Articles written in Journal of Earth System Science

    • Effect of uncertainty in $V_{\mathrm{S}}{-}N$ correlations on seismic site response analysis

      Narayan Roy Ravi S Jakka Sahu R B Amit Shiuly

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      This paper studies the reliability of the calculated shear-wave velocity $(V_{\mathrm{S}})$ from different available $V_{\mathrm{S}}$ and SPT-N correlations in terms of seismic site response analysis. In the present study, various $V_{\mathrm{S}}–N$ correlations developed for different regions around the globe have been used to calculate the bound of $V_{\mathrm{S}}$ variations with depth at three different sites in Kolkata city. This bound has later been used to generate the random $V_{\mathrm{S}}$ profiles using the Monte Carlo simulation. Equivalent linear site response analysis has been performed to study the response of those generated profiles under different input motion excitations. Strong-to-weak ground motion records have been used for this purpose. The amplification spectra of the generated $V_{\mathrm{S}}$ profiles using all soil types and specific soil-type $V_{\mathrm{S}}–N$ correlations show significant variations. The study also shows that the $V_{\mathrm{S}}–N$ correlation may result in quite different $V_{\mathrm{S},30}$ values and subsequently it may lead to the different site classes according to the NEHRP 2003 classification. So, the random choice of the $V_{\mathrm{S}}–N$ correlation, where the direct measurement of $V_{\mathrm{S}}$ is not available, may affect the outcome of seismic hazard analysis significantly. The study points out the need for accurate estimation of the $V_{\mathrm{S}}$ profile either from in-situ determination or using site-specific correlation.

    • Influence of trapped soft/stiff soil layer in seismic site response analysis


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      As the ground response analysis serves an integral part of site specific seismic hazard study to obtain design ground motion, a proper and accurate estimation should be of prime importance. The paper presents a study on the effect of trapped soft and stiff soil layer on equivalent linear ground response analysis implemented in computer program STRATA. For this purpose, the concept of normally stiff and inversely stiff soil profiles have been introduced. The study clearly indicates the higher impact of a trapped soft soil layer profile, i.e., inversely stiff soil profile with soft layer, in comparison with trapped stiff layer profile, i.e., inversely stiff profile with stiff layer. For low to moderate ground motions, as the depth of the trapped soft layer increases, peak amplification and peak frequency reduces, and for high intensity input ground motion, significant reduction only in the peak frequencies is observed. On the other hand, as the depth of trapped stiff soil layer increases the outcome of ground response analysis remain quite similar. Peak transfer function, peak frequency, peak spectral ratio and peak spectral acceleration are found to exhibit a COV $\leq$ 60 to 100% for all combinations of IS-Soft profiles, whereas, IS-Stiff profiles exhibit a maximum COV $\leq$ 15 to 25% for all the considered input ground motions. Computed normalized-root mean square error (Norm-RMSE) values also clearly indicate the higher deviations in the ground response analysis for different combinations of trapped soft soil profiles, from that of normally stiff profile, whereas, trapped stiff layer profiles show lower deviations in the Norm-RMSE values. The profiles with trapped soft layer exhibit a Norm-RMSE value  $\leq$ 0.8, whereas, trapped stiff layer profiles exhibit a Norm-RMSE value  $\leq$ 0.2, which clearly demonstrate the large deviations in the outcome of ground response analysis in case of trapped soft layer profiles.

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