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Three Dimensional Structure of the BAM Region from Local Earthquake Tomography

Document Type : Original Research Paper

Authors

1 M.Sc. Student of Geophysics, International Institute of Earthquake Engineering and Seismology, Tehran, Iran

2 Associate Professor, International Institute of Earthquake Engineering and Seismology, Tehran, Iran

3 Assistant Professor, Institute of standard and Industrial research - Khorasan-e Razavi, Mashhad, Iran

Abstract

The catastrophic December 26, 2003 Mw 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. This earthquake attracted much attention, and has been far more studied that which would be expected from a moderate magnitude earthquake. Nevertheless, there are doubtful results related to geometry and location of the causative fault that produced the Bam earthquake. As very little is known about the 3-D structure of the region and in order to define the 3-D velocity structure and the geometry of the active fault in the Bam area travel time data from the aftershock series of the Bam earthquake  are  inverted simultaneously for both hypocenter locations and 3-D Vp structure. The data used for this purpose are 6201 P-wave arrival times from 544 selected local earthquakes recorded by temporary 23 short-period seismic stations. 3-D P-wave velocity variations down to 20 km depth were obtained. The acquired tomographic images show that the 3-D velocity structure beneath the region is heterogeneous in that low velocity appears throughout the region down to ~10 km depth, and high velocities occur in western part from ~14 km depth. Velocity structure of seismogenic region is well resolved to a depth of 20 km. Difference in observed velocities in two sides of  the Bam fault in depth of 10-20 km is clear in tomographic images and on cross sections. We relate this difference in velocity to the effect of the reverse Bam-Baravat fault which seems influenced the deeper layer down to 20 km depth. This implies that the pre-existing reverse Bam-Baravat fault is a major active structure in the region that could be caused the December 2003 Bam earthquake.

Keywords

References
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Scientific Quarterly Journal of Geosciences
Volume 22, Issue 88
December 2013
Pages 31-40
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