Host‐To‐Target Site Response Adjustments for Site‐Specific Probabilistic Seismic Hazard Analysis

An indispensable component of any site‐specific probabilistic seismic hazard analysis (PSHA) is incorporating local site effects through site response analyses, rather than relying on generic amplification factors in ground‐motion models (GMMs). Traditionally, the seismic hazard is calculated at a buried rock horizon and then convolved with amplification factors for the overlying layers, which requires the definition and characterization of the buried rock profile (without which neither the model for rock motions nor the site response analyses can be developed). An alternative approach is to retain the host reference rock profile in the GMM for the hazard calculations and then perform the host‐to‐target site response adjustment for the full profiles in a single step. This approach is illustrated by its application to a site‐specific Senior Seismic Hazard Analysis Committee Level 3 PSHA for Idaho National Laboratory. The site adjustment factors are calculated using a logic‐tree approach that captures the full epistemic uncertainty in these factors. This is not necessarily achieved when the logic tree only captures uncertainty in the near‐surface shear‐wave velocity profile. The application illustrates the use of borehole and noninvasive shear‐wave velocity measurements, in conjunction with local recordings of weak earthquake motions, to constrain the logic‐tree branches and their associated weights.