Abstract
To assess the long-term earthquake history, we studied paleotsunami deposits on the Pacific coast of Misawa, Aomori Prefecture, northern Japan, using fossil diatom assemblage analysis and radiocarbon dating. We found two sand layers interbedded with fluvial mud beneath the coastal lowland. The upper sand sheet was deposited after the fallout of B-Tm tephra (10th century). Because of artificial disturbance, the source of the upper sand layer is unclear. The lower sand layer, deposited 4,800-2,900calBP, occurs extensively 0.7-1.7km inland from the present shoreline and generally fines landward. The lower sand layer and its overlying mud contain brackish-marine as well as freshwater diatoms. These geological and biological features suggest that a tsunami possibly formed the lower sand layer. However, it is still possible that the brackish diatoms were derived from marine terraces of the late Pleistocene.
