Presently Active Landslides in an Ancient Primary Slide Mass at Higashinomyo, Niigata, Central Japan

Abstract

The present Higashinomyo landslide, which began about 200 years ago, is a partial reactivation (secondary landslise) in an ancient primary slide mass which was formed before about 50 thousands years in the Late Pleistocene. The primary slide involved a surface area of about 2 km2 and an estimated volume of 108m3. The presently active surface area is 0.34 km2 at the center of the primary slide area with an estimated volume of 107m3. The most recent prior active phase of the secondary landslide at the site is 36 thousands years ago during the jomon period in the Holocene. The history of this slide suggests intermittent activity between relatively long dormant periods.The present slide movement has multiple slip surface layers. The shallow movement in the coluvium is of the creep type, and the deep movement in the ancient slide mass, which is composed of the Miocene marine mudstone, is intermittent activity at intervals of several years.The Higashinomyo landslide has often formed slide dams in the valley of the Aburuma-gawa River throughout its history. The last dam was formed by the 1783 landslide.