Transposing an active fault database into a fault-based seismic hazard assessment for nuclear facilities – Part 2: Impact of fault parameter uncertainties on a site-specific PSHA exercise in the Upper Rhine Graben, eastern France

Abstract

Abstract. We perform a fault-based probabilistic seismic hazard assessment (PSHA) exercise in the Upper Rhine Graben to quantify the relative influence of fault parameters on the hazard at the Fessenheim nuclear power plant site. Specifically, we show that the potentially active faults described in the companion paper (Jomard et al., 2017, hereafter Part 1) are the dominant factor in hazard estimates at the low annual probability of exceedance relevant for the safety assessment of nuclear installations. Geological information documenting the activity of the faults in this region, however, remains sparse, controversial and affected by a high degree of uncertainty. A logic tree approach is thus implemented to explore the epistemic uncertainty and quantify its impact on the seismic hazard estimates. Disaggregation of the peak ground acceleration (PGA) hazard at a 10 000-year return period shows that the Rhine River fault is the main seismic source controlling the hazard level at the site. Sensitivity tests show that the uncertainty on the slip rate of the Rhine River fault is the dominant factor controlling the variability of the seismic hazard level, greater than the epistemic uncertainty due to ground motion prediction equations (GMPEs). Uncertainty on slip rate estimates from 0.04 to 0.1 mm yr −1 results in a 40 to 50 % increase in hazard levels at the 10 000-year target return period. Reducing epistemic uncertainty in future fault-based PSHA studies at this site will thus require (1) performing in-depth field studies to better characterize the seismic potential of the Rhine River fault; (2) complementing GMPEs with more physics-based modelling approaches to better account for the near-field effects of ground motion and (3) improving the modelling of the background seismicity. Indeed, in this exercise, we assume that background earthquakes can only host M < 6.0 earthquakes. However, this assumption is debatable, since faults that can host M > 6.0 earthquakes have been recently identified at depth within the Upper Rhine Graben (see Part 1) but are not accounted for in this exercise since their potential activity has not yet been described.

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

“…Including fault maps in probabilistic seismic hazard analysis can overcome this problem (e.g. Chartier et al, ; Hodge et al, ; Pace et al, ), but this requires estimates of both earthquake magnitude and recurrence interval. In Lake Malawi, where fault slip rates have not been measured, Hodge et al () assigned the plate motion to individual faults in their hazard assessment.…”

Transposing an active fault database into a seismic hazard fault model for nuclear facilities – Part 1: Building a database of potentially active faults (BDFA) for metropolitan France

“…In order to perform an exercise aimed at assessing the fault parameters that influence most the results of PSHA calculations at short distance from a site (developed in Chartier et al, 2017), we propose as a first step to construct a fault source model based on the BDFA,…”

“…Only the closest segments to the NPP are considered here (Fig. 5) and used in the PSHA exercises (see Chartier et al, 2017). Our knowledge of the considered fault segments activity is summarized by robustness indexes varying from RI = 4.4 up to RI = 15.4 (Fig.…”

The Role of Viscoelastic Stress Transfer in Long‐Term Earthquake Cascades: Insights After the Central Italy 2016–2017 Seismic Sequence

“…In addition, benchmark studies performed in fast‐deforming areas dominated by strike‐slip faults (e.g., in California and Turkey; Field et al, ; Gülerce et al, ) are not suitable in slowly deforming regions with normal faulting, like Central Italy. Recently, some test studies considered faults not as isolated features but as existing within a population of faults that may interact through their stress fields (e.g., Pace et al, ; Visini & Pace, ), and new methodological approaches exist (e.g., Chartier et al, ). Mildon et al () made a further step in that direction by modeling Coulomb stress transfer prior to the Central Italy 2016 seismic sequence on strike‐variable fault geometries.…”