Intraplate seismicity in the eastern United States
Causes for intraplate earthquakes in the eastern United States are indeed complex. Spatially, the seismicity occurs in distinct zones superposed on a less active regional background level. Epicentral patterns tend to be linear within the Valley and Ridge Province (Giles County and eastern Tennessee), diffuse in the Piedmont (central Virginia), and cluster in the Coastal Plain Province (Charleston, South Carolina area). Historically, moderate to large earthquakes have occurred within the seismic zones as defined by microseismicity. Recent paleoseismicity studies suggest that these larger earthquakes can reasonably be expected to occur into the future. Earthquake focal depths in the eastern US tend to occur within the upper crust with little relation to the surficial features. Most hypotheses on the causes for seismicity invoke the idea of preexisting zones of weakness favorably oriented with respect to the ambient stress field. Because most eastern US seismic zones are buried in the subsurface, geophysical methods are used to infer the presence of possible causal geologic structures at depth. Gravity data are useful in locating density (lithologic) changes within the crust and seismic activity tends to concentrate within gravity saddles formed at the intersection of northeast and northwest trending gravity anomalies. Seismically important plutons and Triassic basins which are associated with seismic activity have been located by their magnetic signatures. The eastern US is interpreted to be composed of a mosaic of allochthonous suspect terranes. This hypothesis suggests that block boundaries are zones of crustal weakness. Seismicity tends to concentrate along the Avalon-Piedmont boundary in the southeastern US. Many earthquakes in the eastern US occur in paleorift zones and/or at the intersection of multiple tectonic features.
- Research Organization:
- Virginia Polytechnic Inst. and State Univ., Blacksburg (USA); Nuclear Regulatory Commission, Washington, DC (USA). Div. of Engineering
- OSTI ID:
- 6149103
- Report Number(s):
- NUREG/CR-4974; ON: TI87900908
- Country of Publication:
- United States
- Language:
- English
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