The features of the landslide distribution and assessment of landslide susceptibility in Japan

Many landslides occur in the place which the landslide generated in the past, or its surrounding area. The causes are considered to be formation of slipping surface, the moving mass which becomes vulnerable by deformation or destruction and geological structures in which a slipping surface is easily formed. Therefore, it is very important for prevention and mitigation of the landslide damages to create the landslide inventory map which is shown the place which the landslide generated in the past. National Research Institute for Earth Science and Disaster Prevention (NIED), Japan, have published the landslide inventory map "landslide distribution maps" for preventing and mitigating landslide disasters. The landslide distribution map have mapped the 380,000 or more landslide topographies in whole Japan by interpretation of aerial photographs. The individual landslide not less than 150 m wide is drawn in the landslide distribution map. The objects of this research are to clarify geological and geomorphological features of landslide distributions by analyzing the landslide distribution map and to make the landslide susceptibility map for the assessment of landslide in whole Japan. The landside distribution in whole Japan is not equal and there is a difference in the density. I propose the method of the wide area landslide assessment used by the features and distributions according to of geological setting. I calculate the landslide body ratio in each geological unit. The landslide body ratio is that the rate of the landslide body area in each geological unit and the whole area in each geological unit. The landslide body ratio can be considered that landslide susceptibility (occurrence probability of landslides) in each geological unit. As a result, an average of the landslide body ratio is about 5.2 % in whole Japan. The area consist of the accretionary complex based on volcanic rocks and plutonic rocks have comparatively high-risk landslide susceptibility, and the area of Neogene rocks and Paleogene rocks have the high-risk too. On the other hand, the area of plutonic rocks and Quaternary rocks have low-risk landslide susceptibility. The results show that the landslide susceptibility is greatly different according to geological unit. Landslide body ratio map