Seismic Site Characterization through Joint Analysis of MASW and Microtremor data in Dhanbad, Jharkhand, India

We present results of seismic site characterization study using joint modelling of Horizontal-to-Vertical Spectral Ratio (HVSR) based on microtremor data and dispersion curves from Multi-Channel Analysis of Surface Wave (MASW) in part of Dhanbad, Jharkhand, India. The aim is to seismically characterize the subsurface structure using shear wave velocity profiles to mitigate the effect of soil's amplification during an earthquake in the area of Dhanbad. A linear array consisting of 12 seismic stations were deployed to record three component microtremor data at a sample rate of 1024 Hz in the time interval of more than 30 minutes. Then, Fast Fourier transform was applied to 20 sec time series data for each location in order to obtain HVSR curve for each time window. These HVSR curves from each time window were averaged together to estimate the number of impedance contrasts after examining the abrupt changes in peak spectral amplitudes. The major peak frequency in HVSR curves ( 7 Hz) indicate the impedance contrast between bedrock and top soil layers; where Rayleigh wave's average phase velocities varies between 300 - 400 m/s. We iteratively searched 1D shear wave velocity profiles in a predefined model search space beneath each investigation site by joint analysis of HVSR and dispersion curves. Then, a 2D shear wave velocity model was constructed using Nearest Neighbour Interpolation technique. The Electrical Resistivity Tomography (ERT) data were also acquired along the same traverse using dipole-dipole, Wenner and Schlumberger arrays. On the basis of the comparative analyses of resistivity and shear wave velocity profiles, we determined that the bed rock with resonance frequency of 7 Hz may be lying at a depth range of 9-12 meters. Also, the engineering bed rock was found to lie at a depth of 45 meters in south and 35 meters in the north of the profile. Both, resistivity and shear wave velocity profiles are in good agreement with each other and belong to D-type site according to National Earthquake Hazards Reduction Program, 2001 and International Building code 2009.