In situ frictional properties of San Andreas Fault gouge at SAFOD

C. G. Coble, Melodie E. French, Frederick M. Chester, Judith S. Chester, & Hiroko Kitajima

Published 2014, SCEC Contribution #1942

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) 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 < 0.13) and rate-strengthening, consistent with findings of previous studies on the CDZ gouge. We also find that, in a rate and state friction framework, slip history has a negligible effect on strength (b ≈ 0) under both saturated and dry conditions. The CDZ gouge is temperature-weakening from 80 to 120°C and weakens 17 percent when saturated with brine compared to room-dry conditions. Employing the laboratory-derived friction constitutive parameters, and including the temperature weakening and the strain-rate strengthening effects, we determine an approximate in situ friction coefficient of μ ≈ 0.11. For μ ≈ 0.11, aseismic creep under normal pore fluid conditions is permitted for angles up to 79° between the maximum horizontal stress and the plane of the SAF, consistent with nearby stress orientation measurements.

Coble, C., French, M. E., Chester, F. M., Chester, J. S., & Kitajima, H. (2014). In situ frictional properties of San Andreas Fault gouge at SAFOD. Geophysical Journal International,. doi: 10.1093/gji/ggu306.