Neotectonic movement, earthquakes and stress state in the eastern United States

Abstract

A regional pattern of crustal movement across the eastern United States is suggested by relations found by investigations into the cause of earthquakes. The pattern consists of zones of fractures and vertical movement. Most of the seismic source areas appear to be related to northwest-trending fracture zones, that commonly have apparent righ-lateral strike-slip offset, where they cross broad northeast-trending belts of vertical movement. The fracture zones also appear to control the positions of local northwest-trending basins, embayments, on the Atlantic and Gulf Coastal plains. These embayments seem to be relatively subsiding at present and have earthquakes associated with them; intervening arches have very few earthquakes. Some earthquakes are also spatially related to north-trending extensional fault zones that commonly lie north of northwest-trending fracture zones. A slight northward tilt of the entire region and possible lingering effects of glacial rebound in the northeast show no relation with seismicity. Both the broad belts of vertical movement and embayments are apparently those initiated by the Late Cretaceous as the North Atlantic basin widened, and the northwest-trending fracture zones can be directly or indirectly related to the offshore transform fracture zones. The interplate movements in the eastern United States are thus directly related to those in the North Atlantic basin. The fracture pattern is also similar to that of the western United States, but the movements less. These tectonic movements in the eastern United States can be accounted for by a maximum horizontal compressive stress oriented approximately north-northwest-south-southeast to north-south and an extension oriented approximately east-west.