Geomorphologic features of large-scale submarine debris-avalanche along Japan coast-Comparison of Oshima-Oshima Volcano in Hokkaido and off Kaimon Volcano in southern Kyushu-

Abstract

Ninety percent of all tsunamis are generated by earthquakes; however, the remaining 10% are caused by meteorite collisions, volcanic activity, and submarine landslides. Submarine landslides occur due to slope instability, and large-scale submarine landslides can generate very large tsunamis. Therefore, from the viewpoint of disaster prevention, it is important to clarify the generation mechanism and features of submarine landslides. In this research, we compared the features of submarine landslides generated in two different regions, and we calculated their volumes. The first environment is a debris avalanche that occurred by the collapse of a volcanic body; the debris flowed into the sea and generated the Kanpo Tsunami along the Sea of Japan in 1741 in the case of Oshima-Oshima Volcano southwest off Hokkaido in the Japan Sea. A hummocky topography was found from the sea floor at the northern part of Oshima-Oshima, it is assumed to be the debris avalanche in 1741. In the second place, a submarine landslide exists from the continental shelf margin southwest of the Satsuma Peninsula in the southern part of Kyushu up to a depth of 320 m. We calculated the volume of the submarine landslide, and the volumes of the two landslides were similar (-2km3) . However, those distributions and morphology are significantly different. We clarified that these differences are caused by the difference in the environment (topography, width of head wall, collapsed materials, etc.) .