Identification of 1771 Meiwa Tsunami deposits using radiocarbon dating and oxygen isotope microprofiling of emerged massive coral boulders

The Meiwa Tsunami, one of the largest tsunamis recorded in historical documents in Japan struck Ishigaki Island and neighboring islands of the Ryukyu Arc in April 1771 AD. An enormous number of massive Porites coral boulders are scattered on the shore and in the reef moat of eastern Ishigaki Island. These boulders likely were cast ashore by the Meiwa Tsunami, but a detailed examination has not yet been conducted. The range of calibrated radiocarbon ages indicates that the transport of the boulders cannot be ascribed solely to the tsunami. The results of the oxygen isotope microprofiling suggest that the skeletal growth of most coral colonies was interrupted in summer or autumn; hence, tropical storms and typhoons are also very likely to be agents of transport. Thus, by combining radiocarbon dating with oxygen isotope microprofiling to investigate Porites coral boulders, it is possible to separate paleotsunami boulders from those transported by storm events as far as tsunamis occurred during the non-storm season.