The 2003 December 26 Bam earthquake (Iran), Mw 6.6, aftershock sequence

From 2003 December 29 to 2004 January 30, a dense seismological network of 23 stations was installed in the epicentral area of the 2003 December 26 Bam earthquake to study the aftershock seismicity. We select the 331 earthquakes recorded at a minimum of 10 stations, with rms less than 0.1 s and uncertainties less than 1 km, to infer the precise geometry of the seismicity in the fault region. We also process the data with the Double Difference technique to confirm the results. The aftershock cluster is 25 km long, trends north-south, and is located 5 km west of the Bam-Baravat escarpment, exactly beneath the observed surface breaks. At depth, aftershocks are concentrated between 6 and 20 km, beneath the upper layer of relatively low velocity that experienced the maximum slip, and they dip slightly westward. The southernmost part of the aftershock cluster is narrow and defines the rupture zone that is likely the Bam-Baravat fault at depth. However, it is unlikely that it is connected at surface to the Bam-Baravat escarpment but more likely to the co-seismic ruptures south of Bam. On the contrary, the distribution of the northernmost aftershocks spread into a more complex pattern, which is consistent with a northward propagation of the rupture along the fault plane. The focal mechanisms are consistent with right-lateral strike-slip faulting on N-S trending faults, parallel to the Bam-Baravat escarpment.