Surface Deformation and the Fault Responsible for the 2003 Bam,Iran,Earthquake

The Bam fault zone is a major active fault zone in southeastern Iran. Geomorphic evidence indicates that it has been responsible for repeated faulting events since the late Pleistocene. The 26 December 2003 Bam earthquake was associated with a 14 km fresh surface rupture trending north-south along the preexisting Bam fault zone. However, an en echelon rupture pattern trending N 15° E developed in the surface of alluvial deposits 5 km west of the Bam fault, in an area where no fault trace is visible in the geomorphology. The slip along the surface ruptures ranged between 0.5 and 20 cm. Rather than being a direct manifestation of the earthquake fault that does not surface, the fresh surface ruptures associated with the Bam earthquake are secondary structures such as synthetic (Reidel) shears and mole tracks, which indicate right-lateral motion along the Bam fault zone. This is compatible with both the focal mechanism solutions of the earthquake and fault displacements during the late Pleistocene. Fresh surface structures indicate areas of dispersed strain not recognized on SAR interferometry.