New techniques for the robust detection and quantification of seismic anisotropy in the lowermost mantle

Abstract

The lowermost mantle — D″ — is a crucial thermochemical boundary layer within the Earth, exerting control over dynamic processes in both the overlying mantle and the core below. Constraining seismic anisotropy (the variation of seismic velocity with direction) within this layer could give us key insights into its dynamics and composition. In this thesis I collect a new broadband dataset of SKS-SKKS shear-wave splitting, where ca. 17% of global observations are discrepant representing a significant increase compared to previous analysis. Using this dataset I develop new techniques to study discrepant SKS-SKKS shear-wave splitting and to invert of shear-wave waveforms for the orientation and strength of seismic anisotropy. I apply both these new techniques to study seismic anisotropy in D″ beneath the Eastern Pacific. Analysis of SKS-SKKS shear-wave splitting discrepancies highlight a significant region of azimuthal anisotropy in D″ which is most plausibly explained by the lattice preferred orientation of post-perovskite. I then test these qualitative interpretations, which are typical of SKS-SKKS shear-wave splitting studies, by jointly inverting ScS, SKS and SKKS waveforms for seismic anisotropy in a fast shear-wave velocity anomaly beneath the Eastern Pacific. I evaluate four candidate mechanisms for seismic anisotropy in D″: elliptical transverse isotropy (TI; representing for example anisotropy due to melt or layering), bridgmanite, and post-perovskite (for fabrics dominated by either [100](001) or [100](010) slip). Elliptical TI and both post-perovskite fabrics reasonably explain the input data, with predictions of horizontal flow for elliptical TI and post-perovskite [100](001) agreeing with recent mantle flow models. The techniques I have developed give seismologists powerful new tools to constrain lowermost mantle anisotropy.

Date of Award	2 Dec 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	James M Wookey (Supervisor) & Michael Kendall (Supervisor)