Lower crustal intraplate seismicity in Kachchh region (Gujarat, India) triggered by crustal magmatic infusion: Evidence from shear wave velocity contrast across the Moho

The genesis of intraplate seismicity remains a topic of debate among seismologists. Here, we address this issue by investigating shear wave velocity contrast across the Moho (δβM) in the Deccan volcanic-covered Kachchh seismogenic region, located in the Gujarat state of western India. This region, having complex geology, has witnessed several large damaging intraplate earthquakes since historical times. Most of the shallow focused earthquakes clustered in the eastern part of Kachchh often occurred due to the fluid pressure in the fractured rock matrix at the intersection of a fault network consisting of mainly three prominent faults, viz. Gedi, Kachchh Mainland, and North Wagad. However, the causes of lower crustal earthquakes are not well understood – whether it is due to the reactivation of faults or processes related to the fluid-filled fracked rock matrix of mafic/ultramafic material or magmatic infusion from the underlying mantle. Our results show that the δβM in the study area varies widely from ∼0.15 to 0.86 km/s. The lowest value of δβM (∼0.15 km/s) coincide with lower crustal seismicity. Further, the scaling relation between δβM and crustal thickness is negative, i.e., the thick crust has a low δβM, implying that the magmatic material has enveloped the lower crust, which ascended through the deep rifts during Deccan volcanism at ∼66 Ma. Higher values of δβM below a few stations have been ascribed to the pockets of trapped magmatic material or fluid below the lower crust. We suggest that the presence of mafic/ultramafic material in the crust-mantle transition zone, together with the fluid-filled fractured rock matrix in the intersecting fault geometry might accumulate enough strain to generate seismic activity in the lower crust. The presence of volatile influx or fluid pressure facilitates the triggering of deep-seated faults, and CO2-rich carbonatite-type magma percolated from the shallow asthenosphere.