Talc-Bearing Fault at Coseismic Velocities: an Attempt to explain the San Andreas Fault Behavior

Talc is one of the few minerals not following the Byerlee law: friction coefficient of ~0.25 at low velocities and poorly known frictional properties, especially at seismic velocities. Talc has recently been identified in the San Andreas Fault (Moore and Rymer, 2007), and is expected to occur along fault zones within subduction zone. Talc can then affect their seismic behaviour significantly. The properties of the talc were studied by high-velocity rotary-shear experiments carried out at 1.31 m/s, at normal stresses of 0.6 to 1.9 MPa, using the frictional testing apparatus at Kochi/JAMSTEC. The experimental gouge is a pure natural talc schist powder (99.9 wt.%). The samples were sheared under dry (room moisture conditions) or wet (saturated) conditions. During experiments, we measured the evolution of shear stress, simulated fault thickness, moisture release and temperature through the whole simulated fault. Main results are as follows: 1) At the onset of slip, a slip weakening is observed for all experiments with a higher slope value in wet conditions, and a larger Dc value for dry conditions: a value comprised between 0.05 and 6.06 m for wet tests, and 5.72 and 30.87 m for dry tests. 2) The weakening behaviour is strongly dependent on normal stress. 3) The dynamic friction is lower in wet conditions: a peak value of 0.98-0.65 to a steady state value of 0.17-0.26 for dry tests, and a peak value of 0.25-0.52 to a steady state value of 0.08-0.22 for wet tests. 4) Dry samples only experienced large fault dilation comprised between 0.14 and 1.86 mm, as applied normal stress increases. 5) Wet samples only exhibited a large release of water steam. 6) A higher maximum temperature for dry conditions at 1 MPa normal stress: 570 °C for dry tests and 350 °C for wet tests. Microstructural observation with an optical microscope and SEM showed evidence that shear localization processes are different between wet and dry experiments, but a Principal Slip Zone formed by with strong Shape Preferred Orientation is always observed at the shear zone boundary in both cases. In absence of neither evidence of amorphous material nor decomposition product of talc, this preliminary work suggests thermal pressurization and powder lubrication as weakening mechanisms for wet and dry conditions, respectively. Considering now the 28 September 2004 rupture of the San Andreas Fault, we argue the absence of important high stressed patches in the creeping segment part of the SAF to be explained by talc mineral as a weaker component of the SAF gouge, for which very low stress is required to initiate slip.