During earthquakes slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults suffers anomalous strain and damage during large quakes. Data collected along several faults including the Hector Mine rupture, San Andreas Fault at Parkfield, and the Calico Fault show damage zones extending from 100 m to 1 km around the active slip plane. Recent seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic travel times, trapped waves, and interferometric Synthetic Aperture Radar observations, we document seismic velocities reduced by 40 - 50% and shear moduli reduced by 65% compared to wallrock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures, but can sustain further damage in successive events These findings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the portion of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.