The prominent role of barriers is now well recognized in both the initialization and the extension of the earthquake rupture. The mapping of barriers is therefore important in understanding the physical processes by which earthquakes occur. We invert 800 P wave travel times from an aftershock sequence following a moderate earthquake (mb=5.1) in the Pyrenean area (France) using a stochastic inverse technique to derive a three-dimensional velocity image of the structure. We have carried out two scales of imagery. Both scales reveal the presence of a high velocity contrast (+8%) at the eastern limit of the aftershock zone. This velocity anomaly corresponds to a geometric discontinuity in the trace of the major fault and can be mapped down to a depth of 8 km.