Spatial Extent of Mid-To Late-Holocene Sedimentary Record of Tsunamis Along the Southern Kuril Trench, Hokkaido, Japan

With their complex cycles and rupture modes, infrequent megathrust earthquakes require a high-resolution spatiotemporal record of tsunami inundations over thousands of years to provide more accurate long-term forecasts. The geological record suggests that Mw > 8 earthquakes in the Kuril Trench occurred at intervals of several hundred years. However, uncertainties remain regarding the rupture zone, owing to the limited survey areas and chronological data. Therefore, we investigated the tsunami deposits in a coastal wetland of southeastern Hokkaido, Japan, to characterize the tsunamis that originated from the Kuril Trench over the last 4,000 years. On the Erimo coast, more than seven sand layers exhibited common features of tsunami deposits, such as sheet distributions of several hundred meters, normal grading structures, and sharp basal contacts. According to numerical tsunami simulations, the 17th-century sand layer could be reproduced using a multiple rupture zone model (Mw ∼ 8.8). We used high-resolution radiocarbon dating and tephras to correlate the tsunami deposits from the last 4,000 years with those reported from regions ∼100 km away. The tsunami history revealed here shows good agreement with the histories of adjacent regions. However, the paleotsunamis reported to have occurred in regions >200 km away include events that differ from those in this study, suggesting a diversity of Mw > 8 earthquakes in the Kuril Trench. We clarified the history and extents of earthquake-generated tsunamis along the southwestern end of the Kuril Trench, which were previously unknown. Our results provide a framework for magnitude estimations and long-term forecasts of earthquakes.