Low strength of deep San Andreas fault gouge from SAFOD core

Abstract

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.

Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

“…In addition, also the results of laboratory strength measurements of SAFOD core material provide strong evidence that the deformation of the creeping portion of the SAF is controlled by the presence of weak clay minerals (here saponite; Lockner et al, 2011;Van der Pluijm, 2012). Other friction experiments show that fault weakness can occur in cases where weak mineral phases (phyllosilicates) constitute only a small percentage of the total fault rock assemblage (Collettini et al, 2009).…”

“…For instance, clay minerals from the SAFODeSDZ (samples S11, S12s) and SAFODeCDZ (sample S13) are identified as serpentineesaponite mixed layers; saponite, chlorite and illite (Fig. 3e, Table 2; see also Holdsworth et al, 2011;Lockner et al, 2011;Moore and Rymer, 2012;Schleicher et al, 2012). Besides clay minerals, serpentine (chrysotile) is a further main constituent of some samples (samples S10, S13; Fig.…”

“…Many previous studies suggest that the growth of Mg-rich phyllosilicates may play a key role in the mechanical behavior of the SAF and can be directly related to fault weakening (e.g. Rymer, 2007, 2012;Schleicher et al, 2010;Holdsworth et al, 2011;Bradbury et al, 2011;Lockner et al, 2011). Janssen et al (2011) suggest that the presence of abundant clay minerals is often related to the occurrence of nanoscale porosity that may indicates high fluid pressure.…”

Frictional properties of the active San Andreas Fault at SAFOD: Implications for fault strength and slip behavior

“…Experiments performed on powdered core from within and surrounding both the SDZ and CDZ show that abundant magnesium-rich clay (saponite and corrensite) localized within the faults results in low frictional strength (μ < 0.15), whereas the surrounding rock is stronger (μ = 0.3-0.6) [Carpenter et al, 2011;Lockner et al, 2011;Coble et al, 2014]. This is consistent with several studies that have documented the important role of gouge composition-and clay content in particular-on frictional properties [e.g., Lupini et al, 1981;Logan and Rauenzahn, 1987;Brown et al, 2003;Ikari et al, 2009].…”

“…These initiatives have recovered samples of fault rock and gouge and conducted measurements of in situ stress, fluid chemistry, and temperature at depths where earthquakes nucleate. Samples and data emerging from these drilling projects provide an unparalleled opportunity to gain new insight into absolute fault strength [e.g., Brune et al, 1969;Zoback et al, 1987;Scholz, 2000;Carpenter et al, 2012], causes of apparent fault weakness [e.g., Rice, 1992;Faulkner and Rutter, 2001;Brantut et al, 2011;Gratier et al, 2011;Lockner et al, 2011], and the laws that govern fault slip and failure [e.g., Dieterich, 1979;Marone, 1998a;Ikari et al, 2009;Carpenter et al, 2011;Sone et al, 2012].…”