Analysis of P- and S-waves from shallow microearthquakes in the vicinity of The Geysers geothermal area, California, recorded by a dense, telemetered seismic array operated by the U.S. Geological Survey (USGS) shows that these phases are easily recognized and traced on record sections to distances of 80 km. Regional average velocities for the upper crust are estimated to be 5.49 + or - 0.07 and 2.98 + or - 0.07 km/sec for P- and S-waves, respectively. Poisson's ratio is estimated at 23 locations using Wadati diagrams and is found to vary from 0.13 to 0.32. In general, the Poisson's ratio is found to be lower at the locations close to the steam production zones at The Geysers and Clear Lake volcanic field to the northeast. The low Poisson ratio corresponds to a decrease in P-wave velocity in areas of high heat flow. The decrease may be caused by fracturing of the rock and saturation with gas or steam.