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.
Online Material: Tables of nonlinear site model parameters.