Nonlinear Site Models Derived from 1D Analyses for Ground-Motion Prediction Equations Using Site Class as the Site Parameter

Nonlinear site models are an important part of ground-motion prediction equations (GMPEs) and can be constructed in a number of ways. If numerous soil-site strong-motion records contain the effect of strong nonlinear soil response, the parameters for the nonlinear model can be a part of the regression parameters for GMPEs. It is also possible to derive nonlinear site models by numerical simulation. However, the number of strong-motion records from Japan that contain the effect of strong nonlinear soil response is still too small to derive nonlinear site terms. We present a model of nonlinear site terms using site class as the site parameter in GMPEs based on a 1D equivalent linear model. The 1D model was constructed based on the shear-wave velocity profiles from the KiK-net strong-motion stations with a wide range of site periods, soil depth, and impedance ratios. The rock-site strong-motion records were from different earthquake categories in Japan and the Pacific Earthquake Engineering Research Center dataset. Those records had a wide range of earthquake magnitudes, source distances, and peak ground accelerations. A random effects regression model was fitted to the calculated spectral amplification ratios, accounting for the effect of site impedance ratios, earthquake magnitudes, and source distances of the rock-site records. We also designed a method to adjust the 1D model so it can be used in a GMPE, accounting for the fact that a 1D model is an overly simplistic assumption for many real strong-motion recording stations in many parts of the world.