Abstract The Bam earthquake (2003 December 26, MW = 6.6) was one of the largest earthquakes that occurred in southeast of Iran during last century. It took place along an N–S trending right-lateral strike-slip fault, almost near the southern end of Nyband–Gowk fault. In this study, we mapped the frequency–magnitude distribution of aftershockevents spatially across the Bam aftershock zone. The b-valuevaries between 0.6 and 1.1 across the Bam rupture zone. Theoverall depth distribution of b-value in Bam aftershock zonereveals two distinct increases in b-value: (1) at depths of8–10 km and (2) shallower than 4 km beneath the Bam city.There is no correlation between high b- value anomaliesfound in this study and the region of largest slip, whereas thespatial correlation between high b-value anomalies and thezone of low Vs and high r (in earlier tomography study) isobvious. This correlation reveals that material properties andincreasing heterogeneity are more important in controllingb-value distribution in Bam earthquake rupture zone. Thehigh b-value anomaly near the surface of northern part ofrupture zone may be related to unconsolidated and water-richquaternary alluvial sediments and probable low-strengthrocks beneath them. The high b-value anomaly at depthrange 8–10 km can be correlated with fractured and fluid-filled mass, which may result from the movement of magmaduring Eocene volcanism in the Bam area. In this study, theinduced changes in pore fluid pressure due to main shock resuggested as a mechanism for aftershock generation.

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