The 2000 and 2003 hydraulic stimulations carried out at the Soultz-sous-Forets Hot Dry Rock (HDR) site, Alsace, France, generated a continuous, albeit of small magnitude, seismic activity. For each experiment, the Ecole et Observatoire des Sciences de la Terre (EOST) installed a seismic network of 18 stations (in 2000) and 23 stations (in 2003). More than 3500 seismic events were detected and located in both cases. Changes in the physical properties of the reservoir due to fluid circulation have been estimated by means of a 4D tomography in which the 3D seismic velocity structure is calculated for different time windows. The entire data set has been apportioned to different temporal subsets with a fixed number of events. We used the tomographic algorithm of [Thurber, C.H., 1983. Earthquake locations and 3D crustal structure in the Coyote Lake area, central California. J. Geophys. Res. 88, 8226-8236] for the 2000 data and a new, more accurate method that is based on a double-difference tomography [Zhang, H., Thurber, C.H., 2003. Double-difference tomography: the method and its application to the Hayward fault, Califomia. Bull. Seismol. Soc. Am. 93 (5), 1875-1889] for the 2003 set. The latter approach gives a good resolution within the reservoir volume, and one that is slightly less so in its vicinity. This tomographic study points out the changes in reservoir velocity structure occurring during the stimulation. We discuss the variation of the seismic velocity with time during each of the hydraulic stimulations and compare the velocity changes observed in 2000 and 2003. This analysis has allowed us to define the region of influence of hydraulic activity and determine the effects of water circulation on geothermal reservoir properties. (c) 2006 CNR. Published by Elsevier Ltd. All rights reserved.