Historic 1771 Meiwa tsunami confirmed by high-resolution U/Th dating of massive Porites coral boulders at Ishigaki Island in the Ryukyus, Japan

The chronicles of the Ryukyu Kingdom describe at least four coastal inundation events at Ishigaki Island during the seventeenth through nineteenth centuries: the 1771 Meiwa earthquake tsunami, which killed 12,000 people; another earthquake tsunami (1687); and two events with unrecorded causes (1625 and 1714). We used a high-resolution U/Th method to date well-preserved surface parts of massive Porites spp. coral head boulders that had been cast ashore and found that the ages of two boulders correspond to the 1771 event. One large Porites boulder (diameter > 4 m) was dated to 1625, suggesting that that event was also a large inundation event. Calibrated C-14 ages, based on a newly estimated local value for the radiocarbon marine reservoir effect, clustered around 1625 and 1771, suggesting that these events were larger than the others. Our study demonstrated that Porites boulders have enormous potential value for studies of paleoinundation events such as tsunamis and extreme storms.