Region-Specific Correlations Between $$\varvec{V}_{{{\mathbf{S}}30}}$$ and Time-Averaged $$V_{\text{S}}$$ and SPT-N Values at Different Depths for the Indo-Gangetic Basin

Abstract

Time-average shear wave velocity at top 30 m depth ($$V_{{{\text{S}}30}}$$) is extensively used for site characterization and amplification parameters estimation. However, due to various reasons, the shear velocity profile ($$V_{\text{S}}$$) does not reach to 30 m depth. In this study, using the multichannel analysis of surface waves, $$V_{\text{S}}$$ profiles at 275 locations have been determined in the Indo-Gangetic Basin (IGB). Further using $$V_{\text{S}}$$ profiles, new correlations between $$V_{{{\text{S}}30}}$$ and $$V_{{{\text{S}}Z}}$$ (where $$z$$ is the average depth) have been developed for $$z 30$$ m. For $$z 30$$, both linear and second-order polynomial equations have been used. Based on the variation of residual with $$V_{{{\text{S}}30}}$$, it has been observed that second-order polynomial equation derived using orthogonal regression is performing better in both cases. Additionally, new correlations between $$V_{{{\text{S}}30}}$$ and SPT-NZ, with $$z$$ equals to 5, 10, 20, 30, 40 and 50 m, have been derived. This correlation is further used in deriving the $$N_{30}$$ map of the IGB as per BIS:1893 useful for Indian scenario. Further, the depth corresponding to one-quarter of wavelength for various periods has been studied. The velocities known up to 30 m depth are relevant for site amplification at period up to 0.5 s. This is the first time such extensive study has been carried out in the IGB for correlation of $$V_{{{\text{S}}30}}$$ and $$V_{{{\text{S}}Z}}$$, for both deep and shallow depths.