LOCAL EARTHQUAKE TOMOGRAPHY IN NORTHERN CHILE USING FINITE-DIFFERENCECALCULATIONS OF P-TRAVEL TIMES

The velocity in the crust near Antofagasta in northern Chile is obtain ed by a simultaneous inversion of local earthquake arrival-time data f or velocity and hypocentral parameters. The most characteristic tecton ic feature of this region is the subduction of the Nazca plate under S outh America associated with the high level of background seismicity. The specific display of the events along the Wadati-Benioff zone and t he reduced number of stations used in this study imply the development of a new method of simultaneous inversion. The ray tracing is solved by a finite difference calculation of P-travel times. Hypocentral coor dinates are obtained, using a full nonlinear approach, as the maximum value of a 3D probability-density function. The velocity model is obta ined by iterative inversion using the parameter-separation method. Syn thetic data inversions show that this method gives accurate results in the case of strongly heterogeneous media. This result allows us to ap ply this method to the Andean subduction zone around Antofagasta and t o obtain a 3D-velocity model. The inverted model confirms existing ide as and yields new details on the structure of the subducted plate, esp ecially a thin low-velocity layer roofing the upper face of the slab. (C) 1997 Elsevier Science B.V.