2016 Volume 122 Issue 12 Pages 637-652
Event deposits were identified in the Shonai Sand Dunes, which are situated in northeastern Japan on the coast of the Sea of Japan. The deposits reach a maximum height of 37.9 m at the Old Dune. Using carbon-14 dating with the known volcanic ash “To-a” (AD 915) as a key bed, two distinct events were identified: a northern event that occurred between the late 700s and the 800s, and a southern event (including the Sakata-kita port event) that occurred between the 1000s and the first half of the 1100s. A number of structures peculiar to tsunami deposits are observed in these beds, indicating that they formed by tsunamis. The main muddy sections of the event deposits settled during the period of standing water after the run-up flow, while the sandy sections were deposited by the return flow. Historical records indicate that the northern event corresponds to the AD 850 “Dewa earthquake”. Although the southern event does not have a matching historical record, it may correspond to an earthquake that caused deformation shortly after AD 915 to some remains in the Shonai Plain. The event deposits were well protected by the New Dune, because a sudden environmental change after the events resulted in rapid deposition of eolian sands. The growth of the New Dune of the Shonai Sand Dunes began at the time of these events and continues today; it is now the country's largest class of coastal sand dunes.
