GPS-derived deformation of the central Andes including the 1995 Antofagasta M-w=8.0 earthquake

In order to study both the interplate seismic loading cycle and the distrib ution of intraplate deformation of the Andes, a 215 site GPS network coveri ng Chile and the western part of Argentina was selected, monumented and obs erved in 1993 and 1994. A dense part of the network in northern Chile and n orthwest Argentina, comprising some 70 sites, was re-observed after two yea rs in October/November, 1995. The M-w = 8.0 Antofagasta (North Chile) earth quake of 30th July, 1995 took place between the two observations. The city of Antofagasta shifted 80 cm westwards by this event and the displacement s till reached 10 cm at locations 300 km from the trench. Three different def ormation processes have been considered for modeling the measured displacem ents: (1) interseismic accumulation of elastic strain due to subduction cou pling, (2) coseismic strain release during the Antofagasta earthquake and ( 3) crustal shortening in the Sub-Andes. Eastward displacement of the sites to the north and to the south of the are a affected by the earthquake is due to the interseismic accumulation of ela stic deformation. Assuming a uniform slip model of interseismic coupling, t he observed displacements at the coast require a fully locked subduction in terface and a depth of seismic coupling of 50 km. The geodetically derived fault plane parameters of the Antofagasta earthquake are consistent with re sults derived from wave-form modeling of seismological data. The coseismic slip predicted by the variable slip model reaches values of 3.2 m in the di p-slip and 1.4 m in the strike-slip directions. The derived rake is 66 degr ees. Our geodetic results suggest that the oblique Nazca-South American pla te convergence is accommodated by oblique earthquake slip with no slip part itioning. The observed displacements in the back-are indicate a present-day crustal shortening: rate of 3-4 mm/year which is significantly slower than the average of 10 mm/year experienced during the evolution of the Andean p lateau.