Moderate Tsunamis, Great Storms Leave Little Sedimentary Record on Tokachi Coast, Hokkaido, Japan

Neither the 2003 Tokachi-oki tsunami nor a great storm that struck the Tokachi coast in February 2007 left sedimentary traces distinguishable from normal marine sedimentation in coastal sediments near Otsu, a town with a clear record of prehistoric tsunamis. The lack of preservation of moderate-sized tsunamis and great storms suggests that either favorable circumstances are required for preservation, or that this area has experienced larger events than those that occurred in 2003 and 2007.

The 2003 Tokachi-oki earthquake (Mw = 8.3) spawned a tsunami that reached heights of up to 4 m near the fishing port of Otsu. Nevertheless, a ~5 m high sand dune along the coast prevented inundation landward of the dune in all but the most favorable circumstances. In one area, an artificial cut through the dunes allowed the tsunami to reach an inland marsh, but even here no sedimentary record of the tsunami remained five years after the event.

A very large storm struck the same area in February 2007. The storm surged higher than 4 m, and inundated dunes nearby that were mantled with ash from the 1739 eruption of Tarumae, suggesting that this was the first time in more than 350 years that such waves reached the dune. Nevertheless, the storm did not breach the 5 m dune near Otsu, and in a location 1.5 km northeast of Otsu, a lower dune of ~2 m allowed inundation of no more than 200 m inland, as marked by a line of debris still visible one year later. Deposition from this storm was not distinguishable from normal marine deposition during our survey.

Because the Tokachi coastline is protected by a low dune that effectively limits events smaller than about 5 m from entering the sedimentary record, preservation of smaller events requires favorable circumstances, and is unlikely.