Velocity dependent friction of granite over a wide range of conditions

Direct shear sliding experiments on bare ground surfaces of Westerly granite have been conducted over an exceptionally wide range of sliding rates (10⁻⁴ µm/s to 10³ µm/s) at unconfined normal stresses (σ_n) of 5, 15, 30, 70, and 150 MPa. A new sample configuration was developed that permitted measurements at normal stresses of 70 and 150 MPa without immediate sample failure. Measurements of steady-state velocity dependence of friction at velocities between 10⁻⁴ and 1 µm/s show similar velocity weakening behavior at all normal stresses, with more negative dependence at lower slip rates. However, at rates above 10 µm/s, velocity weakening is observed only at σ_n=30, 70 and 150 MPa, while velocity neutral behavior is observed at σ_n=15 MPa and velocity strengthening is observed at σ_n=5 MPa. The greater velocity weakening observed at velocities below 10⁻² µm/s may suggest a transition in competing deformation mechanisms, or the influence of additional mechanisms. The transition to velocity strengthening at high velocity and low normal stress implies that rapid slip on shallow faults could be arrested before resulting in true stick-slip behavior. Stable fault creep and creep events observed at shallow levels on some natural faults may result from this transition in velocity dependence.