Correlation of major eastern earthquake centers with mafic/ultramafic basement masses

Extensive gravity highs and associated magnetic anomalies are present in or near seven major eastern North American earthquake areas as defined by Hadley and Devine (1974). The seven include the five largest of the eastern North American earthquake .centers. The immediate localities of the gravity anomalies are, however, relatively free of seismicity, particularly the largest events. The anomalies are presumably caused by extensive mafic or ultramafic masses embedded in the crystalline basement. Laboratory experiments show that serpentinized gabbro and dunite fail under stress in a creep mode rather than in a stick-slip mode. A possible explanation of the correlation between the earthquake patterns and the anomalies is that the mafic/ultramafic masses are serpentinized and can only sustain low stress fields thereby acting to concentrate regional stress outside their boundaries. The proposed model is analogous to the hole-in-plate problem of mechanics whereby stresses around a hole in a stressed plate may. reach values several times the average.