A parametric study for the characterization of site amplification

In earthquake engineering, the approximation of site amplification by using practical ways has been an important issue. Various site parameters were proposed and applied in the engineering practice. Among these, time averaged shear wave velocity for the top 30 m, Vs30, and fundamental frequency, f0, have been used widely. In this study, we investigated the reliability of Vs30 parameter, and the performance of alternative time averaged shear wave velocities (e.g., Vs40, Vs50, etc.) and shear wave travel times (Ttz) at various depths for the estimation of site amplification. For the same bedrock depth, we considered 17 shear wave velocity profiles, changing from convex (i.e., the velocities changing faster near the surface and slower near the bedrock) to concave (i.e., the velocities changing slower near the surface and faster near the bedrock). We divided the soil media, first into layers with equal thickness, and then into layers with equal wave travel times. For each layering type and soil profile, we calculated the site amplification factors and fundamental frequencies, and studied their correlations with time averaged shear wave velocities (Vsz) and wave travel times (Ttz) for different depths, z. We have also investigated the correlation of site amplification factors, surface PGAs (Peak Ground Accelerations), and fundamental soil frequencies (f0) for each case. We have identified the optimal averaging depths for the averaged shear wave velocity and the wave travel time to characterize site amplification. The study showed that there is a sharp change in the correlations when switching from convex to concave profiles. By gradually increasing the bedrock acceleration levels, we have also studied the nonlinear soil response and its correlations with linear soil response. We presented guidelines to estimate nonlinear soil amplification factors and fundamental frequency from the linear ones. Considering that the linear fundamental frequency and amplification can easily be calculated from field tests (e.g., ambient noise measurements for f0 detection), these guidelines provide a useful tool to estimate nonlinear ones.