Investigation on recurrence intervals of the Toukai earthquakes by using tsunami deposits in coastal marsh in the Shima Peninsula, Mie Prefecture, Central Japan

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Great interplate earthquakes have repeatedly occurred along the Nankai trough and caused tsunami damage along the Pacific coast. The coastal lowland areas in the Shima Peninsula have been often struck by the tsunami after the Toukal and Tonankai earthquakes. The non-disturbed sampling with the Geo-slicer was carried out at three sites in the coastal marsh in Osatsu, Toba City, in order to investigate its environmental change and the relation with the event deposits.The sediments mainly consist of unconsolidated silt and clay intercalated thin sand layers. The several layers in these sands have good horizontal continuity and similar lithology. These sand layers include fragments of shell and foraminifer and its base of these sand layers have evidence of erosion. Because the surrounding inland area of investigation site can not supply coarse grained crusts under the geomorphologic environments, these sand layers are considered to event deposits such as tsunami.Possible tsunami deposits (named OS-1 to OS-12) are found in Holocene marsh sediments and dated by the AMS ^<14>C method. We calculated that recurrence intervals of large tsunami are almost 300-500 years in this area.