After an earthquake, hospital emergency departments need to provide continuous healthcare services to respond to the eventual sudden increase in injured people. Service performance of an emergency department is influenced by internal factors, such as physical damage and staff availability, and external factors, such as an increased patient arrival rate and disruptions in its supply chain. This research presents a quantification methodology for the performance of the emergency department. The novelty of the proposed approach lies in the explicit integration of the inelastic structural and non-structural response of the building and damage, with its loss of functionality, downtime, and emergency patient treatment rate. A discrete event simulation model is used to model the flow of patients within the different units of the emergency department. Seismic risk is expressed as return periods of exceeding different levels of patient waiting times. Results show that 1, 000 and 30, 000 accumulated waiting hours correspond to return periods of 100 and 1, 000 years, respectively. It is concluded that this model may contribute to improve risk management of critical emergency department infrastructure.