Fault strength in a strike-slip setting from finite element models: California, USA

The strength of secondary faults within plate-boundary zones and that of master faults like the San Andreas has been controversial for decades. Here we use a global finite element code with a variable-resolution grid and mantle-derived driving forces to determine whether the effective friction μast on the San Andreas fault is high (μast = 0.6 - 1), intermediate (μast = 0.3 - 0.1) or low (μast < 0.1), whether a single value of μast can be used for all mapped faults within the region, and whether weakening of the ductile lower crust directly below faults is important. We compare our model results with existing data on fault slip-rates, GPS velocity field, stress field, and depth of earthquakes. This comparison indicates that all faults are weak (μast ≤ 0.1), and that slip-dependent weakening is important. All viable models show that weakening of major faults in the lower crust is necessary to avoid excessive weakening in the brittle crust and therefore unrealistic depths to the brittle-ductile transition. The strongest faults in California have μast in the range 0.1-0.03. The San Andreas fault is a very weak fault among weak faults, with μast values of about 0.01. Our success in modeling this region also shows that a global code with appropriate grid-refinement and mantle-driving forces can reproduce the tectonics of local areas while being driven by global mantle circulation models.