TRACES OF TSUNAMI PRESERVED IN INTER-TIDAL LACUSTRINE AND MARSH DEPOSITS - SOME EXAMPLES FROM NORTHEAST JAPAN

The Japan Sea Earthquake of May 26, 1983 triggered a large-scale tsunami that surged over the coast of NE Japan and caused considerable damage. The tsunami resulted in peculiar sedimentary processes on the coast. Depending on the hydraulic scale of the tsunami, three kinds of sedimentary processes were recognized, each of which left unique deposits. Such deposits are expected to remain in sedimentary sequences, preserving remarkable sedimentary facies. Borehole samples drilled at different localities in coastal regions have been examined to detect traces of ancient tsunamis. Sedimentological and geochemical studies of the samples reveal that layered deposits, similar to those developed by the Tsunami of the Japan Sea Earthquake, are common in the drilled cores. Estimated ages of these deposits are exactly equivalent to the dates of ancient tsunamis documented in historical records. These results show that the invasion of tsunamis is recorded in coastal depositional sequences. From analyses of long borehole cores we conclude that large-scale tsunamis produced by submarine earthquakes have invaded the coastal regions of the Tsugaru and Sendai Plains at intervals of 250 to 400 yr and about 800 yr, respectively.