Three-dimensional seismic velocity imaging of the Kachchh rift zone, Gujarat, India: Implications toward the crustal mafic pluton induced intraplate seismicity

Abstract

Three-dimensional tomographic images of seismic velocities have been constructed for the 2001 Mw7.7 Bhuj earthquake region using data from a dense 58-station seismograph network, during 2001–16. Two distinct, prominent high-velocity anomalies (10–14% increase in Vp and Vs, and 10–11% decrease in Vp/Vs) have been mapped at 2–40 km depths below the Banni and Wagad uplift regions, which could be attributed to mafic plutons. We also image several high-velocity zones (intrusive mafic rocks) and some low-velocity patches (meteoric water/metamorphic fluids/volatile CO2) within the main rupture zone of the 2001 Bhuj earthquake, where maximum confinement of seismicity is noticed. We propose that stresses induced by mafic plutons in response to the N-S compression of the Indian plate might have resulted in a stress regime near to critical for thesub-parallel rift associated fault system within the main rupture zone, thereby, generating small to large earthquakes through small stress perturbations caused by crustal fluid flows and stress transfer.