<p>Achieving realistic seismic behavior of dam structures in numerical dynamic analyses relies heavily on the accurate representation of a semi-infinite foundation, which is conventionally truncated within a finite element mesh. During the forced-vibration phase, continuous seismic input complicates the isolation of inherent damping characteristics. Tracking energy decay during the free-vibration phase provides a clearer mechanism for quantitatively validating soil-structure interaction (SSI) models. Accordingly, this study introduces an alternative numerical methodology for evaluating SSI modeling strategies based on their energy discharge capacities during the free-vibration phase. Utilizing a comprehensive 3D model of a double-curvature concrete arch dam subjected to two earthquake records with contrasting frequency contents, the energy decay coefficient was derived from the total mechanical energy attenuation, and the dominant structural frequency was extracted via Fast Fourier Transform (FFT) analysis of the displacement histories. These fundamental mechanisms were then used to quantify the effective damping ratio and energy half-life across three foundation stiffness classes. The results demonstrated that incorporating viscous boundaries yielded field-consistent effective damping ratios ranging from 7.77 to 10.72% and increased the energy decay coefficient by 18% to 51% compared with the traditional massless foundation model. The findings confirm that seismic damping is not a constant material parameter but a highly dynamic response governed by foundation impedance and earthquake characteristics. Consequently, this methodology provides engineers with a robust tool for validating the true dynamic characteristics of numerical models, ensuring secure and optimized structural designs.</p>