Holocene Tsunami deposits associated with earthquakes along Pacific coast, northeast Japan

We investigated Holocene tsumami deposits along the Pacific coast of northeast Japan in order to define the ages and source areas of earthquakes generating large tsunamis. Sediment cores were collected by using geoslicers and hand augers at alluvial lowlands interpreted by aerial photographs, and ages of deposits were dated by radiocarbon methods and tephrochronology. Pacific coast of northeast Japan faces the Japan trench where Pacific plate subducts beneath the Eurasian plate, so that the coast has repeatedly experienced some large tsunamis following historical interplate earthquakes (the 1896 Meiji Sanriku Tsunami, the 1793 Kansei Tsunami, the 1611 Keicho Sanriku Tsunami and the 869 Jogan Tsunami). While the southern part along the Pacific coast (from the Sendai Plain to the Joban coastal region) has nearly straight shorelines and well developed coastal lowlands, the northern part (the Sanriku coastal region) has typical ria shorelines and poorly developed coastal lowlands. Multiple sand layers are identified between muddy or peaty sediments in drilled core samples to a depth of 1-5.5 m at several coastal areas, which is located at backlands of beach ridges and natural levees. In the southern part along the Pacific coast, sand layers corresponding to the 869 Jogan Tsunami are identified, and the older sand layers suggest that tsunamis which are as large as the 869 tsunami have occurred at few hundred years intervals over the past 5000 years. On the other hand, in the northern part along the Pacific coast, multiple sand layers which indicate the arrival of large tsunamis are also found at the similar intervals to the southern coast during 6000-2000 years ago, but most of the depositional ages of the sand layers do not coincide with those of the southern coast. We suggest that earthquakes generating large tsunamis along northern and southern Pacific coast of northeast Japan have both occurred at few hundred years intervals during the late Holocene, but in most cases, the earthquakes which generate tsunamis attacking northern Pacific coast and those attacking southern Pacific coast have different source areas.