Large bedform generated by the 2011 Tohoku-oki tsunami at Kesennuma Bay, Japan

The 11 March 2011 MW 9.0 Tohoku megathrust earthquake off the Pacific coast of Japan was a salient event in the history of Japan. The resultant huge tsunami (the 2011 Tohoku-oki tsunami) inundated a vast coastal area of northeastern Japan, causing widespread devastation. Twenty days after the tsunami, we analyzed the impact of the tsunami on the sea bottom of the Kesennuma inner bay using side-scan sonar to explore the damage and bathymetric change in the harbor. Herein we present the first direct evidence that the sea bottom sediments of around 10–15 m were largely reworked by the tsunami to thickness of a few meters, and that large dunes were formed by the tsunami. Considering that the sea wave influence is as weak as it is inside the inner bay, the potential exists that even meter-thick paleo-tsunami deposits are preserved in shallow sea bottoms with large bedforms. This finding will be a stepping-stone to future geological studies of tsunami effects in shallow sea regions.

► The first direct evidence showing the tsunami reworked sea bottom sediments.
► Large dunes were formed by the tsunami.
► Meter-thick paleo-tsunami deposits might be preserved in shallow sea bottoms.
► Asymmetric dune shapes suggest their formation by the landward incoming flow.