Large boulders scattered around Hashigui-iwa are evidence of past giant tsunamis

The coastal areas of the Nankai Trough in western Japan have been repeatedly hit by devastating tsunamis, including that of the 1707 Hoei earthquake. In this study, we investigated the distribution of large boulders around the Hashigui-iwa dacite intrusion located in the town of Kushimoto, Wakayama Prefecture (Fig. 1). These large boulders (Fig. 2) originated from the intrusion, but they are distributed as far as 100 m from the intrusive rock, and we can assume that they were carried by storm surges or powerful tsunamis. We carried out field surveys including measurements of the location and size of these large boulders and calculated the magnitude of the tsunami that would cause them to move, and found that some of them did not move by the 1707 Hoei earthquake, the largest earthquake in history along the Nankai Trough. This means that a tsunami larger than the 1707 tsunami once hit this area and moved the large boulders.

＊This figure is based on the press release, “Evidence of extraordinary tsunamis around the Hashigui-iwa dacite intrusion, south of the Kii Peninsula, Japan -Based on the distribution of large boulders, tsunamis larger than that caused by the 1707 Hoei earthquake along the Nankai Trough have been inferred.”