Diatom assemblages within tsunami deposit from the 2011 Tohoku-oki earthquake along the Misawa coast, Aomori Prefecture, northern Japan

This paper reports variations in the diatom assemblages within the deposit from the 2011 Tohoku-oki tsunami along the Misawa coast, Aomori Prefecture, northern Japan. The landward extent of the tsunami deposit was as much as 400 m inland. The deposit contained mixed assemblages of marine-brackish and freshwater diatoms. The mixed assemblages indicated that the tsunami deposit was composed of not only beach and dune sand but also soil from the coastal forest. Marine-brackish species were predominant in the seaward tsunami deposit. However, the relative abundance of marine-brackish species rapidly decreased in the samples about 150 to 250 m inland from the shoreline. A decrease in marine-brackish diatoms at inland locations suggests that the eroded terrestrial soil was incorporated into the tsunami deposit. The composition of marine-brackish species in the tsunami deposit was different from that of modern inter- to supra- tidal sediments. One possible interpretation for this discrepancy is that the 2011 tsunami deposit were sourced from multiple environments, including supra-, inter- and sub- tidal locations.