Evidence for reactivation of new faults and seismicity migration away from the causative fault of the 2001 MW 7.7 Bhuj earthquake, western India

Abstract

SUMMARY The Kachchh Rift Basin of western India that hosted the devastating 2001 MW 7.7 Bhuj earthquake has been witnessing minor to moderate seismicity since then. The genesis of these intraplate earthquakes and geometry of the causative faults is still elusive. In this study, we relocated all the earthquakes recorded between 2006 and 2018 and utilized a set of 4285 best located earthquakes to provide rare insights into the orientation and depth distribution of the currently active faults. We also analysed previously computed source mechanisms, in perspective of our location results. It is revealed that the present seismic activity is primarily aligned along two planes—one steeply dipping (∼60°) to the northeast with a NW–SE strike and the other gently (∼34°) dipping to the SSW, striking WNW–ESE. The traces of these fault surfaces coincide with the Kachchh Mainland fault (KMF) and the North Wagad Fault (NWF) when extrapolated to the surface, respectively. Activity along the NWF has been shown in earlier studies, however, clear evidence of activity along the steep north-dipping KMF is presented for the very first time. Thrust earthquakes dominate the NWF while strike-slip earthquakes are seen across the KMF. Our results show that the two fault surfaces converge between 70.30°E and 70.43°E longitudes in the depth range 22–32 km, however, there is large E–W offset between the northern and southern extremities of the fault-system. This convergence zone hosted the largest earthquakes (ML 4.7–5.1) between 2006 and 2018 and the hypocentre of 2001 main shock also coincide with it. The earthquakes occurring in the interfault region show major strike-slip motion and are probably influenced by the relative motion between the NWF and KMF. Consistent seismicity, assisted probably by high conducting material, is seen across the NWF while intermittent seismicity is revealed along the KMF. Scrutiny of earlier studies revealed that the 2001 main shock and first few weeks of aftershocks were hosted on another south-dipping fault, the South Wagad Fault (SWF), however, activity along the SWF is found to be meagre during 2006–2018. Instead, seismicity has currently migrated to faults north and south of the SWF. These currently active faults most likely were reactivated during/after the 2001 main shock.