長野県小諸市南方の山浦崩壊 : 千曲川ぞいの古期大規模マスムーベメント  [in Japanese] The Yamaura Collapse south of Komoro City, Nagano Prefecture : A Large Prehistoric Mass Movement along the Chikuma River  [in Japanese]

The Yamaura Collapse was discovered during a geological survey of the Komoro Group in 1980 by the present authors. The width of the Collapse is approximately 1.5 km with the estimated aerial extent of the collapse debris being 1.8 km^2. The debris which resulted from the mass movement extend from the eastern margin of Mimakigahara Hill to the eastern side of the present Chikuma River and is preserved as a thick deposit of sediment. The volume of the debris is estimated to range from 3.6×10^7 to 5.4×10^7 m^3 and is therefore regarded as a large scale debris collapse. This Yamaura Collapse is considered to be of primary mass movement origin, and its time of collapse is estimated to have occurred during the Heian period (1000-1200 AD). The cause of the collapse is interpreted to have been the direct result of gravitational unstability initiated by the uplift of Mimakigahara Hill (elevation 750-850 m). The mass movement mechanism is thought to be similar to the Shuroku-san landslide described by Fujita (1984). Some evidences supporting this mass movement mechanism hypothesis are -1. the fact that the mass movement is located just east margin of Mimakigahara Hill, 2. the main scarp of the collapse is high and nearly vertical, and 3. the inclination of the basal plane of the collapse is at a low angle.