COMPARISON OF TWO PURELY EMPIRICAL APPROACHES FOR NON-LINEAR SITE RESPONSE PREDICTION: RESULTS FOR THE ESG6-BLIND TEST (ESG6)

We use two different purely empirical approaches to estimate the non-linear transfer function betweena reference rock and a sedimentary site from recordings of weak ground motions and site-conditionproxies (SCPs). The modulus of the linear transfer function is first computed from weak motions andthen modulated with a correction to consider non-linear soil behavior. Afterwards, a minimum-phaseassumption is used to derive a complex transfer function, which then allows to recover the predictionof acceleration time series a(t) at the sedimentary site.The first approach uses the non-linear to linear site response ratio RSRNL-L proposed in Derras et al.(2020). This ratio considers both the amplitude changes and the frequency shift between non-linear(strong motion) and linear soil (weak motion) behavior. The data-driven model to predict RSRNL-L isbased on various SCPs and ground Motion Intensity Measures (GMIMs). Five SCPs were considered:Vs30 (time-averaged shear-wave velocity in the top 30 m), B30 (vertical velocity gradient in the top30 m), Vsmin (the minimum shear wave velocity), f0 (the fundamental resonance frequency, picked onH/V ratio) and A0HV (the corresponding peak amplitude), while only one GMIM loading parameterwas considered (estimated PGA at KUMA site). In the second approach (Castro-Cruz et al. 2020), thenon-linear transfer function is obtained from the linear transfer function (using weak motions recordedat both sites) applying a loading-dependent frequency shift (fsp). The fsp is predicted also using allweaker motion recordings available and the value of the predicted PGA at the sedimentary site.We apply these two approaches to the material provided for step 3 of the blind prediction exerciseorganized by the ESG6 Conference. We find that the discrepancy between the two methods was mainlyconcentrated on the 0.8 to 2 Hz bandwidth for energy content criteria, which is around the linearpredominant frequency of the transfer function.