Tsunami boulders and their implications on the potential for a mega-earthquake along the Ryukyu Archipelago, Japan

The Ryukyu Archipelago is a chain of islands situated on the Ryukyu Arc that consists of six major islands and 55 smaller islands of various size. Compared to other parts of Japan, the documented seismic history of Ryukyu Archipelago is not well known. However, massive tsunami boulders can be found in the area, which definitely imply that mega-earthquakes have occurred in the vicinity of the Ryukyu Archipelago. Here we describe a number of massive tsunami boulders and our estimation of the magnitude of the respective potential causative mega-earthquake. We also compared our results with those from other independent studies. Taken together, all results lead us to conclude that a mega-earthquake with a magnitude of > 9 is likely to occur in the vicinity of Ryukyu Archipelago. The moment magnitudes of mega-earthquakes estimated from the tsunami boulders range between 8.6 and 9.7. These results are consistent with estimations from the segmentation approach and from the Japan Nuclear Regulation Authority. Therefore, the disaster-prevention measures for Ryukyu Archipelago should take the findings from this study into account and urgently begin to implement counter-measures against the possible damaging effects of mega-earthquakes.