Finite element models of stress orientations in well-developed strike-slip fault zones: Implications for the distribution of lower crustal strain

Finite element models are used to examine the effects of strike-slip earthquakes on stresses in an elastic layer overlying a finite width viscoelastic shear zone in the lower crust. The overall dimensions of the model are 300 km wide, 400 km long, and 50 km deep. Three geometries for the lower crustal shear zone are considered: (1) a viscoelastic half-space approximation, with a shear zone that extends to the model boundaries (300 km in width); (2) a wide shear zone model (70 km in width); and (3) a narrow shear zone model (10 km in width). Earthquakes are simulated with a fault plane that slips without friction at the desired time step and is centered above the shear zone, extending to a depth of 15 km and running the length of the mesh. Far-field plate velocity boundary conditions are enforced at the model edges so that stress on the fault evolves naturally. A Coulomb-type failure criterion based on the average shear stress on the fault is set such that the earthquake cycle is similar to 250 years. The models are run until a limit cycle is reached and transient stresses have decayed away. We focus on the maximum changes in the stress field during the earthquake cycle by examining stresses before and immediately after each earthquake. In addition to comparisons of the separate components of the stress tensor we present results in the form of maximum compressive stress orientations, plunge angles of the principal stress axes, and beach ball diagrams that facilitate visualizing the full tensor. Stresses are concentrated in the upper crust where it overlies the finite width viscoelastic shear zone, which causes the plunge angles of the principal stress axes to rotate from Andersonian orientations of 90 degrees and 0 degrees to angles that approach 45 degrees in the lower crust. Our results suggest that an examination of stress orientations in the upper and middle crust from borehole breakouts and focal mechanisms may provide insight as to the distribution of strain in the lower crust and may eventually allow us to distinguish between localized and distributed deformation models for the lower crust in active strike-slip zones.