Characteristics and hydrodynamics of tsunami boulder were one of the problematic issues, because there was no example of the tsunami boulders that were investigated immediately after the tsunami event. Our research group conducted field survey as well as the numerical modeling for the boulders at Pakarang Cape, Thailand, which were displaced by the 2004 Indian Ocean tsunami, and published the results in Sedimentary Geology in 2007. The paper was awarded by the Sedimentological Society of Japan in 2009. Here, I briefly introduced the contents of this paper as well as the recent researches of boulders deposited by the tsunami and the storm waves. I also discussed identification criteria and future development of the research for disaster prevention countermeasures. The boulders of tsunami or storm wave origins could be discriminated based on the difference of the transport distance of each type of boulders, because tsunami can transport boulders with greater distance than the storm waves. Moreover, it is expected that the size of boulders displaced by the storm waves would be fining landward in an exponential manner on the reef. If we can estimate the transport limit of storm wave boulders at the studied area, the limit will be useful to identify the tsunami boulders. Series of our recent researches further showed that the local wave height and period of the tsunami might be estimated numerically from the distribution of boulders. Such contribution will be useful to make a disaster prevention plan for local residents.

View full abstract

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley fills in the Arakawa Lowland are revealed based on sedimentary facies, fossil shell assemblages, physical properties, chemical element contents and radiocarbon dating in the GS-TKT-1 core, Toda City, Saitama Prefecture.
The sedimentary successions are composed of Pleistocene Shimosa Group (Unit 1), gravelly fluvial deposits (Unit 2), sandy fluvial to salt marsh deposits (Unit 3), inner bay to delta front deposits (Unit 4) and sandy fluvial deposits (Unit 5) in ascending order. Unit 2 and upper units are the latest Pleistocene to Holocene incised valley fills, indicating a transgression (Unit 2-3) followed by a regression (Unit 4-5). Depositional ages are 10000 cal yBP or before for Unit 2, 10000-9000calyBP for Unit 3, 9000-5000calyBP for Unit 4 and 5000calyBP to present for Unit 5.
In a transverse section, the gravelly fluvial deposits and the sandy fluvial deposits fill the narrow axis of the incised valley under the Arakawa Lowland, the salt marsh deposits and lower part of the bay deposits extend over a buried terrace south of the valley axis, and upper part of the bay deposits, the delta front deposits and the fluvial deposits cover the entire lowland.

View full abstract