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The purpose of this map is to show the location of selected spatially associated positive magnetic and gravity anomalies in the eastern United States, and the spatial relationship of these anomalies, which we infer to represent mafic and ultramafic bodies, to seismically active areas. The geophysical anomalies are present in all physiographic provinces except the Valley and Ridge Province (fig. 1), and earthquake epicenters are present in all provinces, but there are few places where the anomalies are within 10 km of the epicenters of earthquakes with body wave magnitudes larger than 5.5. More major earthquakes are within 10 km of the centers of positive anomalies than are clusters of smaller earthquakes. Other major earthquakes occur at the edges of the plutonic complexes that contain the mafic bodies, and these complexes may be as large as several tens of kilometers in diameter. This map is one product of a study of the possible causal relationship between incompletely cooled mafic (includes ultramafic) plutons and major historic earthquakes in the eastern United States. The hypothesis was suggested by McCartan and Gettings (1991) to explain major earthquakes such as the one in Charleston, S.C. in 1886, for which no large fault has been found. The concept is outlined at the end of this text, and the theoretical basis for the idea, which is a modification of Kane's (1977) holein-the-plate concept, is presented by Gettings (1988).

Map showing the relationship of selected mafic and ultramafic bodies in the crust of the eastern United States to seismically active areas