We examine 16 moderate-size earthquakes that have occurred since 1990 in the northeastern United States and southeastern Canada. Source parameters, such as focal mechanism, focal depth, and seismic moment, are determined using a regional waveform inversion technique in which the best-fitting double-couple mechanism is obtained through a grid search over strike, dip, and rake angles. Most of the 16 events and 10 previous ones with well-determined focal mechanisms show horizontal compression with near-horizontal P axes striking northeasterly, which is consistent with the maximum horizontal stress orientation for midplate North America. Four shocks along the St. Lawrence River, however, have P axes trending southeast (100°-130°), which is not compatible with the regional stress field and requires a local stress perturbation close to the St. Lawrence River. We examine several generating mechanisms for this localized stress variation, including that a “rift pillow” structure may exist in the lower crust below the St. Lawrence River. All of the 11 shocks in the United States occurred at shallow depths from 2 to 8 km; the 15 Canadian events are systematically deeper, with foci between 5 and 28 km. This geographic variation in focal depth is well correlated with the difference in surface heat flow values between the older Grenville and younger Appalachian provinces in the study area. We also examine apparent discrepancies between MW and mb(Lg) magnitude for 17 events. Five earthquakes of very similar MW show variations in mb(Lg) as large as 0.6, which implies that for seismic events with similar seismic moment in the study area, high-frequency seismic radiation behavior differs considerably.