Relocations and local earthquake tomography: Implications toward the mafic pluton induced crustal seismicity in Kachchh, Gujarat, India, for last 18 years

Abstract

The study presents new probable crustal model based on precise double difference relocations of 2420 aftershocks of the 2001 Bhuj earthquake (Mw7.7) during 2001–16 and combining other geophysical and geological studies in the region, which will lead to a better understanding of the cause of uninterrupted aftershock activities in the region. Relocations of aftershocks delineate two main seismic clusters in the central aftershock zone (CAZ), i.e. Eastern (A1) and Western (A2). The alignments of relocated hypocenters of the aftershocks delineate three main fault segments viz. the south-dipping north as well as south Wagad faults, and north dipping Kachchh mainland fault. Relocations also detect some zones of devoid of aftershocks with a typical dimension of 3–9 km within the CAZ, which could be related to locales of future earthquakes. Seismic velocity tomograms depict that both seismic clusters A1 and A2 are associated with high velocity anomalies (inferred to be mafic plutons) below the Wagad and Banni regions, respectively. The E-W depth section of relocated hypocenters delineates an 18-km variation of the seismogenic base, which could be attributed to the increase in crustal maficness due to the intrusion of mafic plutons below the CAZ. Stress inversions of selected focal mechanisms depict a rotation of 44° in the σ1-orientation within the CAZ, which could be attributed to large local stresses induced by mafic plutons. The continued seismicity at Kachchh is proposed to be caused by the presence of large stresses associated with mafic plutons owing to the N-S plate compression associated with northward movement of the Indian plate wherein crustal fluid flows (aqueous/metamorphic fluids or volatile CO2) might provide the triggering to generate continued aftershock activities at Kachchh, India.