Potential field and surface geologic data may be used to subdivide the Precambrian and Phanerozoic crust of the eastern and central United States into several blocks. These subdivisions are made by using the trends, amplitudes, and frequencies of magnetic anomalies in conjunction with gravity and surface geologic data. Most blocks appear unrelated to historical seismicity, but between New Mexico and Georgia, the edges of several blocks may be connected to form the northern boundary of a broad zone of very low earthquake activity. We speculate that this southern zone has been decoupled from the stress regime affecting the remainder of the central and eastern United States and should experience few, if any, large earthquakes in the future as long as the present stress configuration is maintained.

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