Frictional behavior of materials in the 3D SAFOD volume

We report on frictional properties of rocks within the 3‐D crustal volume surrounding the San Andreas Fault Observatory at Depth (SAFOD). Samples include lithologies adjacent to the San Andreas Fault (SAF) in the subsurface, SAFOD borehole rocks, and synthetic fault gouge composed of talc, serpentinite, and quartz. Granodiorite, arkosic sandstone, and siltstone samples from the SAFOD borehole are frictionally strong (μ = 0.56 − 0.66). Sand and clay‐rich lithologies from outcrop exhibit friction in the range μ = 0.56 − 0.68. Natural serpentinite thought to abut the SAF at depth exhibits low friction (μ = 0.18 − 0.26). Our results indicate that 1) serpentinite exhibits low strength, but is not weak enough to completely satisfy weak fault models, 2) all other samples are consistent with a strong fault and crust and, 3) if the SAF is weak (μ ≤ 0.2) due to the presence of serpentinite or talc, these minerals would likely need to constitute over 50% by weight of the shear zone.