- Tokyo Geographical Society
- 地学雑誌 (ISSN:0022135X)
- vol.86, no.6, pp.346-363, 1977-12-25 (Released:2009-11-12)
Detailed and intensive investigations have been carried out on the abrupt failure of submarine slope of a small island, Shin-jima, in Kagoshima Bay, Kyushu on April 1, 1975 by the joint research group of Geographical Survey Institute of Japan, Japan Maritime Safety Agency and Geological Survey of Japan under the sponsorship of Japan Science and Technology Agency.The Shin-jima island emerged from water at the great eruption of adjacent Sakurajima Volcano in 1779 to 1780 (An-ei Eruption) as one of central corns of Aira Caldera (MATSUMOTO, 1933). The island gradually reduced its exposed area by continuous wave erosion to form truncated configuration, being surrounded by broad, shallow, flat tidal bench.Geologically the island is composed of the latest Pleistocene to the earliest Holocene thick loose pumice deposits with intercalations of silt and clay showing inner bay facies. These formations are strongly faulted by eastwest faults. The surface of the island is covered with thin shell bed dominantly composed of Ostrea and andesitic lava. The flat bench around the Shin-jima consists of recent, loose pumiceous drift sand deposits derived from the island by wave erosion.The failure of the slope now in concern occurred at the spit of the southern tip of the island, where thickness of drift sand deposits attains at most about 35 meters according to sonic prospecting and borings at the spit. It was caused by submarine landslide in the thick, loose drift sand deposits. Volume of the slide mass amounts approximately 265, 000 cubic meters.The spit had formed steep submarine slope near critical angle by abundant supply of very loose drift sand from northern part of the island before the failure occurred. Neither sensible earthquakes nor volcanic activities of the Sakurajima which might have given significant effects to submarine landslide are observed and recorded in seismometer around the crucial moment of the failure.Therefore, the cause of the landslide is considered that the increase of load resulted from the deposition of drift sand attaining about 8, 400 tons in volume atop of the slope brought about decrease of safety factor of the slope to make it slide. Wave pressure seems to have triggered the slide because safety factor decreases about 20 % when fluctuation of pressure at the bottom caused by wave agitation is taken into account.