Abstract

Since the advent of plate-tectonic theory over 30 years ago(1,2), the geometries of subduction zones have been constrained mainly by the spatial distribution of earthquake hypocentres, known as Wadati-Benioff zones. This is due to the fact that, despite the existence of a wealth of shallow seismic reflection and refraction data, very few high-resolution images of deep subduction-zone structure have been obtained. The few attempts to image these structures (see, for example, refs 3-5) were restricted to marine surveys, with the exception of one experiment(6,7), and none of these studies was successful at mapping structure to more than 30-40 km depth. The association of intermediate-depth earthquakes with a given layer of subducting oceanic lithosphere has therefore remained largely unresolved(8). Here we report seismic reflection and refraction data across the central Andean subduction zone, which image the subduction boundary to a depth of 80 km, We show, by comparing the location of this boundary with earthquake hypocentres precisely located by a temporary seismic array in the region, that most of the intermediate-depth seismicity is offset into the subducting oceanic mantle, rather than lying within the crust or on the subduction-zone boundary itself, as has often been assumed.