A potential post-perovskite province in D″ beneath the Eastern Pacific: evidence from new analysis of discrepant SKS–SKKS shear-wave splitting

Abstract

SUMMARY Observations of seismic anisotropy in the lowermost mantle—D″—are abundant. As seismic anisotropy is known to develop as a response to plastic flow in the mantle, constraining lowermost mantle anisotropy allows us to better understand mantle dynamics. Measuring shear-wave splitting in body wave phases which traverse the lowermost mantle is a powerful tool to constrain this anisotropy. Isolating a signal from lowermost mantle anisotropy requires the use of multiple shear-wave phases, such as SKS and SKKS. These phases can also be used to constrain azimuthal anisotropy in D″: the ray paths of SKS and SKKS are nearly coincident in the upper mantle but diverge significantly at the core–mantle boundary. Any significant discrepancy in the shear-wave splitting measured for each phase can be ascribed to anisotropy in D″. We search for statistically significant discrepancies in shear-wave splitting measured for a data set of 420 SKS–SKKS event–station pairs that sample D″ beneath the Eastern Pacific. To ensure robust results, we develop a new multiparameter approach which combines a measure derived from the eigenvalue minimization approach for measuring shear-wave splitting with an existing splitting intensity method. This combined approach allows for easier automation of discrepant shear-wave splitting analysis. Using this approach we identify 30 SKS–SKKS event–station pairs as discrepant. These predominantly sit along a backazimuth range of 260°–290°. From our results we interpret a region of azimuthal anisotropy in D″ beneath the Eastern Pacific, characterized by null SKS splitting, and mean delay time of $1.15 \, \mathrm{ s}$ in SKKS. These measurements corroborate and expand upon previous observations made using SKS–SKKS and S–ScS phases in this region. Our preferred explanation for this anisotropy is the lattice-preferred orientation of post-perovskite. A plausible mechanism for the deformation causing this anisotropy is the impingement of subducted material from the Farallon slab at the core–mantle boundary.

SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

“…Because SKS and SKKS have similar raypaths in the upper mantle, but diverge significantly in the lowermost mantle, an observation of discrepant splitting between SKS and SKKS phases from the same event (measured on the same seismogram) suggests a contribution from the deep mantle to one or both phases (e.g., Deng et al., 2017; Reiss et al., 2019). SKS‐SKKS splitting discrepancies have been previously documented beneath North America (e.g., Asplet et al., 2020; Lei & Wen, 2020; Lutz et al., 2020), although these studies have also shown that seismic anisotropy in the upper mantle, not the lower mantle, represents the primary contribution to SK(K)S splitting observations. Of the 78 SKS‐SKKS pairs we measured, a minority (18%) showed discrepancies between SKS and SKKS splitting intensity measurements (such that the 95% confidence regions for the measurements did not overlap).…”

Modeling of Seismic Anisotropy Observations Reveals Plausible Lowermost Mantle Flow Directions Beneath Siberia

“…Before estimating the shear wave splitting due to the lowermost mantle, we corrected each PKS waveform by using the SKS upper mantle modeling from Figure 3. There is some scatter in the observed PKS splitting measurements, as is typical for shear wave splitting studies (e.g., Asplet et al, 2020;Creasy et al, 2017;Deng et al, 2017;Grund & Ritter, 2020;Long, 2009;Lutz et al, 2020;Niu & Perez, 2004). Therefore, we stacked each PKS error surface (after upper mantle correction) using Stacksplit (Grund, 2017) and found best-fitting PKS splitting parameters (due to D″ seismic anisotropy) of ϕ = −85° ± 10° (ϕ = −41° in ray-centered coordinates, using the convention of Wookey et al, 2005 andNowacki andWookey, 2010) and δt = 0.4 s ± 0.1 s (Figure 5).…”

Delay Time (δt) and Polarization Direction (φ) Analysis Based on Shear Wave Splitting (SWS) Method

“…In other words, S waves that propagate perpendicularly with the fracture will come slower, while the S wave that is parallel to the fracture will come faster [28]. With further development, delay time (δt) will be bigger if the S wave propagates perpendicularly to the fracture, and it will be smaller if it propagates parallel or in line with the fracture [29].…”