Tomographic inversion is applied to delay times from local earthquakes to image three dimensional velocity variations near Parkfield, California. The 25 × 20 square km region is represented by nearly cubic blocks of 0.5 km per side. Arrival times of P waves from 551 local earthquakes, with depths of 0 to 15 km, were used as sources producing 3135 rays covering the target region. The data were recorded on low-noise downhole seismographs. The results of the inversion show correlation with some of the local geological and geophysical features. The correlation of higher-velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower-velocity, less competent rocks
