Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone, Gujarat, India

In view of an anomalous crust-mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF1, SWF2), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle-ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10-20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle-ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust-mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO2 at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.