Loal network data from San Juan, Argentina, provides new information about crustal seismicity in the Andean foreland above a horizontal segment of the subducted Nazca plate. We find two areas of foreland seismicity, one associated with the Sierras Pampeanas basement uplifts, and the other beneath, but not within, the Precordillera foreland fold-thrust belt. The Precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation. In both areas, events are concentrated between 15 and 35 km depth and have volumetric, rather than planar, fault-like distributions. The depth distribution is unusually deep for intraplate earthquakes and suggests a brittle-ductile transition near 30--35 km. It also suggests the upper plate may have a geotherm similar to stable cratonic regions, rather than a steeper one that may be expected due to a lithospheric thinning implies by the location of the subducted plate. This can be understood as a transient effect of a Neogene flattening of the dip of the subducted plate, where a rapid thinning of the upper plate was followed with underplating by the cool subducted plate, leaving the upper plate geotherm unchanged while producing a thinned, mechanically weaker lithosphere. Finally, a spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31 ¿S may be related to subduction of the Juan Fernandez ridge. ¿ American Geophysical Union 1990