Abstract

Probabilistic Seismic Hazard Analysis (PSHA) is a fundamental tool for mitigating earthquake risks, particularly in seismically active regions like the Iranian plateau. The Gowk fault, a major active right-lateral strike-slip structure with a reverse component located in southeastern Iran, poses a significant seismic threat. A critical challenge in PSHA is managing epistemic uncertainties associated with fault parameters, most notably the fault slip rate. This study investigates the sensitivity of the seismic hazard assessment of the Gowk fault to variations in geodetic slip rates and site-to-source distances, aiming to quantify how these uncertainties propagate into the final design accelerations. To systematically capture epistemic uncertainties, a comprehensive logic tree framework was developed and implemented using the OpenQuake engine. The logic tree incorporated five distinct geodetic slip rate scenarios for the Gowk fault (3.5, 3.8, 4.2, 4.6, and 7.4 mm/yr), derived from recent literature. The seismicity of the fault was modeled using the Youngs and Coppersmith (1985) magnitude-frequency distribution, assuming a composite model of 6% exponential and 94% characteristic earthquake behaviors. Furthermore, the logic tree integrated three Next-Generation Attenuation (NGA-West2) Ground Motion Prediction Equations (GMPEs)—Boore et al. (2014), Campbell and Bozorgnia (2014), and Chiou and Youngs (2014)—to account for uncertainties in ground motion scaling. Peak Ground Acceleration (PGA) was calculated for return periods of 475, 2475, and 10000 years across different distances from the fault trace.

The results demonstrate a high sensitivity of the hazard outputs to the input slip rates. For a 10% probability of exceedance in 50 years (475-year return period), the estimated PGA values at the fault trace ranged from 0.218 g for the minimum slip rate scenario (3.5 mm/yr) to 0.428 g for the maximum scenario (7.4 mm/yr). While absolute PGA values for a 2475-year return period naturally escalated (ranging from 0.487 g to 0.904 g), quantitative analyses revealed a highly non-linear behavior. Contrary to linear assumptions, the relative sensitivity of PGA to slip rate variations actually decreases at longer return periods due to the “hazard saturation” effect. Furthermore, spatial analysis demonstrated that the impact of this epistemic uncertainty diminishes progressively as the site-to-source distance increases. The findings underscore the critical importance of accurately determining fault slip rates in active tectonic environments. The substantial and non-linear variation in design accelerations based on slip rate uncertainties highlights the necessity of using a robust logic tree approach rather than relying on a single deterministic value to ensure infrastructural resilience and public safety.