The stimulation of 2000 and 2003 carried out on the HDR site of Soultz-sous-ForÍts
(Alsace, France) have generated a small magnitude earthquake activity. For each experiment, a seismological network composed of about 25 stations were installed by the Ecole et Observatoire des Sciences de la Terre (EOST). More than 3500 seismic events have been pointed and located in both cases.

In order to estimate the variation of the physical properties of the reservoir due to
the fluid circulation, we have performed a four dimensional tomography: the 3D velocity structure for different time windows. The whole data set has been apportioned into different temporal sets with a fixed number of events. We use the tomographic algorithm of Thurber (1983) for 2000 and a new, more accurate method for the set of 2003 based on a double difference tomography (Zhang & Thurber, 2003). The latter method gives a good resolution on the reservoir volume and allows a relatively good resolution in the vicinity of this volume. It enlights the velocity structure variation of the reservoir all along the stimulation.

In this study, we propose a dicussion based on the variation of the velocity in time during each stimulation and a comparison of the variation of the velocity in 2000 and 2003. This analysis allows us to appreciate the volume of influence of the hydraulic activity and points out the effect of the water circulation on the reservoir properties.

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