The Dynamics of Eastern North America: Intraplate Strain and Stress

Intraplate areas, such as central and eastern North America, sometimes experience large earthquakes. These earthquakes, such as the recent Virginia quake, have the potential of causing major damages. However, the causes of these earthquakes are not well understood. The stresses causing these earthquakes could be due to tectonic effects as well as glacio-isostatic adjustment (GIA). Here, we attempt to explore some of the causes behind these enigmatic earthquakes by computing the deviatoric stresses and strain rates resulting from tectonic forces. These global calculations have influence of topography and lithosphere structure (Crust 2.0 and ocean cooling model), and coupling with a whole mantle convection model (history of subduction + residual tomography in upper mantle and tomography in the lower mantle). Calculations of lithosphere response are run on a 1x1 degree global finite element grid, where coupling with mantle flow is applied as a lower boundary condition. The lithosphere model has lateral variations in effective lithosphere viscosity (mainly at the plate boundary zones). The best model gives a global RMS velocity misfit of 9 mm/yr (63000 points compared to an NNR velocity field). We studied the effects of lateral viscosity variations within the plates. We compared models where (1) intraplate areas have all one effective viscosity, (2) intraplate areas have variable viscosities due to stiff cratonic keels as well as varying viscosities due to old vs young oceanic lithosphere. Models with both cratonic keels and old stiff ocean lithosphere provide the best match to the orientation of earthquake focal mechanisms in eastern North America. Focal mechanisms show a transition of maximum compressive horizontal strain from W-NW on the eastern seaboard areas to NE-SW in areas to the west. The dynamic model that has an effective viscosity within the Appalachian-Coastal Plain regions that is an order of magnitude lower than adjacent craton and old ocean regions matches this transition. The match to focal mechanisms within and south of the hinge line suggests that GIA has little effect on the overall orientation of maximum shortening direction associated with large-scale dynamics.