A temporary seismological network, which was located in the Antofagasta (northern Chile) region, recorded a large part of the aftershocks from a MW = 8.0 earthquake in 1995. Results from a tomographic inversion show that the aftershocks were concentrated in a plane, approximately 3 km thick spatial zone. Comparison of our results with earlier processed wide-angle reflection data, shows clearly that this zone is positioned at the top of the oceanic crust. Therefore, it represents the shearing boundary between the subducting oceanic and the continental plate, and gives an in-situ estimation of the thickness of the boundary. Outside of this zone, events were detected in the continental crust, as well as within the oceanic mantle.