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/).

Our last report (CSLP 19-69) discussed a summit eruption at Ebulobo stratovolcano, near the S coast of Central Flores island, that in 1969 had emitted ash and steam as well as "fire" (generally taken as incandescence but also possibly flames). CVGHM (Center for Volcanology and Mitigation of Geologic Disasters), issued a report on Ebulobo on 26 August 2013 informing readers that during August 2013, observers noted one or more hot emissions escaping from the crater. The resulting plume was of sparse consistency, white in color, under weak pressure, and it rose to 5-30 m above the peak. "Smoke" was noted.

The CVGHM report noted that on the night of 21 August 2013, observers on the volcano's N side saw incandescence at the summit area. Observations during the night of 22-23 August revealed points of glowing remained unchanged. The glowing was considered anomalous, having not been seen since 2011. The exact cause of the incandescent regions was not reported No new fissures, lava flows or pyroclastic flows were reported. The glowing later terminated as discussed in an October follow up report.

During June 2013, the system recorded the earthquakes shown in table 1.

Table 1. A summary of seismicity recorded at Ebulobo. Dashes signify cases without reported data. Extracted from the 26 August and 17 October CVGHM reports.

During 1-22 August 2013, the seismic system also recorded tremor with maximum amplitudes in the range of 0.5-15 mm.

Ebulobo (figure 1) has a dedicated observation post and two seismic instruments as discussed further below.

Figure 1. Ebulobo as seen in a photo taken 9 June 2009. Copyrighted photo by Andrzej-Muda.

Glow diminishes and Alert Level drops (to I). During September-October white plumes rose as high as 100 m above the crater. Despite that, the glowing area had remained absent after 27 August. On 17 October CVGHM scaled back the Alert from II to I (Normal, on a scale that reaches IV).

More background. The following was extracted from CVGHM reporting.

"Ebulobo Volcano is located in the district of Nagekeo, province of Nusa Tenggara Timur. Eruptions of Ebulobo generally have consisted of lava streams that quickly formed mounds but have never so far resulted in sudden eruptive outbursts that produced a symmetrically shaped mass to the volcano. Ebulobo's eruptions have occurred between 3 and 58 years. In its historical record, its latest eruptive activity took place in 1941 and consisted of a lava stream.

"Observation of Ebulobo's activity is carried out from its monitoring post in the village of Ekowolo, sub-district of Boa Wae and is done visually and according to tremor events. The monitoring is done by means of a Type VR-60 seismograph and a Type L4C seismometer. The readings are transmitted by a telemetric system."

Geologic Background. Ebulobo, also referred to as Amburombu or Keo Peak, is a steep-sided symmetrical stratovolcano in central Flores Island. The Watu Keli lava flow traveled down the N flank to 4 km from the summit in 1830, the first of only four recorded historical eruptions of the volcano.

Information Contacts: Center for Volcanology and Mitigation of Geologic Disasters (CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); and theNational Agency for Disaster Management (BNPB), Gedung Graha 55 Jl. Tanah Abang II No. 57 Postal Code: 10120, Jakarta Pusat, Indonesia (URL: http://www.bnpb.go.id/).

In the first nine months of 2011, Ibu was the scene of frequent avalanches and at least one weak explosion that generated minor white-to-gray plumes (BGVN 36:08). Seismic activity decreased during September 2011, prompting the Center of Volcanology and Geological Hazard Mitigation (CVGHM) to lower the Alert Level to 2 (on a scale of 1-4) on 8 September (the Level rose again later). This report discusses activity from 9 September 2011 through March 2014. The location of Ibu is shown in BGVN 36:08.

According to CVGHM, seismicity increased and volcanic tremor was detected during May through 6 June 2013. The lava dome grew, especially the N part, and by early June had grown taller than the N crater rim. White-to-gray plumes rose 200-450 m above the crater rim. Based on visual and instrumental observations, as well as the hazard potential, CVGHM increased the Alert Level to 3 on 7 June. The public was warned to stay at least 3 km away from the active crater.

CVGHM reported that during 7 June-9 December 2013, the lava dome continued to grow, and incandescent material from the dome filled the river valley in the direction of Duono village, about 5 km NW. The seismicity remained relatively stable. Observers saw occasional weak white-to-gray plumes. On 10 December 2013, the Alert Level was lowered to 2; however, the public was warned to stay at least 2 km away from the active crater, and 3.5 km away from the N part.

Between 1 September 2011 and March 2014, MODVOLC thermal alerts were issued on 70 days, or an average of almost one day every two weeks. Such alerts are consistent with dome growth such as that noted above. (Those alerts are derived from satellite data collected by the MODIS instrument and processed by the Hawai'i Institute of Geophysics and Planetology.) For comparison, between 1 January 2011 and 13 September 2011, these alerts only appeared about once every 2.4 weeks on average.

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

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Saut Simatupang, 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); 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/).

A new island emerged on 20 November 2013 out of the ocean as the result of a Surtseyan eruption on the S flank of Nishinoshima, a small volcanic island in the Izu-Bonin arc, ~940 km S of Tokyo (figure 1). The new island, originally called Niijima ('new island') by the Japan Coast Guard (JCG), eventually merged with Nishinoshima on 24 December 2013. We continue to describe the now merged islands under the name 'Nishinoshima.'

Figure 1. Location of Nishinoshima island shown on an annotated topographic map of the Izu-Bonin arc; the insert shows the area of the main map and the larger regional geography. The map highlights the location of Nishinoshima (Nsi). Other features located respectively from N to S are: Os–Oh–shima; Nij–Nii–jima; Myk–Miyake–jima; Mkr–Mikura–jima; Krs–Kurose hole; Hcj–Hachijo–jima; Shc–outh Hachijo caldera; Ags–Aoga–shima; Myn–Myojin knoll; Sms–South Sumisu; Ssc–South Sumisu caldera; Tsm–Torishima; Sfg–Sofugan; G–Getsuyo seamount; Ka–Kayo seamount; S–Suiyo seamount; Kn–Kinyo seamount; D–Doyo seamount; Nsi–Nishinoshima; Kkt–Kaikata seamount; Ktk–Kaitoku seamount; and Kij–Kita Iou-jima. After Kodaira and others (2007).

Niijima emerges. Niijima emerged by 20 November 2013 from the ocean surface at an area ~0.5 km SSE off the coast of Nishinoshima. The latter is a small (700 m²), uninhabited volcanic island that last erupted and expanded in during 1973-74. Additional background information is included at the end of this report.

Based on satellite images, the Tokyo Volcanic Ash Advisory Center (VAAC) reported that at 0717 UTC on 20 November 2013 a plume rose 600 m over a new island which emerged ~500 m S of Nishinoshima (figure 2). At 0630 UTC on 22 November, a plume rose 900 m. MODVOLC satellite thermal alerts were measured almost daily from 1635 UTC on 23 November and continued through the latest alert noted at 0120 UTC on 7 April 2014.

Figure 2. Niijima produces a plume as it emerges from the ocean to form a new island off the coast of Nishinoshima on 20 November 2013. Courtesy of Kurtenbach (2013); image from the JCG.

On 21 November JCG and the Japan Meteorological Agency (JMA) noted that the island formed was by then ~200 m in diameter. A warning of dense black emissions from the eruption was issued by JCG on 20 November, and television footage (Frisk, 2013) showed on 21 November ash and rocks exploding from the crater as steam billowed out of the crater (figure 3). On 24 November, JCG reported lava flows coming from the newly-formed crater. They extended to the coastline of the island, and bombs continued to be ejected.

Figure 3. A photograph of Niijima from 21 November 2013 shortly after it emerged from the ocean . Note the large airborne rock erupting from the crater. Courtesy of Kurtenbach (2013); picture provided by JCG.

The Advanced Land Imager (ALI) on NASA's Earth Observing-1 (EO-1) satellite captured a natural-color image on 8 December 2013 (figure 4). JMA reported that by early December the area of the new island had grown to 56,000 m², about three times its initial size, and was 20 to 25 m above sea level.

Figure 4. NASA Earth Observatory satellite image acquired on 8 December 2013 from the EO-1 ALI sensor. The discolored water around the island was attributed to material included volcanic minerals, gases, and seafloor sediment stirred up by the ongoing volcanic eruption. The faint white puffs above the center and SW portion of the island are likely steam and other volcanic gases associated with the eruption. Courtesy of NASA Earth Observatory web site.

Niijima merges with Nishinoshima. NASA's EO-1 ALI satellite again captured a natural-color image of Nishinoshima and Niijima islands on 24 December 2013 and shows only a narrow channel of water appearing to separate the two (figure 5). The water around the islands continued to be discolored by volcanic minerals and gases, as well as by seafloor sediment stirred up by the ongoing eruption. A faint plume, likely steam and other volcanic gases associated with the eruption, drifted SE. Infrared imagery from the same satellite on the same date showed intense heat from the fresh lava, which continued to build the new island. A strip of isolated, discolored (orange) seawater appeared at the junction of the two islands (figure 6).

Figure 5. NASA Earth Observatory satellite image acquired 24 December 2013. Courtesy of NASA Earth Observatory; satellite image by Jesse Allen using EO-1 ALI data from the NASA EO-1 team.

Figure 6. An aerial photograph just prior to the merger of the two islands, taken on 24 December 2013, with Niijima on the right and Nishinoshima on the left. Seawater trapped at the junction has been discolored to orange, attributed to the presence of particulate matter and biochemical activity of organisms in the water. Courtesy of the JCG.

Figure 7 is a drawing by the Japanese Coast Guard (JCG) showing the location of the coastline and the growth of the new island (Niijima) from 20 November 2013 to 26 December 2013. It is striking how much of the island expanded during 13-24 December 2013.

Figure 7. Scale drawing of the merged islands showing the changing coastlines as the new island grew. Colored enclosing lines during the current eruption of Nishinoshima as shown for the following dates: 20, 21, 22, 26, and 30 November 2013, and 1, 4, 7, 13, 24, and 26 December 2013 (note legend translated from Japanese for dates and color of mapped shorelines). Image and interpretation courtesy of JCG.

According to JCG's aerial observation on 20 January 2014, the new part of Nishinoshima island had an area of 0.3 km² (750 m E to W, and 600 m N to S) (figure 8).

Figure 8. An aerial photograph, looking W, of Nishinoshima island taken on 20 January 2014. The newly merged island, Niijima, on the left, continued to expand NW. White and brown plumes rose from vents on the new land, and the water around the SW portion was discolored. Photo courtesy of the JCG.

New images from an overflight on 3 February (figure 9) confirmed that the activity on the former new island continued steadily. Over the past weeks, the vent fed several active lava flow fronts that enlarged the land in more or less all directions. In particular, there are two active flows relatively close to the vent which had been traveling E and formed a small, almost closed bay with green-orange discolored water inside. The previous shorelines for 20 January 2014 (yellow enclosing line) and 21 November 2013 (white enclosing line) are superimposed over the image to show the growth of the island.

Figure 9. Aerial photography of the island on 3 February 2014. For comparison, the previous shorelines on 20 January 2014 (yellow enclosing line) and 21 November 2013 (white enclosing line). Image courtesy of JCG.

According to Pfeiffer (2014), the island continued growing with lava flows traveling in several directions (figure 10). Its highest peak, formed by the most western of the two active vents, was measured at 66 m. The new addition has more than doubled the size of the island by 16 February. A black-sand beach formed on the NE shore of the old part of the island, as a result of lava fragments washed up by currents and waves.

Figure 10. Direction of lava flow from the western side of two active vents is show by vectors superimposed on the image of the island. North is to the top of the photo. The flow arrows were drawn by JCG over an aerial photograph of the island taken 16 February 2014. Courtesy of JCG.

In summary, the new addition to Nishinoshima grew ~500 m SSE of the island's S flank, beginning ~20 November 2013, from a depth of ~50 m to a height of ~65 m from an originating time no earlier than 1974, the time of the latest addition to the island. Based on continued emissions and satellite-based thermal alerts, it is apparent as of 13 March 2014 that Niijima was still expanding outward in all directions from the vents, and that Nishinoshima had grown to over three times its original size.

Further background. The new island was located in the Volcano Islands, a group of three Japanese active volcanic islands that lie atop the Izo-Bonin-Mariana arc system (Stern and Bloomer, 1992) that stretches S of Japan and N of the Marianas (figure 1).

According to the Geological Survey of Japan, Nishinoshima was an emerged submarine volcano in 1974 with a height of ~3,000 m from the surrounding ocean floor and ~30 km wide at its base.

For further details on earlier Nishinoshima activity refer to our earlier reports in predecessor publications, CSLP 93-73 (eight cards issued during 1973-1974), SEAN 04:07, and BVE 25. The latter (BVE 25) is a 1985 Smithsonian report called the Bulletin of Volcanic Eruptions noting that aerial observations on 2 December 1985 disclosed pale green water SW from the island.

The Geological Survey of Japan reported that Nishinoshima is of andesite to basaltic-andesite composition; Aoki and others (1983) classified the volcano's rocks as high-alkali tholeiite. Nishinoshima is surrounded on all sides by cones, vents, pillars, and parasitic seamounts, and its local bathymetry from surveys in 1911 and 1992 are shown in figure 11.

Figure 11. Comparison of bathymetric maps (depths in meters) around Nishinoshima before and after 1973 eruption. The emerged island is shown in green. Depths of 0-100 m are in white, 100-400 m in light blue, 400-700 m in medium blue, and 700-1,000 m in darker blue. The map on the right shows a survey conducted in 1992, after the eruption, based on 1:50,000 basic map of "Nishino-shima" by the Japan Coast Guard (1993). The map on the left shows a survey conducted prior to the eruption, based on mapping in 1911 (Ossaka, 1973). The new island of Niijima first appeared above the sea surface ~500 m SSE of the S coast of Nishinoshima island shown in the 1992 map. Courtesy of the Geological Survey of Japan (2013).

From the 1992 bathymetric map seen at right on figure 11, it is apparent that the ocean depth from which Niijima erupted in 2013, was ~50 m. A sketch of the setting showing a cross sectional view (roughly NNW-SSE) appears in figure 12.

Figure 12. A sketch depicting an approximately NNW (to the left) to SSE (to the right) cross-section across Nishinoshima (blue indicates sea water) portraying some historical stages of growth. The label "Current Nishinoshima" refers to the pre-existing island prior to and in the early stages of the 2013 eruption. Other labels indicate (a) "Nishinoshima before 1973" (also see 1911 bathymetric map in figure 11), (b) flanking material added to Nishinoshima as it "Emerged during the 1973-74 eruption" (also see 1992 bathymetric map in figure 11), and (c) Niijima "Emerging during ongoing eruption" (red area emerging from the sea early in the 2013 eruption). Original drawing courtesy of The Asahi Shimbun (2013).

References. Aoki, H., and Tokai University Research Group for Marine Volcano, 1983, Petrochemistry of the Nishinoshima Islands, La mer, v. 22, pp. 248-256.

Earth of Fire: Actualité volcanique, Article de fond sur étude de volcan, tectonique, récits et photos de voyage [Volcano News, Feature Article on study of volcanos, tectonics, travel stories and photos], 2013, Evolution of Nishino-shima's eruption, Earth-of-Fire web site (URL: http://www.earth-of-fire.com/page-8837676.html).

Frisk, A., 2013 (21 November), WATCH: Incredible video, photos show new island forming off Japan after volcanic eruption, Global News (URL: http://globalnews.ca/news/981245/watch-incredible-video-photos-show-new-island-forming-off-japan-after-volcanic-eruption/ ).

Geological Survey of Japan, 2013, Nishinoshima (URL: https://gbank.gsj.jp/volcano/Quat_Vol/volcano_data/G22.html).

Japan Coast Guard, 1993, 1:50,000 basic map of "Nishino-shima."

Kodaira, S., Sato, T., Takahashi, N., Miura, S., Tamura, Y., Tatsumi, Y., and Kaneda, Y., 2007, New seismological constraints on growth of continental crust in the Izu-Bonin intra-oceanic arc, Geology, v. 35, no. 11, pp. 1031-1034 (doi: 10.1130/G23901A.1).

Kurtenbach, E., 2013 (21 November), Volcano raises new island far south of Japan, AP (Associated Press) (URL: http://news.yahoo.com/volcano-raises-island-far-south-japan-054228644.html).

Ossaka, J., 1973, On the submarine eruption of Nishinoshima, Bulletin of the Volcanological Society of Japan, v. 18, no. 2, p. 97-98, 173-174.

Pfeiffer, T., 2014 (21 February), Nishinoshima volcano (Izu Islands, Japan): island has doubled in elevation, Volcano Discovery web site (URL: http://www.volcanodiscovery.com/nishino-shima/news/42781/Nishino-Shima-volcano-Izu-Islands-Japan-island-has-doubled-in-elevation.html).

Shun, N., 2014, Kaitei chikei (bottom topography), Nishinoshima Kazan (in Japanese), Geological Survey of Japan web site (URL: https://gbank.gsj.jp/volcano/Act_Vol/nishinoshima/page3.html).

The Asahi Shimbun, 2013 (22 November), Japan counts on survival of new island to expand territorial waters (URL: https://ajw.asahi.com/article/behind_news/social_affairs/AJ201311220084).

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 Coast Guard (JCG) (URL: http://www.kaiho.mlit.go.jp/); MODVOLC, 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 Earth Observatory (URL: http://earthobservatory.nasa.gov); ANN (All Nippon News Network) (URL: https://www.youtube.com/user/ANNnewsCH); VolcanoCafe web site (URL: http://volcanocafe.wordpress.com); Earth of Fire web site (URL: http://www.earth-of-fire.com/); Demis web site (URL: http://www.demis.nl/home/pages/Gallery/examples.htm.).

The last Bulletin report (BGVN 35:11) detailed an explosive eruption that began with gas-and-ash explosions in September 2010 and ended in mid-October 2010. Renewed activity began in February 2011 and continued through June 2011. In this report, we highlight the significant ash events from early-to-mid 2011 as well as the continuous monitoring efforts of Servicio Nacional de Geología y Minería (SERNAGEOMIN) during 2011-2013.

During 17 February-27 June 2011, unrest was detected from Planchón-Peteroa and significant meteorological information (SIGMET) notices were distributed by the Buenos Aires Volcanic Ash Advisory Center (VAAC) (table 3). Ash plumes were reported once or twice a month during this time period, although satellite images were not able to detect many of the events. Ash and gas plumes became continuous during late April, and ash plumes rose as high as 5.8 km altitude on 26 April. On 29 April, SERNAGEOMIN raised the Alert Level to 3 (Yellow).

Table 3. Emissions from Planchón-Peteroa during 18 February-27 June 2011. The Observatorio Volcanológico de los Andes del Sur (OVDAS) maintained a web-camera that contributed to numerous direct observations of emissions and is frequently listed as a source. Courtesy of Buenos Aires VAAC.

Seismicity in April 2011 was dominated by volcano-tectonic (VT) events; 405 were detected, and locations were primarily concentrated in an area 25 km NE of the volcanic complex as well as along the N flank, ~6 km from the crater. Earthquakes were M_C less than 2. Some tremor (RD of 3 cm²) and 30 long-period (LP) (RD 4 cm²) events were also detected that month. SERNAGEOMIN frequently reported seismic data in terms of RD, which is the value calculated from reduced displacements.

SERNAGEOMIN reported that ash emissions on 17, 18, and 29 April correlated with episodes of tremor with RD oscillating between 1 and 3 cm². Overflights conducted on 26, 27, and 29 April determined that the active crater had not changed geometry and also appeared structurally stable (figure 7). The observers noted that tephra deposits from the previous explosions were notable SE and SW of the volcano. Deposits from the 29 April explosion were particularly easy to define during the overflight.

Figure 7. This photo of Planchón-Peteroa was taken during one of a series of overflights during 26, 27, and 29 April 2011. A column of ash rose from the active crater and tephra had visibly covered much of the snow immediately SE and SW of the crater. Courtesy of Orlando Rivera, Exploraciones Mineras Andinas S.A.

Buenos Aires VAAC reported a significant ash plume detected by satellite images on 2 May 2011. The plume drifted between 4.9 and 5.5 km altitude toward the NE at ~7.7 m/s. The OVDAS web-camera also captured images of the plume appearing diffuse and ~3.7 km wide. The VAAC noted that the plume rapidly dissipated during 1315-1845 local time. The following day, continuous emissions of ash, steam, and gas were reported by SIGMET and the VAAC, although satellite images were not able to detect any emissions. By 1000, the VAAC reported SIGMET data for a plume that rose to 4.6-5.5 km altitude, moving E. At 1500, satellite images captured a diffuse and ~15 km wide ash plume. The plume drifted E at 5 m/s and had risen to 5.5 km altitude.

Elevated activity during 4-5 May produced ashfall that reached the towns of Minera Río Teno (about 70 km NW) and Las Leñas (in Argentina, 45 km ENE). An overflight conducted by SERNAGEOMIN confirmed continued ash emissions and explosions that occurred approximately every 30 seconds. The explosive activity rarely produced plumes higher than 1,000 m above the crater. Gray ash deposits were visible downwind of the crater; the wind tended to disperse tephra widely and the observers noted that wind directions were frequently directed to the E, NE, NNE, NNE, and NW. During 30 April-8 May, SERNAGEOMIN noted that seismicity included tremor (RD of 2-3 cm²) and VT earthquakes (M_L less than 1.8).

Geologists from SERNAGEOMIN conducted an overflight of Planchón-Peteroa on 13 June 2011. RedMaule interviewed the observers who were on the helicopter which included representatives of SERNAGEOMIN and Oficina Nacional de Emergencia del Ministerio del Interior y Seguridad Pública (OMENI) as well as the mayor of Maule, Chile. The observers noted that persistent degassing continued; a low-level white plume (less than 400 m high), associated with fumarolic activity, was visible rising from the active crater (figure 8). SERNAGEOMIN concluded from the overflight that Alert Level 3 (Yellow) could be reduced to Alert Level 2 (Green/Normal); Alert Level 3 had been in effect since 29 April 2011.

Figure 8. During an overflight of Planchón-Peteroa on 13 June 2011, few bare rocks were visible around the active crater due to snow-cover and ice; a low-level plume of white vapor rose from the crater. These six photos are stillshots taken from a video interview camera; note that the look direction varies in each photo with the approximate direction noted in the upper left corner of each photo. The tall peak of Planchón is visible in the background of the photo looking N. Courtesy of SERNAGEOMIN and RedMaule.

The Buenos Aires VAAC released an ash advisory on 29 October 2011. Satellite images could not detect ash, but a SIGMET was available.

Activity during 2012. An approximate average of 400 earthquakes per month was detected in 2012, and roughly 75% of the events were VT while 25% were cataloged as LP events. The VT events were rarely larger than M_L 3.0 and depths were in range of 4-10 km; these earthquakes were frequently clustered in groups that correlated with local faults. LP earthquakes were typically M_D ≤ 2.0 and RD ≤ 2.9 cm². SERNAGEOMIN reported tremor in April, May, November, and December (table 4). One notable seismic swarm occurred on 5 April. Approximately 123 VT earthquakes were detected during 0230-0730; these events were located ~20 km NE of the crater with depths less than 10 km. The largest earthquake was M_L 1.7.

Table 4. Tremor was detected during four months in 2012. RD is the value calculated from the reduced displacements of seismicity. Courtesy of SERNAGEOMIN.

On 30 October 2012, the Oficina Nacional de Emergencia del Ministerio del Interior y Seguridad Pública (OMENI) released a report highlighting several communities that would be included in the early warning system designed to report flood risks. The towns included Curicó, Romeral, and Teno, in the region Maule, which are especially vulnerable due to proximity to Planchón-Peteroa's major drainages (figure 9).

Figure 9. This Google Earth image includes the location of Planchón-Peteroa (lower right-hand corner), major towns, and primary roads. The background image is a composite of Landsat images from 2014. Note that the yellow line crossing through the volcanic center is the international border for Chile and Argentina. The scale is approximate. Courtesy of Google Earth.

On 6 November 2012, the network of web-cameras captured images of a white plume rising from the crater. At 1620, the persistent plume rose to ~1.3 km and drifted NE. SERNAGEOMIN noted that this activity was related to fumarolic emissions.

Activity during 2013. During 2013, an approximate average of 200 earthquakes was detected per month. Of these events, ~80% were VT and ~20% were LP. Magnitudes and depths of the VT earthquakes were comparable to the previous year, although ML values were sparsely reported. LP seismicity was reported in M_L, instead of M_D and values were in range of 0.3 to 2.0. The reduced displacements (RD) of LP events were frequently reported on a monthly basis with values in range 0.3-8.4.

Tremor was rarely detected in 2013. SERNAGEOMIN reported six episodes of tremor, but these only occurred in January and the calculated RD was 0.5 cm².

Geologic Background. Planchón-Peteroa is an elongated complex volcano along the Chile-Argentina border with several overlapping calderas. Activity began in the Pleistocene with construction of the basaltic-andesite to dacitic Volcán Azufre, followed by formation of basaltic and basaltic-andesite Volcán Planchón, 6 km to the north. About 11,500 years ago, much of Azufre and part of Planchón collapsed, forming the massive Río Teno debris avalanche, which traveled 95 km to reach Chile's Central Valley. Subsequently, Volcán Planchón II was formed. The youngest volcano, andesitic and basaltic-andesite Volcán Peteroa, consists of scattered vents between Azufre and Planchón. Peteroa has been active into historical time and contains a small steaming crater lake. Historical eruptions from the complex have been dominantly explosive, although lava flows were erupted in 1837 and 1937.

Information Contacts: Observatorio Volcanológico de los Andes del Sur-Servicio Nacional de Geologia y Mineria (OVDAS-SERNAGEOMIN), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC) (URL: http://www.smn.gov.ar/vaac/buenosaires/productos.php); and Oficina Nacional de Emergencia del Ministerio del Interior y Seguridad Pública (OMENI) (URL: http://www.onemi.cl/index.html).

A partial dome collapse took place at Soufrière Hills on 11 February 2010 (BGVN 35:03), an event followed by a lack of easily measured dome growth during an interval that continued into at least April 2014. Despite a lack of significant extrusion into the dome, pyroclastic flows continued, as did rockfalls and volcano-tectonic (VT) earthquakes. MVO describes intervals of this nature as extrusive pauses or more simply pauses. Pauses have been diagnosed as a prevalent behavior since they began following an extrusive phase starting in mid-1995. Our last issue (BGVN 36:08) covered part of the still-ongoing pause.

The various phases of activity at Soufrière Hills Volcano (SHV) during 1 January 1992 to 30 April 2013 are summarized in table 72. The table comes from a Montserrat Volcano Observatory (MVO) report providing a synthesis of activity during ~6 months ending in April 2013, and making authoritative and instructive comparisons to the overall eruption (table 72).

Table 72. Inventory of behavioral phases observed at SHV between 1 January 1992 and 30 April 2013. Pause 5 continued into at least April 2014. Taken from the MVO Scientific Report for Volcanic Activity between 13 October 2012 and 30 April 2013.

In brief, table 72 documents that an increase in seismicity occurred from 1992 to 1995, followed by a phreatic eruptive phase starting in mid-1995. That episode was followed by intervals of extrusion, transition, and pause. Extrusive phases included dome growth and frequent pyroclastic flows. During transition phases, dome growth slowed, but the risk to areas near the volcano continued.

As noted above, pauses are characterized by much slower dome growth (if at all), yet residual activity. The current pause is the longest yet recorded since the eruption began in 1995. Pause 5 began on 12 February 2010, and as of March 2014 was over 50 months long.

MVO established three criteria that indicate the potential for future activity. These criteria include low frequency seismic swarms and tremors, daily SO₂ fluxes above 50 tons/day, and significant ground deformation. Most of the data reported in this Bulletin came from MVO Scientific Reports from 1 November 2011 to 30 April 2012, 1 May 2012 to 12 October 2012, and 13 October 2012 to 30 April 2013.

Short, intense swarms of VT earthquakes have occurred at Soufrière Hills since late 2007. The smaller swarms are often described by MVO as strings.

The most notable activity since September 2011 included intense Volcanic Tectonic (VT) earthquake swarms during 22-23 March 2012. Two small strings of VT events occurred in early August 2012, a brief VT string occurred on 24 December 2012, and a few VT strings of earthquakes took place during 4-6 February 2013.

The seismic events of 22-23 March 2012 and August 2012 were followed by ash venting. The venting in March resulted in the formation of two new craters. One developed inside the 11 February 2010 dome collapse scar; the other was outside the collapse scar to the (figure 91).

Figure 91. The craters at Soufrière Hills that formed following the intense VT earthquake swarms during 22-23 March 2012 are labeled in the above aerial photographs, taken by the Montserrat Volcano Observatory (MVO). The upper photo looks S into the 11 February 2010 collapse scar, and the lower photo looks E from above Gage's Mountain. Courtesy of MVO.

On 20 November 2012, images of the S flank of the dome revealed a pervasively fractured area below the S rim of the explosion crater. That area was considered a potential source for large rockfalls or pyroclastic flows.

During the increased fumarole activity on 4-5 February 2013, a new crater was excavated around a gas vent on the floor of the 11 February 2010 collapse scar. This crater was 15 to 20 m across and 5 to 10 m deep.

The Hazard Level remained at 2, indicating daytime (0800 to 1600) access to Zone C and daytime-transit-only in maritime zone W (located W of the volcano; boats may sail through the zone but must not stop). A map of the zones on the island appeared in BGVN (22:05) and is found as figure 22 above.

Activity during 1 November 2011 to 30 April 2012. Throughout the entire reporting period, seismicity remained comparable to previous pauses in lava extrusion. Four strings of VT events, in this case referred to as "spasmodic bursts," occurred in the course of the interval 1 November 2011 to 30 April 2012. In early December 2011, 10 events were recorded in a 3 minute span; the largest in terms of local magnitude (M_L, discussed further below) was 3.2. The 10 events were interpreted as a sequence of triggered events.

Two intense VT swarms occurred on 23 March 2012, with almost 50 VT earthquakes in each swarm. The largest VT earthquake ever recorded at Soufrière Hills, with M_L of 3.9, was recorded during these swarms. The second more intense swarm was followed by mild ash venting, seven hybrid earthquakes, and three long-period (LP) earthquakes. Topics such as ML are discussed in a subsection below on seismicity.

On 30 March 2012, MVO detected unusually low-level VT seismicity sustained over several hours. This was atypical activity, as seismicity at Soufrière Hills is normally characterized by the occasional appearance of short bursts of VT strings.

November-December 2012. Seven lahars were seismically detected in the Belham Valley region during 1 November 2011 to 30 April 2012. Five took place during November-December 2011. They were associated with rainfall above 10 mm/hr.

A pyroclastic flow occurred in Gages Valley on 9 March 2012. The flow originated close to the summit of Chance's Peak and traveled 1.5 km down the W flank into Spring Ghaut. Although direct volume measurements couldn't be made, an empirical relationship between runout and flow volume suggested the pyroclastic flow deposit volume to be 104 m³.

A slight increase in rockfall activity occurred before the VT swarms of 23 March 2012. There were minor rockfalls on the steep N, E, SW and W sectors of the dome, averaging to less than one rockfall per day. The SW side of the dome above Gingoe's Ghaut was unstable with noticeable rockfall activity.

SO₂ flux averaged 420 tons/day, a value below the multi-year eruption's average. Following the March VT swarms, a daily flux of 4,600 tons was observed, the third highest recorded by the optical spectrometer (DOAS) since its installation in 2002. After 2010, SO₂ cycle fluctuations were dominated by variation with timescales on the order of weeks to months.

On 17 February 2012, a fumarole at the E base of the 2006-2007 dome was observed for the first time by MVO staff. An area with yellow and white sulfur deposits was also discovered on this cliff. Around January 2012, this site had temperatures near 60°C, but temperatures in February ranged from 90° to 275°C.

Ground deformation recorded by a GPS network continued to show a trend of ongoing inflation, a behavior similar to previous pauses.

Activity from 1 May 2012 to 12 October 2012. Among 21 bursts of small earthquakes, the most notable occurred on 11 September 2012. Over the course of 13 hours, a low amplitude VT swarm resulted in 17 events, with the maximum M_L around 1.3. Eight rockfalls and two hybrid earthquakes were noted alongside typical seismic activity.

On 13 and 14 October 2012, tropical storm Rafael triggered eight seismically detected lahars in this region. The most noteworthy were those in the Belham Valley. Also, the SO₂ flux was slightly decreased from the previous reporting period, with an average of 280 tons/day.

As of October 2012, the E and W flanks had been determined to be the most unstable areas of the edifice, based on the presence of fresh rockfall deposits and pyroclastic flows. A large pyroclastic flow from the W flank could travel into Plymouth, the former capital destroyed by previous pyroclastic flows.

On 29 August 2012, a large pyroclastic flow originated at the 2006-2007 dome. This has been the largest pyroclastic flow in Tar River since the end of Phase 5 extrusion. Another pyroclastic flow occurred on 19 September 2012 in Gage's Valley. It originated from the steep slope adjacent to Chance's Peak and traveled about 1 kilometer. The sources of these pyroclastic flows can be viewed in figure 92.

Figure 92. Two photographs showing features at Soufrière Hills. The photograph on the left shows the source of the 19 September 2012 pyroclastic flow. The photograph on the right shows the source and flow direction of the 29 August pyroclastic flow. Courtesy of MVO.

A 10-minute exposure photo taken on 6 September 2012 determined no changes in location and number of incandescent areas on the N flank. However, the large fumarole in the floor of the 11 February 2010 collapse scar reached temperatures of ~300°C, and was the source of weak ash venting on 8 August 2012. Thermal IR camera imaging, showed the brightest point of incandescence, which reached temperatures over 400°C, originated from a hole in the rear of the collapse scar.

It should be noted that from August 2012 to November 2012, measurements at three local continuous GPS (cGPS) stations, AIRS, SPRI, and MVO1, had slight shortening of the radial distance between stations and vents, which may indicate short-term reversal of the long term inflation trend. Conclusions remain speculative without testing with more data.

Activity from 13 October 2012 to 30 April 2013. The largest of seven VT strings occurred on 30 November 2012. That swarm had a total of 23 earthquakes, with M_L of 2.1 or less. As mentioned in the introduction, a brief VT swarm occurred on 24 December 2012, but the four swarms of main interest followed on 3-5 February 2013. The most intense, with a total of 36 events in 27 minutes, occurred on 4 February, with a maximum M_L of 2.6. As a result, there was an increase in temperature of fumaroles residing on the 11 February 2010 collapse scar. This escalation continued until later in the evening, and at 1750 loud roaring sounds were heard, accompanied by minor ash venting. Activity and temperature returned to background levels the next day. This activity was noticeably similar to the events of 23 March 2012. Both were preceded by smaller VT strings, about 11 hours earlier, and the most intense phase had a 10-minute duration. There followed a VT string on 5 February associated with minor ash venting from the main gas vent in the floor of the 11 February 2010 collapse scar, as shown in figure 93.

Figure 93. Two thermal images, viewed from MVO and Jack Boy Hill, show the source of the ash venting on 5 February 2013, as well as a newly observed incandescence. Courtesy of MVO.

The next prominent seismic activity occurred on 15 and 19 April 2013. The earthquakes had M_L of 3.0 and 2.9, respectively, and neither were part of a VT string. The last time isolated VT earthquakes occurred was 28 June and 9 October 2011. Beside VT strings, 15 low-frequency earthquakes, which encompassed long-period and hybrid events, were observed during the October 2012 to April 2013 recording period. As of April 2013, 51 VT strings have occurred, and 13 have directly preceded surface activity.

Heavy rainfall on 28 and 30 March 2013 generated large lahars, lasting several hours, in various valleys around Soufrière Hills, including Belham Valley. The average daily SO₂ flux, as of April 2013, was 511 metric tons/day, with a high of 2,381 tons on 6 February 2013. This was the highest value observed since the ash venting of 23 March 2012. The connection between SO₂ flux and VT activity is still not thoroughly understood, but there seems to be an increase of SO₂ a few days before seismic events at Soufrière Hills.

Pyroclastic flow activity had followed the trends of previous pauses. On 28 March 2013, a pyroclastic flow traveled 1.5 km E through Tar River Valley. This pyroclastic flow began at a peeled-away slab of lava on the near-vertical E face of the dome. This was one of the largest pyroclastic flows since the start of Pause 5, and it removed a large portion of the lava slab on the 2006-2007 dome. This flank became heavily fractured as a result of weather and erosion, continued cooling, and contraction of the E flank of the dome above Tar River. Consequently, the Tar River side of the dome will likely be the source of future pyroclastic flow activity. Rockfall activity has been at its lowest since 10 February 2010, consistent with the stabilization of the dome over the past three years.

After 5 February 2013, temperatures in the collapse scar were ~100°C higher than previously recorded. That increase may be due to MVO's use of a new more sensitive IR camera (a FLIR T650sc), replacing their old (Mikron) camera. The new camera records temperatures that are corrected for atmospheric conditions.

Figure 94 emphasizes the difference in sensitivity between the two cameras. However, the distance at which these images were captured, about 5.7 km from the dome, results in unreliable temperature readings. This is because infrared light is absorbed, scattered, and refracted by dust, air, and water (in solid, liquid, or gaseous states). Variables such as solar reflection, heat from direct sunlight, condensates, and high concentrations of SO₂ in the atmosphere can also result in errors in image readings.

Figure 94. For Soufrière Hills, a juxtaposition of thermal images to highlight the difference in resolution and displays between the old infrared-detecting (IR) camera (left) and the more sensitive and accurate new one (right). Although there are temperature scales to the right of each image (22.4-32.4 on the scale at right), they are not applicable in this instance owing to multiple factors (see text). Even at this distance, IR images give scientists greater clarity on dome behavior. Despite the loss of the temperature scale, the images serve as an important tool for monitoring the state of the dome. Both IR photos taken during early 2013. Courtesy of MVO.

According to Adam J. Stinton, a volcanologist at MVO, the new camera produces images twice the size of the older camera due to a larger internal sensor, and therefore the right-hand image was scaled down to a comparable size. Thermal imaging technology works by recording the intensity of radiation in the infrared part of the electromagnetic spectrum and converting it to a radiometric image, with every pixel in the image conveying a temperature measurement.

Using the FLIR camera, a strong fumarole on the summit of the 2006-2007 dome was recorded on 15 March 2013, the first time this fumarole was ever imaged. Its temperature was between 250 and 260°C. No other new thermal features or incandescence had been recorded during this period.

As of April 2013, the trend of long-term edifice inflation continued. This suggested that the magmatic system is still actively deforming surficial areas. MVO observed similar deformation signals during previous pauses in extrusion.

Activity during April 2013 to March 2014. On 14 January 2014, a helicopter assessment of several groups of fumaroles revealed temperatures of 140-340°C within the summit crater. These fumaroles were observed for the first time since 2011. Aside from this detection, there has been a low level of activity at Soufrière Hills, including occasional rockfalls and seismic activity.

Background on seismicity. According to Druitt and Kokelaar (2002), hybrid earthquakes are long-period earthquakes located at (shallow) depths of less than 2 km. LP earthquakes, on the other hand, are widely interpreted as earthquakes associated with the movement of pressurized fluids (eg., BGVN 20:08).

According to MVO, using M_L offers possible advantages when calculating cumulative VT energy. The Gutenberg-Richter magnitude-energy relationship portrays an earthquake's size based on the amplitude of the resulting waves recorded on a seismogram. The concept is that the wave amplitude portrays the earthquake's size once the amplitudes are corrected for the decrease in magnitude with distance owing to geometric spreading and attenuation (Stein and Wysession, 2003). Local magnitude (often also termed Richter magnitude or the Richter scale). MVO employs the following (base 10) logarithmic equation, which associates ML to cumulative VT energy, E, as follows: Log E = 1.5 × ML + 11.8

MVO notes that this equation is a reliable calculation of cumulative energy, as opposed to amplitude measurements at a single station. Amplitude measurement data are easily affected by variables such as data gaps. As further background, magnitudes can be negative for very small displacements (eg. a small rockfall). Stein and Wysession (2003, p. 263) make the point that seismic magnitude scales are logarithmic, ". . . so an increase from magnitude "5" to "6," indicates a ten-fold increase in seismic wave amplitude. Measured displacements range more than 10 units because the displacements measured by seismometers span more than a factor of 1010." In practice, the amplitude is measured in microns of displacement after the effects of the seismometer are removed. Different magnitude scales (eg., M_L, mb, M_s, M_w, etc.) yield different values (Stein and Wysession, 2003).

References: Cole, P., Bass, V., Christopher, T., Melander, S., Pascal, K., Smith, P., Stewart, R., Stinton, A., and Syers, R., undated, MVO Scientific Report for Volcanic Activity Between 1 May 2012 and 12 October 2012, Open File Report OFR 12-02; Montserrat Volcano Observatory, 47 pp. (URL: http://www.mvo.ms/pub/Open_File_Reports/MVO_OFR_12_02-MVO_Scientific_Report.pdf)

Cole, P., Bass, V., Christopher, Odhert, H., Smith, P., Stewart, R., Stinton, A., Syers, R., and Williams, P., undated, MVO Scientific Report for Volcanic Activity Between 1 November 2011 and 30 April 2012. Montserrat Volcano Observatory, (URL: http://www.mvo.ms/pub/Open_File_Reports/MVO_OFR_12_01-MVO_Scientific_Report.pdf)

Druitt, T. and Kokelaar, B., 2002, The Eruption of Soufriere Hills Volcano, Montserrat, form 1995 to 1999, Issue 21. Geological Society Memoir No. 21. UK: The Geological Society Publishing House, 2002.

Stein, S. and Wysession, M., 2003, An Introduction to Seismology, Earthquakes and Earth Structure, 2003, Blackwell Publishing, Oxford, 498 pp. [ISBN 0-86542- 078-5]

Stewart, R., Bass, V., Christopher, T., Cole, P., Dondin, F., Higgins, M., Joseph, E., Pascal, K., Smith, P., Stinton, A., Syers, R., and Williams, P., (27 May) 2013, MVO Scientific Report for Volcanic Activity Between 13 October 2012 and 30 April 2013, Open File Report, OFR 13-06. Montserrat Volcano Observatory. (URL: http://www.mvo.ms/pub/Open_File_Reports/MVO_OFR_13_06-Six_monthly_report.pdf )

Geologic Background. The complex, dominantly andesitic Soufrière Hills volcano occupies the southern half of the island of Montserrat. The summit area consists primarily of a series of lava domes emplaced along an ESE-trending zone. The volcano is flanked by Pleistocene complexes to the north and south. English's Crater, a 1-km-wide crater breached widely to the east by edifice collapse, was formed about 2000 years ago as a result of the youngest of several collapse events producing submarine debris-avalanche deposits. Block-and-ash flow and surge deposits associated with dome growth predominate in flank deposits, including those from an eruption that likely preceded the 1632 CE settlement of the island, allowing cultivation on recently devegetated land to near the summit. Non-eruptive seismic swarms occurred at 30-year intervals in the 20th century, but no historical eruptions were recorded until 1995. Long-term small-to-moderate ash eruptions beginning in that year were later accompanied by lava-dome growth and pyroclastic flows that forced evacuation of the southern half of the island and ultimately destroyed the capital city of Plymouth, causing major social and economic disruption.

Information Contacts: Montserrat Volcano Observatory (MVO), Fleming, Montserrat, West Indies (URL: http://www.mvo.ms/); Washington Volcanic Ash Advisory Center (VAAC); and Adam Stinton, MVO.

Our previous report from May 2013 (BGVN 38:05) noted Strombolian activity, including volcanic bombs in July 2012 and ashfall and volcanic bombs in April and May 2013. The Vanuatu Geohazards Observatory (VGO) bulletin from 28 May 2013 noted that Yasur's explosive activity had increased slightly, compared to the recent past. The activity included Strombolian explosions (figure 43) and ash and steam plumes. This report discusses activity from June 2013 through February 2014, along with photographs taken in May 2013. A map of Vanuatu and nearby countries was provided in BGVN 35:06.

Figure 43. Strombolian activity from Yasur recorded during May 2013. Courtesy of Volcano Discovery (Dietmar Berendes).

Observations and seismic data from early to mid-August 2013 suggested that explosive activity of the volcano had decreased slightly during that time. Explosions were weaker and less frequent. Therefore, on 29 August 2013, the VGO decreased the Alert Level from 2, where it had been since early April 2013, to 1. Level 1 (on a scale of 0-4) indicates "increased activity [but] danger near crater only". From 29 August 2013 until at least February 2014, the Alert Level has remained at 1.

Hazard zones at Yasur are indicated in figure 44. VGO has warned visitors that ejected volcanic bombs could hit the summit area, the tourist walk, and parking area.

Figure 44. This danger map ('Denja Map') of Tanna Island containing Yasur volcano shows Red, Yellow, and Green zones to warn visitors and civilians of ashfall and other hazards. Yasur volcano is near the eastern end of the red, highest-risk zone. Map key and title are in a language with phonetic similarities to English that evolved with contact from traders (a lingua franca) but many other languages also remain in use in Vanuatu. Ash could likely fall well W of Yasur due to trade winds from the ESE. This image is of low to moderate resolution and some symbols are illegible. Courtesy of Vanuatu Geohazards Observatory.

Observations and seismic data from early to mid-August 2013 suggested that explosive activity of the volcano had decreased slightly during that time. Explosions were weaker and less frequent. Therefore, on 29 August 2013, the VGO decreased the Alert Level from 2, where it had been since early April 2013, to 1. Level 1 (on a scale of 0-4) indicates "increased activity [but] danger near crater only". From 29 August 2013 until at least February 2014, the Alert Level has remained at 1.

According to John Search, who has led tours of the volcano since 1998, activity increased beginning October 2013. A large ash emission caused widespread damage to vegetation on Tanna Island, and ashfall was reported on Erromango Island, 150 km N of Yasur. On the evening of 3 November 2013, Search witnessed large Strombolian explosions. These explosions ejected volcanic bombs, up to 4 m in diameter, 250 m from the vent, putting visitors at risk. According to Search, the explosions were some of the largest at Yasur since 1995.

On 19 November 2013, VGO reported that a new phase of ash emissions began on 3 November. The explosive intensity remained low.

Geologic Background. Yasur has exhibited essentially continuous Strombolian and Vulcanian activity at least since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Located at the SE tip of Tanna Island in Vanuatu, this pyroclastic cone has a nearly circular, 400-m-wide summit crater. The active cone is largely contained within the small Yenkahe caldera, and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide open feature associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions has raised Port Resolution harbor more than 20 m during the past century.

Information Contacts: Vanuatu Geohazards Observatory, Department of Geology, Mines and Water Resources of Vanuatu (URL: http://www.geohazards.gov.vu); John Seach, Volcanolive.com (URL: Volcanolive.com/Yasur.html); and Volcano Discovery (www.volcanodiscovery.com).