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Lithospheric S-Wave Velocity Structure of the Bastar Craton, Indian Peninsula, from Surface-Wave Phase-Velocity Measurements

Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India sn_bhattacharya{at}hotmail.com

G. Suresh

Seismology Division, India Meteorological Department, New Delhi 110003, India, gsureshimd@yahoo.com

Supriyo Mitra

Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India mitra{at}gg.iitkgp.ernet.in

^* Current address: 23B Nivedita Enclave, A6 Paschim Vihar, New Delhi 110063, India

Interstation phase velocities of surface waves are measured through a cross-correlation method across the Bastar craton in the eastern part of the Indian peninsula. The periods of Love waves lie between 15 and 91 sec and those of Rayleigh waves lie between 13 and 104 sec. The observed phase velocities are close to the theoretical dispersion curves for the crust and upper mantle structure IP11 obtained earlier for central India (in the Indian peninsula) through inversion of surface-wave group velocities. However, for periods higher than 70 sec, the theoretical curves remain above the observed data. In IP11, the upper mantle low velocity zone (LVZ) starts downward at a depth of 140 km; the S-wave velocity (V_S) in the LVZ of IP11 is decreased based on the recent studies, and the modified structure IP11L is obtained. The theoretical dispersion curves of IP11L improve the fit to the observed data particularly at higher periods. The lithospheric part of IP11L is then improved through genetic algorithm inversion with more stratification in the structure for a better fit to the observed data. Such inversion generated the model IP11BA for the Bastar craton. The crustal thickness of this craton is found to be 40.1±0.6 km with the thickness of the upper crust as 15.3±0.7 km, which is less than the corresponding thickness of 20 km in southern India. In the upper crust, V_S shows a small increase with depth, having an average value of 3.508 km/sec, and in the lower crust, V_S=3.934 km/sec. These values of V_S for the crust of the Bastar craton are nearly the same as those found in southern India. Below the Moho down to a depth of 70 km, IP11BA shows a constant V_S of 4.577 km/sec and then increases slowly to 4.609 km/sec at the top of the LVZ.