Paleomagnetism reveals the emplacement age of tsunamigenic coral boulders on Ishigaki Island, Japan

We use temperature-dependent viscous remanent magnetization to estimate the emplacement age of tsunamigenic coral boulders along the shorelines of Ishigaki Island, Japan. The boulders consist of the hermatypic coral Porites, and the time of their deposition by tsunamis has been established using radiocarbon dating. Recently deceased corals at reef edges around Ishigaki Island record the Earth's current magnetic field (present Earth field, PEF) as a remanence parallel to the field in the skeleton. Since the time when the coral skeletons were emplaced on the shorelines as boulders by destructive tsunami waves, a new viscous magnetization was partially overprinted in the boulder parallel to the PEF. The results of thermal demagnetization indicated that the boulders were rotated at least once, and their emplacement ages determined from L. Néel's relaxation theory for single-domain magnetite agree well with the radiocarbon ages, although there are traces of multidomain magnetites. New application of Néel's theory to tsunamigenic coral boulders gives us an opportunity to ascertain the age and transportation mode of individual tsunamigenic coral boulders in this area.