Holocene tsunami, storm, and relative sea level records obtained from the southern Hidaka coast, Hokkaido, Japan

Abstract

On the Pacific coast of Hokkaido, historical documents and seismic detections spanning the last 200 years show that large earthquakes (magnitude 7–8) have occurred every 50–100 years along the Kuril Trench. Geological reconstructions of past earthquakes in the region show extend records to the last 6000 years and show that unusually large (>M8) earthquakes occurred at intervals of several hundreds of years. However, the magnitude and extent of tsunamis induced by these unusually large earthquakes are not fully understood. In this study, we found nine anomalous sand layers embedded in peat sediments that dated between 5.5 and 1.2 ka on the southern Hidaka coast, Hokkaido. Paleoenvironments reconstructed from the peat sediments using diatom, chemical, and mineralogical analyses showed an evolution of the site from a saltmarsh, to a supratidal marsh, to a back-barrier wetland environment. Radiocarbon dating of peat sediments showed that these environmental changes correspond to a sea-level fall following the mid-Holocene highstand. The anomalous sand layers showed inland thinning and shape contacts with the underlying peat. Amongst the layers, the most recent layer (1.7–1.2 ka) was designated as a storm deposit due to its limited distribution of less than 100 m from the beach ridge. Eight additional sand layers from 5.5 to 2.2 ka were distributed more than 300 m away from the beach ridge, and displayed a normal grading of sand particles, a characteristic typical of tsunami deposits. Because there was no record of tsunamis striking the Hidaka coast from the observed large earthquakes (M7–8) that recur every 50–100 years, we interpret the sand layers as tsunamis deposited by unusually large earthquakes. Because the eight inferred tsunami sand layers were deposited during the highstand period, prior to 2.2 ka, we infer that the inundation area due to tsunamis in the coastal zone has changed with relative sea-level (RSL) in the past. We highlight the necessity of evaluating the influence of long-term RSL change.