北海道厚真（あつま）川を遡上した2011年東北沖地震津波による堆積物の形成過程および津波波形との対応

Abstract

The Tohoku-oki tsunami inundated the Atsuma-gawa River on the Pacific coast of Hokkaido, Japan, to. km inland from shore line and left tsunami deposits on the sand bars. In this study, we reconstruct the formation of the river tsunami deposits using sedimentology, and observations of the development of the tsunami waves. We divide the tsunami deposits into three main units (Unit to , where Unit is the lowermost deposit). Unit comprises a mixture of shallow marine sand and fluvial deposit, involving fallen plants at the base indicating the current direction towards the sea. We interpret Unit as a return flow deposit; extensive return flow occurred after the initial inundation, following a large drop in the level of the tide and rapid erosion of river mouth bar blocking the river channel. Unit consists of shallow marine sand, which rapidly thins and fines inland, and cross laminae indicate the sand was transported by landward flow. We interpret it mainly as a run-up deposit, which formed from tsunami waves that encroached inland, losing energy, wave amplitude, and velocity. Unit comprises alternating layers of shallow marine sand and silt (mud drapes), and represents small variations of the grain size between seaward and landward. Current directions estimated by cross laminae indicate mainly landward, and partially seaward. We interpret Unit formed from repeated long-period runup and return flow cycles in the latter stages of the tsunami, and mud drapes were deposited from fluid mud. Return flow concentrated in topographic lows and the channel. By documenting the formation processes of tsunami deposits correlate with recorded tsunami waveforms, we aim to improve our understanding of how tsunamis interact with onshore river systems, in Japan and elsewhere.