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 ( Porites sp. of moat origin. The group 2 boulders were remarkably heavier ( 14 C 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.