Report on Aira (Japan) — July 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 7 (July 1983)
Managing Editor: Lindsay McClelland.
Aira (Japan) Explosions, tephra emission, and seismicity
Please cite this report as:
Global Volcanism Program, 1983. Report on Aira (Japan). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 8:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198307-282080.
31.593°N, 130.657°E; summit elev. 1117 m
All times are local (unless otherwise noted)
In June, 33 explosions were recorded from the summit crater of Minami-dake and 31 in July. Although the explosion rate from June to mid-July was above its usual level, rarely was a large amount of ejecta observed in any explosion. Only about 1/8 of the explosions ejected much lapilli, or produced eruption columns that rose to more than 2 km above the summit.
Activity intensified slightly 19-24 July. Most explosions produced large amounts of ejecta and ash frequently fell on the cities of Miyakonojo (40 km ENE) and Miyazaki (80 km NE). The end of explosive activity on 25 July was followed by continuous ash ejection. Bad weather limited visual observation, but volcanic tremor that was assumed to be accompanied by ash ejection was recorded until 29 July. The number of large B-type earthquakes suddenly increased at about 1800 on 29 July and remained high until 0300 the next day. Earthquake size then returned to its usual level, but the recorded events were still more numerous than usual.
Explosive activity resumed on 31 July, accompanied by a decrease in seismic activity. An explosion at 1445 on 2 August ejected large amounts of lapilli, which fell near Kyoto University's Sakura-jima Volcano Observatory (about 1.7 km SW of the summit) and the site of sand trap wall construction, where 1 worker [originally reported as 4] was slightly burned. [Blocks] made many craters [near the University Observatory and the construction site]; the largest was 1.5 m in diameter and 1 m deep [produced by a block 50 cm in diameter].
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.