This study images the P velocity structure of a region in the vicinity of the large artificial reservoir at Oroville, California. The velocity structure of the upper crust (to a depth of 12 km) was obtained by simultaneous version of P-arrivals of 168 aftershocks of the 1975 Oroville earthquake. A decrease in P-wave velocity by 7 to 8% was observed across a known fault zone. The hypocenters of the 1975 sequence are associated with a southwest dipping structure characterized by low velocity, with most of the deeper activity occurring in adjacent regions of higher velocity. Resolution of the velocity structure was examined through inversions of synthetic travel times using different starting velocity models. Although the role of the reservoir in generating the low velocities remains speculative, the structure is comparable to that of some active segments of the San Andreas fault where fluid pressures are thought to be high. ¿ American Geophysical Union 1993