SEISMIC WAVES REFLECTED FROM DISCONTINUITIES WITHIN EARTHS UPPER MANTLE

Precursors to normal seismic waves of the PKPPKP type in the distance range of 55° to 75° 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.