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 Jap an. Almost all of these major landslides were preceded by gravitationa l deformation of rocks (mass rock creep, MRC), as inferred from geolog ic and geomorphic investigations of five rockslide-avalanches in the A kaishi Mountains. The MRC that preceded the rockslide-avalanches in th e Akaishi Mountains was characterized by a slow but steady downslope b owing of steeply dipping foliations, which accompanied intensive defor mation 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 generatio n of debris by the landslide scar of the rockslide-avalanche.