Geological evidence for extreme wave events on a coastal lowland facing the Tokai segment of the Nankai-Suruga Trough

Located close to Japan’s densest concentrations of people and industry, the easternmost region of the Nankai-Suruga subduction zone has long been the focus of attempts to forecast and even precisely predict future earthquakes. While historical records attest to the occurrence of great earthquakes and subsequent tsunamis that may have originated from the Tōkai segment, past rupture zone extents and recurrence intervals remain poorly understood. Coastal stratigraphy has the potential to record the occurrence of both tsunami inundation and coseismic vertical land-level change over timescales far exceeding the historical record, with important implications for refining understanding of future hazards (Garrett et al., 2016). Here we present initial results from an extensive coring survey of the lower reaches of the floodplain of the Sagara River, close to the town of Sagara, Shizuoka Prefecture. The site lies at an altitude of ~1 – 5 m and is within the anticipated inundation zone of future worst-case tsunami scenarios. Typhoon-driven storm surges and river floods are also likely to have inundated the site, complicating the interpretation of potential tsunami deposits. Using CT scans, multi-sensor core logs, diatom assemblages and radiocarbon dates, we evaluate sedimentary processes and make the distinction between extreme wave events and fluvial deposits. Where possible, we assess methods to differentiate between storm surges and tsunami deposits. Finally, we evaluate the potential for the site to provide a long and continuous record of extreme wave events and highlight the probable influence of changing thresholds of evidence creation and preservation over time.

Abstract of an Oral Presentation or a Poster

Palaeotsunami