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Abstract

The wide acceptance of plate tectonics two decades ago caused earth scientists interested in neotectonics and seismicity to focus their attention along plate boundaries with resulting great advances in our understanding of interplate deformation. In contrast, crustal deformation and earthquake hazard in the interior of presumably rigid plates have been considered second-order phenomena and have received relatively little attention. Although the concept of a tectonically quiescent intraplate environment may be justified for some areas, in others, as in parts of eastern North America, the historic record of damaging earthquakes indicates that neotectonics is quite significant, at least from the seismological viewpoint, and needs to be better understood.

Certainly more earthquakes and, particularly, a greater number of damaging earthquakes occur in interplate western North America than in the intraplate rest of the continent. However, the largest known earthquakes in the conterminous U.S. have occurred in the East: the three 1811 to 1812 events in New Madrid, Missouri, and the 1886 event in Charleston, South Carolina. In this discussion the “size” of an earthquake refers to its damage potential, or the size of the area with damaging intensities (which is here arbitrarily taken as the area with Modified Mer- calli intensity VIII or greater). Damage potential depends on the spectrum and attenuation of seismic waves and is not a reliable measure of the total radiated seismic energy or the tectonic strain, which are expressed by the moment or magnitude. Thus, the 300-km-long 1906 rupture of the San Andreas fault produced a smaller area of damaging intensities (Fig. 1), but probably a much larger moment than the 1811 to 1812 earthquakes in the East. Such a systematic difference between eastern and western earthquakes has been attributed in part to differences in seismic attenuation properties (e.g., Nuttli, 1973). High stress-drops may also contribute to this difference.

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