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Report on Suwanosejima (Japan) — 18 August-24 August 2021


Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
18 August-24 August 2021
Managing Editor: Sally Kuhn Sennert

Please cite this report as:

Global Volcanism Program, 2021. Report on Suwanosejima (Japan). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 18 August-24 August 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (18 August-24 August 2021)



29.638°N, 129.714°E; summit elev. 796 m

All times are local (unless otherwise noted)

JMA reported that explosions at Suwanosejima's Ontake crater were detected on 19, 20, and 21 August. The first explosion at 0137 on 19 August produced an ash plume that rose 3 km above the crater and drifted NE, followed by another at 1613 that generated an ash plume 2.2 km above the crater and drifted N. A small amount of ashfall was reported in Yakushima, Nishinoomote, and Nakatane. A third explosion at 2059 that day produced an ash plume that rose 2.5 km above the crater and drifted N; ashfall was reported in Toshima village (4 km SSW). Explosions at 0628 and 0713 on 20 August generated ash plumes that rose 2.5-3 km above the crater and drifted N, resulting in ashfall in Toshima village, with smaller amounts of ash in Yakushima, Mishima, Ibusuki, Minamikyushu, and Makurazaki. On 21 August at 0617 an explosion generated an ash plume that rose 3.2 km above the crater and drifted N. A large amount of ashfall (over 1 mm) was reported in Toshima village and smaller amounts (less than 0.1 mm) were reported in Makurazaki, Minamisatsuma, Minamikyushu, Kagoshima, Ibusuki, and Hioki. A second explosion followed at 0906 that produced an ash plume 3.2 km above the crater that drifted N. The Tokyo VAAC reported ash plumes to 1.5-3.9 km (5,000-13,000 ft) altitude that drifted NE during 18-25 August. The Alert Level remained at 2 and the public was warned to stay 1 km away from the crater.

Geological Summary. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed 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 the horseshoe-shaped Sakuchi caldera, which extends 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.

Sources: Japan Meteorological Agency (JMA), Tokyo Volcanic Ash Advisory Center (VAAC)