Radiocarbon dating of Porites coral boulders cast ashore by paleo-tsunamis at southern Ryukyu Islands, Japan

It is important for future disaster mitigation to evaluate the recurrence period and/or frequency of extreme geohazard events such as earthquakes, tsunamis and severe storms. We focus on Porites spp. coral boulders scattered both along the shore and on the reef at southern Ryukyu Islands, Japan. Our previous works confirmed that Porites boulders on Ishigaki Island were cast ashore by the 1771 Meiwa tsunami, which was one of the largest tsunami disasters in Japanese history, according to radiometric dating [Suzuki et al. 2008; Araoka et al. 2010], thus these fossil coral boulders have possibilities for recording paleo-tsunami events. We performed extensive field survey over Ryukyu Islands, and the biggest Porites colony boulder was identified at the eastern coast of Ishigaki Island, which were cast ashore by 1771 Meiwa tsunami according to multiple radiocarbon dating on the surface of the colony. We also found several Meiwa-tsunami-derived Porites corals on several Islands, according to radiocarbon dating of these boulders. These results could help to constrain the simulation models of the 1771 Meiwa tsunami [Imamura et al. 2008; Goto et al. 2010], and lead to solving the source fault location of 1771 Meiwa tsunami, which has still been controversial. Our latest results from several Islands at southern Ryukyus also suggest that Porites coral boulders cast ashore by not only 1771 Meiwa tsunami but also other various paleo-tsunamis. These results would lead to estimate the recurrence period and/or frequency of tsunami events in the region. Moreover, modern and fossil Porites corals have been used to reconstruct high-resolution records of oceanographic and climatic changes. By splicing paleoenvironmental data reconstructed from several coral boulders which were cast at various dates, it has a potential to reconstruct high-resolution paleoenvironment records for several centuries.