A large region of high horizontal compressive stress is delimited in eastern North America from a combination of fault plane solutions of earthquakes, in situ stress measurements, and geologic observations. Each of these methods, including in situ stress determination by both overcoring and hydrofracturing, yields nearly identical directions for the principal stresses. The maximum compressive stress trends east to northeast over an area extending from west of the Appalachian Mountain system to the middle of the continent, and from southern Illinois to southern Ontario. In eastern North America, intra-plate earthquakes appear to occur in areas of high stress along unhealed fault zones of late Paleozoic or younger age. An example of this is the seismic belt trending from Boston to the northwest through Ottawa. This seismic zone appears to be located along a continental extension of the Kelvin seamount chain which is postulated by others to be a transform fault related to the early opening of the North Atlantic. Similarly, the 1929 Grand Banks earthquake and the Charleston, South Carolina, seismic trend appear to be located along extensions of other oceanic fracture zones.
The relation between high stress and unhealed fault zones may provide a means to assess the earthquake risk within plates. The observed pattern of stresses appears to be post-Mesozoic in origin. This work supports Voight's hypothesis that the compressive stress observed within the North American plate may be generated by the same mechanism that drives the movements of large lithospheric plates. If this is indeed the case, stress measurements may furnish one of the best clues to the driving mechanism of plate tectonics.