For obtaining reliable high-resolution subsurface images in the geothermal testbed of the Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab), we have applied fat ray travel time tomography. To compute a 3D velocity model, we made use of 8 boreholes, which allowed us to compile a large data set including 42’843 manually picked first breaks. We demonstrate that the fat ray approach offers improved image quality compared with traditional ray-based methods. Furthermore, we have validated the 3D model using ground-truth information from wireline logs and geological observations. We succeeded in imaging a major fault zone (MFZ) that has a rather complex structure including considerable heterogeneity. Relocation of passive seismic events, generated during hydraulic stimulations, indicate that the 3D velocity model has only a minor influence on the hypocentral parameters, but a comparison of a selection of particularly well-constrained seismic events with the velocity structures revealed that there is a remarkable spatial correlation. Most events occurred in regions of intermediate seismic velocities, thereby "avoiding" high and very low velocity areas. Based on small-scale laboratory studies, we speculate that these observations can be attributed to the occurrence of stress gradients in the intermediate velocity zones.