Volcanic tremor was registered for six minutes starting at 1810 on 5 March by the JMA station 4.1 km WSW of the crater. During this activity, two main vents opened on and near the S side of Showa 4-nen (1929) crater. A line of vents extending ~200 m N-S formed on the S part of the crater floor. Strong eruptive activity was observed until 7 March and then decreased. Volcanic earthquakes had increased somewhat prior to the eruption, but seismicity remained low afterwards through mid-April.

Geological Summary. Much of the truncated Hokkaido-Komagatake andesitic volcano on the Oshima Peninsula of southern Hokkaido is Pleistocene in age. The sharp-topped summit lies at the western side of a large breached crater that formed as a result of edifice collapse in 1640 CE. Hummocky debris avalanche material occurs at the base of the volcano on three sides. Two late-Pleistocene and two Holocene Plinian eruptions occurred prior to the first historical eruption in 1640, which began a period of more frequent explosive activity. The 1640 eruption, one of the largest in Japan during historical time, deposited ash as far away as central Honshu and produced a debris avalanche that reached the sea. The resulting tsunami caused 700 fatalities. Three Plinian eruptions have occurred since 1640; in 1694, 1856, and 1929.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.