地震によるランドスライド発生予測図 : その現状と問題点

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The method of mapping for seismic slope stability was introduced by Wilson et al. (1979). They wrote critical acceleration (ac) ac=g[(c/γh)+(cos θ tan φ-sin θ)], where slope angle (θ) in the equation can be obtained on slope maps and cohesion (c), friction angle (φ), unit weight of landslide material (γ) and thickness (h) are estimated on engineering geological maps.This method was applied to Nashimoto area in Izu peninsula. The area consists of three rock types: tuffaceous sandstone, andesite of Tertiary, and basaltic lava of Quaternary Period. The slopes in this area were classified into four ranges (>50°, 50°-40°, 40°-30°, <30°). Slopes steeper than 40° in the region of tuffaceous sandstone and more than 50° in andesite region have high susceptibility. Moderate susceptibility appears at slopes from 40° to 50° in andesite region and more than 50° in basaltic lava region. These estimates almost accord with the acutual data. The improvement of engineering geological maps, the problem of knick points and so on are suggested as mooted points of this method.
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