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Gagnon, K, Chadwell D, Spiess FN.  2005.  Evolving method to measure seafloor plate tectonic motions. Sea Technology. 46:49-52. AbstractWebsite
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Gagnon, K, Chadwell CD, Norabuena E.  2005.  Measuring the onset of locking in the Peru-Chile trench with GPS and acoustic measurements. Nature. 434:205-208.   10.1038/nature03412   AbstractWebsite

The subduction zone off the west coast of South America marks the convergence of the oceanic Nazca plate and the continental South America plate. Nazca - South America convergence over the past 23 million years has created the 6-km-deep Peru - Chile trench, 150 km offshore. High pressure between the plates creates a locked zone, leading to deformation of the overriding plate. The surface area of this locked zone is thought to control the magnitude of co-seismic release and is limited by pressure, temperature, sediment type and fluid content(1). Here we present seafloor deformation data from the submerged South America plate obtained from a combination of Global Positioning System (GPS) receivers and acoustic transponders. We estimate that the measured horizontal surface motion perpendicular to the trench is consistent with a model having no slip along the thrust fault between 2 and 40 km depth. A tsunami in 1996, 200 km north of our site, was interpreted as being the result of an anomalously shallow interplate earthquake(2). Seismic coupling at shallow depths, such as we observe, may explain why co-seismic events in the Peruvian subduction zone create large tsunamis.

Gagnon, KL, Chadwell CD.  2007.  Relocation of a seafloor transponder - Sustaining the GPS-Acoustic technique. Earth Planets and Space. 59:327-336. AbstractWebsite

Rigid seafloor arrays of three to four precision acoustic transponders have been repeatedly positioned with the GPS-Acoustic technique to measure horizontal plate motion. In the event that one transponder becomes inactive, a replacement transponder must be precisely located relative to the existing array. Here we present a technique to determine the geodetic azimuth and baseline between the inactive and replacement transponders. We include three examples of relocations between 2002 and 2003 on the Juan de Fuca plate and near the Peru-Chile trench, which add 16-29 mm uncertainty to the GPS-Acoustic estimated position. A simulation of optimal network geometry shows that the relocation's contribution to uncertainty can be as low as 10 mm.