Normal mode observability of radial anisotropy in the Earth’s mantle

Abstract

Summary Observations of seismic anisotropy provide useful information to infer directions of mantle flow. However, existing global anisotropic tomography models are not consistent, particularly in the lower mantle. Therefore, the interpretation of seismic anisotropy in terms of mantle dynamics and evolution remains difficult. While surface and body waves are commonly used to build radially anisotropic tomography models, they provide heterogeneous data coverage and the radial anisotropy structure retrieved using these data may be biased by the use of imperfect crustal corrections. Normal modes, the free oscillations of the Earth, automatically provide global data coverage and their sensitivity to shear-wave (vs) and compressional-wave (vp) velocity makes them suitable to study both vs and vp anisotropy in the mantle. In this study, we assess whether current normal mode splitting data have sufficient sensitivity to lower mantle anisotropy to potentially constrain it. We consider the uncertainties in the data and the effect of inaccuracies in crustal thickness corrections and the assumed scaling between vp and vs. We perform forward modelling of normal mode data using six different 3-D global radially anisotropic tomography models to document how strong and widespread anisotropy has to be to be observable in current normal mode data. We find that, on average 50 per cent of the spheroidal and 55 per cent of the toroidal modes investigated show significant sensitivity to vs anisotropy, while roughly 57 per cent of the spheroidal modes also have strong sensitivity to vp anisotropy. Moreover, we find that the normal mode data fit varies substantially for the various anisotropic tomography models considered, with the addition of anisotropy not always improving the data fit. While we find that crustal thickness corrections do not strongly impact modes that are sensitive to the lower mantle, we observe a trade-off between radial anisotropy and vp scaling for these modes. As long as this is taken into consideration, our findings suggest that existing normal mode data sets can provide valuable information on both vs and vp anisotropy in the mantle.

Robust estimates of the ratio between S- and P-wave velocity anomalies in the Earth's mantle using normal modes

“…Journal Pre-proof While surface topography and water depth can safely be assumed to be known, Moho depth variations have larger uncertainties. Restelli et al (2023) demonstrated that predictions for normal modes sensitive to the lowermost mantle are not affected by the use of different crustal models. However, we want to verify that the way we account for the crust does not influence the results significantly in any part of the mantle.…”

“…In order to study anisotropy using SOLA applied to normal modes, good-quality measurements of toroidal modes are vital. We have recently demonstrated that current data sets of toroidal mode measurements (including the new measurements of Schneider & Deuss (2021)) contain sufficient sensitivity to both shear-wave and compressional-wave anisotropy in the mantle (Restelli et al 2023). It will be interesting to see whether SOLA inversions applied to these data are able to constrain the anisotropic structure of the Earth's mantle.…”

The sensitivity of lowermost mantle anisotropy to past mantle convection

“…Global-scale observations such as 1-D seismic anisotropy observations (De Wit and Trampert, 2015), current body-wave seismic tomography (e.g. Simmons et al, 2021;Auer et al, 2014) or normal-mode observations (Restelli et al, 2023) should be sensitive mainly to present-day flow only in the lower mantle. Note that in the case of seismic tomography improvements in resolution may lead to sensitivity to past flow.…”