Atsuhiko KINOSHITA, Keita YAGI, Tsutomu YAMAZAKI, Takayuki MAYUMI, Tatsuya SHIBASAKI, Masateru KAWASHIMA and Hidetoshi HASHIMOTO
In the Rokko Mountains, located in Southwest Honshu Island in Japan, the crush zone of the active faults are 1.5kilometers in maximum width. The fault process zone in the surrounding slopes is 2km in maximum width. Furthermore, the moderately]sloping upper mountains consist of rock mass with very little effect of faulting. Severely weathered granite has hardly developed in the crush zone as well as in the process zone. Instead, block failures and topples are the dominant slope failure modes developed within the weathered rocks where the fault system predominates displacement processes in the Rokko Mountains. Dominant fault patterns found in the Rokko Mountains are an active right]lateral fault and a subsidiary Riedel shear fractures propagating out of the main fault. Hence, vertical joint system parallel to the main fault has developed in the vicinity. In fact, the weathered rock mass becomes structurally unstable when steep slopes in the crush zone or process zone have a gradient of 35degrees or more which is highly vulnerable to failure. In the crush zone and process zone , slopes with gradient of 35degrees are widely distributed because the rock mass adjacent to the faulting is relatively weak. Accompanied by the abrupt uplifting on the northwestern side of the main fault, frequent downcutting of antecedent streams has formed steep slopes in this area. This is indicated by the rock mass near the faulting being structurally vulnerable and the joint system having densely developed which in turn creates favorable conditions for development of fault valley. In such slopes, weathered rock mass consisting the crush zone or the process zone is easily unstablized and difficult to form weathering crust while sediment production is potentially greater in weathered rock mass.
Key wordsFRokko Mountains, character of basin, rock failure, active fault, Riedel shear