Seismic array detection of subducted oceanic crust in the lower mantle

Abstract

We analyze short‐period precursory energy to PP that can be observed in seismograms in the distance range from ∼95° to 105° to infer the behavior of subducted slabs beneath western Pacific subduction zones. PP is a P wave once reflected at the free surface between the source and receiver. Using high‐resolution seismic array techniques, we analyze the incidence angle, timing, and azimuth of the PP precursors. The precursory energy is resolved to originate from off great circle path azimuths and is consistent with scattering by small‐scale heterogeneities. Assuming single scattering, upper mantle‐ and midmantle‐derived scatterer locations show a strong geographical and depth correlation to high seismic velocities in tomographic studies. Scattering locations beneath the Tonga and Mariana subduction zones outline continuous dipping structures to a depth of at least 1000 km, consistent with scattering associated with subducted former oceanic lithosphere. Scatterer locations uniquely explain the timing, slowness, and back azimuth of the PP precursors at the array. The observed reflections can be explained with the velocity impedance variations expected for high‐pressure basalt juxtaposed with pyrolite or harzburgite and thus may be due to the paleo‐Mohorovičić discontinuity within subducted slabs. These results are consistent with basaltic crust penetrating into the lower mantle. This method provides a means for tracking the location of geochemically enriched former oceanic crust in the lower mantle by using recordings of globally distributed seismic arrays and is complementary to longer‐wavelength constraints on high seismic velocity slabs inferred from tomography.