Rockmagnetic characterization of event deposits induced by March 2011 Tohoku-oki Earthquake in the Japan Trench

A study on differences in bathymetric data between before and after the 11 March 2011Tohoku-Oki earthquake revealed a large coseismic displacement (up to 50 m horizontally) of the overriding plate. The study also identified that a topographic high had appeared in the trench axis after the earthquake. This bulge is interpreted as a possible mass of remobilized deposit derived by the landsliding (Fujiwara et al., 2011). Accompanied by this mass transportation, event deposits (e.g. slump, debris, or turbidite) could be formed in the seafloor as evidences of the mega-earthquake. In order to characterize the sediment deposition or disturbance by the earthquake, six sediment coring using a piston corer were conducted and collected the surface sediment around the topographic high in the deep trench axis. Intervals in the upper several ten-cm of recovered cores consist of graded units, which are considered to have been formed just after the earthquake. Other graded units are also recognized in the deeper intervals than those of 2011 event. However, there is no interval indicating a mass transportation in cores, even in the core taken from the top of topographic high. Rock magnetic studies on samples were carried out to analyze their depositional process. Magnetic susceptibility profiles show several upward decreasing patterns, and they can be interpreted as upward fining cycles. Anisotropy of magnetic susceptibility were measured to estimate paleo-current directions which controlled the top sequence depositions. The results show that Kmin axes are normal to the horizontal plane. It indicates that all cycling sequences were not formed by chaotic deposition (e.g. debris flow), and that the topographic high appeared after the earthquake was not formed by landsliding. On the other side, Kmax directions are parallel to the horizontal plane and those alignments generally show clusters in two major directions. It is interpreted that such variation was formed by different paleo-currents occurred during the event. A combination of trench parallel and down slope currents, which are almost perpendicular each other, might make this pattern. A variation in lineation parameter of anisotropy of magnetic susceptibility suggests different current speed during deposition of the top sequences. These data indicates that the depositional process of 2011 event sediment in the trench system was not simple. Figuring out this process is important not only for understanding the deposition of 2011 event but for understanding the older event deposits took place previously in the Japan trench.