In this study, we have utilized our understanding of the physics of elastic wave propagation to simulate ground motions within about 200 km from earthquakes in eastern North America. Detailed crustal velocity structure models were developed for sources located in the Grenville and Appalachian provinces. The former consists of a single gradient in the crust, while the latter contains a mid-crustal discontinuity and a deep Moho. Semi-empirical synthetic seismograms were computed by convolving the generalized ray response of the structures with an empirical source function obtained from a recording close to the source. Peak ground motions derived from the synthetic seismograms adequately interpolate the sparse empirical data, while providing information on the wave propagation phenomena that define ground-motion attenuation. They suggest that peak ground-motion amplitudes are controlled by (in order of increasing range) the upgoing, direct S-wave, the diving S-wave, post-critical reflections from the mid-crustal or Moho discontinuities, and finally, Lg.