Genesis of the Landslide Topography at Kamiya, Higashi-Kubiki Area, Niigata Prefecture

Abstract

In the Higasi-Kubiki Mountains, Niigata prefecture, the well-known Kamiya Landslide about 20 km east of Takada City of which miocene Teradomari black shale is the only constituent has been active for the last several centuries, although the exact year of the beginning of the movement is not known. The crown of the slide has been regressing year after year by sliding and/or fall mainly in the period of snow melting. The size of the mass which continues to move is about 1400 m long, 400 to 500 m wide and about 20 m deep at the present time.
On the upper half of the slide, there are three scarps (upper, middle and lower scarp) and two terraces (upper and lower) between those scarps, as shown in Fig. 1. Each terrace surface dips toward mountain side, being different from the general slope of the slide, as shown by the cross section in Fig. 2.
Without any fixed evidence have many authors believed that the topographic feature of terraces was formed by rockslides which are termed “sô-suberi” in Japanese based primarily on the assumption that the valleyward-dipping bedding plane is coincident with topographic surfaces of the middle and fower scarps. In other wards, they have assumed that the two scarps are themselves true surfaces of rupture (or planes of sliding). This article concerns with revision of such imagination.
From the results of geologic survey at the slide and its vicinity, dip of the bed rock which is composed of miocene black shale including very thin tuff layers was expected to be about 30 degrees or more for WNW at the middle scarp of the slide. The field evidence at the spot, however, sohows a remarkably lower dipping plane (15°N) in the direction of N 60°W against many authors' conjecture and the above-mentioned expection, as shown in Fig. 3.
This fact suggests that the mass below terraces took backward rotations with little internal disturbance.
With the aim of confirming this suggestive fact of the slump, physical prospecting on the surfaces of rupture below the upper and lower terraces was made by means of electro-resistivity method. As a result of the prospecting, the surfaces of rupture are presumably concave below these terraces, as illustrated by dotted lines in Fig. 2. Furthermore, the slump blocks seem to take little downslope movement. Consequently, the terrace features of this fandslide were concluded to be formed by backward rotation to which the terms “slump (Sharpe, 1938, Crandell, 1954), slump-earthflow (Jones et al, 1961, Vernes, 1958), treva block (Reiche, 1937), shearing slide (Terzaghi, 1950) and Felsstürze (Heim, 1930)” have been already given.
It could not be clarified whether the movement at the present time (4-5m in a year at the lower half) which is considered as a plastic flow has a relation to the movent forming the terrace topography, This slump was not caused directly by excavation from the toe of original valley wall, because lateral erosion by the revulet Hirakatagawa takes piece far away from the slump blocks.