# Improved Simplified Engineering Fault Displacement Hazard Evaluation Method for On-Fault Sites
## 1 Introduction  
Vulnerability and safety of critical infrastructure, such as long gas pipelines, bridges or high-potential risk facilities including nuclear power plants and dams, concerning fault displacement hazard is a significant engineering and design issue associated with high economic risks and enhanced social sensitivity. This is especially true if the susceptibility of fault displacement hazard is revealed during the operation of the hazardous facility. For example, cases are reported for the nuclear power plants in [1; 2; 3; 4; 5] and the dam in [6]. Different methods have been developed for evaluating hazards. These methods are rather case-oriented and correspond to the traditions and routines of industries. In the case of pipeline design and safety evaluation, the state of the art is presented, for example, in [7; 8; 9]. Traditionally, the design practice is based on the simple deterministic approach to identifying the maximum considered earthquake and calculating the displacement using empirical scaling correlations. More recently, probabilistic fault displacement hazard analysis (PFDHA) has been proposed to account for the uncertainties in the expected magnitude of displacements, e.g., [7; 10]. A similar approach was followed by the transport industry; see [11; 12]. Regarding the state of the art, fault displacement hazard analysis has been published for four nuclear sites. While presenting methodologies and cases in nuclear practice, the history of investigations into potential sites for new facilities and operating plants should be considered. This will highlight the motivation for developing simple engineering fault displacement hazard methodologies. Generally, the probabilistic approach is preferred in nuclear applications, since it provides hazard characterisation that complies with the definition of design basis and probabilistic evaluation of safety metrics. Unlike other industries, an essential feature of the nuclear practice is considering the hazard up to the very low annual exceedance frequencies of \(10^{-4}/\text{a}\) to \(10^{-7}/\text{a}\). Probabilistic fault displacement hazard analysis was first developed to assess hazards from numerous normal active faults that affect the site of the proposed repository for high-level nuclear wastes at Yucca Mountain, Nevada. A rather specific methodology has been applied to the Yucca-Mountain Project [13] to characterise fault displacement potential for PFDHA. This analysis was part of the most extensive Probabilistic Seismic Hazard Analysis (SSHAC Level 4; see in [14]), which began in 1994 and still is not considered as completed [15]. Another nuclear site investigation and fault displacement hazard evaluation case was the PFDHA study performed for the new plant at Krsko, Slovenia [16; 17]. Time pressure plays a much lesser role in the case of a site investigation for a new facility (especially for the repository) than in operating a potentially hazardous facility (nuclear power plants, dams or gas pipelines). A well-documented and accessible probabilistic distributed fault displacement hazard study has been carried out for the Diablo Canyon Power Plant (DCPP) due to the identified 2008 Shoreline Fault [2]. The urgency of completing the hazard evaluation was related to safety issues and regulatory requirements but stimulated by the need to prolong the operational license. A tailor-made probabilistic method has been applied to select the characteristic earthquake based on the update of Probabilistic Seismic Hazard Analysis (PSHA). Regarding the U.S. nuclear practice, a high-level American Nuclear Society document guides the investigation and evaluation of fault displacement hazards [18] and prefers probabilistic methods. Like [18], high-level comprehensive guidance has been developed for the Japanese nuclear industry, including fault displacement hazard investigation, evaluation methods and plant responses [19]. Besides PFDHA, this document acknowledges the implementation of deterministic numerical simulation of fault rupture and rupture propagation.