Evidence of long-term weakness on seismogenic faults in western North America from dynamic modeling

We investigate the long-term strength of faults within the plate boundary zone of western North America by quantifying the depth-integrated deviatoric stress field acting within the seismogenic portion of the crust. Forcings in the depth-integrated force balance equations are the horizontal gradients in gravitational potential energy per unit area (GPE). Seismic velocity data define the densities we use to determine GPE. We also solve for stress field boundary conditions that, when added to the contribution from GPE differences, provides a best fit to stress indicators. We estimate that the long-term depth-integrated total stress differences within the approximately 20 km thick seismogenic layer are of the order of 0.1-1.4 X 10(12) N m(-1). Using these stress differences as a proxy for depth integrals of fault strength within the actively deforming regions, we infer that the long-term values of coefficients of friction on faults within the Basin and Range of Nevada and Utah, and most of California, are 0.1-0.2 under long-term hydrostatic pore pressure conditions. We test the sensitivity of these results by considering a range of maximum depths of integration. We show that for depths of integration in excess of 20 km below sea level, there is diminishing contribution to the depth-integrated stress differences, and by proxy depth-integrated fault strength. This is consistent with a brittle-ductile transition in the plate boundary zone at depths less than 20 km below sea level, and with a weaker lower crust.