VELOCITY STRUCTURE IN NORTHERN CHILE - EVIDENCE OF SUBDUCTED OCEANIC-CRUST IN THE NAZCA PLATE

2-D P-wave velocity models were determined for the subduction zones near Iquique and Antofagasta in northern Chile, simultaneously inverting P- and S-wave arrival times from locally recorded earthquakes for velocity structure and hypocentral locations. A 2-D parametrization was used because of the paucity of data, but is justified by the lack of significant variations along the strike of the subduction zone observed from both refraction profiles and simple 3-D inversions. The crust and upper mantle are parameterized by constant velocity regions of irregular shape, with the size and boundaries of these regions governed by prior information about the structure and by the ability of the data to resolve P-wave velocities. Beneath the Antofagasta region there is evidence of an approximately 10 km thick layer of oceanic crust attached to the top of the subducting Nazca plate. This crust has a P-wave velocity of 7.3 +/- 0.1 km s-1 and is observed down to a depth of 60 +/- 10 km. This depth also corresponds to the maximum depth of seismogenic coupling in the Chilean subduction zone. The subducted oceanic crust overlies an oceanic upper mantle with a P-wave velocity of 8.0 +/- 0.1 km s-1. Apparently, oceanic crust is being subducted beneath Iquique as well. However, this feature is less constrained by the data available from this region.