Characteristic activity and migration of episodic tremor and slow-slip events in central Japan

Continuous seismic data and tilt measurements in the Aichi and Mie regions of central Japan indicate significant migration of episodic non-volcanic deep tremors and short-term slow-slip events on the subducting plate interface at the deeper extension of the mega-thrust earthquake seismogenic zone. These episodic tremor and slip (ETS) events occur in two concentrated regions on either side of Ise Bay at recurrence intervals of approximately every 6 months throughout 2004 and 2005, separated by a distinct tremor gap. A migrating tremor episode from the southern edge of the Mie region in January 2006, however, is found to have crossed aseismic Ise Bay along a continuous 200 kin long strike of the subducting Philippine Sea Plate. Coincident with the migration of tremors, very low frequency earthquakes with a predominant frequency of 0.05 Hz and tilt changes lasting for a few days were detected. Inversion analysis of the tilt data indicates that at least five reverse faultings with an average slip length of I cm took place on the plate boundary, migrating from southwest to northeast, equivalent to a total moment release of M-w 6.2. The tremor seismicity gap at Ise Bay is attributed to the presence of a small ridge in the Philippine Sea Plate that blocks most propagating ETS events. The 2006 episode propagated orthogonal to the majority of ETS event in this region, and was found not to be obstructed by the ridge structure.