The M7.1 May 26, 2003 off-shore Miyagi Prefecture Earthquake in northeast Japan: Source process and aftershock distribution of an intra-slab event

A M7.1 intra-slab earthquake occurred on May 26, 2003 at a depth of 68 km off-shore Miyagi Prefecture, northeastern Japan. The epicenter of this event was located about 80 km NNE from the epicenter of the 1978 offshore Miyagi earthquake with magnitude of 7.4, whose possible recurrence in near future is a source of concern. We relocated the 2003 M7.1 event and its aftershocks by the homogeneous station method. Aftershocks are distributed along a plane dipping steeply to the WNW, which coincides with one of the nodal planes of the moment tensor solution. They are distributed within both the subducting oceanic crust and the mantle of the slab. The mainshock is located near the center of the aftershock zone and probably occurred near the bottom of the oceanic crust. We also determined the slip distribution of the 2003 M7.1 event by a waveform inversion method based on the empirical Green’s function. We inverted broad-band waveform data observed at stations with epicentral distance less than about 100 km. Results show that the spatial extent of the rupture area is almost consistent with that of the aftershock area. There exist two areas where large amount of slip occurred (asperities). One is located along the northeastern edge of the rupture area and the other is located in the southwestern portion. Aftershock activity is low within those asperities.