Large-scale landslides strongly governed by the Himekawa melange

Abstract

The Himekawa melange is composed of serpentine sheets overthrusted into Mesozoic-Paleozoic rock. This melange has been classified as geotectonic and is associated to a serpentine matrix. The O'dokoro landslide is found within this geological setting. The slip surface of this landslide was formed by transformation of the existing deep seated overthrust fault at a depth of about 60 to 100 m from ground surface. The slickensided slip zone surface is formed by a mixture of the original fault-clay gouge and the clay resulting from transformation along the fractured zone. Fault-clay material is dominating and showed a low residual strength in the presence of high effective normal stress, at preferred clay mineral orientation. Fractured zone-clay in turn, exhibited high residual strength. The presence of preferred-oriented clay minerals originated from nonserpentinized rocks is considered indispensable to the occurrence of large-scale landslides.