Non-ergodic correction factors for hard rock sites in Western Canada

For short-period structures situated on hard rock, accurate characterization of ground motions at high frequencies is critical. Predictions based on empirical ground motion models (GMMs) for generic site conditions carry large uncertainties, especially because the datasets used to develop GMMs are often lacking in observations for hard rock sites. The current state-of-practice is to perform a V S -[Formula: see text] host-to-target correction, where V S is the site shear-wave velocity and [Formula: see text] is the near-surface high-frequency attenuation parameter. However, in the absence of site-specific ground motion data, the modeling parameters ( V S and [Formula: see text]) and their reliability in predicting the correct adjustment factors are not well quantified and are subject to large epistemic uncertainty. We apply an alternative framework to characterize hard rock site response in probabilistic seismic hazard analysis (PSHA) using site-specific ground motion data. Our methodology relies on estimation of the non-ergodic hard rock site response at a given site, as derived using site-specific Fourier amplitude spectra. The use of site-specific data allows us to remove the site-to-site variability component from PSHA and greatly reduce epistemic uncertainty of the hard rock site response. We demonstrate the applicability of the proposed methodology for a critical facility in British Columbia (B.C.), Canada. Comparison of the non-ergodic and generic hard rock correction factors reveals the importance of compiling and using site-specific data in PSHA. For competent hard rock, we suggest that amplification at high frequencies is often controlled by factors other than impedance effects, such as topographic effects or near-surface high-frequency attenuation; such effects are not well modeled through the traditional V S -[Formula: see text] approach.