We characterize the SAFOD Phase 3 core samples from the San
Andreas Fault zone at ~3 km depth as compositionally heterogeneous
and structurally complex at the meter scale, i.e., at a scale that is
important to earthquake rupture nucleation and propagation (Sibson,
2003). The ~41 m of core is comprised of a mixture of fractured
arkosic sandstones, penetratively sheared siltstones and shales,
cataclasite to ultracataclasite, and foliated serpentinite-bearing clay
fault gouge, alternating with blocks of less-deformed fine-grained
sandstone and siltstone. Over 60% by volume of the core is comprised
of sheared phyllosilicate-rich layers, gouge and ultracataclasite, and
lenses of other fault rocks