Seismic Crustal Structure of Saurashtra (India) Using Ambient Noise Analysis

ABSTRACT

In western India, the Deccan Volcanic Province (DVP) is known for its geological complexity, a result of extensive flood basalt eruptions that took place ~ 65 million years ago, due to the interaction of the Indian plate with the Reunion plume. The Gulf of Cambay is considered a potential zone of plume-facilitatelithosphere interaction, supported by the anomalies of low shearwave velocity and thin lithosphere observed in the surrounding area, including Saurashtra and Cambay regions. In the present study, we have obtained the crustal shear-wave velocity (Vs) structure beneath the Saurashtra region, the Gulf of Cambay, the Gulf of Kachchh, and the surrounding coastal areas utilising ambient noise. Continuous seismic ambient noise recorded by sixteen broadband stations of the Institute of Seismological Research (ISR) between January 2015 and December 2017 was processed to retrieve Rayleigh wave dispersion curves through cross-correlation of the vertical-component recordings. A genetic algorithm-based inversion technique is used to retrieve 1D Vs structure across 12 different profiles from the dispersion data. The model delineates multiple crustal layers, including a high-velocity basaltic top layer, an underlying sub-basaltic layer, and distinct upper and lower crustal layers. In the Saurashtra region, high Vs of 2.6 to 3.3 km/s correspond to basaltic composition of upper crustal rocks, Vs between 3.6 and 4.0 km/s represent the middle to lower crust composed of metamorphic and more mafic rocks, while velocities exceeding 4.0 km/s indicate the lower crust to upper mantle, typically consisting of mafic to ultramafic rocks. Crustal thickness in the study area varies from about 33 to 38 km beneath. Our results indicate that magmatic activity may have influenced the fault reactivation in Saurashtra and nearby gulf regions, providing insights relevant to seismic hazard assessment.