Data from a digitally recording seismic network in San Juan, Argentina, provide the first images of crustal scale basement faults beneath the Precordillera. This seismicity is near the boundary between the Precordillera (a thin‐skinned thrust belt) and the Sierras Pampeanas (a region of thick‐skinned basement deformation), two seismically active tectonic provinces of the Andean foreland. The seismicity data support models for this region in which crustal thickening, rather than magmatic addition or thermal uplift, plays the dominant mountain building role. The Precordillera seismicity occurs in three segments distributed north to south. The southern segment is an area of diffuse activity extending across the Precordillera and eastward into the Sierras Pampeanas that shows no patterns in map or cross section. The northern and central segments have well‐defined dipping planes that define crustal scale faults extending from 5 to 35 km depth. It is clear from the relative fault geometries that the overlying Precordillera is not simply related to the basement activity. The seismicity here may result from reactivation of an ancient suture between the Precordillera and Pampeanas terranes or be occurring in basement of unknown affinity west of the suture. The seismicity provides the first constraints on basement fault geometries, and we present models integrating this information with the surface geology. These basement faults may have been responsible for the 1944 Ms 7.4 earthquake that destroyed the city of San Juan. The imaging of these faults suggests that seismic risk estimates for San Juan made on the basis of surface geologic studies may be to