Seismic Waves Reflected from Discontinuities within Earth's Upper Mantle.

Precursors to normtial seismic waves of the PKPPKP type in the distance range of 55 degrees to 75 degrees are ascribed to reflection of this phase from within the earth's upper mantle. The new observations confirm the existence of a sharply defined transition zone, probably worldwide in extent, at a depth of approximately 650 kilometers. These data are shown to be a useful tool for the study of upper mantle structure on a global basis.