In this study we prepared a 3D velocity model suitable for modeling long-period wave propagation in the Puget Sound region. The model is based on products of the Seismic Hazard Investigation in Puget Sound (SHIPS) and geophysical information from other studies of the region. The adequacy of the velocity model was evaluated based on analyses of goodness of fit between recorded and simulated ground-motion velocity from the M 6.8 Nisqually earthquake. The earthquake was located about 60 km south of Seattle with a hypocentral depth of 59 km. The analyses were performed in the frequency range of 0.02-0.5 Hz, using data from 40 stations. Although our model covers a wide area of the Puget Sound region, its quality is assessed in the Seattle region in which the distribution of stations that recorded the Nisqually earthquake was denser. Our 3D finite-difference ground-motion modeling suggests that the propagation of long-period waves (periods longer than 3 sec) in the Seattle basin is mostly affected by the deep basin structure. The tomographic velocity model of Parsons et al. (2001), combined with the model of depth to the basement of the Seattle basin of Blakely et al. (1999), was essential in preparing and constraining geometrical features of the proposed velocity model.