Local earthquake seismic tomography of the Kachchh rift zone (India) using arrival times of P- and S-waves detect a semi-circular region of lower seismic velocities (8–15% drop in Vp and 10–20% drop in Vs) down to 10 km depth and increased seismic velocities (10– 17% increase in Vp as well as Vs) between 10 and 40 km depths, which spatially correlates well with the areas of earliest alkaline magmatism related to the Deccan plume, in the SW part of the Kachchh rift. The occurrence of ultramafic xenoliths and tholeiitic basalts in the region suggests a direct link of this magmatism with the initial stage of Deccan plume. Thus, the mapped anomaly might be indicating the presence of imprints of a crystallized magma-conduit related to the 65 Ma Deccan mantle plume. This plume model gets further support from the modelled crustal and asthenospheric thinning below this anomaly, from earlier studies. Relocations of majority of M ≥ 5 events (including the 2001 mainshock and the 1956 Anjar event) fall within this semi-circular crustal anomaly. Most of these M≥5 events are observed to occur near the contacts between high- and low-velocity zones while some earthquakes (including the 1956 Anjar earthquake) also occurred in the low-velocity zones (8–18% drop in Vp and Vs, indicating the presence of aqueous/metamorphic 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.
Citation: Prantik Mandal, 2021. Natural Hazards