Surface wave studies for shear wave velocity and bedrock depth estimation over basalts

Shear wave velocity (V (S)) estimation is of paramount importance in earthquake hazard assessment and other geotechnical/geo engineering studies. In our study, the shear wave velocity was estimated from ground roll using multichannel analysis of surface wave (MASW) technique making use of dispersive characteristics of Rayleigh type surface waves followed by imaging the shallow subsurface basaltic layers in an earthquake-prone region near Jabalpur, India. The reliability of MASW depends on the accurate determination of phase velocities for horizontally traveling fundamental mode Rayleigh waves. Inversion of data from surface waves resulted in a shear wave velocity (V (S)) in the range of 200-1,200 m/s covering the top soil to weathering and up to bedrock corresponding to a depth of 10-30 m. The P-wave velocity (V (P)) obtained from refraction seismic studies at these locations found to be comparable with V (S) at an assumed specific Poisson's ratio. A pair of selected set of V (S) profiles over basalt which did not result in a hazardous situation in an earthquake of moderate magnitude are presented here as a case study; in other words, the shear wave velocity range of more than 200 m/s indicate that the area is highly unlikely prone to liquefaction during a moderate or strong earthquake. The estimated depth to basalt is found to be 10-12 m in both the cases which is also supported by refraction studies.