Relationship between depositional environments and preservabilities of Holocene tsunami deposits on the Hidaka coast, Hokkaido, Japan

Rising sea levels and associated coastal topography changes are expected to increase coastal vulnerability to tsunamis. Reliable records of the periodicity of the most damaging tsunamis, with recurrence intervals of several centuries, are often incomplete. To understand the likelihood of coastal disasters that would be worsened by sea-level rise, as well as their possible occurrence leading to the actual extent of inland inundation, it is useful to examine the geological record of the mid-Holocene highstand periods to supplement the written archives. However, the formation and preservation potential of event layers depends heavily on the depositional environment resulting from coastal evolutions. This study attempted to reconstruct the depositional environment changes using diatom and chemical analyses on the Hidaka coast of Hokkaido, Japan, to examine the preservation of event layer archives. Following field investigations, we found three sand layers formed by extreme waves from field investigations. The relationship between the depositional environment and the timing of the event layer formation over the past 7000 years along the central Hidaka coast indicates that supratidal to freshwater marsh environments are the most suitable for preserving tsunami deposits in this region. Since sand layers in the distant areas from wave sources were not well preserved in upland (as at elevations above marsh environment) and saltmarsh environments, the time windows of preservation were very limited depending on the depositional environments with relative sea-level changes. On the other hand, in areas closer to the Kuril Trench, sand layers were preserved not very sensitive to the depositional environment, suggesting that the event archive length is related to the magnitude of tsunamis.