Evidence from boulders for extraordinary tsunamis along Nankai Trough, Japan

Abstract

The coastal area along the Nankai Trough, western Japan, has been repeatedly hit by catastrophic tsunamis. The largest known earthquake, the 1707 CE Hoei earthquake (Mw 8.7), ruptured the plate interface along the entire trough. We present evidence for even larger tsunamis from boulders scattered on the landward side of the Hashigui-iwa dacite intrusion (the HID intrusion), Kushimoto, Japan, which faces the Nankai Trough. Although the boulders derive from the intrusive rock, their locations, which are up to 100 m from the intrusive rock, suggest their transport by tsunamis and/or storm surges. Comparisons of laser-scanned topographies before and after a typhoon in 2012 and aerial photographs taken in 1976 and 2007 indicate that storm surges did not move any of the heavier boulders. A comparison of the fluid forces caused by simulated tsunamis acting on the boulders with the static frictional force indicates that the 1707 CE earthquake tsunami did not move all the boulders. The distribution of the boulders is thus evidence that tsunamis larger than that caused by the 1707 CE earthquake occurred in the past. We also show that large slip on a splay fault or the plate boundary is a potential source of earthquakes larger than the 1707 CE earthquake that could have set the boulders in motion.