Koichi Mochizuki

Landslide and Geological Zoning in the Kanto District and itsvicinity

Attempts have been made from previously by a number of investigators to classify geologically the occur rence conditions and the characteristics of landslides in Japan. In this report, landslides in the Kanto dis trict and its vicinity (Kanagawa, Gunma, Chiba, Yamanashi and Nagano Prefectures) are investigated from the viewpoint of the geological zoning and the occurrence conditions of landslides. Landslides triggered by the continual creep movement of earth and avalanche—type landslides suddenly caused by the earthquake are studied simultaneously. Landslides due to the earthflow induced by a localized torrential downpour are excluded from this study. This report consists of a part of the Geological Classification Committe reports in the Japan Society of Landslides.

Subsurface Valleys and Landslide

The hill-land along the Sai River in Nagano Pref. is frequent in landslide. The upper part of the hill-land often contains the pyroclastic rock layers. When the valleys dissecting the hill-land is buried by dissolved pyroclastic rock, the landslide activities on and around the subsurface valleys are as characteristic as observed at Nishigawara and Chausuyama landslide areas in Nagano-city.1) Deposits filling the subsurface valleys scarcely slide secondarily; the landslide often transit and extend outward of subsurface valleys.2) Subsurface valleys may be the veins of ground-water, which promotes the sliding activity in and around subsurface valleys; especially, large-scale landslides are liable to occur at the terminal of subsurface valleys.3) Around subsurface valleys, the mountain streams show such unnatural phenomena as the discontinuity of valleys, the disappearance of the heads of former valleys or the new tributaries formed on both sides of subsurface valleys.