Rupture process of the July 2003 northern Miyagi earthquake sequence, NE Japan, estimated from double-difference hypocenter locations

Abstract

A shallow M6.4 inland earthquake occurred on July 26, 2003 in the northern part of Miyagi Prefecture, northeastern Japan. We precisely located hypocenters of this event, its foreshocks and aftershocks by applying the double-difference method to data from temporal seismic stations and the seismic stations of Tohoku Univ., JMA and Hi-net. Aftershocks thus located are distributed in an area with about 15 km × 15 km and their depths range from 2 to 13 km. They are distributed along a curved plane dipping westward to north-westward with a dip of about 50 degrees. Its strike is about N-S in the northern part and about NNE-SSW in the southern part. The M5.6 foreshock, which occurred 7 hours before the main shock, is located near the center of the whole aftershock area at which the strike of the aftershock alignment changes abruptly. Aftershocks for the M5.6 foreshock are distributed in the southern part of the whole aftershock area. The main shock is located in the southern part of the aftershock area. It occurred near the edge of the area of aftershocks for the M5.6 foreshock. Aftershocks following the main shock are mainly distributed in the northern part of the aftershock area. The largest aftershock (M5.5) is located at the northern end of the whole aftershock area. After the occurrence of the main shock, few aftershocks occurred in the area of aftershocks for the M5.6 foreshock. We also determined fault plane solutions for the events that occurred during the sequence from the M5.6 foreshock to the largest (M5.5) aftershock. The spatial variation of focal mechanism is consistent with the curved geometry of the fault plane estimated from the aftershock distribution.