Historical tsunami and storm deposits during the last five centuries on the Sanriku coast, Japan

Nine event deposits in a small alluvial valley along the Sanriku coast, Japan, were correlated with historical tsunamis and storms that have been recorded in this region since the 15th century. We identified nine sandy layers in 15 geo-slices collected at distances ranging from 140 to 260 m from the coast in a lowland back marsh protected from the sea by a high sandy ridge. Based on their sedimentary characteristics, grain-size distribution, and marine microfossil assemblages, namely Coccoliths, these event layers, which were well preserved in their order of deposition, were probably either tsunami or storm deposits. Diatom analysis revealed four stages of paleo-environmental change from tidal marsh to wetland. The uppermost event layer covering the ground surface is probably the deposit from the 2011 Tohoku-Oki tsunami. Ages for other event deposits were radiometrically dated using 14C, 137Cs and 210Pb with Bayesian estimation. The second event layers can be correlated with the 1960 Chilean or 1968 Aomori-Oki earthquake tsunamis. The candidates for the third event layer are the 1947 Catherine and 1948 Ione typhoons as well as the candidates for the second event layer. The fourth and fifth event layers are likely associated with the 1933 and 1896 Sanriku-Oki earthquake tsunamis. Four lower event layers can be correlated with historical tsunamis and storms that occurred during the 15th to 18th centuries, such as the 1611 Sanriku-Oki earthquake tsunami.