Frictional strength of cuttings and core from SAFOD drillhole phases 1 and 2

We investigated the frictional properties of drill cuttings and core obtained from 1.85-3.1 km true vertical depth in the SAFOD scientific borehole in central California. Triaxial frictional sliding experiments were conducted on samples from primary lithologic units and significant shear zones, including the inferred active trace of the San Andreas fault. The samples were deformed at room temperature under constant effective normal stresses of 10, 40, and 80 MPa with axial shortening rates of 0.01-1.0 mm s(-1). The weakest samples were from shale, claystone, and siltstone units with friction coefficient mu=0.4-0.55. Stronger samples were from quartzo-feldspathic rocks with mu >= 0.6. Materials tested from two shear zones at 2560 and 3067 m measured depth had mu=0.4-0.55 and velocity strengthening behavior consistent with fault creep at depths <4 km. The coefficient of friction for bulk samples from the inferred trace of the San Andreas fault was similar to 0.6.