A new empirical method for obtaining horizontal site amplification factors with soil nonlinearity

Abstract Currently, the theoretical prediction of soil amplification considering nonlinear soil behaviors can be implemented through nonlinear analysis of soil layers in time domains. However, in such theoretical methods, we need detailed velocity information down to the seismological bedrock and nonlinear properties of sediments, which are challenging to obtain in a wider area for a scenario‐type seismic risk evaluation. Recently, using diffuse‐field theory, we found that the horizontal‐to‐vertical spectral ratio of Earthquake (EHVR) is the ratio of the S‐wave horizontal transfer function with respect to the P‐wave vertical transfer function with the vertical incidence from the seismological bedrock with a coefficient for HVRs at the bedrock. Therefore, if there is no significant nonlinearity in the P‐wave vertical transfer function during strong shaking, we can regard soil nonlinearity emerging on the EHVR as the nonlinearity of the S‐wave horizontal amplification. In this study, we tried to distinguish the difference between the EHVRs in the linear and nonlinear cases, quantify the difference, and utilize a creative empirical approach to acquire the S‐wave horizontal amplification factors with soil nonlinearity to improve the quality of strong motion prediction in the near field in the future.