Report on Aira (Japan) — February 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 2 (February 1983)
Managing Editor: Lindsay McClelland.
Aira (Japan) Increased explosive activity; rain-caused debris flows
Please cite this report as:
Global Volcanism Program, 1983. Report on Aira (Japan). In: McClelland, L (ed.), Scientific Event Alert Network Bulletin, 8:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198302-282080.
31.593°N, 130.657°E; summit elev. 1117 m
All times are local (unless otherwise noted)
The rate of explosions at the summit crater of Minami-dake has gradually increased since December. In January, 53 were recorded and in February, 73 explosions were recorded, one of the larger monthly totals since the eruption began in 1955.
Ashfall and eruption clouds were observed on 11, 14, and 27 February at Miyakonojo Observatory. The air shock from the explosion at 2241 on 5 February was large enough to be felt at Miyazaki Observatory, 80 km NE. On 18 February a hut at Arimura, 3 km SSE of the summit, was set on fire by an incandescent block, 50-100 cm in diameter. The explosion at 1043 on the 21st was not large enough to be accompanied by any observed explosive sound or felt air shock at Kagoshima Observatory but strong NW winds carried lapilli toward the SE foot of the volcano where 4 car windshields were cracked or broken. Two incandescent columns, rising about 200 m above the crater, were observed in February for 10 seconds on the 1st, and for 5 seconds on the 27th.
Rain on 2 February triggered debris flows in S flank valleys. One flowed into nine houses and a hotel after pushing away a 10-m-long sand-trap wall, and covered the adjacent road for about 50 m. The monthly number of recorded seismic events was 4,456 in January, but decreased to 2410 in February.
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 Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical eruption took place during 1471-76.
Information Contacts: JMA, Tokyo.