INTEGRATED GEOLOGIC AND GEOPHYSICAL STUDIES OF NORTH AMERICAN CONTINENTAL INTRAPLATE SEISMICITY

The origin of earthquakes within stable continental regions (SCR's) has been the subject of debate over the past thirty years. Here we examine the correlation of North American SCR earthquakes with five geologic and geophysical data sets: (1) a newly compiled age-province map; (2) Bouguer gravity; (3) aeromagnetic anomalies; (4) the tectonic stress field, (5) crustal structure as revealed by deep seismic reflection profiles. We find that: (1) Archean age (3.8-2.5 Ga.) North American crust is essentially aseismic, whereas post-Archean (less than 2.5 Ga.) crust shows no clear correlation of crustal age and earthquake frequency or moment release; (2) seismicity is correlated with continental paleo-rifts, and (3) seismicity is correlated with the NE-SW structural grain of the crust of eastern North America, which in turn reflects the opening and closing of proto- and modern Atlantic oceans. This structural grain can be discerned as clear NE-SW lineaments in the Bouguer gravity and aeromagnetic anomaly maps. SCR seismicity either: (1) follows the NE-SW lineaments; (2) is aligned at right angles to these lineaments; or (3) forms clusters at what have been termed stress concentrators (e.g., igneous intrusions and intersecting faults). Seismicity levels are very low to the west of the Grenville Front (i.e., in the Archean Superior craton). The correlation of seismicity with NE-SW oriented lineaments implies that some SCR seismicity is related to the accretionary and rifting processes that have formed the North American continental crust during the past 2 Gy. We conclude that the deep structure of the crust, in particular the existence of deeply penetrating faults, is the controlling parameter, rather than temperature, rheology, or high pore pressure. The distribution of SCR earthquakes in eastern North America is consistent with the existence of deeply penetrating crustal faults that have been reactivated in the present stress field. We infer that future earthquakes may occur anywhere along the geophysical lineations that we have identified. This implies that seismic hazard is more widespread in central and eastern North America than indicated by the limited known historical distribution of seismicity.