156 on 17 February possibly showed incandescent material descending the SE flank. Ash plumes rose to 2.1 km altitude and drifted SE during 22-23 February, according to the Darwin VAAC.

Figure 61. Webcam image of summit incandescence and lava flows descending the S, SW, and W flanks of Karangetang on 13 February 2023. Courtesy of MAGMA Indonesia.

Incandescent avalanches of material and summit incandescence at Main Crater continued during March. White gas-and-steam emissions during March generally rose 25-150 m above the summit crater; on 31 March gas-and-steam emissions rose 200-400 m high. An ash plume rose to 2.4 km altitude and drifted S at 1710 on 9 March and a large thermal anomaly was visible in images taken at 0550 and 0930 on 10 March. Incandescent material was visible at the summit and on the flanks based on webcam images taken at 0007 and 2345 on 16 March, at 1828 on 17 March, at 1940 on 18 March, at 2311 on 19 March, and at 2351 on 20 March. Incandescence was most intense on 18 and 20 March and webcam images showed possible Strombolian explosions (figure 62). An ash plume rose to 2.4 km altitude and drifted SW on 18 March, accompanied by a thermal anomaly.

Figure 62. Webcam image of intense summit incandescence and incandescent avalanches descending the flanks of Karangetang on 18 March 2023. Photo has been color corrected. Courtesy of MAGMA Indonesia.

Summit crater incandescence at Main Crater and on the flanks persisted during April. Incandescent material at the S crater and on the flanks was reported at 0016 on 1 April. The lava flows had stopped by 1 April according to PVMBG, although incandescence was still visible up to 10 m high. Seismic signals indicating effusion decreased and by 6 April they were no longer detected. Incandescence was visible from both summit craters. On 26 April the VAL was lowered to 2 (the second lowest level on a 1-4 scale). White gas-and-steam emissions rose 25-200 m above the summit crater.

During May white gas-and-steam emissions generally rose 50-250 m above the summit, though it was often cloudy, which prevented clear views; on 21 May gas-and-steam emissions rose 50-400 m high. Nighttime N summit crater incandescence rose 10-25 m above the lava dome, and less intense incandescence was noted above Main Crater, which reached about 10 m above the dome. Sounds of falling rocks at Main Crater were heard on 15 May and the seismic network recorded 32 rockfall events in the crater on 17 May. Avalanches traveled as far as 1.5 km down the SW and S flanks, accompanied by rumbling sounds on 18 May. Incandescent material descending the flanks was captured in a webcam image at 2025 on 19 May (figure 63) and on 29 May; summit crater incandescence was observed in webcam images at 2332 on 26 May and at 2304 on 29 May. On 19 May the VAL was again raised to 3.

Figure 63. Webcam image showing incandescent material descending the flanks of Karangetang on 19 May 2023. Courtesy of MAGMA Indonesia.

Occasional Main Crater incandescence was reported during June, as well as incandescent material on the flanks. White gas-and-steam emissions rose 10-200 m above the summit crater. Ash plumes rose to 2.1 km altitude and drifted SE and E during 2-4 June, according to the Darwin VAAC. Material on the flanks of Main Crater were observed at 2225 on 7 June, at 2051 on 9 June, at 0007 on 17 June, and at 0440 on 18 June. Webcam images taken on 21, 25, and 27 June showed incandescence at Main Crater and from material on the flanks.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed strong thermal activity during mid-February through March and mid-May through June, which represented incandescent avalanches and lava flows (figure 64). During April through mid-May the power of the anomalies decreased but frequent anomalies were still detected. Brief gaps in activity occurred during late March through early April and during mid-June. Infrared satellite images showed strong lava flows mainly affecting the SW and S flanks, accompanied by gas-and-steam emissions (figure 65). According to data recorded by the MODVOLC thermal algorithm, there were a total of 79 thermal hotspots detected: 28 during February, 24 during March, one during April, five during May, and 21 during June.

Figure 64. Strong thermal activity was detected during mid-February 2023 through March and mid-May through June at Karangetang during January through June 2023, as recorded by this MIROVA graph (Log Radiative Power). During April through mid-May the power of the anomalies decreased, but the frequency at which they occurred was still relatively high. A brief gap in activity was shown during mid-June. Courtesy of MIROVA.

Figure 65. Incandescent avalanches of material and summit crater incandescence was visible in infrared satellite images (bands 12, 11, 8A) at both the N and S summit crater of Karangetang on 17 February 2023 (top left), 13 April 2023 (top right), 28 May 2023 (bottom left), and 7 June 2023 (bottom right), as shown in these infrared (bands 12, 11, 8A) satellite images. The incandescent avalanches mainly affected the SW and S flanks. Sometimes gas-and-steam plumes accompanied the thermal activity. Courtesy of Copernicus Browser.

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: 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); Badan Nasional Penanggulangan Bencana (BNPB), National Disaster Management Agency, Graha BNPB - Jl. Scout Kav.38, East Jakarta 13120, Indonesia (URL: http://www.bnpb.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/); IDN Times, Jl. Jend. Gatot Subroto Kav. 27 3rd Floor Kuningan, Jakarta, Indonesia 12950, Status of Karangetang Volcano in Sitaro Islands Increases (URL: https://sulsel.idntimes.com/news/indonesia/savi/status-gunung-api-karangetang-di-kepulauan-sitaro-meningkat?page=all).

Ahyi seamount is a large, conical submarine volcano that rises to within 75 m of the ocean surface about 18 km SE of the island of Farallon de Pajaros in the Northern Marianas. The remote location of the seamount has made eruptions difficult to document, but seismic stations installed in the region confirmed an eruption in the vicinity in 2001. No new activity was detected until April-May 2014 when an eruption was detected by NOAA (National Oceanic and Atmospheric Administration) divers, hydroacoustic sensors, and seismic stations (BGVN 42:04). New activity was first detected on 15 November by hydroacoustic sensors that were consistent with submarine volcanic activity. This report covers activity during November 2022 through June 2023 based on daily and weekly reports from the US Geological Survey.

Starting in mid-October, hydroacoustic sensors at Wake Island (2.2 km E) recorded signals consistent with submarine volcanic activity, according to a report from the USGS issued on 15 November 2022. A combined analysis of the hydroacoustic signals and seismic stations located at Guam and Chichijima Island, Japan, suggested that the source of this activity was at or near the Ahyi seamount. After a re-analysis of a satellite image of the area that was captured on 6 November, USGS confirmed that there was no evidence of discoloration at the ocean surface. Few hydroacoustic and seismic signals continued through November, including on 18 November, which USGS suggested signified a decline or pause in unrest. A VONA (Volcano Observatory Notice for Aviation) reported that a discolored water plume was persistently visible in satellite data starting on 18 November (figure 6). Though clouds often obscured clear views of the volcano, another discolored water plume was captured in a satellite image on 26 November. The Aviation Color Code (ACC) was raised to Yellow (the second lowest level on a four-color scale) and the Volcano Alert Level (VAL) was raised to Advisory (the second lowest level on a four-level scale) on 29 November.

Figure 6. A clear, true color satellite image showed a yellow-green discolored water plume extending NW from the Ahyi seamount (white arrow) on 21 November 2022. Courtesy of Copernicus Browser.

During December, occasional detections were recorded on the Wake Island hydrophone sensors and discolored water over the seamount remained visible. During 2-7, 10-12, and 16-31 December possible explosion signals were detected. A small area of discolored water was observed in high-resolution Sentinel-2 satellite images during 1-6 December (figure 7). High-resolution satellite images recorded discolored water plumes on 13 December that originated from the summit region; no observations indicated that activity breached the ocean surface. A possible underwater plume was visible in satellite images on 18 December, and during 19-20 December a definite but diffuse underwater plume located SSE from the main vent was reported. An underwater plume was visible in a satellite image taken on 26 December (figure 7).

Figure 7. Clear, true color satellite images showed yellow-green discolored water plumes extending NE and W from Ahyi (white arrows) on 1 (left) and 26 (right) December 2022, respectively. Courtesy of Copernicus Browser.

Hydrophone sensors continued to detect signals consistent with possible explosions during 1-8 January 2023. USGS reported that the number of detections decreased during 4-5 January. The hydrophone sensors experienced a data outage that started at 0118 on 8 January and continued through 10 January, though according to USGS, possible explosions were recorded prior to the data outage and likely continued during the outage. A discolored water plume originating from the summit region was detected in a partly cloudy satellite image on 8 January. On 11-12 and 15-17 January possible explosion signals were recorded again. One small signal was detected during 22-23 January and several signals were recorded on 25 and 31 January. During 27-31 January a plume of discolored water was observed above the seamount in satellite imagery (figure 8).

Figure 8. True color satellite images showed intermittent yellow-green discolored water plumes of various sizes extending N on 5 January 2023 (top left), SE on 30 January 2023 (top right), W on 4 February 2023 (bottom left), and SW on 1 March 2023 (bottom right) from Ahyi (white arrows). Courtesy of Copernicus Browser.

Low levels of activity continued during February and March, based on data from pressure sensors on Wake Island. During 1 and 4-6 February activity was reported, and a submarine plume was observed on 4 February (figure 8). Possible explosion signals were detected during 7-8, 10, 13-14, and 24 February. During 1-2 and 3-5 March a plume of discolored water was observed in satellite imagery (figure 8). Almost continuous hydroacoustic signals were detected in remote pressure sensor data on Wake Island 2,270 km E from the volcano during 7-13 March. During 12-13 March water discoloration around the seamount was observed in satellite imagery, despite cloudy weather. By 14 March discolored water extended about 35 km, but no direction was noted. USGS reported that the continuous hydroacoustic signals detected during 13-14 March stopped abruptly on 14 March and no new detections were observed. Three 30 second hydroacoustic detections were reported during 17-19 March, but no activity was visible due to cloudy weather. A data outage was reported during 21-22 March, making pressure sensor data unavailable; a discolored water plume was, however, visible in satellite data. A possible underwater explosion signal was detected by pressure sensors at Wake Island on 26, 29, and 31 March, though the cause and origin of these events were unclear.

Similar low activity continued during April, May, and June. Several signals were detected during 1-3 April in pressure sensors at Wake Island. USGS suggested that these may be related to underwater explosions or earthquakes at the volcano, but no underwater plumes were visible in clear satellite images. The pressure sensors had data outages during 12-13 April and no data were recorded; no underwater plumes were visible in satellite images, although cloudy weather obscured most clear views. Eruptive activity was reported starting at 2210 on 21 May. On 22 May a discolored water plume that extended 4 km was visible in satellite images, though no direction was recorded. During 23-24 May some signals were detected by the underwater pressure sensors. Possible hydroacoustic signals were detected during 2-3 and 6-8 June. Multiple hydroacoustic signals were detected during 9-11 and 16-17 June, although no activity was visible in satellite images. One hydroacoustic signal was detected during 23-24 June, but there was some uncertainty about its association with volcanic activity. A single possible hydroacoustic signal was detected during 30 June to 1 July.

Geologic Background. Ahyi seamount is a large conical submarine volcano that rises to within 75 m of the ocean surface ~18 km SE of the island of Farallon de Pajaros in the northern Marianas. Water discoloration has been observed there, and in 1979 the crew of a fishing boat felt shocks over the summit area, followed by upwelling of sulfur-bearing water. On 24-25 April 2001 an explosive eruption was detected seismically by a station on Rangiroa Atoll, Tuamotu Archipelago. The event was well constrained (+/- 15 km) at a location near the southern base of Ahyi. An eruption in April-May 2014 was detected by NOAA divers, hydroacoustic sensors, and seismic stations.

Information Contacts: US Geological Survey, Volcano Hazards Program (USGS-VHP), 12201 Sunrise Valley Drive, Reston, VA, USA, https://volcanoes.usgs.gov/index.html; Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kadovar is a 2-km-wide island that is the emergent summit of a Bismarck Sea stratovolcano. It lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the volcano, filling an arcuate landslide scarp open to the S. Submarine debris-avalanche deposits occur to the S of the island. The current eruption began in January 2018 and has comprised lava effusion from vents at the summit and at the E coast; more recent activity has consisted of ash plumes, weak thermal activity, and gas-and-steam plumes (BGVN 48:02). This report covers activity during February through May 2023 using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

Activity during the reporting period was relatively low and mainly consisted of white gas-and-steam plumes that were visible in natural color satellite images on clear weather days (figure 67). According to a Darwin VAAC report, at 2040 on 6 May an ash plume rose to 4.6 km altitude and drifted W; by 2300 the plume had dissipated. MODIS satellite instruments using the MODVOLC thermal algorithm detected a single thermal hotspot on the SE side of the island on 7 May. Weak thermal activity was also detected in a satellite image on the E side of the island on 14 May, accompanied by a white gas-and-steam plume that drifted SE (figure 68).

Figure 67. True color satellite images showing a white gas-and-steam plume rising from Kadovar on 28 February 2023 (left) and 30 March 2023 (right) and drifting SE and S, respectively. Courtesy of Copernicus Browser.

Figure 68. Infrared (bands B12, B11, B4) image showing weak thermal activity on the E side of the island, accompanied by a gas-and-steam plume that drifted SE from Kadovar on 14 May 2023. Courtesy of Copernicus Browser.

Geologic Background. The 2-km-wide island of Kadovar is the emergent summit of a Bismarck Sea stratovolcano of Holocene age. It is part of the Schouten Islands, and lies off the coast of New Guinea, about 25 km N of the mouth of the Sepik River. Prior to an eruption that began in 2018, a lava dome formed the high point of the andesitic volcano, filling an arcuate landslide scarp open to the south; submarine debris-avalanche deposits occur in that direction. Thick lava flows with columnar jointing forms low cliffs along the coast. The youthful island lacks fringing or offshore reefs. A period of heightened thermal phenomena took place in 1976. An eruption began in January 2018 that included lava effusion from vents at the summit and at the E coast.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

San Miguel in El Salvador is a broad, deep crater complex that has been frequently modified by eruptions recorded since the early 16th century and consists of the summit known locally as Chaparrastique. Flank eruptions have produced lava flows that extended to the N, NE, and SE during the 17-19th centuries. The most recent activity has consisted of minor ash eruptions from the summit crater. The current eruption period began in November 2022 and has been characterized by frequent phreatic explosions, gas-and-ash emissions, and sulfur dioxide plumes (BGVN 47:12). This report describes small gas-and-ash explosions during December 2022 through May 2023 based on special reports from the Ministero de Medio Ambiente y Recursos Naturales (MARN).

Activity has been relatively low since the last recorded explosions on 29 November 2022. Seismicity recorded by the San Miguel Volcano Station (VSM) located on the N flank at 1.7 km elevation had decreased by 7 December. Sulfur dioxide gas measurements taken with DOAS (Differential Optical Absorption Spectroscopy) mobile equipment were below typical previously recorded values: 300 tons per day (t/d). During December, small explosions were recorded by the seismic network and manifested as gas-and-steam emissions.

Gas-and-ash explosions in the crater occurred during January 2023, which were recorded by the seismic network. Sulfur dioxide values remained low, between 300-400 t/d through 10 March. At 0817 on 14 January a gas-and-ash emission was visible in webcam images, rising just above the crater rim. Some mornings during February, small gas-and-steam plumes were visible in the crater. On 7 March at 2252 MARN noted an increase in degassing from the central crater; gas emissions were constantly observed through the early morning hours on 8 March. During the early morning of 8 March through the afternoon on 9 March, 12 emissions were registered, some accompanied by ash. The last gas-and-ash emission was recorded at 1210 on 9 March; very fine ashfall was reported in El Tránsito (10 km S), La Morita (6 km W), and La Piedrita (3 km W). The smell of sulfur was reported in Piedra Azul (5 km SW). On 16 March MARN reported that gas-and-steam emissions decreased.

Low degassing and very low seismicity were reported during April; no explosions have been detected between 9 March and 27 May. The sulfur dioxide emissions remained between 350-400 t/d; during 13-20 April sulfur dioxide values fluctuated between 30-300 t/d. Activity remained low through most of May; on 23 May seismicity increased. An explosion was detected at 1647 on 27 May generated a gas-and-ash plume that rose 700 m high (figure 32); a decrease in seismicity and gas emissions followed. The DOAS station installed on the W flank recorded sulfur dioxide values that reached 400 t/d on 27 May; subsequent measurements showed a decrease to 268 t/d on 28 May and 100 t/d on 29 May.

Figure 32. Webcam image of a gas-and-ash plume rising 700 m above San Miguel at 1652 on 27 May 2023. Courtesy of MARN.

Geologic Background. The symmetrical cone of San Miguel, one of the most active volcanoes in El Salvador, rises from near sea level to form one of the country's most prominent landmarks. A broad, deep, crater complex that has been frequently modified by eruptions recorded since the early 16th century caps the truncated unvegetated summit, also known locally as Chaparrastique. Flanks eruptions of the basaltic-andesitic volcano have produced many lava flows, including several during the 17th-19th centuries that extended to the N, NE, and SE. The SE-flank flows are the largest and form broad, sparsely vegetated lava fields crossed by highways and a railroad skirting the base of the volcano. Flank vent locations have migrated higher on the edifice during historical time, and the most recent activity has consisted of minor ash eruptions from the summit crater.

Information Contacts: Ministero de Medio Ambiente y Recursos Naturales (MARN), Km. 5½ Carretera a Nueva San Salvador, Avenida las Mercedes, San Salvador, El Salvador (URL: http://www.snet.gob.sv/ver/vulcanologia).

Semisopochnoi is located in the western Aleutians, is 20-km-wide at sea level, and contains an 8-km-wide caldera. The three-peaked Mount Young (formerly Cerberus) was constructed within the caldera during the Holocene. Each of these peaks contains a summit crater; the lava flows on the N flank appear younger than those on the S side. The current eruption period began in early February 2021 and has more recently consisted of intermittent explosions and ash emissions (BGVN 47:12). This report updates activity during December 2022 through May 2023 using daily, weekly, and special reports from the Alaska Volcano Observatory (AVO). AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

Activity during most of December 2022 was relatively quiet; according to AVO no eruptive or explosive activity was observed since 7 November 2022. Intermittent tremor and occasional small earthquakes were observed in geophysical data. Continuous gas-and-steam emissions were observed from the N crater of Mount Young in webcam images on clear weather days (figure 25). On 24 December, there was a slight increase in earthquake activity and several small possible explosion signals were detected in infrasound data. Eruptive activity resumed on 27 December at the N crater of Mount Young; AVO issued a Volcano Activity Notice (VAN) that reported minor ash deposits on the flanks of Mount Young that extended as far as 1 km from the vent, according to webcam images taken during 27-28 December (figure 26). No ash plumes were observed in webcam or satellite imagery, but a persistent gas-and-steam plume that might have contained some ash rose to 1.5 km altitude. As a result, AVO raised the Aviation Color Code (ACC) to Orange (the second highest level on a four-color scale) and the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale). Possible explosions were detected during 21 December 2022 through 1 January 2023 and seismic tremor was recorded during 30-31 December.

Figure 25. Webcam image of a gas-and-steam plume rising above Semisopochnoi from Mount Young on 21 December 2022. Courtesy of AVO.

Figure 26. Webcam image showing fresh ash deposits (black color) at the summit and on the flanks of Mount Young at Semisopochnoi, extending up to 1 km from the N crater. Image was taken on 27 December 2022. Image has been color corrected. Courtesy of AVO.

During January 2023 eruptive activity continued at the active N crater of Mount Young. Minor ash deposits were observed on the flanks, extending about 2 km SSW, based on webcam images from 1 and 3 January. A possible explosion occurred during 1-2 January based on elevated seismicity recorded on local seismometers and an infrasound signal recorded minutes later by an array at Adak. Though no ash plumes were observed in webcam or satellite imagery, a persistent gas-and-steam plume rose to 1.5 km altitude that might have carried minor traces of ash. Ash deposits were accompanied by periods of elevated seismicity and infrasound signals from the local geophysical network, which AVO reported were likely due to weak explosive activity. Low-level explosive activity was also detected during 2-3 January, with minor gas-and-steam emissions and a new ash deposit that was visible in webcam images. Low-level explosive activity was detected in geophysical data during 4-5 January, with elevated seismicity and infrasound signals observed on local stations. Volcanic tremor was detected during 7-9 January and very weak explosive activity was detected in seismic and infrasound data on 9 January. Weak seismic and infrasound signals were recorded on 17 January, which indicated minor explosive activity, but no ash emissions were observed in clear webcam images; a gas-and-steam plume continued to rise to 1.5 km altitude. During 29-30 January, ash deposits near the summit were observed on fresh snow, according to webcam images.

The active N cone at Mount Young continued to produce a gas-and-steam plume during February, but no ash emissions or explosive events were detected. Seismicity remained elevated with faint tremor during early February. Gas-and-steam emissions from the N crater were observed in clear webcam images on 11-13 and 16 February; no explosive activity was detected in seismic, infrasound, or satellite data. Seismicity has also decreased, with no significant seismic tremor observed since 25 January. Therefore, the ACC was lowered to Yellow (the second lowest level on a four-color scale) and the VAL was lowered to Advisory (the second lowest level on a four-color scale) on 22 February.

Gas-and-steam emissions persisted during March from the N cone of Mount Young, based on clear webcam images. A few brief episodes of weak tremor were detected in seismic data, although seismicity decreased over the month. A gas-and-steam plume detected in satellite data extended 150 km on 18 March. Low-level ash emissions from the N cone at Mount Young were observed in several webcam images during 18-19 March, in addition to small explosions and volcanic tremor. The ACC was raised to Orange and the VAL increased to Watch on 19 March. A small explosion was detected in seismic and infrasound data on 21 March.

Low-level unrest continued during April, although cloudy weather often obscured views of the summit; periods of seismic tremor and local earthquakes were recorded. During 3-4 April a gas-and-steam plume was visible traveling more than 200 km overnight; no ash was evident in the plume, according to AVO. A gas-and-steam plume was observed during 4-6 April that extended 400 km but did not seem to contain ash. Small explosions were detected in seismic and infrasound data on 5 April. Occasional clear webcam images showed continuing gas-and-steam emissions rose from Mount Young, but no ash deposits were observed on the snow. On 19 April small explosions and tremor were detected in seismic and infrasound data. A period of seismic tremor was detected during 22-25 April, with possible weak explosions on 25 April. Ash deposits were visible near the crater rim, but it was unclear if these deposits were recent or due to older deposits.

Occasional small earthquakes were recorded during May, but there were no signs of explosive activity seen in geophysical data. Gas-and-steam emissions continued from the N crater of Mount Young, based on webcam images, and seismicity remained slightly elevated. A new, light ash deposit was visible during the morning of 5 May on fresh snow on the NW flank of Mount Young. During 10 May periods of volcanic tremor were observed. The ACC was lowered to Yellow and the VAL to Advisory on 17 May due to no additional evidence of activity.

Geologic Background. Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains an 8-km-wide caldera. It formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is Anvil Peak, a double-peaked late-Pleistocene cone that forms much of the island's northern part. The three-peaked Mount Cerberus (renamed Mount Young in 2023) was constructed within the caldera during the Holocene. Each of the peaks contains a summit crater; lava flows on the N flank appear younger than those on the south side. Other post-caldera volcanoes include the symmetrical Sugarloaf Peak SSE of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the NE part of the caldera. Most documented eruptions have originated from Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone could have been recently active.

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

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

Activity during October consisted of explosive activity, ash plumes, and occasional thermal anomalies. Visual data by volcanologists from Severo-Kurilsk showed explosions producing ash clouds up to 2.1-3 km altitude which drifted E, N, NE, and SE during 1-8, 10, 16, and 18 October. KVERT issued several Volcano Observatory Notices for Aviation (VONA) on 7, 13-15, and 27 October 2022, stating that explosions generated ash plumes that rose to 2.3-4 km altitude and drifted 5 km E, NE, and SE. Ashfall was reported in Severo-Kurilsk (Paramushir Island, about 7 km E) on 7 and 13 October. Satellite data showed a thermal anomaly over the volcano on 15-16 October. Visual data showed ash plumes rising to 2.5-3.6 km altitude on 22, 25-29, and 31 October and moving NE due to constant explosions.

Similar activity continued during November, with explosions, ash plumes, and ashfall occurring. KVERT issued VONAs on 1-2, 4, 6-7, 9, 13, and 16 November that reported explosions and resulting ash plumes that rose to 1.7-3.6 km altitude and drifted 3-5 km SE, ESE, E, and NE. On 1 November ash plumes extended as far as 110 km SE. On 5, 8, 12, and 24-25 November explosions and ash plumes rose to 2-3.1 km altitude and drifted N and E. Ashfall was observed in Severo-Kurilsk on 7 and 16 November. A thermal anomaly was visible during 1-4, 16, and 20 November. Explosions during 26 November rose as high as 2.7 km altitude and drifted NE (figure 45).

Figure 45. Photo of an ash plume rising to 2.7 km altitude above Ebeko on 26 November 2022. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

Explosions and ash plumes continued to occur in December. During 1-2 and 4 December volcanologists from Severo-Kurilsk observed explosions that sent ash to 1.9-2.5 km altitude and drifted NE and SE (figure 46). VONAs were issued on 5, 9, and 16 December reporting that explosions generated ash plumes rising to 1.9 km, 2.6 km, and 2.4 km altitude and drifted 5 km SE, E, and NE, respectively. A thermal anomaly was visible in satellite imagery on 16 December. On 18 and 27-28 December explosions produced ash plumes that rose to 2.5 km altitude and drifted NE and SE. On 31 December an ash plume rose to 2 km altitude and drifted NE.

Figure 46. Photo of an explosive event at Ebeko at 1109 on 2 December 2022. Photo has been color corrected. Photo by S. Lakomov, IVS FEB RAS.

Explosions continued during January 2023, based on visual observations by volcanologists from Severo-Kurilsk. During 1-7 January explosions generated ash plumes that rose to 4 km altitude and drifted NE, E, W, and SE. According to VONAs issued by KVERT on 2, 4, 10, and 23 January, explosions produced ash plumes that rose to 2-4 km altitude and drifted 5 km N, NE, E, and ENE; the ash plume that rose to 4 km altitude occurred on 10 January (figure 47). Satellite data showed a thermal anomaly during 3-4, 10, 13, 16, 21, 22, and 31 January. KVERT reported that an ash cloud on 4 January moved 12 km NE. On 6 and 9-11 January explosions sent ash plumes to 4.5 km altitude and drifted W and ESE. On 13 January an ash plume rose to 3 km altitude and drifted SE. During 20-24 January ash plumes from explosions rose to 3.7 km altitude and drifted SE, N, and NE. On 21 January the ash plume drifted as far as 40 km NE. During 28-29 and 31 January and 1 February ash plumes rose to 4 km altitude and drifted NE.

Figure 47. Photo of a strong ash plume rising to 4 km altitude from an explosive event on 10 January 2023 (local time). Photo by L. Kotenko, IVS FEB RAS.

During February, explosions, ash plumes, and ashfall were reported. During 1, 4-5 and 7-8 February explosions generated ash plumes that rose to 4.5 km altitude and drifted E and NE; ashfall was observed on 5 and 8 February. On 6 February an explosion produced an ash plume that rose to 3 km altitude and drifted 7 km E, causing ashfall in Severo-Kurilsk. A thermal anomaly was visible in satellite data on 8, 9, 13, and 21 February. Explosions on 9 and 12-13 February produced ash plumes that rose to 4 km altitude and drifted E and NE; the ash cloud on 12 February extended as far as 45 km E. On 22 February explosions sent ash to 3 km altitude that drifted E. During 24 and 26-27 February ash plumes rose to 4 km altitude and drifted E. On 28 February an explosion sent ash to 2.5-3 km altitude and drifted 5 km E; ashfall was observed in Severo-Kurilsk.

Activity continued during March; visual observations showed that explosions generated ash plumes that rose to 3.6 km altitude on 3, 5-7, and 9-12 March and drifted E, NE, and NW. Thermal anomalies were visible on 10, 13, and 29-30 March in satellite imagery. On 18, 21-23, 26, and 29-30 March explosions produced ash plumes that rose to 2.8 km altitude and drifted NE and E; the ash plumes during 22-23 March extended up to 76 km E. A VONA issued on 21 March reported an explosion that produced an ash plume that rose to 2.8 km altitude and drifted 5 km E. Another VONA issued on 23 March reported that satellite data showed an ash plume rising to 3 km altitude and drifted 14 km E.

Explosions during April continued to generate ash plumes. On 1 and 4 April an ash plume rose to 2.8-3.5 km altitude and drifted SE and NE. A thermal anomaly was visible in satellite imagery during 1-6 April. Satellite data showed ash plumes and clouds rising to 2-3 km altitude and drifting up to 12 km SW and E on 3 and 6 April (figure 48). KVERT issued VONAs on 3, 5, 14, 16 April describing explosions that produced ash plumes rising to 3 km, 3.5 km, 3.5 km, and 3 km altitude and drifting 5 km S, 5 km NE and SE, 72 km NNE, and 5 km NE, respectively. According to satellite data, the resulting ash cloud from the explosion on 14 April was 25 x 7 km in size and drifted 72-104 km NNE during 14-15 April. According to visual data by volcanologists from Severo-Kurilsk explosions sent ash up to 3.5 km altitude that drifted NE and E during 15-16, 22, 25-26, and 29 April.

Figure 48. Photo of an ash cloud rising to 3.5 km altitude at Ebeko on 6 April 2023. The cloud extended up to 12 km SW and E. Photo has been color corrected. Photo by L. Kotenko, IVS FEB RAS.

The explosive eruption continued during May. Explosions during 3-4, 6-7, and 9-10 May generated ash plumes that rose to 4 km altitude and drifted SW and E. Satellite data showed a thermal anomaly on 3, 9, 13-14, and 24 May. During 12-16, 23-25, and 27-28 May ash plumes rose to 3.5 km altitude and drifted in different directions due to explosions. Two VONA notices were issued on 16 and 25 May, describing explosions that generated ash plumes rising to 3 km and 3.5 km altitude, respectively and extending 5 km E. The ash cloud on 25 May drifted 75 km SE.

Thermal activity in the summit crater, occasionally accompanied by ash plumes and ash deposits on the SE and E flanks due to frequent explosions, were visible in infrared and true color satellite images (figure 49).

Figure 49. Infrared (bands B12, B11, B4) and true color satellite images of Ebeko showing occasional small thermal anomalies at the summit crater on 4 October 2022 (top left), 30 April 2023 (bottom left), and 27 May 2023 (bottom right). On 1 November (top right) ash deposits (light-to-dark gray) were visible on the SE flank. An ash plume drifted NE on 30 April, and ash deposits were also visible to the E on both 30 April and 27 May. Courtesy of Copernicus Browser.

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

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Home Reef is a submarine volcano located in the central Tonga islands between Lateiki (Metis Shoal) and Late Island. The first recorded eruption occurred in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, a large volume of floating pumice, and an ephemeral island 500 x 1,500 m wide, with cliffs 30-50 m high that enclosed a water-filled crater. Another island-forming eruption in 2006 produced widespread pumice rafts that drifted as far as Australia; by 2008 the island had eroded below sea level. The previous eruption occurred during October 2022 and was characterized by a new island-forming eruption, lava effusion, ash plumes, discolored water, and gas-and-steam plumes (BGVN 47:11). This report covers discolored water plumes during November 2022 through April 2023 using satellite data.

Discolored plumes continued during the reporting period and were observed in true color satellite images on clear weather days. Satellite images show light green-yellow discolored water extending W on 8 and 28 November 2022 (figure 31), and SW on 18 November. Light green-yellow plumes extended W on 3 December, S on 13 December, SW on 18 December, and W and S on 23 December (figure 31). On 12 January 2023 discolored green-yellow plumes extended to the NE, E, SE, and N. The plume moved SE on 17 January and NW on 22 January. Faint discolored water in February was visible moving NE on 1 February. A discolored plume extended NW on 8 and 28 March and NW on 13 March (figure 31). During April, clear weather showed green-blue discolored plumes moving S on 2 April, W on 7 April, and NE and S on 12 April. A strong green-yellow discolored plume extended E and NE on 22 April for several kilometers (figure 31).

Figure 31. Visual (true color) satellite images showing continued green-yellow discolored plumes at Home Reef (black circle) that extended W on 28 November 2022 (top left), W and S on 23 December 2022 (top right), NW on 13 March 2023 (bottom left), and E and NE on 22 April 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. Home Reef, a submarine volcano midway between Metis Shoal and Late Island in the central Tonga islands, was first reported active in the mid-19th century, when an ephemeral island formed. An eruption in 1984 produced a 12-km-high eruption plume, large amounts of floating pumice, and an ephemeral 500 x 1,500 m island, with cliffs 30-50 m high that enclosed a water-filled crater. In 2006 an island-forming eruption produced widespread dacitic pumice rafts that drifted as far as Australia. Another island was built during a September-October 2022 eruption.

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

Ambae, also known as Aoba, is a large basaltic shield volcano in Vanuatu. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas. Periodic phreatic and pyroclastic explosions have been reported since the 16th century. A large eruption more than 400 years ago resulted in a volcanic cone within the summit crater that is now filled by Lake Voui; the similarly sized Lake Manaro fills the western third of the caldera. The previous eruption ended in August 2022 that was characterized by gas-and-steam and ash emissions and explosions of wet tephra (BGVN 47:10). This report covers a new eruption during February through May 2023 that consisted of a new lava flow, ash plumes, and sulfur dioxide emissions, using information from the Vanuatu Meteorology and Geo-Hazards Department (VMGD) and satellite data.

During the reporting period, the Alert Level remained at a 2 (on a scale of 0-5), which has been in place since December 2021. Activity during October 2022 through March 2023 remained relatively low and mostly consisted of gas-and-steam emissions in Lake Voui. VMGD reported that at 1300 on 15 November a satellite image captured a strong amount of sulfur dioxide rising above the volcano (figure 99), and that seismicity slightly increased. The southern and northern part of the island reported a strong sulfur dioxide smell and heard explosions. On 20 February 2023 a gas-and-ash plume rose 1.3 km above the summit and drifted SSW, according to a webcam image (figure 100). Gas-and-steam and possibly ash emissions continued on 23 February and volcanic earthquakes were recorded by the seismic network.

Figure 99. Satellite image of the strong sulfur dioxide plume above Ambae taken on 15 November 2022. The Dobson Units (DU) exceeded 12. Courtesy of VMGD.

Figure 100. Webcam image of a gas-and-ash plume rising above Ambae at 1745 on 20 February 2023. The plume drifted SSW. Courtesy of VMGD.

During April, volcanic earthquakes and gas-and-steam and ash emissions were reported from the cone in Lake Voui. VMGD reported that activity increased during 5-7 April; high gas-and-steam and ash plumes were visible, accompanied by nighttime incandescence. According to a Wellington VAAC report, a low-level ash plume rose as high as 2.5 km above the summit and drifted W and SW on 5 April, based on satellite imagery. Reports in Saratamata stated that a dark ash plume drifted to the WSW, but no loud explosion was heard. Webcam images from 2100 showed incandescence above the crater and reflected in the clouds. According to an aerial survey, field observations, and satellite data, water was no longer present in the lake. A lava flow was reported effusing from the vent and traveling N into the dry Lake Voui, which lasted three days. The next morning at 0745 on 6 April a gas-and-steam and ash plume rose 5.4 km above the summit and drifted ESE, based on information from VMGD (figure 101). The Wellington VAAC also reported that light ashfall was observed on the island. Intermittent gas-and-steam and ash emissions were visible on 7 April, some of which rose to an estimated 3 km above the summit and drifted E. Webcam images during 0107-0730 on 7 April showed continuing ash emissions. A gas-and-steam and ash plume rose 695 m above the summit crater at 0730 on 19 April and drifted ESE, based on a webcam image (figure 102).

Figure 101. Webcam image showing a gas-and-ash plume rising 5.4 km above the summit of Ambae at 0745 on 6 April 2023. Courtesy of VMGD.

Figure 102. Webcam image showing a gas-and-ash plume rising 695 m above the summit of Ambae at 0730 on 19 April 2023. Courtesy of VMGD.

According to visual and infrared satellite data, water was visible in Lake Voui as late as 24 March 2023 (figure 103). The vent in the caldera showed a gas-and-steam plume drifted SE. On 3 April thermal activity was first detected, accompanied by a gas-and-ash plume that drifted W (figure 103). The lava flow moved N within the dry lake and was shown cooling by 8 April. By 23 April much of the water in the lake had returned. Occasional sulfur dioxide plumes were detected by the TROPOMI instrument on the Sentinel-5P satellite that exceeded 2 Dobson Units (DU) and drifted in different directions (figure 104).

Figure 103. Satellite images showing both visual (true color) and infrared (bands B12, B11, B4) views on 24 March 2023 (top left), 3 April 2023 (top left), 8 April 2023 (bottom left), and 23 April 2023 (bottom right). In the image on 24 March, water filled Lake Voui around the small northern lake. A gas-and-steam plume drifted SE. Thermal activity (bright yellow-orange) was first detected in infrared data on 3 April 2023, accompanied by a gas-and-ash plume that drifted W. The lava flow slowly filled the northern part of the then-dry lake and remained hot on 8 April. By 23 April, the water in Lake Voui had returned. Courtesy of Copernicus Browser.

Figure 104. Images showing sulfur dioxide plumes rising from Ambae on 26 December 2022 (top left), 25 February 2023 (top right), 23 March 2023 (bottom left), and 5 April 2023 (bottom right), as detected by the TROPOMI instrument on the Sentinel-5P satellite. These plumes exceeded at least 2 Dobson Units (DU) and drifted in different directions. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The island of Ambae, also known as Aoba, is a massive 2,500 km3 basaltic shield that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone with numerous scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes (Manaro Ngoru, Voui, and Manaro Lakua) is located at the summit within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. That large central edifice is also called Manaro Voui or Lombenben volcano. Post-caldera explosive eruptions formed the summit craters about 360 years ago. A tuff cone was constructed within Lake Voui (or Vui) about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Information Contacts: Geo-Hazards Division, Vanuatu Meteorology and Geo-Hazards Department (VMGD), Ministry of Climate Change Adaptation, Meteorology, Geo-Hazards, Energy, Environment and Disaster Management, Private Mail Bag 9054, Lini Highway, Port Vila, Vanuatu (URL: http://www.vmgd.gov.vu/, https://www.facebook.com/VanuatuGeohazardsObservatory/); Wellington Volcanic Ash Advisory Centre (VAAC), Meteorological Service of New Zealand Ltd (MetService), PO Box 722, Wellington, New Zealand (URL: http://www.metservice.com/vaac/, http://www.ssd.noaa.gov/VAAC/OTH/NZ/messages.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Remeasurement of five EDM lines on 15-16 May yielded no significant changes (>1 cm) since the network was established in September 1990. Two seismometers temporarily operated on the caldera floor recorded no local shallow seismicity. The temperature of the boiling spring in the caldera was 98°C, the same as in 1990. The volume of water issuing from the hot spring was less than in 1990, maybe because of seasonal rainfall variations. The highest measured fumarole temperature was 102°C, 4° higher than in 1990, perhaps related to a drop in the water table.

Geologic Background. The highest of the Marianas arc volcanoes, Agrigan contains a 500-m-deep, flat-floored caldera. The elliptical island is 8 km long; its summit is the top of a massive 4000-m-high submarine volcano. Deep radial valleys dissect the flanks of the thickly vegetated stratovolcano. The elongated caldera is 1 x 2 km wide and is breached to the NW, from where a prominent lava flow extends to the coast and forms a lava delta. The caldera floor is surfaced by fresh-looking lava flows and also contains two cones that may have formed during the only historical eruption in 1917. This eruption deposited large blocks and 3 m of ash and lapilli on a village on the SE coast, prompting its evacuation.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Only two explosions occurred . . . in June, causing no damage. The month's highest ash clouds rose 2,000 m on 9 and 18 June. Two 9-hour swarms of volcanic earthquakes were recorded, a relatively low level of seismicity for the volcano.

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

Information Contacts: JMA.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating.

[At Alamagan] the team measured a temperature of 72°C at one fumarole. No shallow earthquakes or volcanic tremor have been recorded on the Alamagan seismic station since it was installed in September 1990. Charcoal was collected that should date the youngest and one of the oldest eruptions.

Geologic Background. Alamagan is the emergent summit of a large stratovolcano in the central Mariana Islands with a roughly 350-m-deep summit crater east of the center of the island. The exposed cone is largely Holocene in age. A 1.6 x 1 km graben cuts the SW flank. An extensive basaltic-andesite lava flow has extended the northern coast of the island, and a lava platform also occurs on the S flank. Pyroclastic-flow deposits erupted about 1000 years ago have been dated, but reports of historical eruptions were considered invalid (Moore and Trusdell, 1993).

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Remeasurement of the EDM network on 22 May showed no significant changes, consistent with the lack of shallow seismicity since September 1990. Boiling hot springs on the eastern crater floor and solfataras at the base of the nearby crater wall had maximum temperatures of 98°C.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Lava production, tephra ejection, and fumarolic activity continued through mid-July. Most of the W-flank lava moved down a channel feeding the flow's S lobe, which moved into young forest on the WSW flank, an area that had been affected by the 1968 pyroclastic flows. Since mid-May, the S lobe's front had advanced almost 300 m, reaching 665 m elevation on 10 June and 650 m elevation by the 24th. As it advanced, the lava flow continued to start fires that burned well over a hectare of the surrounding woodland. Between 12 and 22 July, the flow front advanced at an average rate of ~20 m/day, reaching ~2.5 km from the new summit crater (C). The lava supply to the N lobe had dwindled, and its front had halted at 830 m elevation.

Explosions were stronger and more numerous in June than in May. Some caused rumbling that vibrated house windows in La Palma, 4 km N of the volcano. An impact crater 1 m in diameter and 30 cm deep was found at 780 m elevation on the W flank, and large blocks frequently reached slightly >1 km from the new summit crater (C) 12-22 July. Some ash columns rose >1 km above Crater C. The rate of explosions varied; during observations on 12 June, an explosion was heard every hour. Ashfall on the observation point at 780 m elevation, 1.8 km W of the active crater, accumulated more rapidly in the 4 weeks ending 10 June than in the succeeding 2 weeks (see table 5). Vegetation on the NE, E, and SE flanks continues to be affected by acid rain and tephra fall, as it has for more than 20 years. Fumarolic activity occurred from the remnants of the old summit crater (D).

Volcanic seismicity recorded at a station (Fortuna) 4 km E of the active crater averaged 30 events/day, with a maximum of 51 on 18 June (figure 48). Conspicuous tremor episodes occurred on 4, 6, 10, 17, and 30 June. The level of both seismic and pyroclastic activity decreased 12-22 July, as did the number of avalanches from the advancing lava flow front.

Figure 48. Daily number of seismic events recorded at a station (Fortuna) 4 km E of Arenal's active crater, June 1992. Courtesy of the Instituto Costarricense de Electricidad.

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

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto, ICE; M. Fernández, Univ de Costa Rica.

Eruptions that occurred from Crater 1 during the night of 30 June-1 July were the first [strong explosions] since . . . December 1990. The daily number of isolated volcanic tremor episodes began to increase in October 1991, and had reached ~100/day by the end of May. Isolated tremor episodes rapidly became more frequent in late June, and the amplitude of continuous tremor also increased through the month.

Ejections of mud and water from the lake in Crater 1 were first noted on 23 April and were sporadically observed later in April and in May. The ejections became more vigorous in late June, increasing in height from 5 m on 24 June to 20 m on the 26th, 50 m on the 29th, and 150 m on the 30th. Surface temperatures of the lake water increased from around 20°C in May 1991 to 78°C in June 1992. Steam plumes also grew to 1,000 m height in late June.

Strong tremor episodes were recorded during the night of 30 June-1 July. During fieldwork at noon on 1 July, the crater was quiet, but many blocks to 0.8 m across had been scattered to 100 m from the crater's NE rim. The eruptions were not seen or heard, but seismic and air-vibration records suggested that they may have occurred at 2349 on 30 June and 0316 on 1 July.

Tremor decreased in early July, but remained at higher levels than in mid-June. Ejections of mud and water to heights of a few tens of meters occurred sporadically through early July, but no additional strong mud/water ejections or eruptions were reported.

Because of the increasing activity, the area within 1 km of the crater was closed to tourists on 24 June, and remained closed as of mid-July.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Vigorous steaming was occurring from several locations in the summit crater [of Asuncion] during observations from a helicopter on 18 May.

Geologic Background. A single large asymmetrical stratovolcano forms 3-km-wide Asuncion Island. The steeper NE flank terminates in high sea cliffs, while the gentler SW flanks have low-angle slopes bounded by sea cliffs only a few meters high. The southern flank is cut by a large landslide scar. The S and W flanks are covered by ash deposits. An explosive eruption in 1906 produced lava flows that descended about halfway down the W and SE flanks, but several other eruption reports are of uncertain validity.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

A eruption . . . had begun by 6 July, when airplane pilots first reported steam and ash rising through low clouds. Similar activity was seen through the week, when satellite images revealed repeated plumes from Bogoslof. Pilots reported a cloud to ~3 km altitude on 14 July at 1815. Satellite images showed the plume extending roughly 100 km SE, to the S side of Unalaska Island. An image from 16 July at 1140 showed another plume extending ~100 km E to Unalaska. That day, a pilot saw a white plume rising to ~4 km altitude. An episode of vigorous steam and ash ejection began on 20 July at about 1700, and material had reached nearly 8 km asl by 1725, drifting NNE. A dark gray cloud that was ~15 km wide at 3 km altitude was moving NW from the volcano several hours later. Poor weather prevented subsequent observations, but satellite images showed no volcanic plumes rising above weather-cloud tops at ~6 km elevation. There have been no reports of ashfall. Cloudy weather has prevented direct observation of the island . . . .

Geologic Background. Bogoslof is the emergent summit of a submarine volcano that lies 40 km N of the main Aleutian arc. It rises 1,500 m above the Bering Sea floor. Repeated construction and destruction of lava domes at different locations during historical time has greatly modified the appearance of this "Jack-in-the-Box" volcano and has introduced a confusing nomenclature applied during frequent visits by exploring expeditions. The present triangular-shaped, 0.75 x 2 km island consists of remnants of lava domes emplaced from 1796 to 1992. Castle Rock (Old Bogoslof) is a steep-sided pinnacle that is a remnant of a spine from the 1796 eruption. The small Fire Island (New Bogoslof), about 600 m NW of Bogoslof Island, is a remnant of a lava dome formed in 1883.

Information Contacts: AVO; SAB.

The following, from José Luís Macías, Arturo Macías, Jean-Christophe Komorowski, Claus Siebe, and Robert Tilling, describes observations during fieldwork 18 April-21 May 1992, ten years after the major 1982 eruption.

Geology. We made several visits to the crater. The very significant erosion that has occurred in the last 10 years allowed us to descend relatively easily into the crater through its SE wall, where the rim's altitude is 1,060 m. The crater floor is at 900 m elevation.

The only changes that we noticed during our visits were caused by frequent rockfalls from the crater walls. Between the first and second visits, on 19 April and 3 May, new crater-floor rockfall deposits had originated from the SE crater wall. Recently exhumed fault planes veneered by secondary mineralization in the crater wall were also quite common. On the SE part of the rim, a fracture system 90 m long, 6-9 cm wide at its SE end, and 0.2-8 cm wide at the NE end, trended N 65°E, and was associated with mild fumarolic activity. The fracture cuts through bedded domal talus breccia mapped by Rose and others (1984) and might evolve to produce rockfalls in the near future. Several other curviplanar slump fractures encompass apparent areas of several hundred square meters on the crater wall. Thus, more vigorous rockfall activity might be expected, particularly during the coming rainy season or periods of heightened regional seismic activity.

People living near the volcano reported an eruption in late March or early April that produced light ashfall near the volcano, and was accompanied by loud, thunder-like noises. We think that the ashfall most likely was dust produced during large rockfalls from the crater walls, and the noise was the sound of the rockfalls. Eruption-like dust clouds produced by rockfall activity have been described at Kīlauea by Tilling (1974) and Tilling and others (1975).

To try to reduce local alarm, J.L. Macías and J.-C. Komorowski described the current activity and their interpretations of it during an informal conference on 19 May with residents of Chapultenango (11 km ESE of the crater), local authorities, and a group of elementary school teachers. Rumors in El Volcán (5 km E of the crater) that the volcano would erupt on its 10th anniversary caused many women and children to leave their homes.

Crater lake. Temperature and acidity of the crater lake were measured three times at two different sites (table 2). Lake temperature had increased from 28.6°C in 1986 to more than 40° in May 1992, nearing the 42° of October 1983 and February 1984. The pH values of 1.8 and 1.9 measured in 1983 and 1984, respectively, were similar to the April 1992 value. Although no heavy rainfall occurred between 18 April and 8 May, brief rains were common at night and may have diluted the lake with meteoric water, raising its pH. Water samples collected on the lake's N shore are being studied by M.A. Armienta and S. de la Cruz-Reyna at the Instituto de Geofísica, UNAM.

Table 2. Temperature and acidity of the crater lake at El Chichón, measured at sites on the SE and N shores.

Fumarolic activity. Gas emission from the crater fed a low-altitude plume visible on clear days. Fumarolic activity was observed throughout the crater but was much more extensive and vigorous in its NNE sector (steaming ground zone of Casadevall and others, 1984). Almost all of the fumaroles showed a steady, audible release of overpressured gas, except for one just N of the crater lake, where frequent noise changes showed that output was distinctly discontinuous. At times, vapor formed only within about 1 m above this vent, suggesting that the gas is initially superheated. All of the fumaroles produced sublimates, primarily native sulfur. A high-temperature fumarole NE of the crater lake contains molten orange sulfur within the orifice of a 1-m-high feature otherwise covered with needle-like amorphous yellow sulfur. Numerous mildly steaming areas were found in the NW and NE parts of the crater, and small fumaroles were active several tens of meters above the crater floor along the path descending from the SE crater wall. Relict portions of altered brecciated trachyandesite described by Rose and others (1984) as remnants of the pre-1982 dome and shown on the map of Casadevall and others (1984) as "altered areas" are still actively steaming.

A few fumaroles on the NE side of the crater are characterized by vigorous geyser activity, sending a constant flux of boiling water to 2-3 m height. In the same area, several boiling springs about 2-3 m above the present crater-lake surface produce boiling streams with a significant discharge into the lake, 50 m away. A similar situation was evident near a boiling mud pit in the NW part of the crater. These boiling streams are sites of mineral precipitation, and active red, brown, and green algae growth. Ferns and grasses have returned to some of these hydrothermal areas. Ponds 1 m in diameter on the NW side of the lake contained vigorously boiling mud (rising <1 m) and water.

The crater lake, which had recovered to November 1982 levels by November 1990, was turquoise-blue and had at least two large zones of intense surface effervescence as described by Casadevall and others (1984).

Although an acrid smell was noted at active hydrothermal areas, H₂S concentrations must have decreased below the 2-6 ppm that forced geologists to take special precautions in 1983 and to leave the crater in 1984. During several 4-hour periods in the crater, we never needed gas masks, even in the most active areas.

Other observations. In the Río Magdalena near Xochimilco (8 km NW of the crater), vegetation has made a strong comeback on pyroclastic-flow deposits, which are now covered by tall grasses and acacia trees up to 2 m high with trunks several centimeters in diameter. In all other areas within 2-3 km of the crater, the 1982 deposits are covered only by moss, lichen, and tall grass. Where pyroclastic flows and surges did not surmount topographic barriers or deposited only a thin veneer of material, vegetation is much more lush, with trees, ferns, and other broad-leafed tropical plants. Trees that were charred but not totally blown down >5 km away have begun to grow again from their stumps. The river that now passes through El Volcán was formed after the pyroclastic flows changed the former drainage pattern. An abundant, rusty colored precipitate (Fe oxides) was sampled for analysis.

Future work. More extensive field observations within the crater are planned for November or December. We will measure temperature and pH, and sample sites of hydrothermal activity. An attempt will be made to overfly the crater with a COSPEC, to bring portable seismometers into the crater and somma flanks, and to make bathymetric measurements.

References. Casadevall, T., de la Cruz-Reyna, S., Rose, W., Bagley, S., Finnegan, D., and Zoller, W., 1984, Crater lake and post-eruption hydrothermal activity, El Chichón Volcano, México: Journal of Volcanology and Geothermal Research, v. 23, p. 169-191.

Rose, W., Bornhorst, T., Halsor, S., Capaul, W., Plumley, P., de la Cruz-Reyna, S., Mena, M., and Mota, R., 1984, Volcán el Chichón, México: pre-1982 S-rich eruptive activity: Journal of Volcanology and Geothermal Research, v. 23, p. 147-167.

Tilling, R., 1974, Rockfall activity in pit craters, Kīlauea Volcano, Hawaii: Proceedings of the Symposium on "Andean and Antarctic Volcanology Problems", IAVCEI, Santiago, Chile, September 1974, p. 518-528.

Tilling, R., Koyanagi, R., and Holcomb, R., 1975, Rockfall seismicity-correlation with field observations, Makaopuhi Crater, Kīlauea Volcano, Hawaii: Journal of Research, U.S. Geological Survey, v. 3, p. 345-361.

Geologic Background. El Chichón is a small trachyandesitic tuff cone and lava dome complex in an isolated part of the Chiapas region in SE México. Prior to 1982, this relatively unknown volcano was heavily forested and of no greater height than adjacent non-volcanic peaks. The largest dome, the former summit of the volcano, was constructed within a 1.6 x 2 km summit crater created about 220,000 years ago. Two other large craters are located on the SW and SE flanks; a lava dome fills the SW crater, and an older dome is located on the NW flank. More than ten large explosive eruptions have occurred since the mid-Holocene. The powerful 1982 explosive eruptions of high-sulfur, anhydrite-bearing magma destroyed the summit lava dome and were accompanied by pyroclastic flows and surges that devastated an area extending about 8 km around the volcano. The eruptions created a new 1-km-wide, 300-m-deep crater that now contains an acidic crater lake.

Information Contacts: José Luís Macías V. and Michael Sheridan, State Univ of New York, Buffalo, NY; Jean-Christophe Komorowski and Claus Siebe, Instituto de Geofísica, UNAM; Robert Tilling, USGS.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. The submarine Clark stratovolcano lies near the southern end of the Southern Kermadec arc. This basaltic and dacitic edifice consists of a basal substrate of massive lava flows, pillow lavas, and pillow tubes overlain by volcaniclastic sediments. Craters are present along the complex crest. Clark is the southernmost volcano of the submarine chain that displays hydrothermal activity. Diffuse hydrothermal venting and sulfide chimneys were observed near the summit during a New Zealand-American NOAA Vents Program expedition in 2006.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Geysers geothermal area report. Film records showed 50 small events in the 24 hours following the M 7.5 earthquake, 46 of which had coda durations

Geologic Background. The late-Pliocene to early Holocene Clear Lake Volcanic Field in the northern Coast Ranges contains lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. The westernmost site of Quaternary volcanism in California, this volcanic field is in a complex geologic setting within the San Andreas transform fault system. Mount Konocti, a composite dacitic lava dome on the south shore of Clear Lake, is the largest volcanic feature. Volcanism has been largely non-explosive, with only one major airfall tuff. The latest eruptive activity, forming maars and cinder cones along the shores of Clear Lake, continued until about 9,000 years ago. A large silicic magma body provides the heat source for the Geysers, a geothermal field with a complex of electrical power plants.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

The following . . . covers activity between 10 April and 30 June 1992, and describes the 25 June 1991 lahar deposits.

Seismicity and rockfall activity. After a brief seismic crisis 4-10 March, activity at Colima remained near background levels. Starting 10 April, seismicity became more frequent. Nine B-type earthquakes were detected by the Red Sismológica de Colima (RESCO) and up to 60 events were recorded 10-20 May at the SW-flank Yerbabuena station (figure 17). Subsequent seismic activity remained near background, with only four B-type earthquakes recorded by RESCO 20-31 May, and three between 1 and 20 June. Seismic activity increased slightly 21-30 June, when 22 B-type earthquakes were recorded and the number of associated seismically detected rockfalls reached 55. Other rockfalls were also noted, probably associated with small diurnal changes in the volcano's hydrothermally altered summit region, which might be related to changes in rock temperature and surface water content. Extraordinary out-of-season precipitation in January, related to the El Niño/Southern Oscillation event of 1991-92, exceeded 700% of the monthly mean of the past 30 years and must have saturated the volcano's upper porous regions.

Figure 17. Sketch map of the summit area and SW flank of Colima, showing major canyons and recent volcanic deposits. Modified from Rodríguez-Elizarrarás, and others, 1991.

Current thermal activity. Fumarolic activity has been steady, with an impressive white plume that can rise several hundred meters above the summit before dissipating. This represents the systematic release of meteoric water accumulated in the upper part of the volcano, not an increase in the magmatic component of the fumarolic activity. Further avalanching of the most precarious hydrothermally altered regions of the summit area is expected during the rainy season, which has just started.

25 June 1991 lahar deposit. Block-and-ash flows emplaced about 1 x 10⁶ m³ of loose pyroclastic debris in the upper Barranca El Cordobán during collapse of the crater dome and rim on 16-17 April 1991, just before the 1991 lava flow began to move down the SW flank (figure 17) (Rodríguez-Elizarrarás and others, 1991). Despite heavy rains in May-September 1991, geologists from the CICT reported that most of the pyroclastic deposits had been washed away without producing sizeable mudflows (Rodríguez-Elizarrarás, and others, 1991). Nevertheless, on 28 March 1992, S. de la Cruz-Reyna and CICT geologists observed a significant laharic mass-flow deposit near El Jabalí, which was studied 5-7 June by J.-C. Komorowski and CICT geologists. A more thorough field and laboratory investigation of this deposit is in progress.

The lahar reached the settlements of La Becerrera and San Antonio, ~12 km SW of the summit (figure 17). Unequivocal non-reworked lahar material was seen at 1,280 m elevation, ~500 m above the confluence of the barrancas El Zarco and El Cordobán. The total thickness was 2 m with a channel width of 30 m. Deposits from this lahar have been identified up to ~1,900 m above sea level, at the bottom of a 20-30-m vertical lava wall in the barranca El Cordobán. The barranca's slope flattens drastically after the lava wall, so deposition probably began below this point. The most distant block-and-ash flow deposits in this barranca reached down to 2,100 m elevation. Upstream, the barranca was significantly eroded by water and debris from a maximum elevation of 2,600 m. Although there is no clear evidence of lahar deposits at San Antonio and La Becerrera, one person reported that the water crossing on the San Antonio-Laguna Verde road was obstructed for two days by lahar material, until machines cleared the debris. Such occurrences are frequent in the rainy season, because several large barrancas draining the upper slopes join there to form a channel 30 m wide.

We estimate the total lahar path at 9.9 km. Based on several measurements at different sites, the lahar deposit averages 25 m wide and 2 m thick. Maximum width was 38 m and maximum thickness 2.9 m at 1,640 m elevation (star on figure 17). Volume was estimated at approximately 0.5 x 10⁶ m³, or about 50% of the material estimated to have been emplaced by the 16-17 April 1991 pyroclastic activity. Field evidence and testimony (see below) unequivocally show that all of the lahar deposit was emplaced during one event. April 1992 field studies of barrancas at higher altitude revealed tremendous erosion since April 1991, leaving ravines incised deeply (to 15 m) into the pre-1991 pyroclastic deposits. A significant volume of loose 1991 debris remains on the mountain, ready to be incorporated into lahars during the rainy season.

Preliminary field investigations showed that the lahar deposit is characterized by a very flat surface, with suspended lava blocks to 1-2 m in maximum dimension protruding through the surface, and abundant pumiceous clasts from eroded 1913 deposits. The deposit is massive, non-stratified, non-graded, poorly sorted, and matrix supported. Its typical massive lowermost zone (0.6 m thick), locally well-sorted, has a concentration of blocks (to 0.5 m size) and wood fragments at the base, a prominent clast-supported medial zone (0.7 m thick) with imbricated sub-rounded boulders (to 0.3 m), and an uppermost massive unit (0.8 m) with a tendency toward reverse grading of lithic cobbles, supported in a sandy matrix. The deposit is typically semi-indurated. Inter-unit contacts are sharply defined in several places, most likely reflecting shear between rheologically different portions of the mass flow. Given the large suspended blocks, the very flat surface, the constant thickness over 9 km of travel distance, the presence of marginal levees, and overturned logs that came to rest vertically, the mass flow clearly had a significant yield strength. However, it must have been relatively swift, as it was able to flow around topographic barriers in the channel, and in some places to leave an elevated deposit on the outside wall when it rounded a sharp curve.

Few people witnessed the lahar. The best testimony came from a farmer (Ramón Aguirre Valencia) who went to Barranca El Cordobán on 26 June 1991 to check his cattle. At 1,600 m altitude, the barranca was filled by a gravel- and boulder-rich deposit with a flat surface. Rocks on the surface were coated with a thin layer of light-colored fine ash. Of the 20 cows killed by the lahar, several could be seen, with horns, heads, and feet protruding from the deposit. Numerous tree trunks several meters long and as much as 30 cm in diameter were also on the lahar's surface. Heavy rains had occurred the previous day, and the lahar apparently began to form after about 2 hours of heavy precipitation, accompanied by loud thunder. The nearest meteorological station (Cofradía de Suchitlán), about 12 km from the lahar's most likely source area, recorded 50 mm of rain on 25 June. By 3 July, a ravine had developed in the new lahar that was as deep (4.6 m) but not as wide as the present channel, which now spans 10.6 m of the 38-m-wide deposit. Five kilometers downstream, the lahar overran and destroyed a 2-m-high stone wall at El Jabalí and clogged the existing channel, but 2 km farther downslope, residents of La Becerrera noticed nothing unusual. Larger sediment flows reported at La Becerrera in January may have been related to breaching of a small earthen dam.

Warnings of future lahar flows and the hazards within Barranca El Cordobán were reiterated to authorities in 1992, as abundant loose material remains from the 1991 eruption and recently exposed 1913 pyroclastic units. The El Jabalí basin is filled with old mass-flow deposits that have traveled down several steep, deeply incised barrancas. On 12 June, CICT organized a meeting that included civil protection authorities to discuss these hazards.

Reference. Rodríguez-Elizarrarás, C., Siebe, C., Komorowski, J.-C., Espindola, J.M., and Saucedo, R., 1991, Field observations of pristine block-and-ash flow deposits emplaced April 16-17, 1991 at Volcán de Colima, México: Journal of Volcanology and Geothermal Research, v. 48, no. 3/4, p. 399-412.

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: Carlos Navarro, Abel Cortés, I. Galindo, José J. Hernández, and Ricardo Saucedo, CICT, Universidad de Colima; Jean-Christophe Komorowski and Claus Siebe, Instituto de Geofísica, UNAM.

Lava production continued from the fissure that opened in the W wall of the Valle del Bove on 15 December. Gas emission from 4 vents in the upper part of the fissure (2,215-2,235 m altitude; figure 52) fluctuated daily, probably with changes in weather conditions. However, gas emission has diminished since the eruption's initial months.

Figure 52. Sketch map of the fissure system and the upper part of the lava field at Etna, June 1992. Contour interval, 50 m. Courtesy of Romolo Romano.

No variation was evident in the movement of lava visible through a skylight high in the main channel, at 2,205 m altitude. Lava was also seen flowing through a skylight in lava tubes that formed in June along the channel into which lava was artificially diverted on 27 May (~ 1,980 m elevation) (17:05). From there, lava advanced through a complex series of tubes past the field that had formed in recent months. Lava again reached the surface around 1,800 m altitude from a changing number (generally 3-4) of ephemeral vents at varying locations representing tube bases. Lava flows extruded from these vents have generally been modest, have remained in the center of the lava field, and have not advanced beyond 1,600 m asl. As of the morning of 9 July, only one flow was active within the Valle del Bove, near the center at around 1,670 m altitude, with a fairly well-fed front. The volume of lava produced during ~7 months of eruption is estimated to be around 165 x 10⁶ m³.

Seismic activity during the period was characterized by low energy release. Significant increases were observed 8-9 July, when events of 2-4 Hz were recorded. The most significant perturbations were detected on 8 July at 1554, for 180 seconds, and at 1601 for 130 seconds. Tremor was almost nonexistent, obscured by seismic noise that characterizes periods of low activity at the volcano.

More or less voluminous gas emissions occurred from two vents at the bottom (~100 m from the rim) of the two central craters (Bocca Nuova and La Voragine). Incandescence caused by superheated gases (>1,000°C) from the vent in La Voragine was sometimes visible. Gas also emerged from a vent that has opened in Southeast Crater. Northeast Crater appeared to have been completely obstructed by internal collapse. COSPEC measurements of SO₂ flux from the summit crater showed relatively high values of ~ 8,000 t/d.

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: R. Romano and T. Caltabiano, IIV; P. Carveni, M. Grasso, and C. Monaco, Univ di Catania; G. Luongo, OV.

When observed from an airplane on 13 May, the volcano continued to fume vigorously, but no active lava was seen.

Geologic Background. The small 2-km-wide island of Farallon de Pajaros (also known as Uracas) is the northernmost and most active volcano of the Mariana Islands. Its relatively frequent eruptions dating back to the mid-19th century have caused the andesitic volcano to be referred to as the "Lighthouse of the western Pacific." The symmetrical, sparsely vegetated summit is the central cone within a small caldera cutting an older edifice, remnants of which are seen on the SE and southern sides near the coast. Flank fissures have fed lava flows that form platforms along the coast. Eruptions have been recorded from both summit and flank vents.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

An explosion on 16 July, the largest since activity began in 1989, ejected large tephra and may have generated a small pyroclastic flow. Partial collapse of the summit crater's lava dome occurred in June, and minor seismicity had been recorded a few days before the explosion.

June activity. The NW portion of the 1991 lava dome collapsed during June, and explosions and ash emissions occurred from the collapsed area. Las Portillas fumarole, formerly just NW of the dome, was larger after the collapse, and a line of new vents had opened nearby. The fracture on the NW crater wall remained active, and it and Las Portillas appeared to be the highest temperature vents in the crater. Gas columns were generally small, and were dispersed to the N and W. The number and energy release of long-period events (figure 55) and high-frequency earthquakes were low. Ten high-frequency earthquakes occurred in the NW part of the crater, with magnitudes of 0.3-1.7. The amplitude and period of background tremor showed small variations on 15 and 30 June. The maximum rate of SO₂ emission measured by COSPEC was ~5,500 t/d.

Figure 55. Daily number of long-period seismic events at Galeras, 1 January 1991-30 June 1992. The first observation of the summit lava dome is marked by an arrow. Courtesy of INGEOMINAS.

Precursory seismicity and tilt. Banded tremor episodes of moderate to high energy occurred 11-12 July, accompanied by a small inflationary tilt event recorded on both instruments near the summit. Between 14 and 16 July, six monochromatic long-period events were recorded, with durations on the order of 80 seconds. On 15 July, there was a small swarm of high-frequency events with magnitudes of 0-0.5.

16 July explosion. The explosion began at 1740 with a strong shock felt in Pasto . . . . More than 90% of the summit lava dome was destroyed as at least 120,000 m³ of blocks were ejected, falling primarily on the E and NE flanks. Blocks 30 cm in diameter fell 2.3 km from the crater, and impact craters to 3.5 m across were found 400 m away. Incandescent blocks started fires 2 km from the crater on the NE flank. The tephra severely damaged a small military facility on the crater rim, and dropped 40-cm blocks on telephone and television facilities near the summit. Roughly 30,000 m³ of ash were dispersed in a narrow band to the W, with the 1-mm isopach extending ~10 km. The dark-gray cauliflower-shaped eruption column reached ~4 km altitude. Reports from observers 10 km WSW of the crater (in Consacá) suggested that small pyroclastic flows may have descended the W flank. A powerful sonic wave generated by the explosion broke windows in Pasto, and reportedly in Consacá.

A seismic signal lasting ~8 minutes accompanied the explosion, saturating instruments for the first 37 seconds. Two distinct signals were recognized, one with a frequency of 1 Hz and a duration magnitude of 3, the other a 1.3-Hz tremor episode that lasted 4 minutes. A high-frequency, M 3.2-3.5 event occurred 26 hours after the explosion, in the S part of the volcano at ~5 km depth.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS-Observatorio Vulcanológico del Sur.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Observations [of Guguan] from an airplane on 13 May and a helicopter on 21 May revealed no gas emission.

Geologic Background. The small island of Guguan, only 2.8 km wide, is composed of an eroded volcano on the south, a caldera with a post-caldera cone, and a northern volcano. The latter has three coalescing cones and a breached summit crater that fed lava flows to the west and NW. The 287-m high point of the island is the south rim of the caldera. Freycinet misidentifed Guguan with Alamagan; reported eruptions in 1819 and 1901 (Catalog of Active Volcanoes of the World) actually refer to solfataric activity on Alamagan (Corwin, 1971). The only known historical eruption of Guguan took place between 1882 and 1884 and produced the northern volcano and lava flows that reached the coast.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Fumarolic activity continued in the main crater. Its lime-green lake had a mean temperature of 28°C and a minimum pH of 4.9 on 3 June. Fumaroles persisted in the area NE of the lake, with temperatures of 84-90°C. Areas of bubbling to the NE remained vigorous, with strong emission of cold gas, perhaps CO₂. Hot bubbling areas were stable at temperatures <=91°C. Fumarolic vents in the sedimentary fan N of the lake were buried by new sedimentation triggered by heavy rains. The resulting zone of steaming ground had surface temperatures of up to 90°C.

Seismicity continued, with 48 events recorded during June at a station (ICR) 2.2 km E of the active crater and 36 low-frequency microseisms registered 5 km WSW of the crater (at station IRZ2). The largest daily earthquake count was 7 on 2 June (at ICR). On 30 June, a M 1.9 event occurred 6.7 km SW of the main crater, at 3 km depth.

Geologic Background. The massive Irazú volcano in Costa Rica, immediately E of the capital city of San José, covers an area of 500 km2 and is vegetated to within a few hundred meters of its broad summit crater complex. At least 10 satellitic cones are located on its S flank. No lava effusion is known since the eruption of the Cervantes lava flows from S-flank vents about 14,000 years ago, and all known Holocene eruptions have been explosive. The focus of eruptions at the summit crater complex has migrated to the W towards the main crater, which contains a small lake. The first well-documented eruption occurred in 1723, and frequent explosive eruptions have occurred since. Ashfall from the last major eruption during 1963-65 caused significant disruption to San José and surrounding areas. Phreatic activity reported in 1994 may have been a landslide event from the fumarolic area on the NW summit (Fallas et al., 2018).

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G.J. Soto, ICE; Mario Fernández, Escuela Centroamericana de Geología, Univ de Costa Rica.

Activity began to increase in February 1992. Glowing rockfalls on 18 May filled the upper Keting river valley to 4 km from the crater. The volume of the deposit was estimated at 1.2 x 10⁶ m³, ~ 20% of the dome (17:04). Since then, the eruption has fluctuated, but a general decrease in intensity was indicated by declines in the height of the ash plume, the behavior of the glowing lava flow, and the vigor of incandescent tephra ejection. In July, glowing rockfalls advanced down the Keting river to 1,500 m from the crater. The number of volcanic and local tectonic earthquakes decreased in June and July compared to previous months. June-July seismicity was dominated by surface activity, such as explosion earthquakes and rockfalls (figure 2).

Figure 2. Tectonic seismicity (top) and volcanic earthquakes (bottom) at Karangetang, June-July 1992. 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: W. Modjo, VSI.

Lava production continued through early July from the E-51 vent . . . (figure 85), but was interrupted by several brief pauses. With each resumption in activity, lava reoccupied tubes on the S flank of the E-51 shield. Flows emerged from the tubes under some pressure, creating small, meter-high dome fountains at their heads. The lava pond at the top of the E-51 shield drained and refilled with changing lava supply, sustaining frequent overflows that did not advance far. Some lava also ponded at the base of the shield before flows advanced S and E. The small lava lake in Pu`u `O`o crater remained active, fluctuating between 38 and 55 m below the crater rim in June. The lake surface rose during pauses in activity at the episode-51 vent and dropped when lava production resumed there. By early July, it had dropped farther, to 65 m below the rim.

Activity resumed on 2 June, after a 3-day pause (17:5), while harmonic tremor began a gradual increase to about twice background levels at 0000. Large flows advanced N along the W flank of Pu`u `O`o cinder cone. These shelly pahoehoe flows formed shallow tubes and stagnated within a few days. The eruption stopped briefly on 5 June, as tremor dropped to near background at 1800, resumed the next day accompanied by a tremor increase at about 0700, and halted again ~24 hours later on the 7th, when lava drained slowly from the pond atop the shield.

Another increase in tremor began early on 9 June, reaching about twice background levels by noon on the 10th. Shallow, long-period microearthquakes (LPC-A, 3-5 Hz) were frequent on 9 June, as were upper east rift events on 9-10 June. Lava started to emerge from the E-51 vent at 1325 on 10 June, re-entering the tube system on the S flank of the E-51 shield. The lava lake in Pu`u `O`o crater had been nearly level with the crater floor when E-51 activity resumed, but had dropped ~9 m by the next day.

A small spatter cone formed 3-11 June over a weak point in the tube on the N flank of the E-51 shield. This tube had fed numerous aa ooze-outs that spread out around the shield's N flank in past months. On 13 June, an aa flow was active on the shield's N flank, appearing to originate from the new spatter cone.

Lava production stopped again on 16 June, the pond at the top of the shield drained, and flows slowed their advance. The eruption restarted during the morning of 21 June, continuing through the end of the month. Pahoehoe flows extended N and SE from the vent. Through 25 June, the shield's pond was full and intermittently overflowing, but by 1 July it had drained to ~15 m depth with a solid crust at the bottom. However, lava continued to ooze into the S-flank tube system and to break out at the base of the shield. Tremor amplitudes gradually declined to near background by 2000 on 29 June, and remained at low levels into early July.

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

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

A M 5.2 earthquake, centered in the sea 8 km SW of the volcano at 9 km depth, occurred on 15 June at 1046. Island residents felt the shock at intensity 5 on the JMA scale of 0-7. Data from 30 stations of the Worldwide Standardized Seismic Network yielded magnitudes of 4.9 (m_b) and 4.7 (Ms). One person was slightly injured by a rockfall, and wallrock collapse at 10 sites closed 5 roads to traffic. Aftershocks continued until 17 June off the island's SW coast. The event was the second largest since . . . April 1991 (figure 1). No surface anomalies were observed on the island or on the sea-surface nearby.

Geologic Background. A cluster of rhyolitic lava domes and associated pyroclastic deposits form the 4 x 6 km island of Kozushima in the northern Izu Islands. The island is the exposed summit of a larger submarine edifice more than 20 km long that lies along the Zenisu Ridge, one of several en-echelon ridges oriented NE-SW, transverse to the trend of the northern Izu arc. The youngest and largest of the 18 lava domes, Tenjosan, occupies the central portion of the island. Most of the older domes, some of which are Holocene in age, flank Tenjosan to the north, although late-Pleistocene domes are also found at the southern end of the island. A lava flow may have reached the sea during an eruption in 832 CE. The Tenjosan dome was formed during a major eruption in 838 CE that also produced pyroclastic flows and surges. Earthquake swarms took place during the 20th century.

Information Contacts: JMA; NEIC.

"A new phase of eruptive activity that started on 30 May lasted until 8 June. From 1 to 4 June, both Crater 2 and Crater 3 produced ash-rich Strombolian explosions to 500-700 m height. A new, short lava flow was emplaced on the NW flank of Crater 3. Emissions from Crater 2 became markedly ash-laden 4-7 June, with a plume rising a few kilometers above the crater and ashfalls on coastal areas 10 km NW. After the 7th, only weak to moderate vapour emissions and occasional Vulcanian explosions were noted from Crater 2.

"Activity at Crater 3 also waned after the first week in June, although more progressively. On the night of 7 June, intermittent explosions projected incandescent lava fragments to 250 m above the crater, while on 8 June there was weak steady glow over the crater. Intermittent explosions still occurred daily until the 24th, producing dark convoluting ash clouds that rose a few hundred meters above the crater.

"Seismic monitoring resumed on 11 June and showed only low-level activity throughout the rest of the month."

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

Information Contacts: P. de Saint-Ours, D. Lolok, and C. McKee, RVO.

"A Landsat TM image recorded the night of 15 April 1992 shows the most intense thermal anomaly of a dataset extending back to December 1984. The thermal signature, in the short-wavelength infrared bands 5 (1.55-1.75 mm) and 7 (2.08-2.35 mm), represents the active lava dome in the central crater. Comparison with the previous image (night of 7 January 1991) shows a marked increase in the anomaly's area (figure 11). In the April 1992 scene, the core of the anomaly occupies an irregular area of ~7 x 6 pixels (equivalent to 210 x 180 m). These dimensions correspond closely with the 180-190 m dome diameter estimated from 20 March airphotos (17:5). The increase in area of the TM anomaly may be explained, at least in part, by the growth of a subsidiary lava dome first sighted on 4 March. The summed thermal radiance from the whole hot spot shows a corresponding increase in the April Landsat image (figure 12).

Figure 11. 15 x 15 pixel maps (equivalent to 450 x 450 m) of the signal recorded in band 7 of the Landsat TM over Lascar at night on 7 January 1991 (left) and 15 April 1992 (right). The vertical axis represents the number between 0 and 255 proportional to the spectral radiance. In each case, several pixels are saturated. Courtesy of C. Oppenheimer.

Figure 12. Summed spectral radiance in bands 5 and 7 for fifteen images acquired over Lascar since December 1984. The dataset includes several processing formats, and images acquired during the day and night. Only pixels with a thermal signal >=10 were included. The total was then converted to spectral radiance using calibration coefficients supplied with the digital data. Arrows mark the explosive eruptions of September 1986 and February 1990 (12:4-5 and 15:2-3). Courtesy of C. Oppenheimer.

"An interesting feature of the two most recent TM acquisitions is the persistence of a discrete hot site ~200 m W of the centre of the main anomaly (figure 11). This is very likely the expression of incandescent fumarole vent(s) beyond the steep margin of the extruded lava."

Reference. Oppenheimer, C., Francis, P.W., Rothery, D.A., Carlton, R.W., and Glaze, L.S., Analysis of Volcanic Thermal Features in Infrared Images: Lascar Volcano, Chile, 1984-1992; Journal of Geophysical Research, in press.

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

Information Contacts: C. Oppenheimer, D. Rothery, P. Francis, and R. Carlton, Open Univ.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Lassen Report. Of the three major Holocene volcanoes in the California Cascades, Lassen (~800 km NNW of the epicenter) had the strongest response to the 28 June earthquake (figure 1). About 10 minutes after the S-wave's arrival and while surface waves were still being recorded, a M 2.8 event occurred south of Lassen Peak. Film records showed 9 more earthquakes in the first hour, and 22 events were identified during the first 24 hours. Although most were M 1 or smaller, at least two and perhaps as many as four were of magnitude greater than or equal to 2. Nine were detected by the RTP system. The best preliminary locations were concentrated ~3 km SW of Lassen Peak at

Figure 1. Seismic events in the Lassen area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of S. Walter.

Geologic Background. The Lassen volcanic center consists of the andesitic Brokeoff stratovolcano SW of Lassen Peak, a dacitic lava dome field, and peripheral small andesitic shield volcanoes and large lava flows, primarily on the Central Plateau NE of Lassen Peak. A series of eruptions from Lassen Peak from 1914 to 1917 marks the most recent eruptive activity in the southern Cascade Range. Activity spanning about 825,000 years began with eruptions of the Rockland caldera complex and was followed beginning about 590,000 years ago by construction of Brokeoff stratovolcano. Beginning about 310,000 years ago activity shifted to the north flank of Brokeoff, where episodic, more silicic eruptions produced the Lassen dome field, a group of 30 dacitic lava domes including Bumpass Mountain, Mount Helen, Ski Heil Peak, and Reading Peak. At least 12 eruptive episodes took place during the past 100,000 years, with Lassen Peak being constructed about 27,000 years ago. The Chaos Crags dome complex was constructed about 1100-1000 years ago north of Lassen Peak. The Cinder Cone complex NE of Lassen Peak was erupted in a single episode several hundred years before present and is considered part of the Lassen volcanic center (Clynne et al., 2000). The 1914-1917 eruptions of Lassen Peak began with phreatic eruptions and included emplacement of a small summit lava dome, subplinian explosions, mudflows, and pyroclastic flows.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

During a previously unreported 26 February climb by David Peterson, Howard Brown, and students from St. Lawrence Univ, activity was continuing from one cone (T20) . . . . Periodic gurgling and rumbling noises from the cone were audible from the crater rim. As Peterson and several students approached the active cone, lava fragments were ejected, one of which struck a student on the leg, causing a small burn. Crater photographs show a small dark vent at the summit of T20, but no dark (fresh) lava was evident on its flanks. However, by . . . 12 March, T20 had extruded a lava flow that covered much of the W part of the crater floor (17:03).

Brown's 26 February photographs show . . . T5/T9 as tall but pale gray, with no fresh, dark patches of lava. T15 was composed of jagged dark-gray pinnacles with medium-brown lower slopes and no sign of fresh lava. T8 and T8A seemed little changed from recent photographs, with slight yellow coloring at T8's summit. T14 appeared to have been surrounded by younger lava, which had turned pale gray to white. Some dark patches were visible around its summit vent. No dark fresh flows were evident on the crater floor.

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

Information Contacts: C. Nyamweru, St. Lawrence Univ; D. Peterson, Arusha; H. Brown, Nairobi, Kenya.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers. No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Long Valley Report. Within eight minutes of the major earthquake's origin time, seismic activity within Long Valley caldera (400 km NNW of the epicenter) increased abruptly (figure 15). Of the >260 events located by the RTP system during the next three days, three were of M 3 or greater. The first event within the caldera located by the RTP system was a M 1.4 earthquake at 1207, but develocorder film from caldera stations provides evidence of local earthquakes beginning at least a minute earlier within the strong coda waves from the M 7.5 event. The P-wave travel-time from the epicenter is just over 1 minute, and the S-wave travel-time just under two minutes, so it appears that local earthquake activity began no later than six minutes after the S-wave arrival.

Figure 15. Earthquakes >M 1.5 in the Long Valley area, 25 June-1 July 1992. Larger events are identified by numbered triangular labels beside earthquake symbols: (1) 25 June, 2143 GMT, M 2.4; (2) 28 June, 1214, 1230, 1232, M 2.6, 3.0, 2.5; (3) 29 June, 0103, M 3.1; (4) 29 June, 0537, 0638, M 3.7, 2.3; (5) 29 June, 0758, M 3.4; (6) 29 June, 0834, 0838, 0839, M 2.0, 2.1, 2.0. Courtesy of D. Hill.

Earthquake activity within Long Valley caldera had persisted, but at relatively low levels, through the first half of 1992, averaging

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

Information Contacts: D. Hill, USGS Menlo Park.

"The eruption . . . ended on 15 June after another paroxysmal phase from Main Crater (on 7 June). Following the paroxysmal phase of 31 May from Southern Crater, the level of activity was moderate in the first days of June. Both craters were emitting white and blue vapours in weak to moderate amounts, with occasional explosions of ash-laden vapour rising a few hundred meters above the craters, weak roaring noises, and weak fluctuating glow at night.

"On the afternoon of 5 June, Southern Crater entered a phase of intermittent Strombolian activity that sprayed incandescent spatter to as much as 300 m above the crater at intervals of 30-40 minutes. At 1600, Main Crater emitted a dark ash column to ~1,000 m above the crater. Strombolian explosions within the crater must have started soon afterwards, as suggested by fluctuating night glow and roaring sounds. On the 6th, the level of activity remained moderate at Southern Crater while it strengthened at Main Crater. The forceful emissions of grey-brown ash from the latter were identified as Strombolian projections at night. From 0025 until about 1830 on 7 June, this crater produced continuous incandescent projections to 600 m above the rim in an ash column that rose 2-3 km. New lava flows were erupted into the NE Valley and followed the path of the previous flows (4-6 May) on the southern side of the valley, down to 110 m asl.

"Pyroclastic flows were also produced, scorching vegetation and some garden areas on the southern side of the NE Valley to about 1 km from Bokure Village. Downwind from the crater, on the NW side of the island, the sustained dark ash cloud overhead, the fall of ash and lapilli, and roaring sounds of the eruption caused some concern to the population.

"This paroxysmal eruption phase ended with loud explosions from 1817 to 1830 on 7 June. In the following days there was hardly any visible activity from either crater, apart from weak-to-moderate vapour emission. However, the seismicity, which had increased dramatically during the eruptive phase of 6-7 June, remained moderately high. On 12 June, occasional dull explosion sounds were heard again from Main Crater with occasional brown ash clouds and incandescent projections at night. This activity lasted until the 14th, becoming more and more intermittent. The last significant event from Main Crater observed in this eruption was a moderately strong Vulcanian explosion at 0800 on 14 June, which projected a convoluting cloud to 2-3 km above the crater. Likewise, Southern Crater was somewhat reactivated 13-15 June, with occasional weak explosions, a fluctuating night glow, and incandescent projections to 250 m above the crater rim. From 16 June onward, the seismicity dropped markedly and neither crater showed further signs of activity apart from weak, fumarolic emission. The Stage 2 volcanic alert that had applied since 13 April was dropped to Stage 1 (i.e. non-threatening, background level) on 25 June.

"This eruption of Manam is among the most significant since 1958, and can be compared with the eruption of 1974 (Palfreyman and Cooke, 1976; Cooke et al., 1976) as it involved both craters, produced pyroclastic flows and lava flows of significant volume, and affected all but one of the main valleys. However, the 1992 eruption appears to have been larger than the 1974 event. A preliminary estimate of the 1992 lava-flow volume is 17 x 10⁶ m³, compared with only 3 x 10⁶ m³ of lava flows in 1974."

References. Cooke, R.J.S., McKee, C.O., Dent, V.F., and Wallace, D.A., 1976, Striking Sequence of Volcanic Eruptions in the Bismarck Volcanic Arc, Papua New Guinea, in 1972-75; in Johnson, R.W, ed., Volcanism in Australasia, Elsevier, p. 149-172.

Palfreyman, W.D. and Cooke, R.J.S., 1976, Eruptive History of Manam Volcano, Papua New Guinea; Ibid., p. 117-131.

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: P. de Saint-Ours, D. Lolok, and C. McKee, RVO.

An explosion on 5 July killed one person and injured five others. Marapi has been erupting since 1987, with explosions typically occurring about once every 1-7 days. Material ejected by the smaller explosions rises 100-800 m, whereas ejecta from larger explosions reach 800-2,000 m above the summit. The recent explosions, which produce ash and lapilli, have originated from Verbeek Crater in the summit complex. Ashfalls have been frequent NW of the volcano in Bukittinggi (roughly 15 km NW of the summit), Sungai Puar (30 km NW), and the Agam district (>30 km NW), depending on wind direction. Fluctuations in Marapi's explosions seem to parallel shallow volcanic earthquakes (figure 2), suggesting that the activity is primarily caused by degassing from a relatively shallow source through an open vent.

Figure 2. Number of explosion, A-, and B-type earthquakes at Marapi, January 1991-June 1992. Courtesy of VSI.

Activity in June began with an explosion on the 1st. Continuous tremor followed, and on 6 June at 0227 another explosion occurred. Repeated explosions then deposited ~0.5 mm of ash on Bukittinggi. On 25 June, witnesses 2 km from the volcano (at the Batu Palano Volcano Observatory) heard a detonation and saw glow. A brownish-black cauliflower-shaped plume rose 1,800 m above the summit. During June, 45 deep and 312 shallow volcanic earthquakes, 108 volcanic tremor episodes, and 2,104 explosion earthquakes were recorded.

The strongest explosion occurred on 5 July at 0912. Bukittinggi and vicinity were covered by 0.5-1.5 mm of ash several hours later, with ash in some areas reaching 2 mm thickness. Ash also extended to Padang, ~10 km SW of the crater. Bombs killed one person, seriously injured three, and caused minor injuries to two others. The victims had climbed to the summit without consultation with the Mt. Marapi Volcano Observatory or local authorities, although a hazard warning had been in effect since 1987.

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

Information Contacts: W. Modjo, VSI.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Aerial observations [of Maug] on 13 May revealed no signs of steaming or other evidence of recent volcanic activity.

Geologic Background. Three small elongated islands up to 2.3 km long mark the northern, western, and eastern rims of a largely submerged 2.5-km-wide caldera. The highest point of the Maug Islands reaches only 227 m above sea level; the submerged southern notch on the caldera rim lies about 140 m below sea level. The caldera has an average submarine depth of about 200 m and contains a twin-peaked central lava dome that rises to within about 20 m of the sea surface. The Maug Islands form a twin volcanic massif with Supply Reef, about 11 km N. The truncated inner walls of the caldera on all three islands expose lava flows and pyroclastic deposits that are cut by radial dikes; bedded ash deposits overlie the outer flanks of the islands. No eruptions are known since the discovery of the islands by Espinosa in 1522. The presence of poorly developed coral reefs and coral on the central lava dome suggests a long period of general quiescence, although it does not exclude mild eruptions (Corwin, 1971). A 2003 NOAA expedition detected possible evidence of submarine geothermal activity.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Medicine Lake Report. Twelve events were detected in the Medicine Lake area (~900 km NNW of the epicenter) in the 30 minutes after the M 7.5 earthquake. All had coda durations less than or equal to 10 seconds. The lack of any S-P separation indicated that they were centered very close to the single seismic station, near the center of the caldera. All known historical seismicity had occurred in the central caldera as part of a mainshock/aftershock sequence during the fall and winter of 1988-89.

Geologic Background. Medicine Lake is a large Pleistocene-to-Holocene, basaltic-to-rhyolitic shield volcano east of the main axis of the Cascade Range. Volcanism, similar in style to that of Newberry volcano in Oregon, began less than one million years ago. A roughly 7 x 12 km caldera truncating the summit contains a lake that gives the volcano its name. A series of young eruptions lasting a few hundred years began about 10,500 years before present (BP) and produced 5 km3 of basaltic lava. Nine Holocene eruptions clustered during three eruptive episodes at about 5000, 3000, and 1000 years ago produced a chemically varied group of basaltic lava flows from flank vents and silicic obsidian flows from vents within the caldera and on the upper flanks. The last eruption produced the massive Glass Mountain obsidian flow on the E flank about 900 years BP. Lava Beds National Monument on the N flank of Medicine Lake shield volcano contains hundreds of lava-tube caves displaying a variety of spectacular lava-flow features, most of which are found in the voluminous Mammoth Crater lava flow, which extends in several lobes up to 24 km from the vent.

Information Contacts: S. Walter and D. Hill, USGS Menlo Park.

Vigorous lava production continued through June . . . . The eruption has built 23 cinder cones along a 2.5-km zone that trends generally NE, ~15 km NE of Nyamuragira caldera and 5 km ENE of the 1957 Kitsimbanyi vent (figure 12 and table 1). The eruption's early phases produced substantial lava flows, but since 20 November activity has been characterized by vigorous ejection of bombs, lava fragments, and ash, with lava flows of only limited extent.

Figure 12. Schematic map of cones built by the 1991-92 eruption of Nyamuragira, in a zone ~15 km NE of the caldera. Vent 20, shown in black, opened on 14 July, and remained active in August 1992. Courtesy of N. Zana.

Table 1. Sequence of activity at Nyamuragira's 1991-92 eruption vents. Locations are shown on figure 12. Some small, short-lived vents removed by subsequent lava flows are not listed.

From 20 September until 5 February, activity was confined to a N32-34°E fissure (cones 1-8). The most persistent activity at a single vent, 25 October-3 February, has made Cone 3 the largest of the eruption, rising ~80 m above the surrounding lava plain. Three new cones developed in February, nos. 9 (6 February), 10a and 10b (26 February). In March, activity resumed at the S end of the fissure along a branch that trended E from the initial vent, successively building cones 11, 12, and 14. Vent 13, 1 km to the N, erupted during the same period.

In early May, activity moved to the N end of the fissure, as a NE branch developed and formed vents 15-17. These vents remained active at the end of May, as did no. 14 at the S end of the fissure, producing intermittent lava fountains. Vent 18, near the middle of the fissure, began to erupt at about 1100 on 24 May. By 8 June it had grown to ~25 m height and its lava flows had extended ~3 km N, eroding away cones 10a and 10b. Activity at the new vent was preceded by an increase in microtremor amplitude recorded at a seismic station (Katale) 12 km E. Amplitude increased significantly from 8 June, indicating movement of new magma from a deeper source. As of 1 July, there was no indication that the eruption was nearing its end. Lava production remained vigorous, with high lava fountains, and strong emission of bombs and other tephra.

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: N. Zana, CRSN, Bukavu.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Reports from brief visits to Pagan indicate that the most recent small ash eruption occurred on 13 April. Continuing seismicity was dominated by short bursts of long-period earthquakes and volcanic tremor. The highest measured steam temperature was 76°C; solfataras that are probably hotter are inaccessible deep within the crater. Episodic fuming, marked by periods of relatively high SO₂ outgassing followed by quiescence, was observed continuously 13-21 May. EDM lines from the coast to reflectors on the flanks had shortened by as much as 11.3 cm since September 1990. These lines had shown no significant changes between 1983 and 1990, a period characterized by frequent small ash eruptions following the large Plinian eruption of 15 May 1981 (Banks and others, 1984). After the first remeasurement on 17 May, no large changes in line lengths were detected during the next 3 days.

The team collected three charcoal samples on Pagan. Two of the units to be dated are relatively old, and their ages should help to constrain the age of the caldera.

South Pagan . . . has several steaming fumaroles, but no temperatures were measured. No shallow earthquake swarms have been recorded since the installation of the seismic station in 1990.

Reference. Banks, N.G., Koyanagi, R.Y., Sinton, J.M., and Honma, K.T., 1984, The eruption of Mount Pagan volcano, Mariana Islands, 15 May 1981: JVGR, v. 22, p. 225-269.

Geologic Background. Pagan Island, the largest and one of the most active of the Mariana Islands volcanoes, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7 and 4 km in diameter, respectively. North Pagan at the NE end of the island rises above the flat floor of the northern caldera, which may have formed less than 1,000 years ago. South Pagan is a stratovolcano with an elongated summit containing four distinct craters. Almost all of the recorded eruptions, which date back to the 17th century, have originated from North Pagan. The largest eruption during historical time took place in 1981 and prompted the evacuation of the sparsely populated island.

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Increased seismicity preceded the emergence of a lava dome into the center of the caldera lake. Moderate steam-and-ash emission was associated with the lava extrusion.

Long-period earthquakes and tremor began to be recorded on 6 July. An aerial survey during the morning of 7 July showed no visible change in steaming from crater vents, although the caldera lake was convecting and somewhat muddier than normal. A small island was reported in the caldera lake early on 9 July. An overflight that day at 1500 revealed a mud cone about 100 m in diameter near the center of the lake, protruding about 5 m above the lake surface. Small phreatic explosions to about 100 m height occurred near the side of the island. PHIVOLCS raised the official alert level to 3, indicating the possibility of an eruption within weeks. The announcement described possible activity as quiet extrusion of a lava dome or moderately explosive phreatomagmatic eruptions. A danger zone of 10-km radius was being enforced.

The cone had reportedly reached 200-300 m in diameter by 12 July. A lava dome 100-150 m in diameter was visible near the center of the island during an aerial survey on 14 July at 0900-1000. The island had grown to around 250-300 m across and was 8-10 m above lake level. A continuous dirty white steam column that included some ash was emerging from the dome and drifting SW during the overflight. Ashfall was reported on two towns ~30 km SW of the summit (San Marcelino and Castillejos) at about 0600 and 1300. The alert level was raised to 5 (eruption in progress).

On the flanks of the volcano, monsoon rains triggered secondary explosions and lahars that forced the evacuation of thousands of people living along rivers. Two people were reported killed by lahars on 12 July. The Department of Social Welfare said that about 70,000 people remained in evacuation centers and resettlement sites in the aftermath of the June 1991 eruption.

Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.

Information Contacts: PHIVOLCS; UPI; Reuters; AP.

Water level in the crater lake had dropped at least 3 m since April, shrinking it substantially by early June (figure 41). Its color was lime green to sky blue, and the temperature in accessible areas reached 85.8°C. Numerous cones and miniature mud volcanoes were visible within the lake. The nine main fumaroles emitted water vapor with yellowish and bluish gases (sulfur and SO₂). Bluish gases and orange flames, probably caused by combustion of sulfur, emerged from the northernmost fumarole. The fumaroles to the SE occurred among collapsed sulfur-and-mud cones, as in the past 3 years.

Figure 41. Sketch map of the crater at Poás, 10 June 1992. Courtesy of the Instituto Costarricense de Electricidad.

As the rainy season began, fumaroles exposed by the shrinkage of the crater lake were covered by water. The resulting continuous phreatic activity produced plumes 1-2 m high. As the lake rose, it cooled to 64-73°C, with a pH of 1.1. Weak fumarolic activity continued on the 1953-55 dome, with a maximum measured temperature of 89°C and a condensate pH of 4.4.

A daily average of 200 low-frequency events and 24 A-B-type (medium-frequency) events were recorded 2.7 km SW of the summit (by station POA2) in June (figure 42). Highest seismicity was on 2 June.

Figure 42. Daily number of seismic events recorded at a station (POA2) 2.7 km SW of the summit of Poás, June 1992. Courtesy of the Univ Nacional.

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

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSCIORI; G. Soto, ICE; M. Fernández, UCR.

"Seismic activity . . . has shown a slight increase over the last 2 months (June: 410 caldera earthquakes, May: 425) compared with activity over the last 2.5 years (100-300 events/month). Less than 1% of the recorded earthquakes in June could be located. Most were from the NW part of the caldera seismic zone. Similarly, levelling measurements showed a slight uplift of the central part of the caldera during the last two months (20 mm, 11 May-4 June; and an additional 13 mm by 8 July)."

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: P. de Saint-Ours, D. Lolok, and C. McKee, RVO.

Fumarolic activity continued through June in the active crater, where it had fed a plume more than 100 m high during May fieldwork. Chemical analyses of water collected 13 May showed pH values of less than 3 in two of the three N-flank rivers sampled, and some enhancement in sulfate and chloride concentrations (table 2). A seismographic station 5 km SW of the crater (RIN3) registered seven low-frequency earthquakes in June.

Table 2. Chemistry of water collected 13 May 1992 from three rivers on the N flank of Rincón de la Vieja. Data courtesy of the Univ. de Costa Rica.

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

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI; G. Soto, ICE; Mario Fernández, Univ. de Costa Rica.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. Rumble III seamount, the largest of the Rumbles group of submarine volcanoes along the South Kermadec Ridge, rises 2,300 m from the seafloor to within about 200 m of the surface. Collapse of the edifice produced a scarp open to the west and a large debris-avalanche deposit. Fresh-looking andesitic rocks have been dredged from the summit and basaltic lava from its flanks. It has been the source of several submarine eruptions detected by hydrophone signals.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. The submarine Rumble IV volcano was thought to have been active from April to December 1966, based on hydrophone signals (Kibblewhite, 1967), but later evidence indicated that the hydrophone array had been damaged and the signals originated from Rumble III (Hall, 1985). Fresh, glassy andesitic lava was dredged from the summit in 1992 during a New Zealand Oceanographic Institute cruise, and gas bubbles were acoustically detected rising from Rumble IV.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. Rumble V was discovered in 1992 at the southernmost end of the Rumble seamounts on the southern Kermadec Ridge, 17 km ESE of Rumble IV. Andesitic and basaltic-andesite rocks have been dredged from this volcano, which rises more than 2,000 m to nearly 400 m below the ocean surface and shows a pristine morphology. A large plume of gas bubbles was acoustically detected rising from the summit in 1992, and subsequent expeditions detected evidence of vigorous hydrothermal activity.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. Gas emission [from Sarigan] was not evident during overflights in an airplane on 13 May and a helicopter on 21 May.

Geologic Background. Sarigan volcano forms a 3-km-long, roughly triangular island. A low truncated cone with a 750-m-wide summit crater contains a small ash cone. The youngest eruptions produced two lava domes from vents above and near the south crater rim. Lava flows from each dome reached the coast and extended out to sea, forming irregular shorelines. The northern flow overtopped the crater rim on the north and NW sides. The sparse vegetation on the flows indicates they are of Holocene age (Meijer and Reagan, 1981).

Information Contacts: R. Moore, USGS; R. Koyanagi, M. Sako, and F. Trusdell, HVO.

Southern California's largest earthquake since 1952, M 7.5 on 28 June, appeared to trigger seismicity at several volcanic centers in California. It was centered roughly 200 km E of Los Angeles. In the following, David Hill describes post-earthquake activity at Long Valley caldera, and Stephen Walter discusses the USGS's seismic network, and the changes it detected at Lassen, Shasta, Medicine Lake, and the Geysers.

In recent years, the USGS northern California seismic network has relied upon Real-Time Processors (RTPs) to detect, record, and locate earthquakes. However, a film recorder (develocorder) collects data from 18 stations in volcanic areas, primarily to detect long-period earthquakes missed by RTPs. The film recorders proved useful in counting the post-M 7.5 earthquakes, most of which were too small to trigger the RTPs.

The film record was scanned for the 24 hours after the M 7.5 earthquake, noting the average coda duration for each identified event. Some events may have been missed because of seismogram saturation by the M 7.5 earthquake. Marked increases in microseismicity were observed at Lassen Peak, Medicine Lake caldera, and the Geysers (table 1). No earthquakes were observed at Shasta, but the lack of operating stations on the volcano limited the capability to observe small events.

Table 1. Number of earthquakes at northern California volcanic centers during 24-hour periods following major earthquakes on 25 April (40.37°N, 124.32°W; M 7.0) and 28 June (34.18°N, 116.47°W; M 7.5) 1992. Events with coda durations less than or equal to 10 seconds and greater than 10 seconds are tallied separately. Earthquakes were identified from film records of seismograms from nearby stations. Courtesy of Stephen Walter.

Film was also scanned for the 24 hours following the M 7.0 earthquake at 40.37°N, 124.32°W (near Cape Mendocino) on 25 April. Although smaller than the 28 June earthquake, its epicenter was only 20-25% as far from the volcanoes. Furthermore, both the 25 April main shock and a M 6.5 aftershock were felt at the volcanic centers, but no felt reports were received from these areas after the 28 June earthquake. Only the Geysers showed any possible triggered events after the 25 April shock. However, background seismicity at the Geysers is higher than at the other centers, and is influenced by fluid injection and withdrawal associated with intensive geothermal development.

Shasta report. The film record showed no earthquake activity beneath Shasta (~900 km NNW of the epicenter), although telemetry problems limited the ability to detect events below M 2. Of the six earthquakes in the 24 hours following the M 7.5 shock, two were large enough to be recorded by the RTP system. These were centered about 60 km SE of Shasta and about equidistant from Lassen (figure 1). Because the arrival times and S-P sequences of the other four events were similar to those of the two located shocks, it is likely that all had similar epicenters. Occasional M 2 earthquakes have previously occurred in this area, which includes several mapped N-trending normal faults with Quaternary movement. Three days after the M 7.5 earthquake, a M 2.0 shock occurred beneath Shasta's SE flank, followed by a M 2.7 event the next day. Both were centered at about 15 km depth, similar to most earthquakes beneath Shasta in the last decade.

Figure 1. Seismic events in the Shasta/Medicine Lake area that were apparently triggered by the M 7.5 southern California earthquake of 28 June 1992 (circles) compared to 1978-90 seismicity in the region (crosses). Squares mark seismic stations. Courtesy of Stephen Walter.

Geologic Background. The most voluminous of the Cascade volcanoes, northern California's Mount Shasta is a massive compound stratovolcano composed of at least four main edifices constructed over a period of at least 590,000 years. An older edifice was destroyed by a large debris avalanche which filled the Shasta River valley to the NW. The Hotlum cone, forming the present summit, the Shastina lava dome complex, and the SW flank Black Butte lava dome, were constructed during the early Holocene. Eruptions from these vents have produced pyroclastic flows and mudflows that affected areas as far as 20 km from the summit. Eruptions from Hotlum cone continued throughout the Holocene.

Information Contacts: Stephen Walter and David Hill, MS 977, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 USA.

Increased seismicity preceded a brief eruption of Spurr that began on 27 June at 0704, producing an eruption cloud that was carried rapidly NNE. Seismic data suggested that the eruption ended at about 1100, after apparent eruptive pulses at 0814 and 0904. By 1049, shortly before feeding of the plume stopped, data from the Nimbus-7 satellite's TOMS showed its leading edge roughly 500 km from the volcano, near Fairbanks (figure 3), with an apparent SO₂ content of 35 kilotons. The next day, the cloud was detached from the volcano but still clearly visible on weather satellite imagery, extending in a 2,000-km arc E and SE over NE Alaska and NW Canada (figures 3 and 4). As the plume elongated, SO₂ detected by the TOMS instrument increased to a maximum of 185 kilotons on 28 June at 1125, then decreased slightly to 160 kilotons as it started to dissipate on 29 June. The cloud remained visible on both TOMS data and weather satellite imagery for several more days.

Figure 3. Three overlain images of the SO2 cloud from Spurr, as detected by the Total Ozone Mapping Spectrometer on the Nimbus-7 satellite. Values of SO2 in each 50 x 50-km pixel are shown on a relative scale of 0-9, then upward through alphabetic characters with increasing concentration. The cloud slowly dispersed until 3 July, when it could no longer be distinguished above background. Courtesy of Gregg Bluth.

Figure 4. Image from the NOAA 11 polar-orbiting weather satellite on 29 June at about 0600, showing the plume from Spurr over the Beaufort Sea and western Canada. Courtesy of NOAA/NESDIS.

The maximum eruption cloud altitude reported by pilots was about 12 km. However, radar installed on the Kenai Peninsula after the Redoubt eruption, to monitor nearby volcanic activity, measured higher altitudes. At 0803, radar detected a vertical cloud to about 9 km altitude; at 0840, strong returns to 9 km and some material to 14.5 km; at 0950 and 1004, columns to 16 km altitude; and at 1018, to 18 km (figure 5).

Figure 5. One of several radar images of the eruption column from Spurr on 27 June. This image, at 1018, shows echoes from the plume to about 18 km altitude. The instrument, an Enterprise Electronics WSR74C, 5-cm radar, is at Kenai, Alaska, about 80 km away. Vertical scans were used to maximize detection of the vertical cloud; the plume extending downwind is not visible. Courtesy of Joel Curtis and Dale Eubanks.

Because the plume was carried northward, major air routes to Asia that extend along the Aleutian chain from Anchorage were not affected. A Notice to Airmen warned aircraft to avoid the immediate vicinity of the volcano. No routes were officially closed, but airlines avoided using routes N and NW of the volcano (J501, 111, 133, 120, and 122; and V319, 444, and 480) during the eruption. Flights arriving in Anchorage, 120 km E of Spurr, were routed along normal approaches from the south.

Geologic Background. Mount Spurr is the closest volcano to Anchorage, Alaska (130 km W) and just NE of Chakachamna Lake. The summit is a large lava dome at the center of a roughly 5-km-wide amphitheater open to the south formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an older edifice. The debris avalanche traveled more than 25 km SE, and the resulting deposit contains blocks as large as 100 m in diameter. Several ice-carved post-collapse cones or lava domes are present. The youngest vent, Crater Peak, formed at the southern end of the amphitheater and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash in Anchorage.

Information Contacts: AVO; G. Bluth, NASA GSFC; SAB, NOAA/NESDIS; Joel Curtis and Dale Eubanks, NWS Alaska Region, Anchorage; Darla Gerlach, Air Traffic Division, FAA, Anchorage.

Fieldwork during the first week in June revealed that eruptive activity was mainly concentrated in craters C1 (vent 1) and C3 (vent 4), which fed black plumes no more than 100 m high. Seismicity remained high in June (figure 26), near the 180 events/day reached in the last third of May. A minimum of 108 events was recorded on 24 June. After declining rapidly about 20 May, tremor energy returned to levels characteristic of the period since November 1991.

Figure 26. Seismicity at Stromboli, June 1992. Open bars show the number of recorded events per day, black bars those with ground velocities exceeding 100 mm/s. The curve represents the each day's average of tremor energies on hourly 60-second samples. Courtesy of M. Riuscetti.

Geologic Background. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: M. Riuscetti, Univ di Udine.

Three previously unknown submarine arc stratovolcanoes have been identified at the S end of the Kermadec Ridge: Rumble V (36.140°S, 178.195°E, summit 700 m below sea level); Tangaroa (36.318°S, 178.031°E, summit 1,350 m below sea level); and Clark (36.423°S, 177.845°E, summit 1,150 m below sea level) (figure 1). All three have basal diameters of 16-18 km and rise from the seafloor at ~2,300 m depth. The first evidence of the volcanoes was from GLORIA side-scan mapping of the southern Havre Trough-Kermadec Ridge region in 1988 (Wright, 1990). Later investigations, including a photographic and rock-dredge study during the 3-week Rapuhia cruise (early 1992), confirmed previous interpretations. Side-scan and photographic data show a complex terrain of lava flows and talus fans on the flanks of all three volcanoes, with the most pristine-looking morphology at Rumble V. During the 1992 cruise, gas bubbles were detected acoustically, rising from the crests of Rumble III, IV, and V. No gas bubbling was evident from Tangaroa or Clark. Bathymetric surveys indicated that the summits of the shallowest volcanoes, Rumble III and IV, were at ~140 and 450 m, respectively, below the sea surface.

Figure 1. Sketch map of New Zealand's North Island and the southern Kermadec Ridge area, with locations of young volcanoes. Courtesy of Ian Wright.

Reference. Wright, I.C., 1990, Bay of Plenty-Southern Havre Trough physiography, 1:400,000: New Zealand Oceanographic Institute Chart, Miscellaneous Series no. 68.

Geologic Background. Tangaroa submarine volcano in the southern Kermadec arc rises to within 600 m of the ocean surface. The volcano is elongated in a NW-SE direction and contains smaller cones on its SE to eastern flanks. A larger edifice lies further to the SE. Tangaroa lies between Clark and Rumble V submarine volcanoes near the southern end of the Kermadec arc and is one of more than a half dozen volcanoes in this part of the arc showing evidence for active hydrothermal vent fields.

Information Contacts: I. Wright, New Zealand Oceanographic Institute, National Institute of Water and Atmospheric Research, Wellington.

A telemetering seismic station (VTU) 0.5 km E of the active crater recorded 17 events in June. The maximum daily number, 4, occurred on 13 June.

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

Information Contacts: E. Fernández, J. Barquero, and V. Barboza, OVSICORI.

Growth of the lava dome continued through early July. Partial collapses of the dome complex frequently generated pyroclastic flows. Dome 7, which had begun to emerge in late March, grew exogenously against dome 6 (figure 43), which was buried and eroded by dome 7's lava blocks. Frequent rockfalls from the front and margins of dome 7 reduced its length (to ~ 200 m) and height (to ~ 50 m). Petal or peel structures, which had always appeared on the dome's surface during periods of rapid lava extrusion, were not evident, perhaps indicating a declining magma supply rate. The cryptodome, including dome 5, grew endogenously, frequently generating small rockfalls that were probably triggered by earthquakes within or beneath the dome complex.

Figure 43. Sketch of the dome complex at the summit of Unzen, 8 July 1992. Courtesy of Setsuya Nakada.

Volcanic gas was emitted continuously from the E part of dome 3, as well as from the depression between domes 3 and 7. The depression divides the cryptodome area into a conical NE section that includes the dome's summit, and a lower SW section with a flat top.

Deposits of the pyroclastic flows that cascade down the SE flank continue to bury the Akamatsu valley. The lowest saddle of the valley's southern cliff remains ~ 10 m high. On 23 June, the ash-cloud surge from a pyroclastic flow struck the saddle, but the main flow did not reach the cliff. The surge toppled brush on the saddle and to ~ 100 m distance, but small cedar trees remained standing. Bark and leaves were not burned, but leaves in the area died. About 10 cm of ash was deposited on the saddle. Thin lead foil, set in a stainless-steel hole to detect the pressure of the ash-cloud surge, was hollowed, and aluminum foil was broken.

Debris flows that have occasionally occurred during the current rainy season eroded pyroclastic flow deposits in the valley. Pyroclastic-flow material was deposited along the valley's N side and in its upper reaches. This deposition pattern, erosion by debris flows, and the declining magma-supply rate delayed the overflow of the lowest part of the saddle by southern-cliff pyroclastic flow deposits. In early July, the Nagasaki prefectural government began to construct a steel fence, 35 m wide and 10 m high, in a stream originating from the saddle, hoping to prevent ash-cloud surges from entering the stream.

JMA reported that the daily number of seismically detected pyroclastic flows ranged from 6 to 21 in June. The total of 373 in June was almost unchanged from previous months. The longest June flow extended 3 km SE from the dome. Most ash clouds generated by the flows rose about 1,000 m, with the highest, to 1,200 m, on 13 and 17 June.

Small earthquakes continued to occur within and beneath the dome complex, at rates of 50-200/day through mid-July. The June total, 3,671 recorded earthquakes, was similar to previous months.

Evacuated areas . . . were somewhat reduced on 11 July, decreasing the number of evacuees from 6,746 to 6,064.

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

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