The Long-Term Dynamics of the Seismogenic Layer in Western North America

We investigate the dynamics of the seismogenic layer within the plate boundary zone of western North America. We use a forward dynamic modeling approach, where the body force distributions, inferred lateral variations in effective viscosity, power-law behavior, and the known far-field velocity boundary conditions are defined. Body forces are the differences in gravity potential energy per unit area (GPE), obtained by performing depth integration of vertical stress from the surface down to a common depth reference level (the brittle-ductile transition). In our treatment of the seismogenic layer, the effective viscosity is proportional to the long-term friction coefficients of the faults. Solutions to force-balance equations define self-consistent velocity, strain rate tensor, and stress tensor fields of the seismically active upper crust. We investigate models using a range of long-term fault friction coefficients from 0.02 - 1.0 under hydrostatic pore pressure conditions. The precise long-term friction on the faults, along with the distribution of fault fabric, control the relative influence that internal crustal buoyancies and plate boundary forces have on the deformation field. We score our dynamic model velocity and strain rate fields with estimates of the long-term deformation field constrained using Quaternary fault observations. Faults given a long-term friction coefficient of 0.1 yield a best-fit to deformation indicators. For low fault friction cases, the contribution to the total deviatoric stress field from differences in GPE is approximately equal to the contribution from the accommodation of plate motions. However, for higher fault friction (µ > 0.5), plate motion accommodation mechanisms dominate over GPE differences. These solutions with higher fault friction yield poor fits to deformation indicators. As well, we investigate use of fault orientations of published block models, assuming weak shear zones for the faults and high effective viscosity for the blocks. These models also fail to adequately match the long-term deformation indicators.