Aira caldera, located in the northern half of Kagoshima Bay, contains the active post-caldera Sakurajima volcano near the southern tip of Japan’s Kyushu Island. Eruptions date back to the 8th century and have deposited ash on Kagoshima, one of Kyushu’s largest cities, 10 km W from the summit. The Minamidake summit cone and crater has had persistent activity since 1955; the Showa crater on the E flank has also been intermittently active since 2006. The current eruption period began during late March 2017 and has more recently consisted of explosions, ash plumes, and ashfall (BGVN 48:01). This report covers activity during January through June 2023, characterized by intermittent explosions, eruption events, eruption plumes, and ashfall from both summit craters, according to monthly activity reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity remained at low levels during this reporting period; less than ten thermal anomalies were detected each month by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system (figure 139). Occasional thermal anomalies were visible in infrared satellite images mainly at the Minamidake crater (Vent A is located to the left and Vent B is located to the right) and during May, in the Showa crater on the E flank (figure 140).

Table 29. Number of monthly explosive events, days of ashfall, area of ash covered, and sulfur dioxide emissions from Sakurajima’s Minamidake crater at Aira during January-June 2023. Note that smaller ash events are not listed. Ashfall days were measured at Kagoshima Local Meteorological Observatory, and ashfall amounts represent material covering all the Kagoshima Prefecture. Data courtesy of JMA monthly reports.

Figure 139. Thermal activity at Sakurajima in the Aira caldera was relatively low during January through June 2023, according to this MIROVA graph (Log Radiative Power). Three anomalies were detected during January, six during February, seven during March, nine during April, six during May, and none during June. Courtesy of MIROVA.

Figure 140. Infrared (bands 12, 11, 8A) satellite images showed occasional thermal anomalies mainly at the Minamidake crater at Aira’s Sakurajima volcano on 1 January 2023 (top left), 20 February 2023 (top right), 1 May 2023 (bottom left), and 16 May 2023 (bottom right). Vent A is located to the left and Vent B is to the right of Vent A; both vents are part of the Minamidake crater. On 16 May the image showed a weak anomaly in the Showa crater to the E of the Minamidake crater. Courtesy of Copernicus Browser.

JMA reported that during January 2023, there were 14 eruptions, nine of which were explosion events. Accompanying eruption plumes rose 2.4 km above the crater rim. Large blocks were ejected 800-1,100 m from the Minamidake crater. Nighttime incandescence was observed in the Minamidake crater using a high-sensitivity surveillance camera. No eruptions in the Showa crater were reported, though there was a gradual increase in the amount of white gas-and-steam emissions beginning around mid-January. Seismicity consisted of 121 volcanic earthquakes, which was higher than the 78 earthquakes in December. The Kagoshima Local Meteorological Observatory reported a total of 2 g/m² of ashfall was observed over the course of two days of the month. According to field surveys, daily sulfur dioxide emissions ranged from 1,000-2,800 tons/day (t/d); emissions have remained at comparable, elevated, levels since July 2022. Explosions were reported on 3 January at 1615, 8 January at 0642 and 1955, 18 January at 1215, 19 January at 0659, 21 January at 0307, and 28 January at 2342 where eruption plumes rose 1-2.4 km above the Minamidake crater and drifted SE and S. The explosion at 0307 on 21 January generated an eruption plume 1.6 km above the crater rim and ejected large blocks 800-1,100 m from the crater rim; crater incandescence was also visible (figure 141). On 28 January at 2342 an explosion produced an eruption plume that rose 2-2.2 km above the Minamidake summit crater and drifted SE.

Figure 141. Webcam image of the explosion at the Minamidake summit crater of Aira’s Sakurajima at 0307 on 21 January 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, January 2023).

There were 26 eruptions reported during February, 11 of which were explosion events. Eruption plumes rose 2.4 km above the crater rim. Large blocks were ejected 800-1,100 m from the Minamidake summit crater, and daily nighttime crater incandescence continued. Occasional eruptive activity was observed in the Showa crater starting on 8 February, which included four eruptions (figure 142). The last time activity was reported in the Showa crater was early April 2018, according to JMA. There were 130 volcanic earthquakes detected during the month. Sulfur dioxide emissions ranged from 1,900-3,500 t/d. On 8 February large blocks were ejected 300-500 m from the Showa crater and an accompanying eruption plume rose 1.5 km above the crater rim. Summit crater incandescence was also visible at night during 8 and 21-26 February at the Showa crater. Weak crater incandescence was also reported on 8 February at the Minamidake summit crater. Explosions were recorded at 1815 on 9 February, at 1007 on 11 February, at 1448 on 14 February, at 0851 on 16 February, at 0206 on 19 February, at 2025 on 20 February, at 0937 and at 1322 on 21 February, and at 0558 on 28 February. Volcanic plumes rose 300-2,000 m above the Minamidake crater and drifted N, E, S, SE, and NE. An explosion at 1448 on 14 February at the Minamidake summit crater ejected large blocks 800-1,100 m from the crater. The eruption plume rose 800-1,200 m above the crater and drifted S. A field survey conducted on 14 February showed that the ejected volcanic clasts measured up to 3 cm in diameter, though most were smaller in size, and were deposited in Arimura, Kagoshima City (3 km SE) (figure 143). An aerial survey conducted by the Japan Maritime Self-Defense Force Air Group (JMSDF) on 21 February confirmed white gas-and-steam plumes rising from the N side of the Showa crater and water was visible at the bottom of the crater. Ashfall measurements showed that a total of 6 g/m² fell over seven days during the month at the Kagoshima Local Metrological Observatory.

Figure 142. Webcam images showing the initial white gas-and-steam plume rising above the Showa summit crater of Aira’s Sakurajima at 0701 on 12 January 2023, at 0701 on 18 January (top left and right), and at 0708 on 5 February 2023 (bottom left). The amount of white gas-and-steam emissions gradually increased from mid-January leading up to the eruption at 1052 on 8 February 2023 (bottom right). Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, February 2023).

Figure 143. Photo showing the size of the deposits found in Arimura, Kagoshima City, after an eruption on 14 February 2023 at the Minamidake summit crater of Aira’s Sakurajima. The maximum diameter of these clasts was about 3 cm. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, February 2023).

During March, 22 eruptions were reported, eight of which were explosion events. Volcanic plumes rose 2.8 km above the crater rim. There were four eruptions recorded at the Showa crater, for a total of eight eruptions during February and March. Large volcanic blocks were ejected 1,000-1,300 m from the Minamidake crater and nighttime incandescence remained visible at night, based on webcam images. Blocks ejected from the Showa crater traveled 500-800 m and accompanying eruption plumes rose 2.7 km above the crater rim. Nighttime crater incandescence was reported during 4-5 March at the Showa crater, based on webcam images. Seismicity included 97 volcanic earthquakes detected throughout the month. According to the Kagoshima Local Meteorological Observatory, a total of 9 g/m² ashfall was observed over six days of the month. A field survey reported that 2,100-3,500 t/d of sulfur dioxide was released during the month. An eruption was detected at the Showa crater at 1404 on 6 March, that ejected blocks 500-800 m from the crater, accompanied by an eruption plume that rose 2.7 km above the crater rim (figure 144). Explosions were detected at 0116 on 3 March, at 2157 on 4 March, at 1322 on 8 March, at 2228 on 11 March, at 0418 on 14 March, and at 0035 on 22 March. Eruption plumes rose 1-2.8 km above the Minamidake crater and drifted SE, NE, NW, S, and SW. At 0035 on 22 March an explosion generated an eruption plume that rose 1.2 km above the Minamidake crater and drifted SW. Material was ejected 1-1.3 km from the Minamidake crater.

Figure 144. Webcam image of an eruption plume rising 2.7 km above the Showa crater rim of Aira’s Sakurajima at 1412 on 6 March 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, March 2023).

Two eruption events were reported in the Minamidake summit crater during April, neither of which were explosions; no eruptions occurred at the Showa crater. Eruption plumes rose 1.5 km above the crater rim and nighttime crater incandescence persisted nightly at the Minamidake crater. The number of volcanic earthquakes deceased to 38 and according to the Kagoshima Local Meteorological Observatory, a total of 3 g/m² of ash fell over a period of four days during the month. The amount of sulfur dioxide released during the month ranged 1,800-2,700 t/d. An eruption event at 0955 on 17 April generated an eruption plume that rose 1.5 km above the crater rim (figure 145).

Figure 145. Webcam image of an eruption plume rising 1.5 km above the Minamidake crater rim of Aira’s Sakurajima at 1004 on 17 April 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, April 2023).

Eruptive activity during May consisted of 17 eruptions, 10 of which were explosion events. Volcanic plumes rose 2.3 km above the crater rim and large ejecta traveled 800-1,100 m from the Minamidake summit crater. Activity at the Showa crater was characterized by 11 eruption events and material was ejected 300-500 m from the crater. Nighttime crater incandescence was observed at both summit craters. The number of monthly volcanic earthquakes increased to 88 and the amount of ashfall recorded was 10 g/m² over a period of 13 days during the month. According to a field survey, the amount of sulfur dioxide released ranged 1,800-3,900 t/d.

Explosions were recorded at 0422 on 2 May, at 0241 and at 1025 on 3 May, at 1315 on 9 May, at 2027 on 17 May, at 0610 on 24 May, at 1327 on 25 May, at 0647 and 1441 on 26 May, and at 1520 on 28 May. Resulting eruption plumes rose 400-1,800 m above the Minamidake crater and drifted SW, W, and N. On 14 May an eruption plume was visible above the Showa crater at 0859 that rose 1.7 km above the crater rim (figure 146). An eruption event at the Minamidake summit crater occurred at 1327 on 25 May; the eruption plume rose 2.3 km above the crater rim (figure 147).

Figure 146. Webcam image showing an eruption plume rising 1.7 km above the Showa crater rim of Aira’s Sakurajima at 0903 on 14 May 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, May 2023).

Figure 147. Webcam image showing an eruption plume rising 2.3 km above the Minamidake crater rim of Aira’s Sakurajima at 1331 on 25 May 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, May 2023).

JMA reported four eruptions occurred during June, two of which were explosion events. Eruption plumes rose as high as 2.5 km above the Minamidake crater rim and large volcanic blocks were ejected 500-700 m from the crater rim. At the Showa crater, seven eruptions occurred, one of which was an explosion event. Eruption plumes rose 1.5 km above the Showa crater rim and large material was ejected 500 m from the crater rim. Nighttime incandescence was reported for both summit craters. There were 73 volcanic earthquakes detected during the month and a total of 3 g/m² of ashfall during eight days of the month. According to a field survey, the amount of sulfur dioxide emissions released ranged 1,400-1,900 t/d. On 5 June at 0012 an explosion generated an eruption plume that rose 400-1,000 m above the Minamidake crater and drifted SE. An explosion at the Minamidake crater occurred at 1401 on 7 June that generated an eruption plume that rose 2.5 km above the crater and drifted SE (figure 148). A single explosion was reported at the Showa crater at 0438 on 22 June. The eruption plume rose 600 m above the crater rim and large blocks were ejected 500 m from the crater rim. This is the first report of an explosion at the Showa crater since October 2017, according to JMA.

Figure 148. Webcam image of an explosion and the accompanying plume that rose 2.5 km above the Minamidake crater rim of Aira’s Sakurajima at 1410 on 7 June 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, June 2023).

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: 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/).

Suwanosejima is located in the northern Ryukyu Islands, Japan, and is an 8-km-long island that consists of a stratovolcano and two active summit craters. Volcanism during the 20th century is characterized by Strombolian explosions, ash plumes, and ashfall. The current eruption began in October 2004 and has more recently consisted of intermittent explosions, eruption plumes, ashfall, and incandescent ejecta (BGVN 48:01). Similar activity continued during this reporting period of January through June 2023, based on monthly report from the Japan Meteorological Agency (JMA) and satellite data.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph of the MODIS thermal anomaly data showed low thermal activity throughout the reporting period (figure 76). Three anomalies were detected during February, four during March, three during April, one during late May, and two during early June. A single thermal hotspot was detected by the MODVOLC thermal alerts system on the NE flank on 7 February. There were only two clear weather days in infrared satellite imagery that showed a thermal anomaly on 7 March and 5 June (figure 77).

Figure 76. Low thermal activity was detected at Suwanosejima during January through June 2023, based on this MIROVA graph (Log Radiative Power). Three anomalies were detected during February, four during March, three during April, one during late May, and two during early June. Courtesy of MIROVA.

Figure 77. Infrared (bands B12, B11, B4) satellite imagery showing two thermal anomalies at the Otake crater of Suwanosejima on 7 March 2023 (left) and 5 June 2023 (right). Courtesy of Copernicus Browser.

Activity in the Otake crater during January 2023 was relatively low, which prompted JMA to lower the Volcano Alert Level (VAL) from 3 to 2 (on a 5-level scale) on 24 January. The number of explosions recorded during the month was 13. There were 50 volcanic earthquakes detected on the W side of the island, which was roughly comparable to December (44), although near the Otake crater, there were 188 earthquakes recorded, which excluded earthquakes associated with explosions. An aerial overflight conducted on 11 January by the Japan Maritime Self-Defense Force Air Group (JMSDF) reported a gray-white plume rising from the Otake crater. During 26-30 January there was a brief increase in the number of explosions. An eruption at 0331 on 26 January generated an eruption plume that rose 1.7 km above the crater rim and ejected large blocks 400 m S from the crater. Nighttime crater incandescence was visible in a highly sensitive surveillance camera starting on 26 January. According to the Toshima Village Office, Suwanosejima Branch Office, ashfall was occasionally observed in the village (3.5 km SSW). According to observations conducted by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Toshima Village, and JMA, the amount of sulfur dioxide emissions released during the month was 200-600 tons per day (t/d).

Eruptive activity in the Otake crater continued during February; the total number of explosions increased during this month from 13 to 56. There were 119 volcanic earthquakes detected on the W side of the island and 449 near the Otake crater, excluding earthquakes associated with explosions. During 15-21 February there was a brief increase in the number of explosions, and large blocks were ejected as far as 1 km from the crater. An explosion at 2131 on 15 March ejected material 900 m SE (figure 78). Eruptions on 18 and 27 February generated plumes that rose 2 km above the crater (figure 79). By 21 February the number of explosions reached 42, though no large-scale volcanic earthquakes were reported. Nighttime crater incandescence continued from late January through February. Ashfall was also occasionally observed in Toshima Village. The amount of sulfur dioxide emissions released during the month was 700 t/d.

Figure 78. Webcam image of the explosion at Suwanosejima’s Otake crater at 2131 on 15 February 2023. Crater incandescence was visible, and large blocks were ejected 900 m from the crater (white dashed line). Courtesy of JMA (Volcanic activity commentary for Suwanosejima, February 2023).

Figure 79. Webcam image of the explosion at Suwanosejima’s Otake crater at 1606 on 18 February 2023. The eruption plume rose 2 km above the crater rim. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, February 2023).

The number of explosions at the Otake crater increased during 2-5 March; 28 explosions were detected during this time. Large volcanic blocks were ejected 500 m from the crater. As a result, the VAL was increased to 3 on 5 March. There were 65 explosions recorded throughout the month. On the W side of the island, 63 volcanic earthquakes were reported, and closer to the Otake crater, 422 were detected, excluding earthquakes associated with explosions. Nighttime crater incandescence continued, as well as occasional ashfall in Toshima Village. On 16 March an eruption produced a volcanic plume that rose 2.4 km above the crater rim (figure 80). The amount of sulfur dioxide emissions released during the month was 200-1,100 t/d.

Figure 80. Webcam image of an eruption plume rising 2.4 km above the Otake crater at Suwanosejima at 0644 on 16 March 2023. Photo has been color corrected. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, March 2023).

Eruptive activity continued at the Otake crater during April. Eruption plumes rose as high as 2 km above the crater rim and large blocks were ejected as far as 500 m from the crater. The number of explosions decreased to one throughout the month, although nighttime crater incandescence remained visible in the surveillance camera. Rumbling and ashfall continued intermittently in Toshima Village. There were 32 volcanic earthquakes detected, and 129 volcanic earthquakes near the Otake crater, not including those associated with explosions. According to JMA, the amount of sulfur dioxide released during the month was 200-1,400 t/d. On 16 April at 0402 an eruption ejected incandescent material 500 m S from the crater.

Activity continued at the Otake crater in May. An eruption plume rose 1.8 km above the crater rim and large volcanic blocks were ejected 300 m from the crater. The number of explosions remained low throughout the month (7) and nighttime crater incandescence persisted. Occasional ashfall was reported in Toshima Village. As many as 44 volcanic earthquakes were recorded on the W side of the island, and 205 were recorded closer to the Otake crater, which was higher compared to the previous month. Generally, the amount of sulfur dioxide released during the month ranged 400-700 t/d, but on 19 May the amount increased to 2,600 t/d. On 16 May an eruption produced a volcanic plume that rose 1.8 km above the crater rim.

Eruptive activity was relatively low in June; the number of explosions generally decreased and on 9 June the VAL was lowered to 2. Nighttime crater incandescence continued, and according to the Toshima Village Office, rumbling and ashfall were also noted occasionally. There were 31 explosions throughout the month and 28 volcanic earthquakes detected on the W side of the island and as many as 722 volcanic earthquakes were recorded near the Otake crater. During 13-19 June, JMA reported a brief increase in the number of explosions. On 15 June at 2200 an eruption generated a volcanic plume that rose 2 km above the crater rim. An eruption on 16 June at 2147 ejected material 400 m SE from the crater. The amount of sulfur dioxide emitted was relatively low, at 100 t/d on 27 June.

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

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/); 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/).

Semeru contains the active Jonggring-Seloko vent at the Mahameru summit and is located in East Java, Indonesia. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano. The current eruption began in June 2017 and more recently has been characterized by intermittent gas-and-ash plumes and incandescent avalanches (BGVN 48:01). This report updates activity such as ash plumes, incandescent avalanches, and pyroclastic flows from January through June 2023, based on information from daily, VONA, and special reports from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, and various satellite data.

Activity during January and February mainly consisted of frequent ash plumes and white-and-gray emissions. The ash plumes during January rose 200-1,000 m above the crater and drifted in different directions. The white-and-gray emissions rose 200-1,000 m above the crater. A photo was posted on social media that showed an incandescent lava flow extending 500 m from the summit crater on the SE flank at 0027 on 8 January (figure 83). Video posted to social media on 5 February showed a pyroclastic flow descending the SE flank and ash plumes rising along the path and drifting N. Ash plumes rose 1 km above the crater at 0802 on 13 January, at 0536 on 17 January, at 0628 on 19 January and drifted SW, W, and SE, respectively. White, gray, and brown emissions were reported on 15 and 17 January that rose 300-1,000 m above the crater. During February, ash plumes rose 200-1,500 m above the crater and drifted mainly N and NE. White-and-gray emissions rose 100-1,000 m above the crater.

Figure 83. Photo showing an incandescent lava flow descending 500 m on the SE flank of Semeru at 0027 on 8 January 2023. Photo has been color corrected. Courtesy of Info Semeru.

Similar activity consisting of frequent ash plumes and gas-and-steam emissions continued through March and April. During March, ash plumes rose 300-1,200 m above the crater and drifted in multiple directions. On 25 March at 0738 an ash plume rose 1.2 km above the crater and drifted SE. Occasional white-and-gray emissions rose 50-1,000 m above the crater. Ash plumes in April rose 400-1,200 m above the crater and drifted in different directions. An ash plume on 3 April rose 1.2 km above the crater and drifted SE and S at 0538. On 8 April a photo and videos were posted on social media showing a pyroclastic flow moving 1.5 km down the SE flank, accompanied by an ash plume (figure 84). New material was deposited along the crater, according to a local news source. Another pyroclastic flow occurred at 0710 on 18 April that descended up to 2 km from the crater to the SE (figure 85). White-and-gray emissions rose 100-800 m above the crater during April.

Figure 84. Photo showing a pyroclastic flow descending the SE flank of Semeru on 8 April 2023. Courtesy of Info Semeru.

Figure 85. Photo showing a pyroclastic flow descending 2 km on the SE flank of Semeru on 18 April 2023. Photo has been color corrected. Courtesy of Info Semeru.

Ash plumes and white-and-gray emissions persisted during May and June. During May, ash plumes rose 300-1,200 m above the crater and drifted generally N and S. On 13 May around 1012 a pyroclastic flow was observed moving 1.5 km down the SE flank, accompanied by an ash plume (figure 86). On 27 May an ash plume rose 1.2 km above the crater and drifted S and SW at 0819. White-and-gray emissions rose 100-800 m above the crater. Ash plumes during June rose 200-1,500 m above the crater and generally drifted N and SW. A webcam image showed incandescent material at the summit and on the flanks at 0143 on 23 June that traveled 3.5 km. According to a local news source, a pyroclastic flow traveled 5 km down the SE flank at 1910 on 26 June; the accompanying an ash plume rose as high as 1.5 km above the crater and drifted NE and E. Dominantly white gas-and-steam emissions rose 50-300 m above the crater.

Figure 86. Photo of a pyroclastic flow descending the SE flank of Semeru as far as 1.5 km at 1012 on 13 May 2023. Photo has been color corrected. Courtesy of Info Semeru.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed frequent and moderate-power thermal anomalies during January through June 2023 (figure 87). There was a short gap in activity during late January through late February, followed by low-power and less frequent anomalies through April. During mid-May, there was an increase in both power and frequency of the anomalies. A total of 73 thermal hotspots were detected, based on data from the MODVOLC thermal algorithm. There were 10 detected in January, four in March, two in April, 17 in May, and 40 in June. Infrared satellite images showed persistent thermal activity at the summit crater during the reporting period; strong incandescent avalanches of material were occasionally captured in these images and affected the SE flank (figure 88).

Figure 87. Frequent, moderate-power thermal anomalies were detected at Semeru during January through June 2023, according to this MIROVA graph (Log Radiative Power). There was a short gap in activity during late January through late February, and lower-power anomalies were registered during late February through April; during mid-May there was an increase in both power and frequency of the anomalies. Courtesy of MIROVA.

Figure 88. Infrared (bands B12, B11, B4) satellite images showed strong thermal activity at Semeru on 10 January 2023 (top left), 19 February 2023 (top right), 11 March 2023 (middle left), 20 April 2023 (middle right), 30 May 2023 (bottom left), and 14 June 2023 (bottom right). Incandescent material mainly affected the SE flank from the summit crater, as shown in each of these images. Courtesy of Copernicus Browser.

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

Information Contacts: 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/); 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/); Info Semeru (Twitter: @info_semeru, https://twitter.com/info_semeru).

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/).

Anatahan's third historical eruption began on 5 January 2005 (BGVN 29:12 and 30:02). On 5-6 April 2005, an eruption cloud rose to 15.2 km altitude, the highest yet seen at the volcano (BGVN 30:04). That eruption, estimated to have expelled 50 million cubic meters of ash, caused the temporary closure of Anderson Air Force Base on Guam. An eruption that began on 5 May and produced an extensive ash and steam plume was briefly described in BGVN 30:04, but further details follow. Plumes were frequently visible in satellite imagery; a summary of satellite observations is presented for 16 June-20 July 2005 (table 4).

Table 4. Daily summaries of Anatahan plumes seen in satellite imagery, 16 June-20 July 2005. Satellite abbreviations: DMSP: Defense Meteorological Satellite Program; Feng Yun: "Wind and Cloud"-Peoples Republic of China Earth Observing System meteorological satellite; GOES: Geostationary Operational Environmental Satellites; HIMAWARI: "Sunflower"-Japanese geostationary meteorological satellites; MTSAT: Japanese Meteorological Agency and Japanese Ministry of Transportation satellite; NASA: National Aeronautics and Space Administration; NOAA: National Oceanic and Atmospheric Administration. Courtesy of U.S. Air Force Weather Agency Satellite Applications Branch (Charles Holiday, Jenifer E. Piatt, Mickael A. Archuletta, Brent A. Persinger).

Observations during early May 2005. Activity surged to a moderately high level on 5 May, when an extensive ash-and-steam plume to 4.5 km altitude was visible in all directions. Ash extended 770 km N, 130 km S to northern Saipan, and 110 km W. Vog extended in a broad swath from 3,000 km W, over the Philippine Islands, to 1,000 km N of Anatahan. By 9 May harmonic tremor amplitude had decreased to near background levels, with a corresponding drop in eruptive activity. As of 10 May the Air Force Weather Agency (AFWA) reported ash rising to about 3 km altitude and extending 400 km W, with an area of vog less than half that noted on 5 May.

Anatahan began erupting suddenly from its E crater at about 1700 on 10 May. Within hours of the eruption's onset, a towering column of volcanic ash and gas rose to more than 10 km altitude, and the prevailing wind blew the ash W. An immediate concern was the potential for the tiny abrasive ash fragments to damage aircraft passing nearby and downwind from the volcano. The Washington Volcanic Ash Advisory Center issued an advisory that volcanic ash was present at 11 km altitude moving S at 65 km/hour and at 4.6 km altitude moving W at 20-30 km/hour.

The single seismic station on the island maintained by the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO/CNMI) was not working at the time, but a broadband seismic instrument installed 6.5 km W of Anatahan's crater on 6 May by scientists from Washington University in St. Louis recorded significant earthquake activity in the hours before the eruption began; the instrument was one of many installed to conduct a seismic experiment along the Mariana Trench. A preliminary review of the data shows there was a rapid increase in the number of small-magnitude earthquakes (probably less than M 2) to more than 100 per hour beneath the volcano within a few hours of the eruption onset.

A smaller but nearly continuous eruption column rose from the E crater of Anatahan for several days following 10 May. The resulting eruption clouds were generally below about 6 km altitude. On 11 May AFWA reported thick ash rising to 4.2 km altitude and moving WNW. The ash extended in a triangular shape from the summit 444 km to the WSW through 510 km to the NW. A layer of diffuse ash at 3 km altitude extended beyond the dense ash for another 1,000 km. A broad swath of vog extended over 2,200 km W nearly to the Philippines and over 1,400 km NNW of Anatahan. Although the ash plume diminished over the next few days, it remained significant, rising to 2.4 km altitude and extending 370 km WNW on 13 May. Personnel from EMO/CNMI and the U.S. Geological Survey (USGS) who were repairing and installing equipment on 14 May reported hearing a continuous roaring sound from 2-3 km W of the active vent. They also saw ash and steam rising by pure convection, not explosively, to 3 km altitude.

Observations during later May and June 2005. Following nearly continuous eruption from January through April 2005, on 23-24 May typhoon Chan-hom shifted the prevailing E winds to the S, blowing the eruption column toward Saipan and Guam. Light ashfall resulted in flight cancellations at the Saipan and Guam international airports. Residents of Saipan reported a rotten-egg smell associated with the ashfall. The ongoing explosive activity excavated a deep crater within Anatahan's E crater. Scientists estimated the inner crater was nearly at sea level by about 20 May; before the eruption, the floor of E crater was 68 m above sea level.

The spiny surface of a lava flow was first observed in the inner crater on 4 June. The flow appeared to form a mound-shaped lava dome, but its volume is unknown. New fault scarps and slump features were seen within the E crater, as well as additional faulting W of the E crater. A gradual increase in the number of long-period (LP) earthquakes and tremor began at Anatahan on 5 June. Both LP and tremor events peaked during 2230-0030 on 6 June. During the peak in activity, more than 350 LP events occurred. Tremor amplitudes briefly reached a new high for the current eruptive activity, and an ash column reached ~ 7.9 km altitude. On 6 June, tremor amplitudes returned to low levels. During the rest of the week of 1-7 June, ash plumes reached a maximum altitude of 4.3 km. On 5 June the EMO/CNMI seismic station was repaired and ash samples were collected from the site. Through 12 June, the seismic records showed only continuous ground shaking to varying degrees. The most intense periods of tremor lasted 3 to 10 hours and occurred about every 24-36 hours.

On 12 June, three LP earthquakes were recorded, the largest about M 2. Other earthquakes followed in the late afternoon and early evening of 13 June. During 17-26 June 2005, seismicity was at the highest level since the eruption on 6 April, with real-time seismic amplitude (RSAM) values at ANA2 consistently near 625.

Since 18 May, Anatahan has sent ash and steam continuously to 2.4 km altitude or higher, with seven eruptive pulses to 7.6 km altitude or higher. On 11 June, a 10 minute-long eruptive pulse sent ash and steam to 14 km altitude. On June 19, a 2.6 minute-long eruptive pulse sent a cloud of steam and ash to 15.2 km altitude; the cloud moved E and dissipated after about 7 hours. On 6 July, very high levels of tremor for about 30 minutes accompanied an eruptive pulse to 12.2 km altitude.

On 11 June beginning at 1622 three explosions produced a dense ash cloud that rose to an altitude of ~ 13.7 km. On 12 June, seismicity was at moderately high levels, with periods of strong tremor and frequent small LP earthquakes. Satellite imagery showed an ash cloud at an altitude of ~ 3 km.

Two strong explosions on 14 June removed much of the small new dome in the inner crater. Just before noon on 14 June, earthquakes began to occur at intervals of 1-2 minutes. For the next two days, episodes of intense tremor and earthquakes lasting about 1.5 hours occurred about every 12 hours, accompanying strong ash emissions from the E crater, with eruption columns higher than 2 km altitude. Quiet intervals in which the eruption column contained little ash were accompanied by continuous weak tremor.

On 19 June at 1525 a brief eruption produced a steam-and-ash cloud that reached an altitude of ~ 15.2 km (figure 18). Guam Meteorological Weather Office radar showed that the cloud drifted E. No seismic signal was clearly associated with the eruption. Two days before the eruption, the amplitude of continuous tremor was relatively high. During the days before and after the eruption, ash reached 3-4.6 km and drifted W.

Figure 18. NOAA satellite image of the ash plume at FL500 (15.2 km) from the Anahatan eruption of 19 June 2005. This was one of the highest plumes ever recorded from the volcano. Its position SE of Anahatan is unusual; the usual direction of the ash and other emissions is W. Courtesy of US Air Force Weather Agency.

During 22-27 June AFWA observed, on satellite imagery, a moderately dense cloud of ash and steam that rose to a maximum altitude of ~ 3 km, and drifted W. Additional thin ash and vog were visible to the W and NNW of the island. On 26 June AFWA identified, on satellite imagery, a dense cloud of ash and steam rising to ~ 3.7 km, moving towards the W, and vog to the W, N and NE of the island (figure 19). No particular seismic signal was associated with the eruptions. By 28 June the seismicity level dropped by about 80% from the continuously high levels of the last week.

Figure 19. GOES-9 image of 30 June 2005 showing the extent of the atmospheric injection of Anahatan ash and gas. The emission reached the island of Kyushu. Courtesy of US Air Force Weather Agency.

On 3 July at 1646 an eruption produced a SSE-drifting plume to an altitude of ~ 12.2 km, according to Guam Meteorological Office radar. Vog briefly drifted S over the islands of Saipan and Tinian. During 29 June to 5 July, steam-and-ash emissions continued to rise to low altitudes. During 6-11 July, eruptive activity continued, with steam-and-ash plumes rising to a maximum altitude of 6.1 km. On 6 July beginning at 1730 tremor at the volcano increased, and an eruption produced an ash plume to an altitude of ~ 12.2 km. During 8-11 July, a thin layer of vog extended over much of the Philippine Sea (figure 20).

Figure 20. GOES-9 image of 8 July 2005 showing the ash plume and vog from Anahatan extending 2,450 km W almost to the Philippines and Taiwan. Courtesy of US Air Force Weather Agency.

As of 1 August 2005, Anatahan was presumed to be in a state of constant eruption. For the first half of 1 August, volcanic tremor levels as recorded at Anatahan's E seismic station (ANA2) were between 40 and 60 % of the peak levels observed during 17-26 June. At 0800, the National Weather Service at Tiyan, Guam, issued a volcanic ash advisory for Saipan and Tinian. A strong sulfur odor from the emitted volcanic gases was reported by numerous residents, and ash was observed on the tips of aircraft at Saipan International Airport. Traces of ash were also apparent on solar panels powering equipment run by the EMO/CNMI on Saipan. According to the Air Force Weather Agency, continued cloud cover caused by a tropical storm inhibited ash detection on METSAT imagery. As of 1252 on 1 August, the ash plume was presumed to be at an altitude of 4.6 km, moving toward the S at 18-27 km/hour.

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: Juan Takai Camacho and Ramon Chong, Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO/CNMI), PO Box 100007, Saipan, MP 96950, USA (URL: http://www.cnmihsem.gov.mp/); Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawaii Volcanoes National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); Charles Holliday and Jenifer E. Piatt, U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA; Randy White and Frank Trusdell, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025-3591, USA (URL: https://volcanoes.usgs.gov/nmi/activity/).

Heavy monsoon rains that fell soon after the beginning of the eruption on 28 May made observations and fieldwork difficult, and the eruption appeared to have ended by 6 July (BGVN 30:05). Based on information from the Indian Coast Guard, Dhanapati Haldar noted that as of 6 June the mode of eruption was Strombolian, the same as that observed during 1994-95, with fire fountains rising ~ 100 m, a dark plume rising 1 km, and lava piling up on the W face of the main cone.

On 13 June an Indian Navy ship transported Geological Survey of India scientists Sumit Kr. Mitra, P.C. Bandopadhyay, Sanjeev Raghav, and Tapan Pal to the island. Prior to the visit the volcano was spewing a gray ash plume charged with water vapor from both the main crater and a subsidiary vent on the SW slope. Around 13 June activity at the subsidiary vent decreased considerably and lava debris formed a mound of loose hot fragments. Forceful ejection of bombs and lapilli continued from the main crater. The proximal accumulations of pyroclasts displayed some incandescence. Red-hot lava fragments were forcefully ejecting from the main crater to heights of more than 100 m, accompanied by loud explosions. Strombolian fire fountains every 15-30 seconds created an eruption column and mushroom-shaped plume that blew to the N. Hand specimen study revealed both jet-black and brownish black basaltic fragments. Both types contained large phenocrysts of plagioclase and pyroxene in a finer black groundmass with a porphyritic texture.

A story in the BBC News-World Edition of 11 July about the volcano becoming a tourist attraction served by charter boats included a statement that lava was flowing into the sea. However, the observation was not dated or attributed to a specific source.

According to a pilot's report described in a Volcanic Ash Advisory, ash was visible near Barren Island on 18 July at 0211 at an altitude of ~ 6.1 km. Ash was observed on satellite imagery at 0755 that day below 4.6 km altitude. MODIS imagery from the NASA Terra satellite at 0930 (0400 UTC) showed a distinct brown plume extending around 4.6 km NNE. A plume was again reported by a pilot on 18 August at an altitude of ~ 3 km, although ash was not visible on satellite imagery.

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

Information Contacts: Geological Survey of India, 27 Jawaharlal Nehru road, Kolkata 700016, India (URL: http://www.gsi.gov.in/); Dhanapati Haldar, Presidency College, Kolkata, India; Jenifer E. Piatt, U.S. Air Force Weather Agency (AFWA), Satellite Applications Branch, Offutt Air Force Base, Nebraska 68113, USA (URL: http://www.557weatherwing.af.mil/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); BBC News World Edition, Room 7540, BBC Television Centre, Wood Lane, London W12 7RJ, United Kingdom (URL: http://news.bbc.co.uk/).

According to the Instituto Nicaraguense de Estudios Territoriales (INETER) an eruption occurred at dawn on 28 July 2005 from Concepción, which lies on the island of Ometepe in W-central Lake Nicaragua (figure 3). Concepción is frequently active at low levels and INETER reports suggested these new events as late as 31 July were not considered major behavioral anomalies indicative of an energetic reactivation of the volcano. A colored diagrammatic map that for the case of larger eruptions included hazard zones, refuges, and escape routes for three contingencies; it appeared in the press several days before the July eruption. Many of the scenarios indicated movement of people to the SE side of the island. The map noted that Concepción has 26 craters, and eruptions could occur from other than the central vent.

Figure 3. A map of the portion of Lake Nicaragua containing Ometepe island, the northern portion of which includes Concepción volcano. Eruptions there in late July led to ash falling on many of the labeled settlements to the W of the summit. Small dots represent epicenters detected during 2003-2005; they mainly centered 10-16 km to the SE and often off the island, but typically closer to the island's other volcano (Maderas). The largest were MR 5.6 (date not given). Courtesy of INETER.

The 28 July eruption cloud deposited ash in the island town of Moyagalpa (~ 9 km W of the summit) and in lesser quantities on the mainland settlements W of the volcano, at San Jorge, Buenos Aires, Potosí, Belén, and in the vicinity of Rivas. Residents also smelled volcanic gases.

INETER recorded seismic tremor at a station N of the volcano, but no large earthquakes occurred. By the afternoon of 28 July ashfall had reduced considerably, or completely ceased, but gas emission continued. No thermal anomalies were observed on satellite imagery. During the night and the following day residents on Ometepe island's W side reported continued presence of ash and gas.

On the morning of 29 July, geodetic measurements determined that significant deformation had occurred, presumably related to magma injected. The seismic station to the N recorded constant tremor; during 0500-0800, a series of volcanic earthquakes may have been associated with small explosions in the crater. At 1025 the seismic station recorded a moderate explosion in the crater.

On 30 July the N seismic station registered tremor, which continued with variations. Significant earthquakes remained absent. On 31 July after 0300 tremor amplitude rose and it remained elevated for an undisclosed amount of time. However, episodes of ashfall diminished or ceased.

Geologic Background. Volcán Concepción is one of Nicaragua's highest and most active volcanoes. The symmetrical basaltic-to-dacitic stratovolcano forms the NW half of the dumbbell-shaped island of Ometepe in Lake Nicaragua and is connected to neighboring Madera volcano by a narrow isthmus. A steep-walled summit crater is 250 m deep and has a higher western rim. N-S-trending fractures on the flanks have produced chains of spatter cones, cinder cones, lava domes, and maars located on the NW, NE, SE, and southern sides extending in some cases down to Lake Nicaragua. Concepción was constructed above a basement of lake sediments, and the modern cone grew above a largely buried caldera, a small remnant of which forms a break in slope about halfway up the N flank. Frequent explosive eruptions during the past half century have increased the height of the summit significantly above that shown on current topographic maps and have kept the upper part of the volcano unvegetated.

Information Contacts: Instituto Nicaraguense de Estudios Territoriales (INETER), Volcanology Department, Apartado 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni//vol/concepcion/concepcion.html).

The most recent reported observations of Erta Ale made during 22-23 January 2005 (BGVN 30:01) described hornitos on a chilled lava lake surface. The following report is courtesy of Tony Waltham, who recently authored an article discussing the Afar Triangle (Waltham, 2005). These observations from January 2004 further illustrate the shrinking of the lava lake previously noted by a February 2004 expedition (BGVN 29:02).

A group of English geologists who visited on 15-16 January 2004 observed an active lava lake estimated at about 25 m across almost in the center of the lower lava floor within the S crater (figure 16) with a turbulent lava surface ~ 3 m below its rim. Crusting was minimal, and there was no development of substantial lava rafts. Modest fountaining occurred mainly over the zone of rising lava under the southern margin, and none was observed to rise more than 3 m to rim level. A hornito just a few meters high was active on the SE side (figure 17), a few meters from the lake, and night viewing revealed incandescence from a few other fissures across the old lava floor. Minimal fumarolic activity within the crater generated some periods of thin blue haze, though there were major emissions of sulphurous fumes from many fumaroles and fissures around the remains of the old northern crater.

Figure 16. Erta Ale's remaining lava lake in the lower floor of the South crater, 15-16 January 2004. Courtesy of Tony Waltham.

Figure 17. Telephoto view of Erta Ale's lava lake, with a hornito barely visible on the left side, 15-16 January 2004. Courtesy of Tony Waltham.

Reference. Waltham, T., 2005, Extension tectonics in the Afar Triangle: Geology Today, v. 21, no. 3, p. 101-107.

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

Information Contacts: Tony Waltham, 11 Selby Road, Nottingham NG2 7BP, United Kingdom.

An eruption in 2002 began on 11 May when discolored plumes were noted (BGVN 28:04). Anomalous seismicity began on 14 May 2002, when about 900 events were recorded (table 1). The number of events dropped to very low levels the next day, but then gradually increased to a peak of 967 on the 28th and almost that many on the 29th. During June 2002, seismicity was high on the 2nd (650 events), 3rd (> 300 events), and 8th (~ 240 events). There were also 117 tremor events during the month, 73 of them on the 15th. Plumes and ashfall were reported through 5 June (BGVN 28:04).

Table 1. Summary of seismicity and plume observations at Kikai, May 2002-January 2005. All reported plumes were described as either white (W), light white (LW), grayish white (GW), or gray (G). Data courtesy of JMA.

Activity for the following year consisted of low-level seismicity of less than 200 events per month, and frequent, almost daily, white plumes. Eruptive activity began again on 7-8 June 2003 when 800-1,000 m ash plumes were recorded. Although plumes were not reported, eruptions also occurred during 10-12 June. Additional eruptions were noted by JMA during 7, 14-17, 26, 27, and 30 July, and 12, 13, and 15-18 August 2003. All of the June-August eruptions caused ashfall. The last grayish white eruption plumes in 2003 were seen on 19 and 22 September.

From March to September 2004, Tokyo Volcanic Ash Advisory Center (VAAC) reports indicated a number of small eruptions at Kikai. Three plumes in March 2004 reportedly rose to 1.5 km altitude, but no ash was visible in satellite imagery (table 2). JMA also reported eruptions on those days, but only indicated plumes 700 m high.

Table 2. Date and time of eruptions from Kikai, the direction and altitude of observed plumes, and whether ash was seen on satellite image. Based on information from the Tokyo VAAC.

Another plume on 1 June did have ash visible to satellites. This eruption was not included in the JMA observations. Plumes were seen again on 13 August and 25 September, again with JMA only reporting 700-800 m plumes compared to 1.2 and 1.5 km plumes, respectively, in the VAAC advisory. No seismicity was detected during 25 September-5 October 2004, the period following the eruption of a grayish-white plume to 700 m. Data from JMA through January 2005 indicate continuing volcanic earthquakes (less than 10/day in December) and almost daily white plumes as high as 700 m, but generally 400 m or below.

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

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: https://ds.data.jma.go.jp/svd/vaac/data/).

Seismicity and regular gas-and-steam plumes related to the eruption during the summer of 2000 continued through August 2003 (BGVN 28:10). From August 2003 through August 2005 gas emissions continued; SO₂ flux remained relatively high and nearly constant (4,000-9,000 tons per day) since October 2002 (figure 21). Eruptions were absent in 2003. Seismicity increased again in May 2003 to more than 700 events/month (table 4), compared to less than 450 the previous four months, a level higher than any recorded since August 2000 (BGVN 28:10).

Figure 21. Daily average SO2 flux from Miyake-jima between August 2000 and July 2005. Courtesy of Kazahaya Kohei, Geological Survey of Japan.

Table 4. Summary of seismicity and plume observations at Miyake-jima, May 2003-January 2005. All reported plumes originated from the summit crater, and were described as white (W) or gray (G). Data courtesy of JMA.

The number of monthly events remained above 500 through February 2004, with counts of 1,449 in December 2003 and 1,353 in January 2004. Seismicity increased significantly during 5-15 March 2004, with more than 400 daily events recorded during 6-10 March (a high of 590 events on the 7th), before gradually declining, but resulting in a monthly total of 3,810. No unusual activity or eruptions accompanied the elevated seismicity. Although seismicity dropped in April 2004, more than 1,000 monthly seismic events were recorded during May-July 2004.

Seismicity was high again in November (1,015 events) and December (1,634 events) 2004, but the December seismicity was primarily due to over 700 events during 2-3 December. The amplitude of the continuous tremor also increased from below 1 ?m/s to around 4 ?m/s in June 2004. Amplitudes remained elevated, though variable, through December 2004.

On 30 November 2004 a minor ash eruption occurred after a 2-year lull. A minor eruption is defined as a small explosion with minor ash emission and plume height of less than 1 km. The Japanese Meteorological Agency (JMA) noted another gray plume on 2 December, and the Geological Survey of Japan (GSJ) listed minor eruptions on 2, 7-8, and 9 December 2004.

As of April 2005, the SO₂ flux was about 2,000-5,000 tons/day. The danger of destructive eruptions was considered to be small, and some residents of the island (~ 3,800 people), who had been evacuated since September 2000 were returning home as of May 2005. However, the GSJ noted minor eruptions again on 12 April and 18 May 2005.

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

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); A. Tomiya, Geological Survey of Japan (AIST), 1-1 Higashi, 1-Chome Tsukuba, Ibaraki 305-856, Japan (URL: https://staff.aist.go.jp/a.tomiya/miyakeE.html); Kazahaya Kohei, Geological Survey of Japan (URL: https://staff.aist.go.jp/kazahaya-k/miyakegas/COSPEC.html); Earthquake Research Institute (ERI), University of Tokyo,Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032, Japan.

Monowai is a frequently active submarine volcano; during April-November 2003, eleven earthquake and T phase swarms from Monowai were recorded by the Polynesian seismic network (Réseau Sismique Polynésien, RSP) operated by the Laboratoire de Geophysique (LDG) (BGVN 28:11). Approximately 260 T phases in 2004 and 365 in January-August 2005 were detected and analyzed by LDG.

In 2004, four short swarms, of 2-3 days duration and 50-80 T phases per swarm (figure 16), occurred on 18-19 February, 31 March-2 April, 27-29 June, and 14 August. Between August 2004 and March 2005, no T phases from Monowai were recorded, indicating a period of quiescence at the volcano. In 2005, T phase swarms from Monowai were recorded on 2-3 March, 16-21 April, and 25-26 May.

Figure 16. Amplitudes of T phases originating from Monowai Seamount recorded at TBI station on Austral Island (see map in BGVN 28:02), versus time. Courtesy of D. Reymond and O. Hyvernaud, LDG.

As of early August 2005, volcanic activity, as indicated by T phases recorded by RSP, resumed, though numerous events have not yet been analyzed.

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

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

As of July 2004 Tavurvur was releasing white vapor in variable amounts, seismicity was at a low level, and ground deformation continued as slow uplift (BGVN 29:07). Eruptive activity had stopped months earlier, in February 2004 (BGVN 29:04).

On 25 January 2005 ash rose to ~ 500 m above the summit and drifted E. Another ash emission on 31 January reached ~ 1 km above the summit but was not visible on satellite imagery. During 1-21 February, frequent eruptions of ash clouds rose a few hundred meters, drifted SE, and deposited ash mainly offshore. However, ashfall was reported in the town of Tokua during 18-21 February. Incandescent lava fragments were visible on several evenings. Between 200 and 350 daily earthquakes were associated with the eruptions. The number of seismic events leveled off around 20 February to between 150 and 200 per day. During 22-24 February ash fell offshore, but there were also reports of fine ash reaching Tokua airport.

Low-level eruptions continued during the first two weeks of March. During 22-28 March, eruptions continued every 10-20 minutes. Ash clouds rose several hundred meters above the summit, and moderate ash fell in Rabaul Town during 25-28 March. There were 100-200 daily earthquakes associated with the eruptions. No changes were recorded in ground deformation.

During April, May, and most of June 2005, low-level eruptive activity consisted of occasional emission of diffuse pale gray to gray ash clouds, which rose a few hundred meters above the summit. On 1-5, 17-22, and 25-30 April the ash clouds were blown NNW; on 6-16 and 23-24 April they drifted ESE. Fine ashfall occurred over Rabaul Town and villages downwind. Occasional roaring noises were heard throughout April. The daily average number of low-frequency seismic events increased from about 40 during the first half of the April to about 100 in the second half. One high-frequency event, on 26 April, was located NE of the caldera. Ground deformation indicated an inflationary trend. The real-time GPS site on Matupit Island, in the center of the caldera, has shown an inflationary trend since January 2005.

Photographs taken by visitors to Rabaul in late May to early June documented activity from two separate vents at Tavurvur. On 25 May there were two distinct plumes, one a very dark, coherent, ash column and the other a more diffuse white or light gray emission; the plumes appeared to mix a short distance above the volcano (figure 40). A single larger gray plume was seen on 5 June (figure 41). On 27 June the Darwin VAAC received a pilot report of an ash plume 37 km to the NW of the volcano. A pilot observed an ash plume from Rabaul on 28 July at a height of 3 km, but ash was not visible on satellite data.

Figure 40. Photograph showing an eruption of the Tavurvur cone at Rabaul looking from the NW across Matupi Harbor on 25 May 2005. Two plumes, one white and the other dark gray, are originating from separate vents. The peak in the background is Turanguna. Courtesy of Roy Price.

Figure 41. Photograph showing an eruption of the Tavurvur cone at Rabaul looking from the SE on 5 June 2005. The single large plume in this view was darker gray in the upper portion and lighter gray in the lower portion. White clouds above the plume appear to be meteorological clouds. Courtesy of Roy Price.

On 9 August, a low-level ash plume at an altitude of 1.5 km was visible on a satellite image of Rabaul. As of mid-August Tavurvur continued to erupt with discrete ash emissions, although their frequency had declined and most were less vigorous. Some of the of ash-laden clouds were also lighter in color, suggesting less ash content. Ash plumes rose between 800 and 1,500 m and drifted N and NW, occasionally depositing ash on the E part of Rabaul Town and in areas farther downwind. Roaring and rumbling noises accompanied the activity. Projections of incandescent lava fragments were visible at night but were less conspicuous compared to the previous week. Seismicity was at a moderate to high level with most earthquakes associated with ash emissions and explosions. However, small low-frequency earthquakes not associated with ash emissions were also recorded. No high-frequency earthquakes were recorded. Ground deformation measurements from GPS and tide gauge instruments fluctuated, but the general trend showed a slow rate of uplift. As a safety precaution, people continue to be discouraged from venturing within 1 km of the erupting vent.

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

Information Contacts: Ima Itikarai and Herman Patia, Rabaul Volcano Observatory (RVO), P. O. Box 386, Rabaul, Papua New Guinea; Darwin Volcanic Ash Advisory Center, Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, Northern Territory 0811, Australia; Roy E. Price, Geology Department, University of South Florida, 4202 East Fowler Ave., Tampa, FL 33620, USA.

Several small eruptions during December 2003 and January 2004 at Suwanose-jima produced ash plumes to unknown heights (BGVN 29:03). Little activity was observed during the first four months of 2004. From the end of April 2004 to the end of July 2005, numerous eruptions and explosions produced plumes reported by the Tokyo Volcanic Ash Advisory Center (VAAC), including some observed by pilots (table 3).

Table 3. Summary of activity at Suwanose-jima from April 2004 to July 2005 based on information from the Tokyo VAAC. "--" indicates data not reported or unknown.

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Tokyo Volcanic Ash Advisory Center, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: https://ds.data.jma.go.jp/svd/vaac/data/).

Long steam plumes during 22-23 August 2004 (BGVN 29:07) were observed on satellite imagery. Additional plumes were seen earlier that month, prompting the Darwin Volcanic Ash Advisory Center to issue advisories on four days.

Ulawun remained quiet from August 2004 until March 2005. During March 2005, weak to moderate volumes of thick white vapor were released from the main crater. On 27 and 28 March light gray emissions were observed, and small continuous volcanic tremor was recorded for six hours. The N vent remained quiet. Seismic activity continued at low levels with low-frequency earthquakes recorded. A tiltmeter was installed on 15 March but no significant movements were detected.

During April-July 2005 white vapor from the main vent was common, and plumes were frequently visible on satellite imagery. On 6 April, a thin plume was visible extending ~ 55 km to the SW. On 19 May a small plume to an unknown height extended W. Plumes to unknown altitudes were again released on 3 and 6 June. Plumes rising to 3 km altitude were seen on satellite imagery on 6 and 21 June. The 21 June plume contained ash, and initially extended W and WSW; imagery about six hours later showed the plume blowing NW. A short plume was visible at ~ 3 km altitude during 22-27 June, and on 27 June a pilot reported that the plume extended 37 km. During 30 June to 1 July, thin ash plumes were visible on satellite imagery, but heights were not given. No noise, night-time glow, or emissions were reported during this time. Small low-frequency earthquakes were recorded. Volcanic tremor was registered on 16-17 June.

On 9 August a plume drifting to the S was visible on satellite imagery (figure 10). During the rest of August, the summit crater released thick white vapor. Seismicity was characterized by small low-frequency earthquakes. One high-frequency earthquake and small periodic volcanic tremors were recorded.

Figure 10. Terra MODIS satellite image of New Britain Island showing a distinct ash plume drifting S from Ulawun (middle right) at 0809 on 9 August 2005 (UTC). Plumes can also be seen originating from Rabaul (NW end of the island, upper right) and Langila (E end of the island, left). Photo courtesy of MODIS Rapid Response Team, NASA Goddard Space Flight Center.

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

Information Contacts: Ima Itikarai, Rabaul Volcano Observatory (RVO), P. O. Box 386, Rabaul, Papua New Guinea; David Innes, Air Niugini, PO Box 7186, Boroko, Port Moresby, National Capital District, Papua New Guinea (URL: http://www.airniugini.com.pg/); Darwin Volcanic Ash Advisory Centre (VAAC), Commonwealth Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

Pago has remained quiet during April-August 2005, with no reports of volcanism since the end of the most recent eruption in early 2003 (BGVN 28:03 and 28:09). Reports since that time have described low-level emissions and seismicity (BGVN 28:12, 29:02, 29:04, 29:07).

In April the upper vents and the summit crater released small amounts of white vapor and occasional thin white vapor was reported from the lower vents. Seismic activity was low; the daily number of low-frequency earthquakes ranged from zero to a few. In June weak emissions of thin white vapor continued to be released from the upper vents but no emissions were noted from the lower vents. Seismicity in June remained low, with no more than 8 small, high-frequency earthquakes recorded per day. Similar activity continued through August. Visual observations on 27 and 28 August revealed emissions of very small volumes of thin white vapor being released from the upper vents of the fissure system. No emissions originated from the lower or main summit vents. Seismic activity was low throughout the month, and some small high-frequency earthquakes were recorded. The greatest number of high-frequency events recorded on any given day was 7 on 25 August. No noises were heard and no glow was observed during the reporting period.

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

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