A high-resolution seismic network at the Coldwater Creek Steam Field (CCSF) in the Northwest Geysers region has provided data with which to estimate three-dimensional P- and S-wave velocity models and to relocate hypocenters. The seismicity distribution and resulting seismic velocity structure, coupled with the known geology, are used to characterize the geothermal reservoir. The velocity structure is estimated using the progressive inversion technique of Thurber [J. geophys. Res. 88, 8226-8236 (1983)]. Hypocenters within the CCSF lie between depths of 1.5 and 2.5 km and appear to be associated with the steam production zone. A deeper cluster of events is located beneath an injection well. High-velocity near-surface anomalies correspond to mapped sections of Franciscan metagraywacke and greenstone. Low-velocity anomalies at depth appear to represent sections of Franciscan melange. High Vp/Vs anomalies at depth suggest a recharge region or a condensation zone. The steam producing zone is marked by low Vp/Vs values, suggesting partial liquid saturation of the reservoir rocks.