Structure of the Seismically Active Kachchh Region

A 3-D structural model for the Kachchh rift zone has been constructed based on the results from our earlier studies using S-to-P converted phase, and joint inversion of P-receiver functions and surface wave dispersion data, which reveals a a seismogenic crustal volume (∼ 50×50×45 km3) with large sediment thickness and marked crustal as well as asthenospheric thinning below the Kachchh rift zone (KRZ). A local earthquake velocity tomography using 24019 P- as well as 23825 S-wave arrival times of 3750 relocated events from 41 three- component seismographs has been performed. Vp and Vs tomograms detect a distinct very high velocity lower crustal anomaly (Vp: 6.8-7.8 km/s; Vs: 3.9-4.3 km/s; Vp/Vs:1.7-1.8) at deeper depths (≥ 20 km) below the central KRZ. Three high velocity anomalies at 5-30 km depths have also been mapped below the Wagad uplift, Kachchh rift and Banni regions. These high velocity anomalies are interpreted as mafic plutons, within the mapped crustal volume, which might have resulted from the 65 Ma Deccan plume episode. Relocations of most of M≥5 events (including 2001 Bhuj mainshock) fall either near the contacts between high and low velocity zones or in the high-velocity zones while some earthquakes (including the 1956 Anjar earthquake) also occurred in the low-velocity zones (8-18% drop in Vs, indicating the presence of fluids). We propose that mapping of such a scenario of crustal and lithospheric structure where resultant tectonic forces encourage seismicity is crucial for the assessment of the intraplate seismic hazard.