Seven years (1977--84) of seismic network monitoring in the southeastern U. S. have resulted in a catalog of 255 earthquakes (0≤M≤4.2) with ERZ≤5 km. Focal depths from the valley & Ridge and Blue Ridge provinces (mean depth 12 km) were combined for analysis as were those for the Piedmont and Coastal Plain provinces (mean depth 8 km). The mean focal depths between these two regions are shown to be statistically different. The 90% depths of earthquakes are also very different between the regions, implying a difference in the depth to the brittle-ductile transition. First order models of crustal rheology based on estimates of composition, temperature and strain-rate, indicate that the observed differences in earthquake focal depths may reflect differences in strain-rate and/or composition.