The three-dimensional P and S velocity (from Vp/Vs ratios) distribution and improved hypocenters for the Yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled-source explosions. The P velocity model has an rms residual of ¿0.09 s, whereas the Vp/Vs ratio model (calculated from 511 earthquakes) has an rms residual of ¿0.29 s. High P and S velocities outside the Yellowstone caldera represent thermally undisturbed basement and sedimentary rocks. A caldera-wide 15% decrease from regional P velocities at depths of 6 to 12 km is coincident with a -60 mGal gravity anomaly and is interpreted as a hot, subsolidus, granitic batholith with a quasi-plastic rheology. Localized 30% reductions from regional seismic velocities and higher Vp/Vs ratios 8 km beneath Yellowstone's resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful (less than 2 Ma) silicic volcanism. Additional low seismic velocities and Vp/Vs ratios and a 20 mGal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt. Hypocenters relocated with the three-dimensional P velocity model show subparallel alignment with NW-SE trending postcaldera volcanic vents and normal faults northwest of the caldera. Focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera, reflecting thinning and heating of the seismogenic crust. ¿ 1999 American Geophysical Union