Modelling emplacement of the world’s largest tsunami boulder

Large tsunami waves have struck the Sakishima Islands of Japan repeatedly. The latest was the 1771 Meiwa tsunami (> 25 m run-up height), which was generated by an earthquake along the Ryukyu Trench. Numerous tsunami boulders lie along the coastlines of these islands. The largest tsunami boulder of the Sakishima Islands, called “Obi-iwa”, is located on the west coast of Shimoji Island on a ca. 12 m high cliff. Historical documents imply that it was transported by the Meiwa tsunami. Nevertheless, whether this boulder could have been transported by the Meiwa tsunami has not been evaluated definitively. Furthermore, the possibility exists that it was transported by a larger and older tsunami generated along the Ryukyu Trench or by an unknown large tsunami originating from the Okinawa Trough. For this study, we conducted 3D measuring of this boulder using LiDAR equipment and used boulder transport models to evaluate whether the Meiwa tsunami could have transported it. From 3D measurements, the weight of this boulder was estimated as approximately 3400 tons: the world’s heaviest tsunami boulder reported to date. The boulder transport calculations suggest that the Meiwa tsunami could have transported this boulder, which implies that the assumption of some unknown large tsunami event is unnecessary to explain this boulder’s deposition.