Depositional process in response to waveform of the 2011 Tohoku oki tsunami inundated the Atsuma gawa River in Hokkaido, Northern Japan

Abstract

The 2011 Tohoku-oki tsunami inundated the Atsuma-gawa River on the Pacific coast of Hokkaido, Japan, to 6.2 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 1 to 3, where Unit 1 is the lowermost deposit). Unit 1 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 1 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 2 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 3 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 3 formed from repeated long-period run-up 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.