Landslidesin the Abashiri-Kitami-Tsubetsu district，Hokkaido，Japan（part 4）；Characteristics of landslide configurations and recent landslides in the Abashiri-Memambetsu district.

摘要

In the topographic maps (1 : 50,000) of Abashiri and Memambetsu, which are situated in the northern part of the Abashiri-Kitami-Tsubetsu landslide concentration region， a total of 497 landslide configurations were identified in aerial photographs of 1 : 40,000 to 1 : 20,000 in scale. Most of these configurations are concentrated in the northern part of eastern hilly area which is underlain mainly by the Miocene Notoro Formation， which consists of shale with interbeds of tuff and tuffaceous siltstone，and in the western mountainous area which is underlain by the Oligocene Tokoro Formation， which consists of shale， siltstone and sandstone associated with tuff，and by the Notoro Formation. This is presumably related to the pelitic rocks which disintegrate easily into sizes from small fragments to as fine as mud by weathering，to the clayey tuff beds which tend to become slip plane，and to the rocks which have been crushed by faulting and folding. Throughout this district most landslide configurations show the forms of small-scale debris and soil slump (A type)，slides(C and F types)，and flow (G type).Moreover, some intermediate-and large-scale landslide configurations, which show the shapes of rock slump (B type)and slides (D and E types)，and the remnants of repeated sliding features, were also found in the areas of the Tokoro and Notoro Formations. The directions of landslide debris movement are mostly ENE to SE and NW to SW. Some recent landslide damage， such as from the Misaki, the Katsura-machi, and the ubaranai Dam landslides, was caused by partial or complete removal of older landslide debris， triggered by natural and artificial factors. Therefore, much attention should be paid to the presence of landslide configurations from the viewpoint of landslide disaster prevention.