Summary
In the northern and central sections of the San Andreas Fault Zone, and along Calaveras and Hayward faults, clay gouges have been found to occur on the surface and at shallow depths.
It is consistent with the available geochemical data that such gouges can exist at depths down to 10 km. If extensive gouge materials exist in a fault zone then their properties will determine, to a large extent, the behavior of the fault. From known properties of clays in the presence of water we can infer that, in such cases, the tectonic stress and the stress drops for earthquakes will be low and substantial creep will take place before earthquakes.
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Wu, F.T., Blatter, L. & Roberson, H. Clay gouges in the San Andreas Fault System and their possible implications. PAGEOPH 113, 87–95 (1975). https://doi.org/10.1007/BF01592901
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DOI: https://doi.org/10.1007/BF01592901