Evidence for Back Scattering of Near-Podal Seismic P'P' Waves From the 150-220 km Zone in 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. P'P' waves are sometimes accompanied by precursors, which were reported first in the 1960s as small-amplitude arrivals on seismograms at epicentral distances of about 50°-70°. Most prominent of these observed precursors were explained by P'P' waves generated by earthquakes or explosions that did not reach the Earth's surface but were reflected from the underside of first order velocity discontinuities at 410 and 660 km in the upper mantle or by forward scattering from the mantle. Here we report the discovery of hitherto unobserved precursors to near-podal P'P' (at epicentral distance < 10°) preceding the main energy by as much as 60 seconds. We interpret these precursors as a back scattered energy from horizontally conected small-scale heterogeneity in the upper mantle, concentrated in a zone between 150 and 220 km depth beneath Earth's surface. From these observations, we identify a frequency dependence of Q (attenuation quality factor) 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, confirming pioneering work by B. Gutenberg.