Researchers from Tohoku University unveiled that tsunami boulders of coral on Ishigaki Island, Japan had been changed place or not by subsequent tsunamis. They employ geomagnetic field as a level, and also the remanent magnetization of corals with Neel's relaxation theory as a clock. The work was published online May 22 in Geology.
You can see lots of tsunamigenic erratic boulders of coral along the eastern sea side of the Ishigaki Island, Japan. Previous radiocarbon dating of coral boulders tells us that the Island has been met by multiple tsunamis at the 200~400 years interval since 5000 years ago. These tsunamis may displace the boulders more than one, but we don't know how many times the boulder displaced to the present position. The latest one is Meiwa tsunami at A.D. 1771. Here the authors applied a paleomagnetic method to solve this problem. If corals have a tiny nanometer-sized magnetite as an inclusion in a skeleton, the coral can record the geomagnetic field direction as a remanence. When the boulder was displaced or rotated, the direction of remanence is also changed and a new remanence will be added as time goes by in the boulder. This is a paleomagnetic clock with a level. The result showed that a huge boulder (>200 t) had displaced in the present position by ancient tsunami, but stayed there by the Meiwa tsunami. Therefore, we concluded that the application of the paleomagnetic method to tsunami boulders unveils a displacement history of tsunami boulders.
This work was supported by the Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (B) (22340146) and a Grant-in-Aid for Priority Project Research of the International Research Institute of Disaster Science (C-12) (Tohoku University, Japan).
Tetsuro Sato (Graduate school student) and Norihiro Nakamura (Associate Professor)
Department of Earth Science
Email: karimero0515*gmail.com and n-naka*m.tohoku.ac.jp (Replace * with @)
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