Uniform automated analysis of Sdiff splitting due to lowermost mantle anisotropy: Caveats and curated global dataset

Alex Sun, Jonathan Wolf , Barbara Romanowicz, Ed Garnero, John D. West

Seismic anisotropy, the dependence of seismic wave speeds on the direction of propagation and/or polarization, places crucial constraints on deformation and convective flow in the lowermost mantle (the D″ layer). Shear waves that diffract along the core-mantle boundary (CMB) are ideally suited for probing this region due to their long horizontal ray paths in the lowermost mantle. However, the inference of D″ anisotropy presents multiple methodological challenges, including the need for accurate upper mantle corrections and the potential for apparent splitting produced by isotropic structures. In this study, we leverage global wavefield simulations to develop an automated measurement procedure that excludes Sdiff waves that are strongly SVdiff-polarized, which can lead to apparent splitting even in the absence of seismic anisotropy. We apply this workflow to a global dataset of ~20 million three-component seismograms. By adhering to strict quality control criteria, we generate a highly regionally consistent dataset of high-confidence measurements. Overall, we obtain ~4,300 Sdiff splitting measurements that constrain the presence or absence of D″ anisotropy. We project these measurements onto the D″ layer and report Sdiff splitting across 237 global geographic bins (approximately 28 per cent of the D″ layer by surface area) with sufficient data coverage. Of these, 55 bins exhibit clear splitting, many of which are located beneath the northern Pacific. Furthermore, we characterize the directional dependence of splitting using a backazimuth binning algorithm for regions with multi-directional coverage. We make this curated dataset of geographically and directionally binned Sdiff splitting measurements, along with all codes necessary to reproduce it, publicly available.