Localized anisotropy in the mantle transition zone due to flow through slab gaps

Abstract:

Seismic anisotropy of Earth’s mantle provides important insights into convective flow and composition. There is abundant evidence for concentrated anisotropy at depths within a few hundred kilometers of the mantle’s top and bottom, but the prevalence of anisotropy at intermediate depths is less clear. The Pacific Northwest of the U.S. is well-suited to investigate the depth distribution of mantle anisotropy because of the large depths-integrated anisotropic signals and densely instrumented broadband seismometers. We used the teleseismic P-to-S (Ps) conversions from the mantle transition zone (MTZ) discontinuities, the P410s and the P660s, to investigate the anisotropy within the layer. Transverse component energy minimization method yields similar estimates on the splitting parameters from the P660s phase when compared with teleseismic shear wave splitting (SWS), suggesting most of the anisotropic signal in SWS comes from depths above the 660. However, the large uncertainties from the two weak Ps phases hinder the detection of the potentially weak anisotropy in MTZ. In contrast, the amplitude ratio between the integrated SH component and the original SV component (P_SH/P_SV) offers better sensitivity to the magnitude of anisotropy. While an isotropic MTZ predicts indistinguishable amplitude ratios from the P410s and P660s, our observations show significant differences even at 95% confidence. Most of the significant differences are observed near slab gaps identified by previous tomographic studies. Synthetic tests using a ray parameter and back-azimuth distribution identical to the observed data show an inclusion of 1-2% anisotropy in the upper MTZ can reproduce the difference of observed amplitude ratios. Our new results support the presence of seismic anisotropy at transition zone depths, which may arise from lattice preferred orientation of wadsleyite. Anisotropy in the mantle transition zone may be restricted to areas of locally high stress such as areas of focused flow through fragmented slabs.