Elucidation of sedimentological mechanism of gravity flows by tsunamis

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Sedimentological and micropaleontological analyses of terrestrial and nearshore to offshore sediments recovered from the coast of the Sendai Bay were performed to clarify the processes of sediment transport and deposition by the tsunami retreat. The distribution pattern of sediment grains and diatom frustules showed seaward migration after the tsunami event. It has been suggested that sediments originally distributed in terrestrial and coastal zones were transported offshore due to the tsunami backwash. The distribution pattern of planktonic and benthic species living in offshore zones showed slight evidence of landward migration by the tsunami. This suggests that landward redistribution of sediments by the tsunami run-up did not occur in the offshore seafloor of the study area. It is possible that tsunami backwashes induce sediment flows that transport a large amount of coastal materials seaward.