Analyses of the composition, properties, and structure of the upper parts of fault zones constrain the mechanical and seismological behavior of faults. We examine macro- to microstructures of fault-related rocks across the San Gabriel Fault zone, California, which was active 5-12 mya and exhibits evidence of Quaternary slip. The steeply plunging ALT-B2 geotechnical borehole encountered an upper ~60 m thick damage zone and a 20-m thick lower damage zone, and two principal slip zones composed of cohesive cataclasite, ultracataclasite, and intact clay gouge within them. The upper ~6.5 m thick principal slip zone separates Mendenhall Gneiss and Josephine Granodiorite, and a lower 11-m thick principal slip is enclosed within the Josephine Granodiorite. Microstructures recorded in core and field samples document overprinted brittle fractures, cohesive cataclasites, veins, sheared clay-rich rocks, and folded foliated horizons in the damage zones. Carbonate veins are common in the lower fault zone, and alteration and mineralization assemblages consist of clays, epidote, calcite, zeolites, and chloritic minerals. Highly altered, mineralized, and deformed rocks in the principal slip zones have very low rock mass rating values. This indicates that portions of the fault zones have very low Young’s moduli and Vs and Vp values as low as 500 and 1000m/sec, respectively, well below the protolith values recorded in a nearby borehole. These data show syntectonic grain-scale alteration, mineralization, and deformation result in low seismic velocities and weak fault-related rocks in the shallow crust, and reduced moduli may impact the distribution of seismic energy and slip to the surface.