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Measuring the onset of locking in the Peru–Chile trench with GPS and acoustic measurements


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 content1. 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 earthquake2. Seismic coupling at shallow depths, such as we observe, may explain why co-seismic events in the Peruvian subduction zone create large tsunamis.

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Figure 1: The GPS–acoustic approach to measure seafloor motion.
Figure 2: Bathymetric map of seafloor geodesy sites off the coast of Peru.
Figure 3: Models of surface deformation and plate organization.


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We thank M. Bevis for comments and suggestions; R. Zimmerman, D. Rimington and D. Price for engineering support; and the Captain and crew of the R/V Roger Revelle. We thank the Instituto Geofisico Del Peru for operating the land GPS stations and the Instituto Del Mar Del Peru, Direccion de Higrografia y Navagacion, for support at sea. This work was supported by the Marine Geology and Geophysics Program of the US National Science Foundation.

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Correspondence to C. David Chadwell.

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Gagnon, K., Chadwell, C. & Norabuena, E. Measuring the onset of locking in the Peru–Chile trench with GPS and acoustic measurements. Nature 434, 205–208 (2005).

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