Three-dimensional velocity imaging of the Kachchh seismic zone, Gujarat, India

To understand the causative mechanism of the continued occurrence of earthquakes in Kachchh, Gujarat for the last six years, we estimated high-resolution three-dimensional Vp, Vs and Vp/Vs structures in the aftershock zones of the 2001 Mw7.7 Bhuj and 2006 Mw5.6 Gedi earthquakes. We used 13,862 P- and 13,736 S-wave high-quality arrival times collected from the seismograms of 2303 aftershocks recorded at 5–18 three-component seismograph stations during 2001–06. Seismic images revealed a marked spatial variation in the velocities (from − 20% to + 14% in Vp, from − 12% to 13% in Vs, and from − 12% to 12% increase in Vp/Vs) in the 0–34 km depth range beneath the Bhuj aftershock zone. Relatively more increase in Vp than Vs, resulting in an increase in Vp/Vs in the crust beneath the seismically active causative fault (North Wagad Fault, NWF) zone of 2001 Bhuj mainshock suggests a rigid, mafic crust beneath the region. They also delineate an increase of 8% in Vp and 14% in Vs, and a decrease of 4% in Vp/Vs in the almost vertical rupture zone of the 2006 Gedi earthquake extending up to 12 km depth. This high velocity body associated with the Gedi mainshock is inferred to be a gabbroic intrusive. The Banni region and the Wagad uplift are found to be associated with high velocity intrusive bodies (inferred to be mafic) extending from 5 to 35 km depth, which might have intruded during the rifting in early Jurassic (~ 160 Ma). Aftershock activity is mainly confined to the zones characterized by high Vp, high Vs and low Vp/Vs ratio, which might be representing the strong, competent and brittle parts of the fault zone/intrusive bodies that could accumulate large strain energy for generating aftershocks for more than six years. It is inferred that the crustal stress concentrations associated with the intrusive bodies are contributing significant perturbation to the crustal stress regime to generate the intraplate earthquakes in the Kachchh rift zone. A few patches of slow (Vp and Vs) and high Vp/Vs between 10 to 30 km depth have also been detected on the causative 45° south dipping north Wagad fault (NWF) for the 2001 mainshock, which may be attributed to the fluid filled fractured rock matrix. Interestingly, the 2001 Bhuj mainshock hypocenter is found to be associated with such a low velocity patch. Thus, it is inferred that the occurrence of 2001 Bhuj mainshock might have facilitated by the presence of fluids at hypocentral depth.