A high-resolution tomographic study, using cubic B-splines parameterization and employing a systematic approach to the choosing of appropriate damping and smoothing parameters, provided a three-dimensional P-wave velocity map of the Loma Prieta area. Used in the inversion were 11,977 high-quality raypaths from 844 aftershocks of the 1989 Loma Prieta earthquake. The velocity model exhibits a low-velocity feature between the San Andreas and Zayante-Vergeles faults in the top 10 km of the crust. This low-velocity feature is interpreted as a sedimentary unit exposed to the northwest and separated from the Salinian block by the Zayante-Vergeles fault. Below 10 km, no consistent change is observed between the Salinian and the Franciscan blocks. There appears to be a high correlation of aftershock activity and localized high-velocity anomalies southeast of the Loma Prieta mainshock. Whereas this anomaly may represent brittle rocks associated with a fault-zone asperity that failed after the mainshock, there is evidence to suggest it may be a body of serpentinite. The serpentinite exhibits high velocities and is potentially less competent than surrounding country rock, thus providing a sector along the fault more likely to be associated with many smaller earthquakes or creep behavior.