A Slope Movement by Gravitational Bending of Hard Shale Beds

Takekawa-Osashi Landslide, Southern Hokkaido, Japan

Abstract

In early September 1997, hillside cut for road construction caused small-scale landslide in the northeastern mountains of Matsumae peninsula, Hokkaido. The slope movement is characterized that bedded hard shale bent valleyward and back-facing steps occurred on the cut-slope. Morphology and geology of the slope indicate that gravitational bending fold had already formed in the Miocene hard shale formation before breaking out of the movement. Therefore, it seems that the landslide was gravitational fold rapidly proceeded by unloading of the lower slope. One of the geological causes of the movement is existence of fine acid tuff, includes smectite clay and been easily changed to slip surface by bending, interbedded in the hard shale. Thick and hard shale intimately developing small joints is relatively flexible to deformation.