Estimation of regression relation between velocity of shear wave and depth: A contribution towards mitigating seismic hazard in Bilaspur and Hamirpur districts of Himachal Pradesh

Himachal Pradesh, a northern state in India, is located in seismic zones IV and V and is affected by several moderate to strong earthquakes. Given the region’s susceptibility to seismic activity, it is always necessary to design structures according to the safety standards available in the country. Shear wave velocity (V_s) averaged up to a depth of 30 m is a crucial factor in designing earthquake-resistant structures in seismically active areas. The current study conducted comprehensive seismic exploration to determine in-situ V_s using active and passive methods at various locations covering the ongoing Bhanupalli–Bilaspur–Beri railway project. This facilitated earthquake hazard assessment and Vs30 calculations. In this work, to estimate a one-dimensional V_s profile, we utilised the joint fit of the horizontal-to-vertical spectral ratio (H/V) technique, a passive approach that utilises ambient noise data and an active seismic technique known as multichannel analysis of surface wave (MASW). This joint fit has provided a 1-D velocity structure. The method employed has demonstrated a consistent correspondence with bore log data and notably, there is a strong correlation between variations in V_s and lithology that is dependent on depth. It has been seen that wave velocity tends to rise with depth within similar formations. For quantifying this relationship, several regression models have been formulated and then validated using different sites, which are excluded in the formulation of the regression relation. We derived shear wave profiles for various stations using lithologs and compared them to existing shear wave profiles and we found a strong agreement between these two datasets.