No abstract

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 identifi ed 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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The occurrence of large earthquakes and tsunamis along the Ryukyu Trench is a subject of continuing interest, the key to which is the long-term geological record. Here we describe the clast size and spatial distributions of ~2900 boulders on the reefs of the Ryukyu Islands, Japan, as markers of paleotsunamis and causative tsunamigenic earthquakes. Boulders of tsunami origin were observed only at a specifi c island group at the southern end, suggesting the local occurrence of tsunamigenic earthquakes there. In contrast, in the central to northern Ryukyu Islands, no evidence exists of tsunamis larger than those at the southern end of the Ryukyu Islands during the past 2000-3000 yr. These islands have numerous boulders deposited by storm waves during the past 2300 yr or earlier. Their spatial distribution has not been disturbed by large tsunamis. This suggests that large tsunamis did not strike this area during that period; nevertheless, these regions are seismically active. Our study shows that coastal boulder deposits present great potential to not only ascertain the histories and effects of paleotsunamis but also to constrain fault models of the causative earthquakes.

Shiikuwasha (Citrus depressa Hayata) is distributed from the South-west of the Japanese archipelago to Taiwan. In this study, re-sequencing against the orange (C. sinensis (L.) Osbeck) chloroplast genome was applied to one superior landrace of Shiikuwasha cultivated in Oku ward, Okinawa, Japan. The chloroplast genome of the landrace was estimated to comprise 160,118 bp, including 48 indels and 71 nucleotide substitutions against the reference genome. The presumptive chloroplast indels were confirmed by subsequent experiments, and these identified multiple maternal lineages among other landraces. Some of the orange SSR markers were available for genotyping of other superior landraces and were able to distinguish among them. These molecular markers were then applied for evaluation of genetic diversity among wild and cultivated Shiikuwasha accessions. Except for Oku ward, the cultivated populations were found to have lost their genetic diversity in comparison with wild populations. Groves in Oku ward maintained, or showed even higher genetic diversity than wild accessions in the surrounding areas by the force of villagers.