The San Andreas Fault (SAF) near Parkfield, CA moves by a combination of aseismic creep and micro-earthquake slip. Measurements of in situ stress orientation, stress magnitude, and heat flow are incompatible with an average shear stress on the SAF greater than approximately 20 MPa. To investigate the micro-mechanical processes responsible for the low strength and creeping behavior, gouge samples from the 3 km-deep scientific borehole near Parkfield (the San Andreas Fault Observatory at Depth, SAFOD) are sheared in a triaxial rock deformation apparatus at conditions simulating those in situ, specifically a temperature of 100?C, effective normal stress of 100 MPa, pore fluid pressure of 25 MPa, and a Na-Ca-K pore fluid chemistry. The 2 mm-thick gouge layers are sheared to 4.25 mm at shear rates of 6.0, 0.6, 0.06, and 0.006 mu m/s. The mechanical data are corrected for apparatus effects and the strength of the jacketing material that isolates the sample from the confining fluid. Experiments indicate that gouge is extremely weak with a coefficient of friction of 0.14, and displays velocity and temperature strengthening behavior. The frictional behavior is consistent with the inferred in situ stress and aseismic creep observed at SAFOD. The low frictional strength likely reflects the presence of a natural fabric characterized by microscale folia containing smectite and serpentinite.
The San Andreas Fault (SAF) near Parkfield, CA moves by a combination of
aseismic creep and micro-earthquake slip. Measurements of in situ stress orientation,
stress magnitude, and heat flow are incompatible with an average shear stress on the
SAF greater than approximately 20 MPa. To investigate the micro-mechanical processes
responsible for the low strength and creeping behavior, gouge samples from the 3 km-deep
scientific borehole near Parkfield (the San Andreas Fault Observatory at Depth,
SAFOD) are sheared in a triaxial rock deformation apparatus at conditions simulating
those in situ, specifically a temperature of 100?C, effective normal stress of 100 MPa,
pore fluid pressure of 25 MPa, and a Na-Ca-K pore fluid chemistry. The 2 mm-thick
gouge layers are sheared to 4.25 mm at shear rates of 6.0, 0.6, 0.06, and 0.006 mu m/s. The
mechanical data are corrected for apparatus effects and the strength of the jacketing
material that isolates the sample from the confining fluid. Experiments indicate that
gouge is extremely weak with a coefficient of friction of 0.14, and displays velocity and
temperature strengthening behavior. The frictional behavior is consistent with the
inferred in situ stress and aseismic creep observed at SAFOD. The low frictional strength likely reflects the presence of a natural fabric characterized by microscale folia
