Monte Carlo Simulation of Probabilistic Fault Displacement Hazard Analysis for Lifelines Crossing Active Faults

Deterministic Fault Displacement Hazard Analysis (DFDHA) commonly provides a straightforward framework to evaluate the potential extent of surface fault displacements at a lifeline crossing for specific magnitude earthquake scenarios. However, this approach is limited in quantifying the aleatory and epistemic uncertainties that come with characterizing the parameters that control the amount of slip occurring across the fault, the frequency of different size events occurring during the design life of the facility, and the distribution of the different amounts of surface displacements that occur along a rupturing fault relative to the location of the lifeline. These uncertainties cannot be captured by assessing different event scenarios in DFDHA, but a more mathematically rigorous assessment of these uncertainties requires a probabilistic approach (PA) to displacement hazard evaluations. This paper discusses a Probabilistic Fault Displacement Hazard Analysis (PFDHA) as a compliment to the DFDHA. The PFDHA accounts for uncertainties in the DFDHA by using the principles of probability and furnishes a fault displacement hazard curve at the lifeline crossing. The PFDHA results are provided in terms of annual frequency of exceedance (AFOE) or its inverse the average return periods (ARPs) versus magnitude range for fault offsets. In addition to DFDHA, the PFDHA is applied to address the recent concerns regarding the seismic vulnerability of the Metropolitan’s Colorado River Aqueduct (CRA), while crossing the South San Andreas Fault System (SSAFS) at San Gorgonio Pass (SGP) due to Mw7.8 earthquake like the 2008 ShakOut Scenario on the SSAFS. The large seismic events along the SSAFS could result a significant component of vertical displacement at SGP, drastically changing the hydraulic gradient of the CRA and potentially disrupting the flow rate at the crossing. The analysis results provide an AFOE or ARP for an uplift in excess of 2 m at SGP due to large seismic events on the SSAFS and are compared with those of DFDHA results.