Seafloor morphology and sediment magnetic fabric in a putative 1771 Meiwa tsunami source region in the southern Ryukyu Islands, SW Japan

Toshiya Kanamatsu,Ayanori Misawa,Ken Ikehara

doi:10.1144/sp501-2019-94

Abstract

AbstractThe Meiwa tsunami of AD 1771 is regarded as an extremely strong tsunami event causing devastating damage in Japan in historical times. Earlier studies explored the possibility that a submarine landslide enhanced the Meiwa tsunami waves. We collected detailed seafloor bathymetry data, sub-bottom structure data and surface sediments in a putative Meiwa tsunami source region to ascertain any signature related to a submarine landslide in the forearc region, which is located south of Ishigaki-jima. The forearc-region seafloor is characterized by its surface submarine landslide morphology. However, the investigated magnetic fabric of surface sediment revealed that there was no landslide mass deposit during historical times. The described landslide morphology in the basin is unrelated to the generation or enhancement of the AD 1771 Meiwa tsunami, although the disturbed relief in the topography of the study area indicates that the forearc region is susceptible to slope failure because of its tectonic setting.

Highlights

The Meiwa tsunami, which struck the Sakishimashoto (Sakishima Islands) in AD 1771, is regarded as one of the largest tsunami events and caused devastating damage in Japan in historical times
Large tsunami boulders distributed on the shore of Ishigaki-jima (‘jima’ means ‘island’ in Japanese) are thought to have been transported from the sea by large tsunami waves such as those of the Meiwa (e.g. Imamura et al 2008; Goto et al 2013)
Submarine landslides have been suggested as contributing to the enhancement of the Meiwa tsunami (Imamura et al 2001)

Summary

Introduction

The Meiwa tsunami, which struck the Sakishimashoto (Sakishima Islands) in AD 1771, is regarded as one of the largest tsunami events and caused devastating damage in Japan in historical times. Nakata and Kawana 1995; Goto et al 2010). Nonearthquake mechanisms have been proposed because such a large discrepancy exists between maximum run-up heights by faulting models and observations (Imamura et al 2001), and between the earthquake and the tsunami magnitudes (Nakamura 2006). Submarine landslides on continental margins are regarded as a general mechanism that generates tsunami waves In the trench landward slope, Okamura et al (2018) recognized a large seafloor depression of an accretionary prism. They proposed that a large area of earthquake-induced rotational sliding generated the Meiwa tsunami

Methods

Results

Conclusion