Numerical modeling studies and a growing number of observations have argued for the propagation of fault-zone guided waves (FZGWs) within a San Andreas Fault (SAF) zone that is 100-200 m wide at seismogenic depths and with 20%-40% lower shear-wave velocity than the adjacent unfaulted rock. Thousands of microearthquakes have been recorded since 1987 by the borehole High-Resolution Seismic Network at Parkfield, California, and they provide a comprehensive data set for characterizing wave propagation in the SAF zone. Using microearthquakes we confirm that FZGWs at Parkfield are generated within the fault zone (FZ) and that they are most prominent late in the coda of S. Numerical waveform modeling and guided-wave amplitude tomographic inversion show clearly that FZGWs are significantly less attenuated in a well-defined region of the FZ. This region plunges to the northwest along the northwest boundary of the region of highest moment release and separates locked and slipping sections of the SAF at depth, as determined independently from geodesy, seismicity, and the recurrence rates of characteristically repeating microearthquakes. We interpret this localized zone of strong FZGW propagation to be the northwest edge of the M 6 asperity at Parkfield. The mechanism for low FZGW attenuation in the zone is possibly due to dewatering by fracture closure and/or fault-normal compression or changes in fracture orientation due to a complex stress or strain field at the boundary between creeping and locked zones of the SAF.