2014 Volume 123 Issue 6 Pages 883-903
The great tsunami that followed the 2011 off the Pacific coast of Tohoku Earthquake caused derangement of superficial sediments in coastal areas of Sendai Plain and Sendai Bay. Tsunami deposits were transported with flooded seawater in the form of mud, sand, or granules, and were deposited in the coastal region. These deposits record important information that facilitate determining the age and frequency of past tsunami events. The origins of tsunami deposits and the derangement of marine sediments are studied based on a chemical composition analysis of tsunami deposits, marine sediments, and samples of subsurface soil collected before the earthquake. The solubility of chemical components, such as of chromium, iron, nickel, copper, zinc, arsenic, and lead, in 1 M HCl was much higher for tsunami deposits than subsurface soils. The metal speciation and correlation of chemical composition indicate that the main origins of tsunami deposits are coastal marine sediments. The transformation of marine sediments to the coastal side, except Arahama offing, is also confirmed from active regional shifts of bacteria. These imply that the tsunami had a large effect on mobilizing marine sediments in the northern area of Sendai Bay, Nanakita River offing, Arahama offing, and Hiroura offing based on the extent of bacterial activity shift and chemical compositions of organic materials.
