Conducting 1D site response analyses to capture 2D V S spatial variability effects

One-dimensional site response analyses (1D SRAs) with shear-wave velocity (V_S) randomization are commonly performed to estimate median site-specific amplification factors (AFs) under the implicit assumption that this approach yields a realistic response. In this work, an investigation is conducted to determine the appropriate amount of V_S randomization (σ_lnVs) needed to capture a median response that accounts for 2D V_S spatial variability effects. Results from 2D SRAs and 1D SRAs with V_S randomization show that the median 2D seismic responses are generally higher than 1D responses at the site’s fundamental frequency, and that higher V_S variability has a mild impact on the median 2D seismic response amplitude at the fundamental frequency, whereas it significantly reduces the median 1D response. Findings indicate that the 84^th percentile AFs based on 1D SRAs conducted with V_S randomization using σ_lnVs = 0.25, approximate well with the more realistic median 2D SRA-based AFs around the fundamental frequency, while the 70^th to 60^th percentiles might be more appropriate at higher frequencies. The benefit of using percentiles of the 1D SRA-based AFs higher than the median is shown for different site conditions and supported by comparisons against empirical data from four downhole sites.