This study examines the three-dimensional velocity structure in a 60- by 80-km region containing the Parkfield segment of the San Andreas fault. The San Andreas fault (SAF), characterized by a sharp across-fault velocity gradient, is the primary feature in the velocity solution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. There may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure. -from Authors