DEEP-SEATED ROCKSLIDE-AVALANCHES PRECEDED BY MASS ROCK CREEP OF SEDIMENTARY-ROCKS IN THE AKAISHI MOUNTAINS, CENTRAL JAPAN

Seven large rockslide-avalanches of sedimentary rocks are known in Japan. Almost all of these major landslides were preceded by gravitational deformation of rocks (mass rock creep, MRC), as inferred from geologic and geomorphic investigations of five rockslide-avalanches in the Akaishi Mountains. The MRC that preceded the rockslide-avalanches in the Akaishi Mountains was characterized by a slow but steady downslope bowing of steeply dipping foliations, which accompanied intensive deformation and fragmentation of the rock mass by shearing along foliations. This deformation and fragmentation of the rock mass provides a basic cause for the occurrence of rockslide-avalanches, which occur when a creeping rock mass has lost its support from the lower part of a slope by stream erosion. This type of MRC continues for a long time after a part of the creeping area has slid and results in long-term generation of debris by the landslide scar of the rockslide-avalanche.