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

Kazushige Obara,Shutaro Sekine

doi:10.1186/bf03353196

Abstract

AbstractContinuous 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 km 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 1 cm took place on the plate boundary, migrating from southwest to northeast, equivalent to a total moment release of Mw6.2. 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.

Highlights

Non-volcanic deep tremors and accompanying transient slow-slip events, which are termed episodic tremor and slip (ETS) have recently been discovered as a new geophysical phenomenon in the transition zone (Hyndman et al, 1997) at a subducting plate interface
For example, tremor epicenters have been found to migrate along the strike of the Philippine Sea Plate at a speed of close to 10 km/day (Obara, 2002) and the location of the source fault of the corresponding slow-slip event has been roughly estimated to move with the tremor activity (Obara et al, 2004)
The location of rupture initiation and the extent of the rupture zone may be controlled by small fluctuations in strain accumulation or fluid content in the patch of the slow-slip event, while the propagation direction of the ETS event may be determined by the configuration of the plate boundary

Summary

Introduction

Non-volcanic deep tremors and accompanying transient slow-slip events, which are termed episodic tremor and slip (ETS) have recently been discovered as a new geophysical phenomenon in the transition zone (Hyndman et al, 1997) at a subducting plate interface. For example, tremor epicenters have been found to migrate along the strike of the Philippine Sea Plate at a speed of close to 10 km/day (Obara, 2002) and the location of the source fault of the corresponding slow-slip event has been roughly estimated to move with the tremor activity (Obara et al, 2004) The extent of such ETS events in the period from 2001 to 2004 in western Shikoku was limited to within 100 km (Obara and Hirose, 2006). From January 8, tremors with large amplitudes became frequent, and the epicenters migrated gradually north-northeastward, crossing Ise Bay, and continuing east-northeastward in the Aichi region at a speed of approximately 10 km/day This is the first record of tremor activity crossing Ise Bay. This is the first record of tremor activity crossing Ise Bay This event had the longest tremor source distribution on record, extending for a length of approximately 200 km along the strike of the subducting Philippine Sea Plate. The synthesized tilt trace is generally coincident with the observed data, the traces diverge somewhat in the Aichi region suggesting that the model geometry or slip parameters employed in the model are not sufficiently representative in this region

Discussion

Conclusion