Western South America is tectonically one of the most active and geodynamically most interesting areas in the world. Aim of the investigation in the framework of the SAGA project is to understand the nature of the present-day deformation (including its temporal and spatial variation) along the Andean subduction zone. In order to achieve this goal we have established a GPS network consisting of about 250 sites covering Chile and the western part of Argentina. GPS observations started in 1993 and include a repeated survey of parts of the SAGA network in the campaign mode and permanent observations at 15 sites located in Chile and Argentina. Surface deformation in Chile is dominated by transient effects related to large earthquake occurring at the interface between the Nazca and South American plates. These effects include interseismic accumulation of elastic strain in the fore-arc region, due to the coupling between plates and coseismic elastic strain release in an area extending from the coast to the Altiplano/Puna. A dense part of the network in northern Chile between 22°S and 26°S and northwest Argentina comprising some 70 sites, was re-observed several times after the Mw= 8.0 Antofagasta earthquake of 30th July 1995. Postseismic relaxation processes are clearly seen after the 1995 Antofagasta earthquake, as well as after the great 1960 Chile earthquake. Additional studies relate structural evidences to the overall deformation behavior in order test how changes in rheology, such as those provided by the magmatic arc, alter the current surface deformation and how interplate deformation is linked to the intraplate and long-term deformation of the crust. Complementary studies concentrate on the modeling of the transient deformation processes related to earthquakes and the connection of the surface deformation with structures in the deeper crust and the upper mantle. Further modeling approaches include the stress transfer at the plate interface, the repeat time of earthquakes and the interaction of seismic segments. Those studies will also help to improve the assessment of seismic hazards. The observed postseismic surface deformation offer the chance to model visco-elastic relaxation processes and to derive rheological parameters of the lower crust and the upper mantle.
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