Studies for subaqueous and submerged landslides and their relation to tsunamis in caldera areas

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Subaqueous landslides and submerged landslides in caldera areas and their outer rims were investigated to know geologic structures and topographies for them. The important results are following three: First, many sublake landslides in Caldera lake Kussharo, east part of Hokkaido were found, and the high possibility that a landslide related to the tsunami disaster of the 1923 Kussharo earthquake was showed by topographic and bottom material analyses and its combined numerical simulation. Second, an example of collapsed outer rims submerging to the sea, the Nebukawa landslide which induced by the 1923 Kanto great earthquake was investigated, and the affected area of landslide and the shape of landslide body that has been scarcely reported was clarified. Third, the newly acoustic investigating method was developed through these above investigations. This method consists of leisure-use fish finders and side-scan sonar, and it can be introduced in narrow and shallow water.