The San Andreas fault accommodates 28-34 mm yr(-1) of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The combination of these measurements of fault core strength with borehole observations yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

Abstract. Recent studies suggest that the tendency of many fault gouge minerals to take on adsorbed or interlayer water may strongly influence their frictional strength. To test this hypothesis, triaxial sliding experiments were conducted on 15 different single-mineral gouges with various water-adsorbing affinities. Vacuum dried samples were sheared at 100 MPa, then saturated with water and sheared farther to compare dry and wet strengths. The coefficients of friction, g, for the dry sheet-structure minerals (0.2-0.8), were related to mineral bond strength, and dropped 20-60% with the addition of water. For non-adsorbing minerals (It = 0.6-0.8), the strength remained unchanged after saturation. These results confirm that the ability of minerals to adsorb various amounts of water is related to their relative frictional strengths, and may explain the anomalously low strength of certain natural fault gouges.

The permeability of both clay-rich and non-clay gouges, as well as several pure clays, was studied as a function of confining pressures from 5 to 200 MPa and shear strain to 10. Permeability ranged over four orders of magnitude, from around 10 -22 to 10 -18 m 2 (1 darcy = 0.987 x 10 -12 m2). The lowest values were characteristic of the montmorillonite-rich and finer grained non-clay gouges. Illite, kaolinite, and chlorite had intermediate permeabilities, while the highest values were typical of the serpentine and coarser grained non-clay gouges. Grain size was an important factor in determining permeability, particularly for the clay-rich samples. The coarse grained gouges were the most permeable and decreased in permeability after sheafing. Conversely, the fine grained gouges had characteristically lower permeabilities that did not vary significantly after various amounts of sheafing. The permeabilities of the non-clay samples were not significantly different than those of the clays. Therefore, comminuted rock flours can be equally as effective in reducing the flow of water as the characteristically low permeability clay gouges. The strengths of the samples were quite variable. The non-clay gouges were consistently the strongest, with yield points (beginning of nonelastic behavior) around 850 MPa, while montmorillonite had an anomalously low strength in relation to all the other gouges at 250 MPa. Strength of the saturated samples under drained (low pore pressure) conditions did not correlate with high or low permeability. However, the low permeabilities of these gouges could be a factor in the measured low shear stresses along fault regions if excess pore pressures were created as a result of sheafing or compaction, and this pressure was unable to dissipate through a thick section of the material. 113, 87, 1975. Zoback, M.D., and J. D. Byedee, Permeability and effective stress, Am. Assoc. Pet. Geol. Bull., 59, 154-158, 1975. Zoback, M.D., and S. Hickman, In situ study of the physical mechanisms controlling induced seismicity at Monticello Reservoir, South Carolina, J. Geophys. Res., 87,6959, 1982 Zoback, M. D., H. Tsukahara, and S. Hickman Stress measurements at depth in the vicinity of the San Andreas fault: Implications for the magnitude of shear stress at depth, J. Geophys. Res., 85,6157, 1980. grained gouges were the most permeable and showed significant Middlefield Road, Menlo Park, CA 94025. L. Q. Shi, State Seismological Bureau, Beijing, People's Republic of decreases in permeability with shearing. Rearrangement of the China. grain fabric through grain rotation, cracking and crushing was observed in thin sections of these coarse gouges layer after shearing. (Received April 8, 1983;The fine-grained clay gouges, on the other hand, had characteris-revised October 6, 1983; tically lower permeabilities that did not vary with sheafing. The

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Montmorillonite is a common mineral in fault zones, and its low strength relative to other common gouge minerals is important in many models of fault rheology. However, the coefficient of friction, μ, varies with degree of saturation and is not well constrained in the literature due to the difficulty of establishing fully drained or fully dried states in the laboratory. We measured μ of both saturated and oven‐dried montmorillonite at normal stresses up to 700 MPa. Care was taken to shear saturated samples slowly enough to avoid pore fluid overpressure. For saturated samples, μ increased from 0.10 to 0.28 with applied effective normal stress, while for dry samples μ decreased from 0.78 to 0.45. The steady state rate dependence of friction, (a − b), was positive, promoting stable sliding. The wide disparity in reported frictional strengths can be attributed to experimental procedures that promote differing degrees of partial saturation or overpressured pore fluid conditions.