Probabilistic seismic performance assessments of engineered structures can be highly sensitive to the seismic input excitation and its variability. In the present study, the scenario-based performance assessment recommended by Federal Emergency Management Agency (FEMA) P-58 guidelines is adopted to estimate seismic fragility of concrete dams for various seismic hazard scenarios. Due to the scarcity of recorded ground motions and thereby their poor representation of uncertainties, stochastic ground motion simulation methods are utilized to obtain the required input excitations. Moreover, to understand the uncertainty in ground motion simulation models, two broadband stochastic simulation models are used to generate input excitations representing six seismic hazard scenarios defined by earthquake magnitude, source-to-site distance, and soil conditions. Optimal intensity measure parameters for each scenario are identified using a systematic procedure that considers criteria such as efficiency, practicality, proficiency, sufficiency, and hazard compatibility. Fragility curves and surfaces are derived using the cloud analysis technique, taking into account various damage measures and limit state functions. The study finds that the derived fragility curves are particularly sensitive to the selection of earthquake scenarios, the choice of records, and the methods used to calculate fragility curves, with less sensitivity observed to different engineering demand parameters. Given this sensitivity, particularly to ground motion selection, the study highlights the necessity of incorporating both model-to- model variability (epistemic uncertainty) and record-to-record variability (aleatory uncertainty), alongside the established material and modeling uncertainties, in the probabilistic seismic assessment.

The Kahramanmaras earthquake sequence that occurred in Turkey on Feb. 6, 2023 caused significant casualties and economic losses, which may relate to its distinct characteristics. This paper establishes optimal intensity measures (IMs) and vector-valued fragility curves for subway station structures considering the Kahramanmaras motion record characteristics. The seismic records are analyzed in terms of time-history, response spectrum, and seismic IMs, and a numerical model of a subway station with two-story and three-span is developed considering the concrete damage and nonlinear behavior of soil. The optimal IMs are selected according to criteria of efficiency, practicality, proficiency and goodness of fit. Based on the optimal IMs, vector-valued fragility surfaces are developed, and the differences in damage probability between the vector-valued fragility curves and scalarvalued fragility curves are analyzed. The results indicate that the velocity time-histories of the 12 mainshock records exhibit pulse-like characteristics with large peak velocities and displacements along with high amplitudes of the response spectra at longer periods. The velocity-based IM, Vsi, is found to be most suitable for probabilistic seismic demand analysis under the Kahramanmaras earthquake, and along with PGA is utilized as elements for vector-valued IM. Finally, the scalar-valued fragility curves may underestimate or overestimate the failure probability, while vector-valued fragility curves can offer a more accurate failure probability for specific characteristics IMs.