Local earthquake tomography in northern Chile using finite-difference calculations of P-travel times

Abstract

The velocity in the crust near Antofagasta in northern Chile is obtained by a simultaneous inversion of local earthquake arrival-time data for velocity and hypocentral parameters. The most characteristic tectonic feature of this region is the subduction of the Nazca plate under South America associated with the high level of background seismicity. The specific display of the events along the Wadati-Benioff zone and the 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 coordinates are obtained, using a full nonlinear approach, as the maximum value of a 3D probability-density function. The velocity model is obtained by iterative inversion using the parameter-separation method. Synthetic data inversions show that this method gives accurate results in the case of strongly heterogeneous media. This result allows us to apply this method to the Andean subduction zone around Antofagasta and to obtain a 3D-velocity model. The inverted model confirms existing ideas and yields new details on the structure of the subducted plate, especially a thin low-velocity layer roofing the upper face of the slab.