Hypocenter and Focal Mechanism Distributions of Aftershocks of July 26, 2003, M6.4 Northern Miyagi Earthquake Revealed by Temporary Observations

We conducted a temporary seismic observation just after the occurrence of July 26, 2003, M6.4 northern Miyagi earthquake, in order to precisely locate aftershock hypocenters. Thirteen portable data-logger stations and one communication satellite telemetry station were installed in and around the focal area of the earthquake. Hypocenters of aftershocks were located by using data observed at those temporary stations and nearby stationary stations of Tohoku University, Hi-net and Japan Meteorological Agency. Obtained aftershock distribution delineates the fault planes of this M6.4 event in the depth range of 3-12km, dipping to the west at an angle of -50 degree in the northern part of the aftershock area and to the northwest again at -40 degree in the southern part. Temporary observation data also allowed us to determine focal mechanisms of many aftershocks. The results show that focal mechanism of reverse fault type is predominant in this earthquake sequence including foreshock (M5.6), main shock (M6.4) and most aftershocks. Directions of P axes, however, are classified into three groups. P axes of M5.6 foreshock and the main shock estimated from P-wave poralities have NW-SE directions. On the other hand, moment tensor solution of the main shock has a P axis of east-west direction. Moreover, the largest aftershock (M5.5), that occurred in northernmost part of the aftershock area, has a P axis of NE-SW direction. Aftershocks with P axis of NW-SE direction occurred mainly in the southern part of the aftershock area where M5.6 foreshock and the main shock ruptures initiated. Many aftershocks with P axes of east-west direction took place in the central part of the aftershock area where large amount of fault slips by the main shock were estimated by wave form inversions. Many aftershocks in the northernmost part of the aftershock area have the same focal mechanisms as that of the largest aftershock.