Viscous remanent magnetization of individual boulders in Ishigaki Island and its application to estimate the paleotsunami histories

Many erratic boulders are distributed along the coasts in the world (Sheffers, 2008). Recent studies investigated the distribution of boulders and their sizes to understand the mechanism for the transportation (e.g. storm, tsunami, erosion). Previous works also conducted the radiocarbon dating to reveal the depositional ages of boulders. In Ishigaki Island, Japan, there are many erratic boulders on the beach emplaced by paleotsunami events. These boulders were originally hermatypic corals that were attached on the reef edge or single colony of coral (e.g. Porites sp.) in the moat. Then, corals were emplaced on the beach and reef flat by tsunamis and dead. The radiocarbon dating can thus estimate the timing of only oldest historical tsunami event when the corals were emerged above the sea water. Their distributions show these boulders were deposited by paleotsunamis (Goto et al. 2010) and the multiple tsunamis were occurred last 3000 years with 150-400 years interval (Araoka et al., 2013). In this study, we propose an alternative method using paleomagnetic viscous dating to reveal the multiple transportation events and their ages. When corals were attached to the reef edge, they magnetized parallel to the Earth's magnetic field. If they are transported by tsunami, its characteristic remanence component has been changed against Earth's magnetic field. Then boulders acquire a secondary magnetic component as a viscous remanent magnetization (VRM) parallel to the different Earth's magnetic field. This magnetization is overprinted to the boulder in each emplacement event, and increased with time. Thus, we conducted progressive thermal demagnetization to reveal the change in magnetic vector component in individual boulders. Also we utilize the Neel's single domain theory to date the acquisition of VRM. This theory shows the magnetization at low temperature over a long time demagnetized at high temperature in short time. Therefore, the turning points of magnetic vector component as VRM and their unblocking temperatures can reveal the multiple paleotsunami ages. In this presentation, we show the result of magnetic vector plot of individual huge boulders and determined their repeated transportation histories by multiple tsunamis by using Neel's single domain theory.