Shear-Wave Velocity Structure of the Koyna-Warna Region in Western India Using Ambient Noise Correlation and Surface-Wave Dispersion

The Koyna-Warna region is a premier site of reservoir-triggered seismicity in the Deccan volcanic province of western India. In the present study, shear-wave velocity structure of this region down to a depth of 10 km is estimated using the ambient noise correlation technique with data from a network of 11 seismic broadband stations. Asymmetric Green's functions are obtained that are suggestive of an anisotropic distribution of noise sources in the Indian subcontinent. Cross correlation of continuous noise data of 12 months duration, recorded on the vertical components, enables computation of group velocity dispersion curves from the Green's functions. This is supplemented by Rayleigh waves from local earthquakes, which, in addition to those from noise data, are inverted for the shear-wave velocity structure using the multiple-filter technique. The study reveals on an average, a 0.8 km thick basaltic layer of the Deccan traps with a shear-wave velocity of about 3.0 km/s on the eastern side of the escarpment. A low-velocity, possibly weathered granitic layer with a velocity of 3.3 km/s is found below the traps and is underlain by the granitic basement with a velocity of 3.6 km/s. Except for the surface topographic undulation, the velocity structure in this region, down to 10 km, is similar on either side.