Investigations of the geology of Matsuhodo slide area consisting of Ogureyama and Sugiyama formations, which belong to sedimentary mineral assemblages of the Neogene Tertiary, have shown that Neogene Tertiary layers are formed from the upper to the Iower by the diagenesis of the two facies;
Facies 1: (montmorillonite)-clinoptilolite-quartz·plagioclase-cristbalite.
Facies 2: montmorillonite (chlorite)-(sericite)-analcite. (clinoptilolite)-quartz·plagioclase-calcite.
The montmorillonite zones, so-called geologic weak zones, are formed from the surface to some dozen meters depth by weathering. And it is clear that the seepage pressure is liable to cause the weak zones to slide along the sliding surfaces.