Seismicity analysis of the Kachchh aftershock zone and tectonic implication for 26 Jan 2001 Bhuj earthquake

We have carried out a detailed analysis of seismicity in the vicinity of the 26 Jan 2001 Bhuj earthquake (Mw 7.7). From the depth sections of 24 parallel profiles, and from the b-value cross section, we claim the existence of a hidden fault which conjugates to the major rupture fault (i.e. North Wagad Fault) of the 2001 Bhuj earthquake. The proximity of the intersection of these faults to the focus suggests a close association Bhuj main shock generation. A circular pattern in the profiles also provides evidence for the existence of an intrusive, consistent with earlier findings from gravity-magnetic modeling and tomography studies. The location of the fault intersection within the intrusive support a model where both play a significant role in the earthquake generation. The intersection of the conjugate faults acts as a stress concentrator, while their presence within a big pluton possibly will facilitate the stress amplification. This mechanism might explain the occurrence of two Mw = 7.7 earthquakes in a relatively short time span of 182 years in the Kachchh rift.The b-value cross section displays high and low b-value patches along the two intersecting faults. This suggests a model of a faulted block that consists of two kinds of segment, the locked and the unlocked. Locked segments do not easily participate in creeping and therefore generate strong magnitude aftershocks (M > 3) while unlocked segments easily creep and result in only weak aftershocks (M < 3). The different fault segments with weak and strong magnitude gathers will result in high and low b-values, respectively.