1978年5月18日妙高災害(1) : 斜面崩壊について

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  • Cause of landslide occurred at Mt. Myoko on 18th May, 1978

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1) At about 6:20 a.m. on 18th May, 1978, the slope area of 1.3ha, located at 1,600 m above the sea level, was suddenly broken and a mass of about 200,000 m^3 were sliden into the valley of river Shirotagiri-gawa (Loc. 1 in Fig. 1). 2) The broken slope consisted mainly of surface materials which can be subdivided into permeable and channel which are composed of altered andesitic tuff breccia, as shown in Figs.11, 19 and 20. 3) The groundwater of landslide site can be subdivided into two types; one is shallow water in buried river deposits (A_2-type) and another is fissure water involved in altered bed rock (A_1-type) as shown in Figs.8 and 20. The tritium content in both types of water is nearly the same as that of recent rain water. 4) The surface materials seem to have been moved sometimes in the past, and as a result of such occasional movement, the crack which coincides with slide surface would be originated in the talus deposits, as shown in Fig.1 of plate III and in Fig.17. 5) It is strongly suggested that the infiltration of large amount of water, which was supplied from rapid snow melting at unusual high temperature in that season, might increased the pore water pressure in the surface deposits to break the above-mentioned unstable state of slope, and the landslide was brought about in succession.

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