Prediction of potential landslide sites on Shirasu steep slopes applying the landslide periodicity

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The purpose of this study is to prepare a practical map for predicting high potential shallow landslide sites on Shirasu (pyroclastic flow deposits from Aira volcano) steep slopes based on the landslide scar chronology and topsoil depth on the slopes and the landslide periodicity in the southern part of Kyushu, Japan. The results are as follows :1. A large scale topographic map of the study area was made based on skeleton and detail topographical surveying using the global positioning system (GPS) and electro-optical survey instrument, respectively. Each map of the shallow landslide scars, topsoil depth and vegetation was superimposed on the topographic map.2. The forest vegetation on younger landslide scar of less than 60 years in age with thinner topsoil and older landslide scar of more than 60 years in age with thicker topsoil differ from each other in tree species composition and plant succession. This suggests that the high potential shallow landslide sites can be indirectly predicted through distinction of the vegetation grown on the slopes.3. It is possible to distinguish between the forest vegetation on the younger landslide scar and the one on the older scar based on the image analysis using large-scale aerial photographs.4. The Shirasu slopes in the study area were divided into six parts based on the slope morphology. Of these parts, a part consisting of steepest slopes of over 40 degrees in gradient with a form of concave or straight was selected as the high potential shallow landslide site. This part with steep slopes are widely distributed on the slopes along ravines and river channels. A most parts of the younger and older shallow landslide have been scarred on the high potential shallow landslide sites predicted based on the division of slope morphology.