What can strike-slip fault spacing tell us about the plate boundary of western North America?

The spacing of parallel continental strike-slip faults can constrain the mechanical properties of the faults and fault-bounded crust. In the western US, evenly spaced strike-slip fault domains are observed in the San Andreas (SA) and Walker Lane (WL) fault systems. Comparison of fault spacing (S) vs. seismogenic zone thickness (L) relationships of the SA and WL systems indicates that the SA has a higher S/L ratio (similar to 8 vs. 1, respectively). If a stress-shadow mechanism guides parallel fault formation, the S/L ratio should be controlled by fault strength, crustal strength, and/or regional stress. This suggests that the SA-related strike-slip faults are relatively weaker, with lower fault friction: 0.13-0.19 for the SA vs. 0.20 for WL. The observed mechanical differences between the San Andreas and Walker Lane fault systems may be attributed to variations in the local geology of the fault-hosting crust and/or the regional boundary conditions (e.g. geothermal gradient or strain rate).