We used dense linear seismic arrays across and along the San Andreas Fault (SAF) at Parkfield, California to record fault zone trapped waves generated by explosions and microearthquakes in 2002. Prominent trapped waves appeared at stations close to the SAF main fault trace while some energy was trapped in the north strand at the array site. Observations and 3-D finite-difference simulations of trapped waves at 2--5 Hz show evidence of a damaged core zone on the main SAF. The zone from the surface to seismogenic depths is marked by a low-velocity waveguide ~150 m wide, in which Q is 10--50 and shear velocities are reduced by 30--40% from wall-rock velocities, with the greatest velocity reduction at shallow depth. We interpret that this distinct low-velocity zone on the main SAF is a remanent of damage due to past large earthquakes on the principal fault plane at Parkfield. A less-developed low-velocity zone may be evident on the north strand that experienced minor breaks in the 1966 M6 event.