Discrimination of boulders deposited by tsunamis and storm waves at Ishigaki Island, Japan

Abstract

The Ryukyu Islands, Japan, where numerous boulders are deposited on the reef and on land, might be ideal research fields of boulders deposited by tsunami and storm waves because many typhoons and several tsunamis attacked the same area during its history. This study investigates the size, position, and the long axis orientation of 626 boulders at Ibaruma and Shiraho Reefs at Ishigaki Island, Japan to elucidate the sedimentary differences of boulders deposited by tsunamis and storm waves in one area. Two distinct groups of boulders were found at these reefs: boulders on the reef crest (group 1) and along the shoreline (group 2). Boulders in group 1 (< 47 t) included reef and coral boulders originating from the reef slope and reef crest. They were deposited within a band from 50 to 210–240 m from the reef edge with an exponentially fining landward trend. They were deposited within the transport limit of the storm waves at the Ryukyu Islands and were deposited at their present locations by storm waves, as inferred from aerial photographs. No boulders were observed on the reef crest from 210–240 to 350 m from the reef edge, suggesting that no storm wave in the past (since the 1771 Meiwa Tsunami) had sufficient energy to displace large boulders more than 240 m from the reef edge at eastern coast of Ishigaki Island. Boulders in group 2 include abundant microatoll-shaped large colonies of massive Porites sp. of moat origin. The group 2 boulders were remarkably heavier (< 216 t) than those in group 1. Boulders were scattered randomly along the shoreline, 390–1290 m from the reef edge, which is far beyond the transport limit of boulders by the storm waves. Hydrodynamically, the source, size, and horizontal displacement distance of boulders in group 2 cannot be explained by storm wave action, which suggests their tsunami origin (probably 1771 Meiwa Tsunami according to 14C age). Based on results of this study, we infer that tsunami boulders in these islands, so-called "tsunami-ishi", can be discriminated from boulders of storm wave origin based on their sedimentary differences. This sedimentological approach is expected to be applicable to other boulder fields throughout the world. Our results further suggest that the spatial and grain size distribution of tsunami boulders are very useful to elucidate local flow characteristics of the tsunami. For example, the group 2 tsunami boulders were deposited below the high tide line irrespective of size. Some were split into several pieces. They were probably deposited below the high tide line by the remarkable reduction of the current velocity of the tsunami because of the local topography. The reduction of the current velocity must have been drastic to have made the boulders hit the ground with sufficient force to split them.