Antipodal observations of global differential times of diffracted P and PKPAB within the D″ layer above Earth's core–mantle boundary

Abstract

SUMMARYAntipodal diffracted, compressional wave (Pdiff) data analysed diametrically opposite three large earthquakes have uniformly sampled 99% of the laterally heterogeneous zone above Earth's core–mantle boundary (D″ in seismic nomenclature). These antipodal data offer a fundamental gross Earth datum—a robust, global constraint on the average compressional velocity at the base of the mantle. We use for the first time the seismic phase PKPAB as a reference, which travels an identical mantle path as Pdiff, thereby cancelling common mantle heterogeneity. Differential traveltimes between Pdiff, PKPAB and PKIKP are measured, appropriately making allowance for the phase shifts acquired in propagation. We have independently confirmed the $\pi /4$ polar phase shift of Pdiff at the antipode. The global mean PKPAB − Pdiff time is 136.5 ± 0.6 s. The global mean apparent velocity (13.05 km s−1) and ray parameter (4.65 ± 0.01 s deg−1) are within the margin of error of prior Pdiff studies—which were dominated by Northern Hemisphere paths—indicating that complementary, southern hemisphere paths have a comparable, mean Pdiff apparent velocity. The seismic velocity constraints afforded by antipodal Pdiff and PKPAB suggest that the heterogeneous processes already observed in D″ may be broadly ascribed where D″ coverage has been lacking or poorly resolved.