## 1 Introduction  
Fault displacement hazard has been a crucial siting issue from the beginning of the construction of commercial nuclear power plants. According to older regulatory approach the site should be abandoned, if a fault near the site judged for capable to cause permanent surface displacement. In the recent siting practice, the hazard should be evaluated and should be accounted for in the design basis, and the safety should be proven (see, International Atomic Energy Agency requirements and guidance SSR-1 [1] and SSG-9 [2] and U.S. Nuclear Regulatory Commission 10 CFR 100 [3]). The attributes for capability of the faults to generate surface displacement are as follows [2, 3]:  
1. Evidence of significant past movement or movements of a recurring nature within such a period that it is reasonable to conclude that further movements at or near the surface may occur. This time interval is fixed by the regulations. Generally, it varies between 10,000 to 100,000 years (see, e.g., [3]). Note, the significance should be qualified considering the effect surface movement on the integrity and function of safety related structures of nuclear power plant, i.e., the significance is matter of nuclear engineering rather than geoscience. 2. Structural relationship exists with a known capable fault such that movement of the one fault may cause movement of the other at the surface. 3. If the maximum potential magnitude of the earthquake, associated with a seismogenic faults at the site vicinity are sufficiently large to cause surface movement. The procedure for assessing the tectonic surface fault rupture hazard for nuclear facilities is given by the ANSI/ANS-2.30-2015 standard [4; 5]. A comprehensive guidance for the justification of plant safety with respect to permanent ground displacement (further indicated as PGD) has been published by the Japan Nuclear Safety Institute [6]. A procedure for screening out the PGD hazard was presented by [PERSON] et al., 2017 [7] that is based on the maximum potential magnitude of the fault considered. This concept has also been discussed by [PERSON] [8]. According to this, for the screening, the probability of non-zero surface rupture \(P(sr\
eq 0|M_{u})\) should be calculated as is as proposed by [PERSON] and Coppersmith [9]. The hazard can be neglected, if the annual probability of non-zero surface rupture is less than \(10^{-7}\), that corresponds to 1% of acceptable annual probability for core damage or to the acceptable limit for severe accidents for new reactors. If the PGD hazard cannot be screened out with high confidence, the annual probability of exceedance should be calculated \(\lambda(D\geq D_{0})\) for the direct movement \(D\), and the \(\lambda(d\geq d_{0})\) for the distributed fault movement \(d\). There are two conceptually different approaches for the PGD hazard analysis: the displacement based and the earthquake-based approaches [5; 6]. For both approaches detailed characterization of fault activity and potential movements on the fault is needed. There are practical cases, where pre-Quaternary faults mapped at the site vicinity revealed to be re-activated in the Quaternary period. However, there are no manifestations of permanent surface movement or the observed PGD is negligible small. Moreover, the available data for characterization of the fault capability to cause surface rupture are insufficient. Difficult to interpret cases have been reported, for example, in [10] for the Bohemian Massive and very recently for the Paks site in Hungary [11]. The practical motivation of this paper was to contribute to the clarification of permanent fault displacement hazard issue for the Paks site in Hungary. The Paks site is in the middle of the Pannonian Basin (46.34 N, 18.51 E). The site was selected for the Paks Nuclear Power Plant (Paks NNPP) more than sixty years ago. The historically credible maximum intensity was defined as MSK-64 grade 6.