First observations of "podal" PKPPKP waves and their precursors:
Evidence for a heterogeneous zone located between 150 and 220 km depths in Earth's upper mantle

PKPPKP (also known as P'P') 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. We report the observations of hitherto unobserved near-podal PP waves (at epicentral distance < 10º) and very prominent precursors preceding the main energy by as much as 60 seconds. 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 Earths surface. From these observations, we identify a frequency dependence of attenuation quality factor Q in the lithosphere that can be modeled by a flat relaxation spectrum below about 0.05-0.1 Hz and increasing with the first power of frequency above this value.