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Abstract

Neotectonic movement in the United States east of the Rocky Mountains results from various superimposed intraplate adjustments related to movement in the North Atlantic Basin and glacial rebound. Some movements, such as slight southward tilt in the north from waning rebound and northward tilt of the entire region, appear aseismic. Others form a rectilinear grid pattern of zones of fractures and vertical movement and coincide with the distribution of seismicity. Most seismic source areas occur along northwest-trending fracture zones, which commonly have some right-lateral strike-slip displacement. They have broad, northeast-trending belts of rising highlands and sinking lowlands and are concentrated at their intersections. These belts are the Atlantic Coast Lowland, Southern and Central Appalachian Highland, Arkansas-St. Lawrence Lowland, and Mid-continent Arch. The northwest-trending fracture zones also apparently control the positions of northwest-trending synclinal basins—embayments—on the coastal plains and continue seaward as the larger transform fracture zones. The embayments are relatively subsiding and have significant associated seismicity. Some earthquakes also occur along north-trending extensional faults, whose movement seems related to lateral displacement on northwest-trending fracture zones, and a few occur along northeast-trending faults at structural intersections. The fracture zones continue into the western United States; the difference between “western” and “eastern” seismicity is more of degree than kind.

Earthquakes in the eastern United States thus are shown to be controlled by neotectonic movement and structural zones despite the lack of recent surface faulting, and a firm basis exists for seismic zoning.

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