The strength of faults in the crust in the western United States

The strength of secondary faults within plate-boundary zones and of master faults like the San Andreas has been controversial for decades. We use a global finite-element code with a variable-resolution grid and global plate-driving forces to determine whether the effective friction µ on the San Andreas fault is high (µ* 0.6–1), intermediate (µ* 0.3–0.5) or low (µ* ≤ 0.2), whether a single value of µ* can be used for all mapped faults within California, and whether weakening of the ductile lower crust associated with faulting is important. We compare our model results with existing data on fault slip-rates, GPS velocities, stress field, and earthquake depth distribution. The comparison indicates that all faults are weak (µ* ≤ 0.2), and that additional weakening of major faults is important. All viable solutions also indicate that weakening of the lower crust below major faults is necessary. The strongest faults in the region have µ* in the range 0.2–0.05. The San Andreas fault is a very weak fault among weak faults, with µ* < 0.05. Our results also show that a global code with appropriate grid-refinement and driven by global plate motions can reasonably reproduce regional tectonics.