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

2011年は東北地方太平洋沖地震により東北地方太平洋沿岸には大規模な津波が押し寄せた.津波堆積物を早期に記録することは過去や現在の津波についての研究には重要である.そこで宮城県において津波堆積物試料をアクリルパイプを用いて採取し堆積構造を検討した.津波堆積物とプレ津波堆積物の地球化学的検討は両者の区別に役立つ.津波堆積物は砂質なものと泥質なものに分類され海岸地域でのそれらの特徴的な分布に関連する.それらの岩相の相違は砂浜や陸地で津波が浸水した地域の地形的な特徴に関連する.砂質と泥質の津波堆積物は一般には15cm以下の薄い層として残されている.砂質堆積物は海岸から5kmの内陸部にまで分布している.泥質津波堆積物は調査地域の北部で湾がより奥まった形状を持つ石巻地域に多く見られる.ここでは泥質津波堆積物は水田や破壊された構造物の上に堆積している.いくつかの砂質津波堆積物は平行葉理や砂鉄の密集層が見られる.津波堆積物はプレ津波堆積物に比較してPb,Zn,Cu,TS(全イオウ),Br,and Clに富む.これらの元素が津波堆積物堆積物で富んでいることは港湾や河口域に蓄積していた有機物に関連することを示す.すでに公表されている海と陸の地球化学図を用いて津波堆積物の後背地の推定を行った.その結果,津波堆積物は海洋堆積物の値よりも河川堆積物の値に近い.このことは津波堆積物が押し波よりも引き波によりもたらされたと考えられる.また,津波堆積物と陸上の堆積物との元素組成の類似性は過去の大規模な津波による混合の結果によることを示す.

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.