A Comparison Between the Interseismic and Long-Term Strain Rate Fields in Western North America

We quantify the full horizontal velocity gradient tensor field associated with the interseismic deformation in western North America, using published GPS velocities and the GPS observations provided by the PBO network of EarthScope. Using Quaternary Fault observations, with far-field plate velocity boundary conditions, we also determine the long-term horizontal deformation field in western North America. The long-term patterns of finite strain reflect the balance of forcings associated with internal body force distributions (gravity potential energy differences), basal tractions associated with mantle convection, and plate interaction. These forces act on a laterally heterogeneous and anisotropic lithospheric rheology. The interseismic strain rate field reflects dominant elastic loading rates, and viscoelastic coupling related to transients associated with past earthquakes. A first step in relating interseismic processes to much longer-term deformation processes is a quantitative comparison between the two deformation fields. We investigate the correlation between the interseismic strain rate tensor field and the long-term strain rate tensor field. Significant differences between the strain rate solutions for the two time scales are found within Cascadia and Yellowstone, as well as for motion of the Colorado Plateau in a North American frame. There is a westward motion of the Colorado Plateau (~2 mm/yr) in a NA frame present in the interseismic solution that is absent in the long-term solution. Long-term kinematic estimates are subject to considerable error due to uncertainties in slip rate. The interseismic solution is also subject to significant uncertainties from both the influence of transients, and also the relatively short time frame over which new PBO stations have been operating within some regions, such as the Colorado Plateau, southern Arizona, and New Mexico. We evaluate the differences between the two solutions, taking into account all quantifiable uncertainties.