A geophysical model of ground-motion simulation is used to generate the statistics of the power spectral density (PSD) of the acceleration at the bedrock level of a site. A nonstationary Poisson model of occurrences is then developed to combine the effects of all the probable sources in the seismic zone of interest. The consistent probability PSD is computed as a basic design parameter. Consistent probability peak response spectra, rms response spectra, and pseudotime histories are also derived as design parameters. A numerical example is given for a site in southern California to demonstrate the applicability of the method.