Information about past tsunami hazards, such as their recurrence interval and magnitude, is needed for future disaster prediction and mitigation. We examined radiocarbon ages of the surfaces of massive coral boulders cast ashore by past tsunamis in the southern Ryukyu Islands, Japan, where few historical and geological records of past tsunamis are available. We selected only non-eroded Porites coral boulders along the shoreline, because their characteristics make it possible to determine the probable timing of their deposition by tsunamis, and we applied a dating method that uses the cumulative probability distributions of large numbers of radiocarbon measurements of those boulders to determine the timing of past tsunamis. The results demonstrate that the southern Ryukyu Islands have repeatedly experienced tsunami events since at least 2400 yr ago, with a recurrence interval of ~150-400 yr. The largest Porites tsunami boulder that we studied (long axis, 9 m), which is probably the largest single-colony tsunami boulder in the world, was displaced by the A.D. 1771 Meiwa tsunami. Although the 1771 Meiwa tsunami was likely the largest event in at least the past 700 yr, calculations of current velocity show that all identified tsunamis occurring before 1771 were probably large enough to cause considerable damage to human-built structures and loss of life. This study demonstrates that by reliably dating large numbers of selected coastal boulders it is possible to ascertain the timing, recurrence interval, and magnitude of past tsunamis in a location where few adequate survey sites of sandy tsunami deposits exist.
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