The M 5.0 earthquake that occurred near Au Sable Forks, New York on 20 April 2002 is the first moderate event in eastern North America (ENA) to be well recorded on modern regional broadband seismographic stations, and is thus important in refining our understanding of source and propagation processes of moderate ENA earthquakes. We analyze the available seismographic recordings to obtain the source spectrum of the Au Sable Forks earthquake, comparing the results obtained using two methods. In the direct method, the Fourier spectra of the records from each station are corrected back to the source by using an empirical regional attenuation model for ENA. In the empirical Green's function method, aftershocks are used as empirical Green's functions to remove path and site effects. Results from these two methods agree closely and are in agreement with a two-corner source model for ENA. A pronounced sag in the source spectrum at a frequency near 2 Hz is suggestive of significant source complexity.
Observed response spectral amplitudes agree well with the predictions of several ground-motion relations for ENA for an event of M 5.0. It is concluded that ground motions from this earthquake were “typical” for ENA earthquakes of this magnitude.