Campaign geodetic observations demonstrate shallow, interplate coupling along the thrust fault between the converging oceanic Nazca and continental South American plates. Delimiting the width of the seismogenic zone contributes to our understanding of the seismic and tsunamigenic nature of convergent plate boundaries. Here we locate the updip limit of the seismogenic zone using a combination of the Global Positioning System (GPS) and acoustic travel time measurements.Two seafloor transponder arrays were positioned in 2001 and 2003, providing plate motion vectors 20 and 50 km landward of the Peru-Chile trench axis at 12_S. Over 100 hours of GPS, acoustic and sound speed profiling data were gathered at each array, in each epoch, providing an estimate of plate motion with an uncertainty on the order of millimeters. I describe each component of the GPS-acoustic technique (GPSA), including static and kinematic GPS, GPS antenna-hydrophone baseline surveys, acoustic travel time measurements, sound speed profiling and moving surveys to determine a priori transponder position. Comparing these geodetic measurements with three dimensional kinematic models reveals coupling at less than 2 km depth. Seismic records, thermal models, and topographic analyses suggest that GPSA array displacements represent shallow, elastic strain accumulation. Transponder relocation, another key component of this technique, consists of surveying a replacement transponder in the event that a transponder becomes inactive. I discuss the methodology and results of a transponder relocation, which contributed ±19 mm uncertainty to the final array vector. Optimal survey network geometry for a transponder relocation was determined in a simulation of the least-squares adjustment. Future relocations could decrease the contribution to array uncertainty to ±10 mm with improved survey geometry. Finally, complimentary investigations such as long-baseline kinematic GPS, transponder depth measurements and near-bottom acoustic surveys are included as necessary asides for the development seafloor geodetic observations
