Analysis of the shallow crustal structure of the Bam Fault Zone, Iran, using the surface wave and first-arrival P-wave tomography approaches

The crustal structure around the Bam fault located southeast of Iran was investigated by applying surface and P-wave tomography to locally recorded aftershocks of the destructive Bam earthquake (Mw 6.6) on December 26, 2003. The aftershock dataset was recorded for 1 month after the main shock by 23 temporary seismic stations operated by the International Institute of Earthquake Engineering and Seismology (IIEES). After applying the selection criteria, 331 well-located events, including more than 1700 waveforms, remained. By calculating the surface wave dispersion measurements in the period range of 0.6-5 s, a tomographic procedure was then performed, and the local dispersion curves were extracted for each geographic grid point (1.8 km x 1.8 km). We applied an iterative damped least-squares inversion procedure to obtain the VSV and VSH models at depths of up to 6.5 km. The discrepancy between these VSV and VSH models also resulted in the radial anisotropy model. Moreover, the P-wave first-arrival times were inverted by the computer program SIMULPS to calculate the VP model for a depth range of 6.5-14 km, while the calculated VP model does not have sufficient resolution at depths of up to 6 km. The tomographic results indicate that a 2 km thick upper sedimentary layer with a shear wave velocity of less than 2.4 km/s and relatively negative radial anisotropy covers the study area (except Bam City, with a thin 500 m positive radial anisotropy layer), which overlay on relatively positive radial anisotropy. Our results indicate that the main shock occurred on the Bam fault escarpment; then the rupturing was shifted to the co-seismic surface rupture (approximately 5 km west of the escarpment) at a depth of-8-10 km. This dividing of energy on the fault plane (an oblique Y shape) leads to damping most of the energy as a deformation in the lower sedimentary layer.