Seismic site characterization and site response study of Nirsa (India)

The Nirsa block, a coal mining area, and an industrial town lies in the southern part of Dhanbad district, at the border between the Indian states of Jharkhand and West Bengal. This area manifests the significant seismic risk posed by earthquakes, natural as well as induced, of high enough seismic intensity VI and VII from regional and local earthquakes. Hence, it is indispensable to seismically characterize the top surficial layers to reduce the risk associated with earthquakes in this region. We characterize these layers, using ambient noise acquired from multichannel simulation with one receiver and single station method to subsequently invert for shear wave velocities, via iterative global search technique of the guided Monte Carlo method by fitting fundamental mode Rayleigh wave phase velocity dispersion curves and horizontal-to-vertical spectral ratios (HVSR). We survey the area to record a sufficient amount of ambient noise within the frequency range of 1–30 Hz at seventy-eight observation points, with uniform spacing of 250 m, covering a total area of about 4 km2. Our results reveal that the bedrock depth lies in the range of 19.5–2.7 m; while Vs30 values vary in the range of 250–950 m/s in the Nirsa region. The variations in fundamental resonance frequencies, obtained from HVSR curves, are found to be more scattered in the range between 2 and 23 Hz. The maps of fundamental resonance frequency, peak HVSRs’ amplitude, bedrock depth and Vs30 derived from this research exhibit good correlation with each other. The Vs30 prediction equation based on the fundamental resonance frequency (fr) and HVSR amplitude (A0) will help future studies to determine Vs30 of unknown regional sites. The efficacy of our results is reliable as there is good concurrence between shear wave velocity profiles, HVSR profiles and litholog data sets. Our results may prove to be important assets to agencies such as National Disaster Management Authorities and Geological Survey of India for construction of seismic microzonation maps in Nirsa.