徳島県川井近傍の地すべりの地質構造規制
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In the present paper, from the geological and geomorphological standpoints we discuss the landslides in the Sambagawa metamorphic belt in the Kawai area, the central part of Tokushima prefecture, which is one of the most famous landslide areas in Japan.In this area, the Sambagawa metamorphic belt is widely ditributed and its stratigraphic sequence in ascending order is as follows:Kashidaira formation; mainly composed of pelitic schist intercalated with basic schist, siliceous schist and calcareous schist.Nonowaki formation; mainly composed of pelitic schist and siliceous schist intercalated with basic schist.This belt is in contact on the south with the Mikabu green rocks or the Kenzan group, the Chichibu Palaeozoic formation, and by the Akui river fault or the Mikabu tectonic line.A large number of landslides in the area occur in the Sambagawa metamorphic belt. Their characteristics can be summarized as follows:(1) Most of the landslides occur in the pelitic schist area, and rarely occur in the siliceous schist area.(2) A sliding body is mainly composed of debris deposites derived from the basement schists.(3) Let θ denote the angle on a horizontal plane between the sliding direction of a landslide and the dip direction of a bedding schistosity plane. Then, most of the landslides move within the range 0°≤θ<90°, and are referred to as the "nagareban" structure type. The landslides within the range 0°≤θ<30° occur in the largest number, and the landslides within the range 60°≤θ<90° have the second highest frequency. On the other hand, there are a small number of landslides moving within the range 90°<θ≤180°, which are of the "ukeban" structure type.(4) The landslides belonging to the "nagareban" structure type are generally large in scale, but the landslides belonging to the "ukeban" structure type are generally small in scale.(5) The slope angles in the area are determined by the Horton method. The slopes in the landslide areas generally range between 25° and 30°, irrespective of the lithofacies of the basement schists.(6) The pelitic schist area often forms slopes of 25° to 30° in slope angle. On the other hand, in the siliceous schist area the slope angles are 35° to 50°, in general. Consequently, from the discussion of slope angle it is concluded that the pelitic schist area has many hazardous slopes appropriate for landslide activity.
- 公益社団法人 日本地すべり学会の論文
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