Geological Survey Research of Past Tsunamis by Detecting Traces of Deposit in Sediment Layers in Lagoon Beds and Sea Shores

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Studies of detection of geological traces of past tsunamis were made. Okamura and Tsuji found out thin sand layers, which were formed historical and pre-historical tsunamis, in the bottom sediment layers on the lagoon bed of two lakes on the cast of the Kii Peninsula : O-ike lagoon in Owase city, and Suwa-ike lagoon in Kii-Nagashima Town. They detected more than ten sand layers in the coring samples obtained at O-ike lagoon and dated them by C-14 method, and clarified that they were formed by a series of major Tokai earthquakes and the oldest one shows the event of 6 century BC.Shiki and Yamazaki detected traces of tsunamis in the layer of the Miocene epoch in the rock shore near the tip of Chita peninsula, Aichi prefecture.Nishimura found out a sand layer formed by the tsunami of 1640 generated by the eruption of Komagatake volcano, Hokkaido, on a coast of the Funka Bay. He also detected sediment layers in swamps besides the coast of the eastern Hokkaido.In addition, we made geological research of tsunami trace detection on the north coasta of Papua New Guinea. Nishimura made field survey of geological traces of the tsunamis generated by an eruption of a volcano near Rabaul, and Tsuji made a study on the tsunami of the 1998 Aitape Earthquake.