The Nature of the 660-Kilometer Upper-Mantle Seismic Discontinuity from Precursors to the PP Phase

Global Seismic data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP Phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lame's constant λ and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lame's constant <~mbada with regard to the shear modulus and constrains the mineralogical composition across the discontinuity.