Hypocenter and focal mechanism distributions of aftershocks of July 26 2003 M6.4 northern Miyagi, NE Japan, earthquake revealed by temporary seismic observation

Abstract

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 satellite communication telemetry station were installed in and around the focal area of the M6.4 event. Hypocenters of aftershocks were located by using data observed at those temporary stations and nearby permanent stations of Tohoku University, National Research Institute for Earth Science and Disaster Prevention (NIED) and Japan Meteorological Agency (JMA). Obtained aftershock distribution clearly delineates the fault plane of this M6.4 event in the depth range of 3-12 km. The fault plane dips westward at an angle of -50 degree in the northern part of the aftershock area and northwestward at -40 degree in the southern part. Data observed at dense temporary stations just above the focal area and nearby permanent stations allowed us to determine focal mechanisms of many aftershocks. The results show that focal mechanism of reverse fault type is predominant in this aftershock sequence. Directions of P-axes, however, varies mainly with locations of hypocenters, and are classified into three groups. Aftershocks with P-axis of NW-SE direction occurred mainly in the southern part of the aftershock area where the M5.6 foreshock and the main shock ruptures were initiated. Many aftershocks with P-axis 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 from waveform inversions. Many aftershocks in the northernmost part of the aftershock area have focal mechanisms with P-axis of NE-SW direction, similar to that of the M5.5 largest aftershock. A few aftershocks with normal fault type occurred close to convex regions of the main shock fault plane or outside of it.