Landslidesin the Abashiri-Kitami-Tsubetsu district，Hokkaido，Japan（part 5）；Characteristics of landslide configurations and recent landslides in the Tsubetsu district

摘要

In the Tsubetsu district which is comprised in a topographic map （1: 50,000）of Honki and located in the southern part of the Abashiri-Kitami-Tsubetsu landslide concentration region，389 landslide configurations were identified in aerial photographs of 1 : 20,000 and 1 : 40,000 in scale. The features used for interpreting landslide configuration from the photographs are the presence of main scarp and its landslide debris. Dissected main scarp and vegetation in the landslide configurations show that most of them were formed in prehistoric times，but some recent landslides have caused damage and recent landslide scars with main scarp lacking in vegetation were also recognized.　Small to large-scale landslide configurations are concentrated in the eastern area of the north-south Futamata Fault which is underlain by the Oligocene Tatsukobu and Miocene Tsubetsu Formations, which consist mainly of alternating beds of shale and soft siltstone and mudstone with interbeds of tuff, and which is featured by the prevalence of folded structures associated with minor faults. This probably results from the pelitic rocks which are prone to disintegrate due to weathering，from the intercalated tuff beds which easily change into clay and probably correspond to slip surface, and from the rocks which have been crushed by faulting and folding. Moreover，small -to intermediate-scale landslide configurations are found along the Futamata Fault and are found sporadically in the western area which is composed of : the Jurrasic to Cretaceous Nikoro Group，which consists of basaltic volcanic rocks with the intercalation of chert and limestone ； the Eocene to Oligocene Rikubetsu Formation，which consists mainly of conglomerate ； and Oligocene andesitic lavas.　Most small scale landslide configurations show forms of debris and soil slump （A type）. In addition to these featurs，large-scale landslide configurations show shapes the rock slump (B type) and slide (D and E types), and show the remnants of repeated sliding features. Large-scale landslide configurations were mostly found along axes of folds and faults on the dip-slope in the area of the Tatsukobu and Tsubetsu Formations. The directions of land-slide debris movement in the district are mostly northeast to southeast and west. Pertial or complete removal of older landslide debris caused recent landslides disasters, such as the Honki landslide and the Route 273 landslide, and landslide configurations, regarded as recent landslide scar. Therefore, it is necessary to be aware of the presence of landslide configurations to prevent landslides and to reduce damages.