S-wave anisotropy in the aftershock region of the 2003 Bam, Iran, earthquake (Mw 6.5)

Abstract

The 2003 Bam earthquake (Mw 6.5) occurred in the southeastern Iran on December 26, 2003. We studied S-wave anisotropy using the aftershock data observed at 5 stations in and around the aftershock region. The fast-polarization direction of shear wave splitting at the stations near the aftershock region is almost NE–SW, which corresponds to the maximum compressive stress trend (the P-axis) of the main shock. It is suggested that the inferred anisotropy might be caused by a systematic alignment of cracks resulting from the present-day stress-field. The fast direction at the station just above the fault zone shows EW direction normal to the fault strike. The fault normal alignment of cracks introduces a significant reduction of shear stress of the fault, which suggests a high stress drop and large slip asperity. Another possible explanation for different result at the station from that of other stations is the effect of inhomogeneous velocity structure in and around the fault damaged zone. The delay time of the split S-waves ranges between 20 and 100ms. Large delay time area is found in the shallower part of the fault zone with high aftershock activity. Assuming a uniform distribution of anisotropy inside and outside the aftershock region, we estimated that the crack densities are 0.013 inside and 0.004 outside the aftershock region. It may show more intense cracks within the high active region of aftershocks.