著者

山岡 耕春 井田 喜明 山科 健一郎 渡辺 秀文

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.34, no.4, pp.263-274, 1989

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Mt. Mihara of Izu-Oshima Volcano exploded on November 16, 1987 after one year's rest of its eruptivity. The eruption followed an intensive activity of earthquakes at the summit which began in January 1987. We studied the earthquakes and their mechanisms with a closely installed seismograph network around the summit. We found that most of the earthquakes occurred very narrow portion just below the summit crater with radius of 300 m. Their focal depths are shallower than 100 m above sea level. Most of the focal mechanisms are of normal types. The seismic activity ceased after this event. These facts show that the earthquakes were caused by small break of the bottom of the crater which was filled with the lava of the 1986 eruption, probably due to the load of the lava. The event on November 16 was the final break of the bottom which supported the load of the lava above it. The explosion on November 16 was caused by the sudden fall of both the bottom and the lava above it. The pressurized gas in the cavity below the bottom moved explosively upward in exchange of the falling body.