Holocene coastal evolution, past tsunamis, and extreme wave event reconstructions using sediment cores obtained from the central coast of Hidaka, Hokkaido, Japan

Investigating coastal areas far from tsunami sources can improve the understanding of the magnitude and origin of giant tsunamis. However, distinguishing tsunami deposits from extreme storm deposits is more challenging since the layers are less prominent compared to the those in the locations near a tsunami source. This study reconstructed the Holocene coastal environments on the central coast of Hidaka, Hokkaido, ~250 km from the Kuril Trench. Sand layers induced by extreme waves were identified by examining the sea-level index points in the late to mid-Holocene by diatom-assemblage analysis through the combination of chemical analyses with the investigation of sediment cores. Detailed facies analyses on sediment cores obtained from several transects showed that sediment preservation differs according to the location of the cores relative to the ambient coastline. In the zone behind the beach ridge, the paleoenvironments changed from a saltmarsh formed by transgression to a swamp closed by a beach ridge formed by sea regression in the mid-Holocene. Conversely, the seaward zone indicated various environmental changes due to meandering rivers and the development of the beach ridge system due to sea regression. We found four volcanic ash layers and seven sand layers with clear basal contact with peat or organic mud. According to their sedimentological features, these sand layers were formed by tsunamis or storm surges; at least four of the sand layers are likely to be tsunami deposits, considering the ages of tsunami deposits in the surrounding region. Dramatic changes in the depositional environment associated with relative sea-level changes resulted in a short period of preservation of the sand layers. In addition, their distributions were limited, ranging from a few tens of meters to hundreds of meters from the beach ridge. Thus, in an area far from tsunami sources, the traces of extreme waves may be overlooked if the coastal evolutional process is not considered.