Definition of the spectrum of ground motion as a function of earthquake magnitude and distance is a crucial component of seismic hazard evaluations of engineered structures. This study uses an empirical approach to investigate the Fourier spectra of earthquake ground motions in southeastern Canada. The primary dataset is comprised of 1500 digital seismograms, from 100 earthquakes of magnitude 3.0 to 6.5 recorded on 30 stations of the Eastern Canada Telemetered Network (ECTN). The distance range of the observations is 10 to 1700 km; focal depths are from 5 to 30 km.;Regression analyses of the ECTN data are used to determine the source spectrum for each event and describe its attenuation with distance. Simulations demonstrate that observed ground motions can be accurately modeled using the regression results, in conjunction with a simple stochastic model.;The attenuation curve has three distance sections. At distances (R) less than 70 km, corresponding to attenuation of direct S-waves, Fourier spectral amplitudes decay as R{dollar}\sp{lcub}-1.1{rcub}{dollar}. Between 70 and 130 km, where the direct-wave is joined by S{dollar}\sb{lcub}\rm m{rcub}{dollar}S, spectral amplitudes are approximately constant. Beyond 130 km, corresponding to the Lg-phase, amplitudes decay at a rate that is consistent with R{dollar}\sp{lcub}-0.5{rcub}{dollar} and Q = 670 f{dollar}\sp{lcub}0.33{rcub}{dollar}. The findings indicate that postcritical reflections from the Moho discontinuity play a significant role in determining the shape of the attenuation curve. However the influence of S{dollar}\sb{lcub}\rm m{rcub}{dollar}S is subtle, allowing the shape to be approximated by simple functional forms. There is little evidence for any dependence of attenuation on focal depth or tectonic province.;A database for source spectral amplitude in eastern North America (ENA) is compiled for earthquakes of 3 {dollar}{dollar} 4. The proposed source model has significant implications for ENA ground motion relations and seismic hazard assessment
