Seismicity in Eastern North America: What Is the Source of Intraplate Strain and Stress?

Abstract:

A comprehensive mechanism to explain seismicity in stable intraplate regions remains enigmatic. Within eastern and central North America, for example, a number of moderate to large-sized earthquakes have occurred, with the New Madrid sequence of 1811 and 1812 being the most noteworthy. The much more recent M 5.8 Virginia earthquake is a fresh reminder that there is a source for strain accumulation within eastern North America. Although there is agreement that these intraplate earthquakes owe their activity to the reactivation of ancient faults created during previous tectonic events, there is no comprehensive model to explain how these faults get reactivated. Several hypotheses exist, including lithospheric sources, such as ridge push, coupling with mantle flow, or even strain release associated with glacial isostatic adjustment (GIA). Here we explore whether tectonic effects, which consist of forces associated with topography and lithosphere structure (gravity potential energy differences – GPE), coupled with the effects of large scale mantle flow, are compatible as a source of stress for seismicity within eastern US. We compare the p-axes of all compiled earthquake mechanisms in this region to the SHmax directions of deviatoric stresses, modeled using a global finite element method. The method we employ couples gravitational potential energy derived stresses with stresses from a global density-driven mantle convection model that incorporates both radial and lateral viscosity variations. Additionally, we also investigate how lateral strength variations within the lithosphere of continental North America and adjacent regions affect these forces, and consequently the occurrence of earthquakes in this region.