Report on Shikotsu (Japan) — March 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 3 (March 1981)
Managing Editor: Lindsay McClelland.
Shikotsu (Japan) Seismicity declines without expected eruption
Please cite this report as:
Global Volcanism Program, 1981. Report on Shikotsu (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 6:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198103-285040.
42.688°N, 141.38°E; summit elev. 1320 m
All times are local (unless otherwise noted)
Local seismicity began to increase in November 1980 and the number of events per month reached 1,211 in February 1981 (figure 3). Seismicity began a gradual decline in early March and by mid-March had reached the usual average of fewer than three recorded earthquakes per day.
|Figure 3. Monthly numbers of days with eruptions (top), tremor events (center), and recorded earthquakes (bottom) at Tarumai, January 1978-March 1981. Courtesy of JMA.|
Only 87 events were recorded in March. Although the December 1978-May 1979 eruption accompanied the last major increase in seismicity, no eruption has occurred during the current, much larger increase [but see 06:04].
Geologic Background. The 13 x 15 km Shikotsu caldera, largely filled by the waters of Lake Shikotsu, was formed during one of Hokkaido's largest Quaternary eruptions about 31-34,000 years ago. The small andesitic Tarumaesan stratovolcano was then constructed on its SE rim and has been frequently active in historical time. Pyroclastic-flow deposits from Tarumaesan extend nearly to the Pacific coast. Two other Holocene post-caldera volcanoes, Fuppushidake (adjacent to Tarumaesan) and Eniwadake (on the opposite side of the caldera), occur on a line trending NW from Tarumaesan, and were constructed just inside the caldera rim. Minor eruptions took place from the summit of Eniwadake as late as the 17th century. The summit of Tarumaesan contains a small 1.5-km-wide caldera formed during two of Hokkaido's largest historical eruptions, in 1667 and 1739. Tarumaesan is now capped by a flat-topped summit lava dome that formed in 1909.
Information Contacts: JMA, Tokyo.