新潟県西頸城郡青海町玉ノ木地すべりの要因  [in Japanese] Tamanoki Landslide in Ohmi-machi, Nishikubiki-gun, Niigata Prefecture  [in Japanese]

On February 15, 1985, a landslide occurred along the steep slope near Tamanoki facing the Japan Sea. The locality is close to the Niigata-Toyama prefectural border. The landslide brought about a great disaster, by which ten people were killed, four were injured either seriously or slightly, and five houses were completely destroyed. The characteristics of this landslide and the significance of the investigation result in view of disaster prevention are as follows: 1. The Tamanoki landslide is different from the landslide frequently occurring in the Grecn Tuff region on the Japan Seaside. In the first place, the former took place where the landform is steep unlike the case of the latter. In the second place, it occurred in the area where mudstone is not much developed. The great disaster, especially in human life, might be ascribed to the steep landform. The landslide in the Green Tuff region, on the other hand, occurs where the landform has a gentle slope, so that the speed of landslide is slow and the casualties are small. 2. The causative factor (internal factor) of the Tamanoki landslide is the plane of cracks left by an older landslide which existed at the back of the slipping cliff. Namely, a re-sliding took place along this plane. From the fact that the trees which stood at the foot of the slope since 260-270 years ago have fallen, it is clear that the older landslide responsible for the Tamanoki landslide had occurred prior to 270 years time. This causative factor has the following background; that is, the slipping cliff is composed of loose talus deposits which had been formed in a much older age and came to be cracked by the old landslide. The old talus deposits have turned to red soil which was formed supposedly 200,000〜300,000 years ago, so the deposits must have been laid down at least earlier than 200,000〜300,000 years ago. 3. It was generally considered that no landslide would take place in the Tamanoki area. This is because a notion of reactivation of older landslides, as stated in 1, was lacking. The area has many traces of old landslides, indicating a possibility of occurrence of new landslides in the fufure.