2011年東北地方太平洋沖地震による東北地方太平洋沿岸の津波堆積物の地球化学的検討

The large-scale tsunami created by the 2011 off the Pacific Coast of the Tohoku Earthquake struck far-flung regions of the Pacific coastline. Description of recently deposited tsunami sediments is significant for future research of both past and modern tsunami. Samples were collected in Miyagi Prefecture of northeast Japan by coring with an acrylic pipe. This allowed observation of sedimentary features. Geochemical analysis of tsunami and pre-tsunami sediments is useful for discrimination of both types of sediments. The tsunami sediments were divided into sandy or muddy sediments, based on their characteristic distribution in the coastal area. Lithologic contrasts were related to the geographic features of the sandy beaches, and the inland areas inundated by the tsunami waves. Both sandy and muddy sediments commonly formed thin layers, less than 15 cm thick. Sandy sediments were distributed along the coastline, up to 5km from the shore. Muddy sediments dominated in the northern side of the Ishinomaki district, where the bay has a more concave shape than the other areas examined. Muddy sediments were deposited on the surfaces of paddy fields and on the foundations of destroyed houses and other structures. Some of the sand deposits exhibited parallel lamination and layered concentrations of iron sands. The tsunami sediments are enriched in Pb, Zn, Cu, TS (total sulfur), Br, and Cl compared to the pre-tsunami sediments. The enrichments in these elements are related to the organic matter which was accumulated in the adjacent bays and/or estuaries. Previously reported geochemical map data for fluvial and marine sediments in this area were used for characterization of original provenance signatures. The results indicate that the tsunami sediments are more similar to those of fluvial sediments, rather than the marine sediments. This suggests the tsunami sediments were deposited mostly by backwash waves, rather than from transportation by run-up waves. The similarity of the geochemical compositions of the tsunami sediments and on-land sediments could be related to historic recurrence of large-scale tsunami in this region.