Inversion of shear-wave waveforms reveal deformation in the lowermost mantle

It is well known that regions of the lowermost mantle — D′′ — exhibit significant seismic anisotropy. Identifying a unique mechanism for seismic anisotropy in D′′ and interpreting results in terms of mantle flow has proved challenging. In an attempt to address this, we outline a method for the direct inversion of shear-wave waveform data for the orientation and strength of seismic anisotropy. We demonstrate our method by jointly inverting SKS, SKKS and ScS shear-wave data for seismic anisotropy in a fast shear-wave velocity anomaly beneath the Eastern Pacific Ocean. Using our inversion method we evaluate four candidate mechanisms for seismic anisotropy in D′′: elliptical transverse isotropy (representing layering or inclusions), bridgmanite, and post-perovskite (for fabrics dominated by either [100](001) or [100](010) slip). We find that all candidate mechanisms can reasonably explain our input data, with synthetic inversions demonstrating that improved backazimuthal coverage is required to identity a single best-fitting mechanism. By inverting for orientation and anisotropic strength parameters we are able to discount bridgmanite as a candidate mechanism as less plausible solution, as our inversion requires an unreasonable ca. 40% of D′′ to consist of aligned bridgmanite crystals. The inversion results for the 4 candidate mechanisms predict two different mantle flow regimes, near vertical upwelling (or downwelling) or predominantly horizontal Southwesterly (or Northwesterly) deformation, both of which are inconsistent with recent mantle flow models. These results show that our new inversion method gives seismologists a powerful new tool to constrain lowermost mantle anisotropy, allowing us to test predictions of lowermost mantle flow.