Arrival-time data from 453 local earthquakes and 6 explosions at Parkfield, CA, are inverted for earthquake locations and three-dimensional Vp and Vp/Vs structure. The structure is dominated by the velocity contrast across the SAF, with the southwest side about 20--25% faster, consistent with previous studies. Nearly all the earthquakes occur almost directly beneath the fault trace. We find high-Vp/Vs anomalies that correlate with low-resistivity features in a magnetotelluric model that are interpreted to represent fluids. We locate a magnitude 2 earthquake that is a potential target event for the final stage of SAFOD drilling, yielding a depth of 3.1 km below surface and an epicenter 100 m southwest of the fault trace. Nonlinear analyses indicate 95%-confidence relative and absolute uncertainties on the order of 500--700 m vertically and 200--300 m horizontally for this target earthquake.