Site-specific partially nonergodic PSHA for a hard-rock critical site in southern France: adjustment of ground motion prediction equations and sensitivity analysis

Modern probabilistic seismic hazard assessment (PSHA) focuses on the separation and different treatment of epistemic and aleatory uncertainties. Recent site-specific PSHA studies have pointed out that, if the site response and its epistemic uncertainties can be appropriately considered by adjustments to median estimates from ground motion prediction equations (GMPEs), the aleatory variability (sigma) of the GMPEs can be replaced by the single-station sigma thus partially relaxing the ergodic assumption employed in the PSHA. The site-specific partially nonergodic approach, correctly applied, provides a more accurate representation of the seismic hazard at a specific site and a more rigorous treatment of uncertainties. This paper presents the strategy followed to apply this relatively recent approach to a critical infrastructure in Southern France located on hard-rock site conditions (Vs30 ≈ 2000 m/s). The target site conditions are defined in terms of shear-wave velocity (Vs) profiles and high-frequency attenuation (κ0) based on the results of site investigations and on the exploitation of earthquake records at seismic stations in the target site area. The host-to-target Vs-κ0 adjustment of median estimates for the selected GMPEs is performed by using the inverse random vibration theory approach (Al Atik et al. in Bull Seismol Soc Am 104:336–346, 2014) considering epistemic uncertainties in target Vs profile and κ0. The single-station sigma model is developed based on Rodriguez-Marek et al. (Bull Seismol Soc Am 104:1601–1619, 2013) due to the lack of local data. The results of the site-specific partially nonergodic PSHA are discussed by means of a sensitivity analysis and are compared to the results from standard ergodic PSHA. We found that, for the considered site, the site-specific approach provides a substantial reduction (up to 50%) of the uniform hazard spectra at 10,000-year return period compared to the ergodic approach.