NORTH AMERICAN INTRAPLATE SEISMICITY: A CASE-BY-CASE PERSPECTIVE

Intraplate seismicity is difficult to reconcile with standard plate tectonics in that it suggests stresses generated by sources other than edge forces. Zones of anomalous weakness and/or high fluid pressure, lithospheric bending stress, stress refraction, variations in lithospheric gravitational potential energy (GPE), and basal shear stress exerted on the lithosphere by asthenospheric convection have all been invoked to explain seismicity in particular regions. I present a series of case studies of North American intracontinental seismicity: the Idaho batholith/Snake River Plain, Yakima fold and thrust belt, New Madrid, and eastern Tennessee. Each seismic zone arises from a unique combination of causes. The dichotomy between extension in the Idaho batholith and aseismicity in the Snake River Plain is readily explained by variations in GPE. Similarly, large N-S variations in lithospheric GPE rotate principal strain from trench-parallel to trench-perpendicular in the Yakima fold and thrust belt. While these areas have topographic gradients and obvious, large lithologic contrasts, the New Madrid seismic zone has a subtle surface expression. Nevertheless, a poorly constrained Markov Chain Monte Carlo simulation of gravity and topography implies that GPE-induced stress of >10 MPa encourages the modern pattern of strain. Finally, analysis of Appalachian-region seismic zones (e.g., eastern Tennessee) suggests that a combination of inherited weakness and lithospheric bending stress promotes seismicity. Taken together, these regions show that the causes of intraplate seismicity are varied and unique to each seismic zone.