Scattering Evidence for an Ancient Subducted Slab Using the Unique Raypath P*PKP

Abstract

We observe high‐frequency scatterers consistent with the interpretation of a tabular high‐velocity structure under the Indian Ocean as an ancient subducted slab. We use a previously rarely used raypath, P waves scattered in the slab into PKP waves (PKP), from 12 earthquakes and explosions in five locations recorded on the antique LASA (Large Aperture Seismic Array) located in Montana, United States. The scatterers concentrate in the mantle transition zone and 1,500 km depths, in the locations where the fast anomalies in the tomography broaden and strengthen. Our inference that the slab lingers in the upper‐ and mid‐mantle despite subducting and detaching more than 130 million years ago suggests that models of slabs sinking into the mantle may have to accommodate such long‐term stagnation.