Coulomb static stress variations in the Kachchh, Gujarat, India:: Implications for the occurrences of two recent earthquakes (Mw=5.6) in the 2001 Bhuj earthquake region

Double difference algorithm was used to relocate aftershock sequences of two M-w = 5.6 Kachchh earthquakes (India), which have occurred on 2006 March 7 and April 6, along almost vertical strike-slip Gedi fault (GF) and south dipping reverse North Wagad fault (NWF), respectively. The relocated focal depths delineate a marked variation of 4 and 7 km in the brittle-ductile transition depths beneath GF and NWF, respectively. We modelled the Coulomb failure stress change (Delta CFS) produced by the 2001 mainshock and 2006 GEDI events on an optimally oriented plane. A strong correlation with occurrences of aftershocks and regions of increased Delta CFS is obtained on both the faults. The predicted Delta CFS on the GF increased by 0.14 MPa at 3 km depth, where the 2006 March 7 M-w = 5.6 event occurred. The variation in brittle-ductile transition depths may be related to the perturbation in geothermal gradient resulted from the presence of intrusives or varying saturation state of fluid-filled fractured rock matrix beneath both the fault zones.