Particle Motion Polarization Properties of Eastern Shikoku Tremor

Deep non-volcanic tremor or slow slip has been observed at many subduction zones around the world. The best observations, which are characterized by both a temporal and spatial coincidence of the two phenomena known as Episodic Tremor and Slip (ETS), are located along the Nankai Trough in Japan and in northern Cascadia. The eastern Shikoku region of southwest Japan experiences repeated episodes of deep non-volcanic tremor with a recurrence interval of a few months. The minor slow slip event is expected to occur coincident with each eastern Shikoku tremor episode. We analyze the polarization properties of eastern Shikoku tremors during the March 2007 event and compare the observed polarizations to those predicted from tremor emanating from the plate interface. Preliminary polarization analysis of 6 NIED Hi-net seismic records in eastern Shikoku show a consistent trend toward shallow dip angles, suggesting horizontal particle motion, when the tremor sources are beneath the stations. Despite low linearity and significant scatter, probably related to considerable heterogeneity of the continental crust, the polarization azimuths observed at most of the stations align approximately with the direction of plate convergence. Predicted polarization directions of near-vertical upgoing S-waves radiated from shear slip on the plate interface align with the relative motion between the Philippine Sea and Eurasian plates. This finding suggests that eastern Shikoku tremors represent shear slip on the plate interface, consistent with similar polarization observations in Cascadia, and with inferences in Japan from focal mechanisms of low- frequency earthquakes and very low- frequency earthquakes.