Assessment of earthquake hazards in the United States is based largely upon knowledge acquired in the seismically active parts of the western United States. Earthquakes in the central and southeastern United States are seismologically and geologically very anomalous, however, compared with those in the western United States. For example, shallow earthquakes of magnitude 7-f in the western United States in general have tectonic surface faulting associated with them. No tectonic surface faulting, however, has been found to be associated with the New Madrid, Mo., earthquakes of 1811 and 1812, several of which had estimated magnitudes of 7-f. The lack of surface faulting is, in part, responsible for the fact that the New Madrid earthquakes have not been related with certainty to any particular geologic structural element, which makes assessment of the seismic hazards associated with possible similar future earthquakes very difficult. Geologic and seismologic data from the central and southeastern United States, however, suggest a spatial correlation between mafic intrusive rocks and seismic activity. An additional supportive correlation is that the trend of many dikes coincides with the trend of nodal planes in many fault-plane solutions. These correlations differ greatly from the direct correlation of earthquakes with faults in the western United States. Consideration of different concepts of earthquake sources in the central and southeastern. United States seems necessary- The preferred concept is that local high-stress concentrations, which may result in earthquakes, occur at or close to the contact of mafic intrusives and their felsic host rocks. Ancient rift zones may have been the primary control of the location of the mafic intrusives. Implications of a causal relation between mafic intrusives and seismicity are that more useful seismo-tectonic maps may be possible and that the source dimensions of the earthquakes would be small. Because of low attenuation, however, effects of earthquakes in the central and southeastern United States, even if relatively small because of smaller source dimensions, must be considered hazardous.
