Report on Asamayama (Japan) — August 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 8 (August 1981)
Managing Editor: Lindsay McClelland.
Asamayama (Japan) Earthquake swarm but no eruption
Please cite this report as:
Global Volcanism Program, 1981. Report on Asamayama (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198108-283110.
36.406°N, 138.523°E; summit elev. 2568 m
All times are local (unless otherwise noted)
On 10 August at about 1000, seismic instruments 1.8 km S of the crater recorded a sudden increase in the number of local earthquakes. The number of recorded events reached 276 on 10 August, and 230 were registered the next day. Seismicity began a gradual decline on 12 August and had dropped to the normal level of about 10 events/day a few days later (figure 3). No eruptive activity was observed, but vapor emission continued and JMA increased monitoring at its [Karuizawa Weather Station].
There have been several periods of increased seismicity at Asama since its last eruption in 1973.
Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the horseshoe-shaped remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asama-yama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has an historical record dating back at least to the 11th century CE. Maekake has had several major plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.
Information Contacts: JMA, Tokyo.