We validate and improve 1D velocity models of the two main crustal provinces in the northeastern United States (NEUS), using seismograms from the 20 April 2002 M 5 Au Sable Forks earthquake, which is the largest earthquake in the region to be recorded by multiple, recently deployed, good-quality, regional broadband stations. To predict and mitigate the effects of future earthquakes in the northeastern United States, more information is needed regarding both the local earthquake sources and how seismic waves travel through the region. We investigate the source and regional wave propagation for the Au Sable Forks earthquake. The earthquake epicenter is located near the boundary of two distinct geological provinces, the Appalachian (New England) and Grenville (New York). We use a forward-modeling approach to study the waveforms recorded at 16 stations located within 400 km of the epicenter. We generate synthetic seismograms using the frequency–wavenumber method, testing several published models for the two provinces. Several models perform well at low frequencies (<0.1 Hz). We refine these models and generate two alternative 1D crustal models for intermediate frequencies (<1 Hz) of engineering interest. Our new Grenville model performs better than previously published models for all six source-station paths modeled in that province according to goodness of fit statistics: variance reduction and correlation coefficient. Our alternative Appalachian model improves the fit of synthetics to data for five of the ten paths modeled in that province. From the results, we identify two specific sources of wave-field complexities that should be investigated in future studies of earthquake ground motions in NEUS: 3% azimuthal anisotropy in the Appalachian Province and complex wave paths along the boundary between the two provinces.