Sedimentary characteristics of the onshore tsunami deposits formed by the 2011Tohoku-oki tsunami in coastal lowlands, Ibaraki and Chiba prefectures

In order to complement regional data of modern tsunami deposits formed in various coastal environments, we described in detail thickness, grain size and sedimentary structures of the deposit formed by the 2011 Tohoku-oki tsunami at some areas in Ibaraki and Chiba prefectures, Japan. We also discussed effects of the tsunami size on the characteristics of tsunami deposits, comparing them to some tsunami deposits formed in other areas. Although the thickness of the tsunami deposit was influenced by local topography, it showed the thinning trend towards the inland distribution limit in all transects. Taking into account the results of previous studies tsunami deposits often show inland thinning toward the distribution limit regardless of tsunami sizes and grain size of source sediments. The thicknesses of the tsunami deposits near the shore were more variable than the deposits left inland. This may have resulted from complex tsunami behavior due to existence of protection forest and other artificial materials around the shore. On the other hand, the inland grain-size fining trend was unclear in all transects. Our data indicate that not all tsunami deposits show this trend. Even if a deposit in geologic stratum does not show an inland grain-size finning trend, the possibility that it might have formed by paleotsunami cannot be completely denied.