Observation of near-podal P′P′ precursors: Evidence for back scattering from the 150-220 km zone in the Earth's upper mantle

P′P′ (PKPPKP) are P waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. Here we report the observations of hitherto unobserved near-podal P′P′ waves (at epicentral distance <10°) and very prominent precursors preceding the main energy by as much as 60 s. We interpret these precursors as a back-scattered energy from horizontally connected small-scale heterogeneity in the upper mantle beneath the oceans in a zone between 150 and 220 km depth beneath the Earth's surface. From these observations, we identify a frequency dependence of attenuation quality factor Q in the lithosphere through forward modeling of the observed amplitude spectra of the main and back-scattered P′P′ waves.