Masafumi Okawara

Landslide Tracks in Kawajiri & Yakeishi-dake Area southwest part of Iwate Prefecture

A large number of landslide track is topographically recognized in the southwestern district of Iwate prefecture, which is situated in the eastern part of the “Green Tuff region”. Understanding the conditions of landslide generation, geological features in the district have been investigated by comparing the distribution map of landslides with the distribution map of gentle slopes, the rostored contour map, and the geological map.Although geologic condition of the district is somewhat peculiar since it had suffered strong mineraliza-tion in addition to the “Green Tuff” alteration, follow ings have been clarified;1) In the case that a flat surface exists on a summit or mountainside of the mountainous area, the flat comes into a recharge area of groundwater. Therefore, where is a slope of low gradient lower in altitude than the flat, the watertable under the slope is raised and become easy to generate landslide.2) In the area in which a potential lifting zone exists, landslides are ready to generate because unstable slopes are formed along the zone.3) In the area where faults or fault zones area are developed, landslides with a linear headscarp, instead of a crescent-shaped scarp, are formed owing to the effect of structural controle in the fractured Neogene strata.4) Strate including layers of muddy rock and/or acidic tuff are ready to cause landslide, because the lalyers are easy to suffer argillization.5) In the studied district, landslides are developed well especially in the area which had suffered deep weathering, geothermal alteration and mineralization, as compared with areas without alteration.