Gas pipelines are considered as lifelines, protection of which is essential for safety, uninterrupted service to the industry and society and preservation of the environment. In Iran, due to the existence of thousands of kilometers of oil and gas pipelines and the extent of active faults, intersection of pipelines with faults is inevitable. Therefore, it is necessary to analyze the hazard of fault rupture for pipelines and take essential design considerations. Through the fault rupture hazard analysis, the permanent deformation of the ground is estimated. Conventionaly, the expected displacement is determined for a certain level of earthquake. This is the so called deterministic approach. Nevertheless, due to the great uncertainties in earthquake parameters (such as uncertainties in magnitude, distance and return period), there is another approach that relates the earthquake-induced force/displacement to its probability of occurrence. This provides insight for the designers to take mindful decisions on the risk level considering the importance of their structure. In this paper, the recent approach, called Probabilistic Fault Displacement Hazard Analysis (PFDHA) is considered for strike-slip faulting and applied to the North Tabriz Fault. The North Tabriz Fault is a major seismogenic fault traversing northwestern Iran with strike slip mechanism. This fault has been responsible for many devastating earthquakes and surface fault ruptures. Many historical earthquakes have occurred in the Tabriz region (e.g., the 858, 1042, 1273, 1304, 1550, 1641, 1717, 1721, 1780 and 1786 earthquakes). Among these arthquakes, the destructive earthquakes of 1042 (Ms 7.3), 1721 (Ms 7.3) and 1780 (Ms 7.4) were accompanied by surface faulting. The surface rupture in 1721 and 1789 earthquakes extended for at least 50 and 60 kilometers, respectively.

Therefore, probabilistic fault displacement hazard analysis has been performed for different locations on the North Tabriz Fault, including the intersection of this fault with Tabriz-Ankara gas pipeline which is of special concern. Herein, earthquake data is gathered for Tabriz region from International Seismological Center (ISC) over a period of 50 years from 1970 to 2020 and organized to derive the recurrence law. Considering the mainly strike-slip mechanism of this fault, fault displacement attenuation functions and magnitude-rupture length relations for strike-slip faults have been used. The present study exclusively focuses on principal fault displacement and does not address distributed displacements.

The analysis results are presented in the form of mean annual rate of exceedance for different fault displacements and the principal fault displacement in the return periods of 475 and 2475 years have been compared. Several site locations, including the intersection of fault and gas pipeline, are considered in this study. The results show that 131 cm and 432 cm of fault displacement is anticipated in the intersection of fault and gas pipeline in the return periods of 475 and 2475 years, respectively. Moreover, fault rupture hazard map of the North Tabriz Fault is illustrated for the return period of 475 years. According to this hazard map, higher fault displacement is expected in the middle part of the fault.