This research involves the study, by means of PASSCAL portable broadband seismometers in Argentina and Chile, of the structure beneath the south central Andes where a major change in subducting slab geometry occurs. The research will be done in conjunction with research institutes from Chile and Argentina. At approximately 33.3 degrees South, the subducting Nazca slab changes from a horizontal slab geometry without any active volcanism to the north, to a 30-degree dipping slab with an active volcanic arc to the south. This north-south change in slab dip is one of the most dramatic bends or warps in a Wadati-Benioff zone geometry anywhere globally. At 31 degrees South, the flattened slab extends 300 km to the east before resuming its descent into the mantle. This segment has high elevations and large amounts of tectonic shortening. Across the dipping slab segment, the elevation decreases, as does the amount of tectonic shortening. Two PASSCAL transects will be deployed: one near 30.5 degrees South above the horizontal slab segment and one at 36 degrees South above the dipping slab segment. The seismic data will be gathered from natural earthquakes and inverted to answer questions involving the structure of the subducting slab across the transition, surrounding mantle structure, mountain building, and earthquake generation processes.