Nonlinear site response from the strong ground-motion recordings in western China

Strong ground-motion records from the mainshocks and aftershocks of the 2008 Wenchuan (Ms 8.0) and 2013 Lushan (Ms 7.0) earthquakes, within 300. km from the faults, were used for horizontal-to-vertical spectral ratio (HVSR) analysis. The HVRSs of the S-wave show that the predominant site frequency decreases with increasing ground-motion level, a characteristic of nonlinear dynamic soil response. We applied diffuse field theory and Monte Carlo search in the model space to produce an inverted shear-wave velocity profile using the HVSRs of weak S-wave motions. The inverted velocity structures are significantly different from the initial ones derived from in-situ measurements. We also applied 1-D equivalent-linear site-response analysis to derive the spectral ratios (i.e., transfer function) for the original and inverted soil models, and compared the results with the observed HVRSs of the S-wave motions. The comparisons showed that the spectral ratios from 1-D simulation for the inverted soil models agree quite well with the observed HVRSs. In other words, this study suggests that the HVSR from observed earthquake ground motion resembles the empirical transfer function of nonlinear site-response.