Sedimentary processes associated with sand and boulder deposits formed by the 2011 Tohoku-oki tsunami at Sabusawa Island, Japan

This paper reports on the sedimentary processes of sand and boulder deposition at Sabusawa Island, Japan as a result of the 2011 Tohoku-oki tsunami. Boulders were composed of tuffaceous rocks and sourced from an earthquake-triggered slope failure as well as concrete fragments of seawall. They were scattered over the ground surface and did not form boulder ridges, although there was some local imbrication. The boulders were deposited on top of a sand layer indicating that the latter, possibly deposited from bed load, covered the ground surface first. This sand layer probably reduced friction allowing boulders to be transported more easily than might be expected across a hard ground with a high bottom friction. Sand deposits showed landward thinning and fining features, while the boulders showed a landward coarsening (tuffaceous boulders) or a landward fining (concrete boulders), indicating that large clasts were not necessarily scattered randomly but rather might have a clast size gradient with distance inland. These features are explained by the local topographic setting that constrained the directions of incoming and returning tsunami flows. Some clasts at the inland extent of the boulder field were covered by an upward fining sand layer. This feature suggests that the boulders were deposited prior to the suspended sands, with the latter subsequently laid down before the water level dropped below the top of the boulders. Such modern investigations of the sedimentary features of various sizes of grains and clasts immediately after a tsunami provide invaluable data for the reconstruction of inundation processes.