In situ frictional properties of San Andreas Fault gouge at SAFOD

• Published in: Geophysical journal international 2014-11, Vol.199 (2), p.956-967
• Main Authors: Coble, C. G., French, M. E., Chester, F. M., Chester, J. S., Kitajima, H.
• Format: Article
• Language: English
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1093/gji/ggu306

Along the central segment of the San Andreas Fault (SAF) near Parkfield, California, displacement occurs by a combination of aseismic creep and micro-earthquake slip. To constrain the strength and parametrize a constitutive relation for the creeping behaviour of the central segment of the SAF, we conducted friction experiments on clay-rich gouge retrieved by coring the Central Deforming Zone (CDZ) of the SAF at 2.7 km vertical depth. The gouge was flaked rather than powdered to preserve the natural scaly microfabric, and formed into 2-mm-thick layers that were sheared using a triaxial deformation apparatus. Experiments were conducted at in situ effective normal stress (100 MPa), pore pressure (25 MPa) and temperature (80–120 °C) conditions using brine pore fluid with the ionic composition of the in situ formation fluid. Velocity-stepping (0.006–0.6 μm s−1) and temperature-stepping experiments were conducted on brine-saturated gouge, and slide-hold-slide experiments were conducted on brine-saturated and room-dry gouge. Results are used to quantify the effects of rate, state, temperature and pore fluid on the strength of the CDZ gouge. We find that the gouge is extremely weak (μ