A finite-element model capable of computing permanent deformations along preexisting sliding surfaces in dams and embankments is used to evaluate the error in the current state-of-practice two-step procedures that are based on the simplifying assumption that the computation of dynamic response and of the resulting sliding displacements can be decoupled. Both sinusoidal and real earthquake ground motions are used as excitation, and Coulomb's friction law governs sliding along any point of the rigid-plastic tensionless interface. It is found that only when the dominant excitation frequencies lie in the proximity of the fundamental dam frequency will the permanent deformation from the decoupling assumption exceed appreciably the "single-step" value. As a validation of the single-step procedure, an analysis is performed and shown to successfully explain the nonsymmetric response of La Villita Dam recorded in the 1985 Mexico Earthquake. © ASCE.