Weakness of the San Andreas Fault revealed by samples from the active fault zone

Understanding the strength and slip behaviour of tectonic faults is a central problem in earthquake physics and seismic-hazard assessment(1-8). Many major faults, including the San Andreas Fault, are weak compared with the surrounding rock, but the cause of this weakness is debated(1). Previous measurements of the frictional strength of San Andreas Fault rocks are too high to explain the observed weakness(1,9-13). However, these measurements relied on samples taken at a distance from the active fault or from weathered surface samples. Recent drilling into the San Andreas Fault(9) has provided material from the actively slipping fault at seismogenic depths. Here we present systematic measurements of the frictional properties and composition of the San Andreas Fault at 2.7 km depth, including the wall rock and active fault. We find that the fault is weak relative to the surrounding rock and that the fault rock exhibits stable sliding friction behaviour. The fault zone contains the weak mineral smectite and exhibits no frictional healing-bonds in the material do not heal after rupture. Taken together, the low inherent strength and lack of healing of the fault-zone material could explain why the San Andreas Fault slips by aseismic creep and small earthquakes in central California, rather than by large, destructive earthquakes.