昭和58年(1983年)7月山陰豪雨による斜面崩壊の地質学的問題  [in Japanese] On Geological Characters of Slope Failure by Heavy Rainfall of July, 1983, in the Shimane Prefecture, West Japan  [in Japanese]

The west region of Shimane Prefecture was swept at the morning of the 23th July 1983 by heavy rainfall with maximum intensity reaching 96 mm per hour, which caused a large number of slope failure. The authors discuss the relationship between the forms of slope failure and the geological conditions on the basis of the data of the Research Group of the Department of Geology, Shimane University. As a result, it is shown that the forms of relatively large slope failure are similar in the area with the same geological condition. And then, slope fallure controlled by the geological structures are discussed. At Nakaba, Hamada City, a large scale of slope failure which is 120 meters long and 40 meters wide occurred and killed fifteen persons. This failure is caused by accumulation of ground water migrating along large shear zones and becomes to be of large scale due to thick breccia on the slope. In the west district of Masuda City, the Plio-Pleistocene Tsunozu Formation, mainly composed of gravels, is widely distributed on the hills, unconformably covered the basement rocks which constitute the lower part of the slope. The surface of unconformity gently inclines toward different directions accordingly to heights of the basement, and many slope failure occur at the greatly altered basement rocks beneath the surface of unconformity. In this district, slope failure show a tendency to increase its occurrence at the slope where the surface of unconformity inclines toward the conformable direction to its dip direction. Also, there is a large possibility that the present topography is controlled by the relief of the basement.