First observations of PKPPKP precursors

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 < 10) 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.

The figure above shows (top left) a schematic representation of PKPPKP podal ray paths (very small epicentral distances) through Earth; (top right) a schematic representation of the interpretation of PKPPKP precursors as back-scattering from the zone of short scale heterogeneities in the upper mantle between 150 and 220 km depth; (bottom) beams of PKPPKP waves and PKPPKP precursors, indicating that they arrive to the station from the same direction.
This is an electronic version of an article published by American Geophysical Union. Copyright (2006) American Geophysical Union:
Tkalčić, H., M.P. Flanagan and V.F. Cormier, Observation of near-podal P'P' precursors: Evidence for back scattering from the 150-220 km zone in the Earths upper mantle, Geophysical Research Letters, 33, L03305, doi:10.1029/2005GL024626, 2006.

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