3D mapping and sedimentary analysis of extensive tsunami deposits near Tokachi, Hokkaido, Japan

Sand sheets that thin and fine landward in a marsh deposit near Otsu, southeastern Hokkaido, suggest that at least nine tsunamis have struck the coast since marsh deposition began more than 3000 years ago. These sand sheets extend landward up to 2 km and appear in a 2 m thick sequence of marsh deposits that begin with peat formation on marine mud. The sand sheets are laterally continuous along at least 1.5 km of coast and correlate to sand deposits in a marsh 40 km southwest of Otsu. This relationship suggests that these tsunami events were regional in extent. Sheet thickness is highly variable in the longshore direction, reflecting changes in micro-topography rather than wave speed or height. Sediment sorting and grain size analysis was done using a Retsch Camsizer. An advective-settling model was then used to estimate tsunami wave height and velocity. Three-dimensional mapping and interpolation of the sand layers shows patterns of deposition characteristic for tsunami flow in this marsh.