Enhancing seismic site response prediction by supplementing shallow shear-wave velocity characteristics

The shallow soil layers play a critical role in affecting the seismic site response. Commonly used site parameters in engineering practice and site response prediction models—such as the time-averaged shear-wave velocity (V_S) in the top 30 m (V_S30) and the site period—may overlook the high impedance contrast and weak stiffness of soft soil layers near the surface. This study examines the complementary role of the uppermost soil layers, characterized by an impedance contrast parameter I_m10 and a velocity parameter V_S10, in enhancing site response prediction using ground motion data from KiK-net stations in Japan. Analysis of the V_S profiles reveals that the commonly used dual-parameter method inadequately characterizes the impedance contrasts and V_S properties of shallow soils. Sites with large I_m10 and small V_S10 would additionally produce larger linear site amplification and stronger nonlinear response. We demonstrate that incorporating I_m10 and V_S10 alongside commonly used parameters improves prediction accuracy for both linear and nonlinear site effects, as evidenced by one-dimensional (1D) site amplification ratios and ground motion observations. The supplementary contribution of V_S10 and I_m10 to site response in empirical ground motion records is much weaker than 1D linear analysis, which may be caused by the inaccuracy of measured near-surface shear-wave velocity, and the 2D/3D effect. Given that I_m10 and V_S10 can be obtained without incurring additional costs in many engineering projects, we recommend their integration into site classification systems and site response prediction models.