Report on Asosan (Japan) — June 1990

Bulletin of the Global Volcanism Network, vol. 15, no. 6 (June 1990)
Managing Editor: Lindsay McClelland.

Asosan (Japan) Ash and block ejection; gradual increase then abrupt decrease in tremor amplitude

Please cite this report as:

Global Volcanism Program, 1990. Report on Asosan (Japan). In: McClelland, L (ed.), Bulletin of the Global Volcanism Network, 15:6. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.BGVN199006-282110.

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Asosan

Japan

32.884°N, 131.104°E; summit elev. 1592 m

All times are local (unless otherwise noted)


Intermittent eruptive activity has continued since 16 July 1989 from Crater 1. Eruptive episodes on 3, 5, 6, 7, 12, 13, and 18 June ejected ash clouds and blocks and were similar to those of previous months. Ash ejection was most active on 13 June, as tephra rose continuously to ~1,000 m throughout the day and the month's highest ash cloud rose 3,000 m above the crater. The 18 June activity was also strong, ejecting blocks 300 m W from the center of the crater. Ash accumulation at AWS . . . was 9,713 g/m2 in June, slightly lower than the May total of 12,837 g/m2 that was the heaviest monthly ashfall since October 1989.

Tremor amplitude gradually increased through the end of June after the abrupt drop that followed a larger eruption on 20 April. Short-period and large tremors recorded between 22 June and 1 July were thought to have been generated by small phreatic explosions. The bottom of the crater was occupied by a water pool after heavy rains 14-16 June and 26 June-4 July. Tremor amplitude was very small 1-9 July. AWS issued an "Extra Volcanic Information" on 4 July, warning of the potential for a sudden explosion following the abrupt decrease in tremor amplitude.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 cu km of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 AD. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.