Precise analysis of a microearthquake swarm occurring within an intrusive rock formation has provided a clear hypocenter distribution that is characterized by a NNW-SSE trending vertical plane consisting of a series of small reverse and strike-slip faults. An empirical Green's function analysis was applied to the largest event of the swarm. The direction of the rupture plane of the event, which is of strike-slip type, was determined to be perpendicular to the trend of the hypocenter distribution. A combination of small reverse faults with small strike-slip faults could result in a permeable reverse fault structure in the deeper parts of the geothermal reservoir. One particular signal was interpreted to be a reflection at the surface of the neo-granitic rock, where the most important fractures for production in the deeper reservoir exist.