Regional calibration of hybrid ground-motion simulations in moderate seismicity areas: Application to the upper rhine graben

This study presents the coupling of the spectral decomposition results for anelastic attenu¬ation, stress drop, and site effects with the Graves-Pitarka (GP) hybrid ground-motion sim¬ulation methodology, as implemented on the Southern California Earthquake Center (SCEC) broadband platform (BBP). It is targeted to applications in the Upper Rhine graben (URG), which is among the seismically active areas in western Europe, yet a moderate seis-micity area. Our development consists of three main steps: (1) calibration of regional high- frequency (HF) attenuation properties; (2) modification of the hybrid approach to add com- pressional waves in the HF computation and examine various strategies to evaluate site amplification factors in the Fourier domain (e.g., VS30-based or site-specific factors); (3) test¬ing of the simulations using earthquake records from the URG (3.7 < Mw < 5). The valida¬tion process of the simulated time histories is performed first on rock sites, and, then subsequently at all stations, whatever their site conditions. The performance of the sim¬ulations for rock sites is assessed through the standard validation technique in the BBP (comparison of the waveforms, intensity measures, and estimation of the response spectra model bias). We additionally compare the Fourier amplitude spectrum of the simulations and observations, and compute their corresponding bias. The results show that the simu¬lated ground motions match the general characteristics of the recorded motions, and that the model bias generally fluctuates around zero across the broadband frequency range. Hence, the hybrid ground-motion methodology implemented in the SCEC BBP can be suc¬cessfully applied outside high-seismicity areas and outside those areas for which it had been generally calibrated. Our results also show that HF modification and calibration were necessary to improve the fits with the observation, and demonstrate the potential benefits of using site-specific amplification factors compared to VS30-based amplification factors.