Paleotsunami deposits from coastal sediments in Hokkaido, Japan

The Japanese archipelago has been subjected to the threat of tsunamis throughout the Holocene. The catastrophic 2011 Tohoku-oki tsunami demonstrated that tsunami hazard assessment in Japan was underestimated, i.e. in wave heights and inundation depths. Paleotsunami studies can provide a deeper understanding of historical tsunami events and deliver crucial information about the frequency, magnitude, and characteristics of past tsunamis, enabling better preparedness for future events. This study focuses on the eastern coast of Hokkaido in the Tokachi region, an area with documented historical tsunami impacts (such as from the Tokachi-oki earthquakes in 1843 CE, 1952 CE, and 2003 CE).

We analyzed 1.5 m-long sediment profiles using an integrated approach combining sedimentological and organic geochemical analyses. At least five distinct tsunami deposits were identified in the stratigraphy and preliminarily dated by volcanic ashes of Mount Tarumae volcanic eruptions, e.g., the 1739 CE (Ta-a), the 1667 CE (Ta-b) and the 2500 cal. BP (Ta-c). These deposits are characterized by distinct sand layers that exhibit landward thinning patterns typical of tsunami deposits intercalated with peats. The application of organic geochemical proxies (e.g., n-alkanes, fatty acids, polycyclic aromatic hydrocarbons and hopanes) allows us to trace tsunami inundation beyond the sand deposits, providing a more comprehensive understanding of the depositional characteristics of these paleotsunamis and the contents of organics.

Our findings contribute to a more comprehensive understanding of the maximum inland extent of these paleotsunamis and demonstrate the effectiveness of multi-proxy approaches in identifying and characterizing these events. This research enhances our understanding of tsunami recurrence intervals, sediment transport processes and inundation patterns along the eastern Hokkaido coast, providing valuable input for regional tsunami hazard assessments and coastal management strategies.