This paper presents a precise determination of the fine structure of a Wadati-Benioff zone based on analysis of analog and digital seismic data recorded by a local network in the San Juan Province of western Argentina. The network is located above one of the subhorizontal segments of subducted Nazca plate. Without the complicating effects of a dipping slab on the locations of earthquakes, the nearly flat geometry of the subducted plate provides an excellent opportunity to determine the thickness and structure of the Wadati-Benioff zone. We found that the depth of the Wadati-Benioff zone beneach the network is 107¿5 km. For a set of digitally recorded events, the distance between the shallowest and the deepest event gives a thickness of 20 km, with 90% of the data concentrated in a zone 12 km thick. The spatial distribution of hypocenters shows no evidence of complex internal structure such as a double Wadati-Benioff zone. The local network data also suggest that in the region near 32¿ S where the dip of the Wadati-Benioff zone changes from flat to steep the Nazca plate flexes rather than tears at depths less than at least 125 km. The intermediate-depth seismicity is separated from an active zone of crustal seismicity by an aseismic region between depths of about 40 and 95 km. The upper surface of the subducted plate, inferred to be at about 90 km, provides an upper limit to the thickness of the South American plate in this region. An apparent westward dip of the Wadati-Benioff zone as determined by the local network data can be accounted for by a 6¿ westward dip of the Moho beneath the San Juan region, a result that is in agreement with regional gravity and topographic data. ¿ American Geophysical Union 1987