Source spectra for earthquakes of 3 ≤ M ≤ 7 in eastern North America (ENA) are determined from regional seismograph recordings, strong-motion records, teleseismic data, and Modified Mercalli Intensity data. The inclusion of MMI data is made possible by the finding that the felt area of an earthquake is an accurate measure of its high-frequency spectral level at the source.
Current ground motion relations for ENA are based on a Brune source model with a stress parameter of 100 bars. This model is a poor fit to ENA source spectra for events of M > 4. At high frequencies (5 to 10 Hz), observed spectral amplitudes are more nearly matched by a Brune stress parameter of 200 bars; intermediate-frequency amplitudes (near 1 Hz) are matched by a stress parameter of approximately 50 bars. An empirical two-corner source model is proposed to describe ENA source spectra for events of 4 ≤ M ≤ 7.