The expression of mantle seismic anisotropy in the global seismic wavefield

Abstract

Summary The dependence of seismic wavespeeds on propagation or polarization direction, called seismic anisotropy, is a relatively direct indicator of mantle deformation and flow. Mantle seismic anisotropy is often inferred from measurements of shear-wave splitting. A number of standard techniques to measure shear-wave splitting have been applied globally; for example, KS splitting is often used to measure upper mantle anisotropy. In order to obtain robust constraints on anisotropic geometry, it is necessary to sample seismic anisotropy from different directions, ideally using different seismic phases with different incidence angles. However, many standard analysis techniques can only be applied for certain epicentral distances and source-receiver geometries. To search for new ways to detect mantle anisotropy, instead of focusing on the sensitivity of individual phases, we investigate the wavefield as a whole: we apply a “wavefield differencing” approach to (systematically) understand what parts of the seismic wavefield are most affected by splitting due to seismic anisotropy in the mantle. We analyze differences between synthetic global wavefields calculated for isotropic and anisotropic input models, incorporating seismic anisotropy at different depths. Our results confirm that the seismic phases that are commonly used in splitting techniques are indeed strongly influenced by mantle anisotropy. However, we also identify less commonly used phases whose waveforms reflect the effects of anisotropy. For example, PS is strongly affected by splitting due to seismic anisotropy in the upper mantle. We show that PS can be used to fill in gaps in global coverage in shear-wave splitting datasets (for example, beneath ocean basins). We find that PcS is also a promising phase, and present a proof-of-concept example of PcS splitting analysis across the contiguous United States using an array processing approach. Because PcS is recorded at much shorter distances than KS phases, PcS splitting can therefore fill in gaps in backazimuthal coverage. Our wavefield differencing results further hint at additional potential novel methods to detect and characterize splitting due to mantle seismic anisotropy.

Advances in Mapping Lowermost Mantle Convective Flow With Seismic Anisotropy Observations

“…While ray-theoretical modeling is still popular due to its computational efficiency, we can now model wave propagation through arbitrary seismic anisotropy using global 3D wave propagation solvers (e.g., Cottaar & Romanowicz, 2013;Nowacki & Wookey, 2016;Tesoniero et al, 2020). AxiSEM3D (Fernando et al, 2024;Leng et al, 2016Leng et al, , 2019 is very efficient in this context and, therefore, frequently used (e.g., Tesoniero et al, 2020;Wolf & Long, 2022;Wolf et al, 2022aWolf et al, , 2022bWolf, Long, Frost, & Nissen-Meyer, 2024).…”

“…For example, S diff can potentially be measured up to a larger distance range using array techniques as opposed to single-station seismograms . Similarly, using array techniques, PcS splitting can be reliably measured for epicentral distances between 45 and ∼63° ( Wolf, Long, Frost, & Nissen-Meyer, 2024), allowing the inference of seismic anisotropy at shorter distances than other commonly applied methods.…”

“…Many new strategies to measure deep mantle anisotropy have been suggested in the past two decades (e.g., H. A. Ford et al, 2015;Kawai & Geller, 2010;Niu & Perez, 2004;Pisconti et al, 2019;Thomas et al, 2011;Wang & Wen, 2004;Wolf et al, 2022a;Wolf, Frost, et al, 2023;Wolf, Long, Frost, & Nissen-Meyer, 2024;. Some of these strategies rely on ray theory, which is a high-frequency approximation of the wave equation.…”