Tsunami deposits in a lacustrine sequence of the Sanriku coast

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Marine sand layr in a succession of lacustrine deposits, together with interstitial carbonates and pore water rich in seawater ingredients within sand layr, is distinct evidence for tunami-genic sedimentation. Thirteen thin sand layrs were found to be intercalated within pond deposits of the Sanriku coast. Detailed sedimentological and geochemical work demonstrates that the sand layr were laid down as a result of tsunamis. The dates of these tsunami deposits are mostly equivalent to the ages of the moderate to severe ancient tusnamis documented in historical materials, suggesting that tsunamis with waves having reached a maximum height of more than 1 m on the coast caused net sediment tranbsport landward and accumulated thin sand beds in the pond.The estimation of potential hazards to coastal zones afforded by tsunamis is especially important, and an accurate knowledge of the frequency of destructive tsunamis is essential for assessment of the hydro-engineering security of high risk facilities such as coastal nuclear power stations. There is a pressing need for learning more about earthquake initiation in and around subduction zones. The methods adopted here can be applied to other coastal areas in the active margin, and may provide useful information on the risks of tsunamis.