1. Introduction
The Taisetsu mountains consists of many quaternary volcanic
mountains over 2000 meters. The eruption of about
30,000 years ago formed Ohachi caldera and the last eruption
is estimated about 500 to 600 years ago forming the
explosion crater at the western flank of the mountains. The
steaming activity of Mt. Asahidake is still active at present.
The 1988 eruption of Mt. Tokachidakewas phreatomagmatic
with a pyroclastic flow of about 1 km. In case of the 1926
eruption the massive debris/mud flow caused a large disaster.
The volcanic activity of Mt. Tokachidake is in the alert
watch level (Monthly Report on Earthquakes and Volcanoes
in Japan, 2000).