Kijiya, ohdokoro, Itoigawa City, Niigata Prefecture, (36*52'N; 137*51'E).
Size of Slide
Length: 1400 m; Width: 250 to 320 m; Average Depth: 48 m; Area: 42 ha; Volume: 2.0x107m3.
Damages
The slide initiated in April of 1974, and although it is relatively minor, the movement continues to date. Damages within the landslide include 800 m of Prefecture Highway, Irino Daira-Shirauma Road, 8 residential structures, 2 check dams, numerous cracks and deformation of transmission towers and telephone poles.
Geology, Mechanism and Causes of Failure
The Ohdokoro River drainage is situated at Western Fossa Magna. From the upper drainage, the region is underlain by crystalline schist and Mesozoic and Paleozoic non-metamorphosed sedimentary rocks. There are numerous intrusive serpentine bodies forming a serpentine melange zone. Two sets of recent pronounced lineaments traverse through the melange structure. One set is oriented in a NW to SE, NNW to SEE direction, and the other set is oriented diagonally and intersects the first set in a NEE to SWW direction. Further, these basement complex materials are overlain by Quaternary Period volcanic clastic rocks originated in Fubiki-Dake located at the southern end of the drainage. The landslide involves the clastic materials. As for the mechanism of sliding, it has been assessed that mud flow deposits had deposited in a depression of about 800 m in diameter, and the slide plane was formed within the basal portion of the depression which contains abundant clay derived from the serpentine. Surficial landslides have formed on the colluvium (which contains serpentine fragments) where the materials had been supplied from alluvial fan deposits and the slopes above. These smaller surface landslides are concentrated at the tension zone along the right side of Ohdokoro Landslide. Topographically, this area represents a conicle shape which tends to collect water. Furthermore, the higher region of the drainage basin is connected by mudflow deposits, thereby supplying a large volume of ground water to this area.
Mitigation Measures
The focus of the mitigation measures was to control the surficial landslides, and to construct drainage wells near Kijiya Village.
