Sedimentary Evidence for Great Tsunamis near Tokachi, Hokkaido, Japan

Sand sheets that fine and thin landward in a sequence of marsh deposits near Otsu, southeastern Hokkaido, suggest that at least nine large tsunamis have struck this coast since marsh deposition began more than 3000 years ago. These sand sheets extend landward up to ~2 km, and extend too far inland to have resulted from either large storms or moderate tsunamis.

The sand sheets 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 extend inland variably, with 4 of the 9 disappearing by 1 km from the coast, and 5 of the 9 disappearing by 1.5 km inland. The deposits are correlative to similar sheets in a marsh ~40 km southwest of Otsu, suggesting that these events were at least regional in extent.

The sand sheets appear to represent a mix of dune sand and offshore sand, and contain minerals indistinguishable from the modern beach (although older units contain significantly more pumice). The sheets thin and fine landward, although they are rarely more than 20 cm thick at their seaward edge. No sedimentary structures were visible in outcrop exposures of the sand sheets or peels taken of the sediments.

The 2003 Tokachi-oki tsunami (~4 m high) left no record of its passage in this marsh, nor did great storms in February 2007 and May 2008. These sheets probably represent, then, the deposits of either storms or tsunamis greater than those recorded historically; the February 2007 storm in particular overwashed dunes unaffected by storms for more than 350 years, making storms an unlikely cause for these sand sheets.