The history of slope failures during the last 300 years was investigated by a tephrochronological method in central Hokkaido. Two air-fall ashes were used for key layers: U
s-
c in A. D. 1663 and Ta-b in A. D. 1667. We researched the topsoil profiles of the slopes in the dissecting valleys of marine terrace in Hidaka district, where numerous slope failures occurred in 1981. The failure slopes were classified into three types with respect to the ash distribution: A, B and C types. The A type represented the failure slopes which lacked ash layers, and the B type represented those overlying ash layers. The C type was complex of the A and B types. Two-thirds of the investigated slopes were categorized as the A and C types. It was obvious that most of the failures occurred on the slopes where previous failures had occurred during the last 300 years. An aerial photo taken in 1947 indicated that most of the failures in 1922 recurred in 1981. These results show that the slope failure has a tendercy to recur in several decades without obtaining a long term immunity.
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