Compressional and shear velocities measured during cross well seismic surveys are found to decrease during operation of a hot dry rock geothermal system. The relative P and S wave velocity decrease is very close to values predicted by O'Connell and Budiansky (1974), who modeled velocities in rocks containing fluid-saturated and dry cracks. Because our data fit their model quite well, we attribute the velocity decrease to an increased density of the fluid-filled fractures in the reservoir. Fracture density parameters of O'Connell and Budiansky range as high as 0.365 near the main heat transfer areas of the geothermal reservior. The dynamic shear and Young's modulus are significantly lower near the principal reservoir. The dynamic shear and Young's modulus are significantly lower near the principal heat transfer surfaces, but the bulk modulus is largely unaffected. These observations suggest that the velocity differences are caused by fully saturated fractures. We demonstrate that the fractures inferred by this survey may have lengths as large as 70 cm. These fractures may have been caused by shear failure, microcracking associated with thermal drawdown, or thermal stress cracking.