EVIDENCE FOR ANISOTROPY IN THE DEEP MANTLE BENEATH ALASKA

We consider the possibility that the velocity structure of D '' is ani sotropic. The data we examined consist of seismograms from 9 deep Japa nese earthquakes recorded at WWSSN receiver stations in North America. The source-receiver combinations span distances of 70 degrees-106 deg rees with associated S waves passing through D '' beneath Alaska. Diff erential travel times of the S, Scd, ScS and SKS phases are used to co nstrain the velocity structure in D ''. Shear waves refracted by D '' are observed beyond 72.2 degrees and provide a sensitive measurement o f the velocity structure in D ''. Beyond 93 degrees, the vertically po larized (SV) and horizontally polarized (SH) shear waves often appear distinctly split, although, at distances less than 89 degrees the comp onents are more nearly synchronous. Near 94 degrees, SH occurs as a do uble arrival. SV in this range, however, remains a single arrival roug hly synchronous with the second SH arrival. We have been unable to rep roduce these effects in isotropic model synthetics. Synthetics for tra nsversely isotropic models have been computed that do match these wave forms. The anisotropy was constrained to be only within D '', with a v ertical symmetry axis. We conclude that these observations may be expl ained by an anisotropic D '' layer. The D '' discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above t he core-mantle boundary.