Seismic reflection image revealing offset of Andean subduction-zone earthquake locations into oceanic mantle

Since the advent of plate-tectonic theory over 30 years ago(1,2), the geome tries of subduction zones have been constrained mainly by the spatial distr ibution of earthquake hypocentres, known as Wadati-Benioff zones. This is d ue to the fact that, despite the existence of a wealth of shallow seismic r eflection and refraction data, very few high-resolution images of deep subd uction-zone structure have been obtained. The few attempts to image these s tructures (see, for example, refs 3-5) were restricted to marine surveys, w ith the exception of one experiment(6,7), and none of these studies was suc cessful at mapping structure to more than 30-40 km depth. The association o f intermediate-depth earthquakes with a given layer of subducting oceanic l ithosphere has therefore remained largely unresolved(8). Here we report sei smic reflection and refraction data across the central Andean subduction zo ne, which image the subduction boundary to a depth of 80 km, We show, by co mparing the location of this boundary with earthquake hypocentres precisely located by a temporary seismic array in the region, that most of the inter mediate-depth seismicity is offset into the subducting oceanic mantle, rath er than lying within the crust or on the subduction-zone boundary itself, a s has often been assumed.