Historical Nankai-Suruga megathrust earthquakes recorded by tsunami and terrestrial mass movement deposits on the Shirasuka coastal lowlands, Shizuoka Prefecture, Japan

Abstract

Geological investigations of coastal sediment sequences play a key role in verifying earthquake and tsunami characteristics inferred from historical records. In this paper, we present a multi-proxy investigation of a coastal lowland site facing the Nankai-Suruga megathrust and appraise evidence for tsunamis and earthquake-triggered terrestrial mass movements occurring over the last 800 years. Combining a high-resolution chronology with X-ray computed tomography and analyses of particle size, diatoms, pollen, non-pollen palynomorphs and aerial photographs, we present the most compelling geological evidence of the 1361 CE Kōan (also known as Shōhei) tsunami reported to date from any site along the megathrust. This finding is consistent with either of two recent hypotheses: a single larger rupture of both the Nankai and Tōnankai regions or two smaller ruptures separated by a few days. Enhancing the site chronology using Bayesian age modelling, we verify evidence for inundation during the 1498 CE Meiō tsunami. While previous investigations identified evidence for historically recorded tsunamis in 1605, 1707 and 1854 CE and a storm surge in 1680 or 1699 CE, we encountered a thick sand layer rather than discrete extreme wave deposits in this interval. The overprinting of evidence highlights the potential for geological records to underestimate the frequency of these events. A terrestrial mass movement also deposited a sand layer at the site; radionuclide dating and aerial photographs provide independent confirmation that this may have been triggered by intense shaking in 1944 CE during the most recent great Nankai-Suruga megathrust earthquake.