Evaluation performance for site amplification factors: S-wave impedance vs. VS30

Simplified regression models to evaluate site amplification are widely adopted for seismic microzonation maps, design specifications, and so forth. In practice, the most popular parameter for controlling the evaluation models is an averaged S-wave velocity of top z m depth, V_Sz, especially V_S30. The S-wave impedance ratio of the uppermost layer to the base layer, I_S1/I_S0, is an alternative evaluation parameter. In this study, we compare the evaluation performance of V_Sz and I_S1/I_S0 for SH wave amplification factors on the basis of numerical experiments on artificially generated soil profiles. Taking the average S-wave velocity, depth z must be selected on the basis of the target period range of the site amplification factors. I_S1/I_S0 is found to perform similarly to V_S10, and it is better than V_S30 for the peak ground acceleration (PGA) and the short-period range (0.1–0.5?s). This implies that I_S1/I_S0 is effective for use in site amplification evaluations because I_S1 only requires a physical parameter on the uppermost surface layer.