Full-waveform modeling of lowermost mantle anisotropy scenarios using AxiSEM3D

Observations of seismic anisotropy at the base of the mantle are abundant. Given recent progress in understanding how deformation relates to anisotropy in lowermost mantle minerals at the relevant pressure and temperature conditions, these observations can be used to test specific geodynamic scenarios. For example, recent observations of a change in fast shear wave splitting direction due to anisotropy in the lowermost mantle beneath Iceland (Wolf et al., 2019) were modeled as reflecting a localized cylindrical upwelling at the base of the Iceland plume. Similarly, Reiss et al. (2019) documented a change in anisotropy at the edges of the African Large Low Shear Velocity Province (LLSVP); forward modeling showed that these observations are consistent with mantle flow deflected at the LLSVP edge. A major limitation in these studies, however, is that the forward modeling was carried out using a ray theoretical framework, and finite frequency effects were not considered. Building on the previous work of Tesoniero et al. (2020), the goal of this work is to develop forward modeling capabilities for lowermost mantle anisotropy models using the AxiSEM3D spectral element solver. This approach enables us to compute seismograms for relatively low periods 5s for models that include arbitrary and fully 3D anisotropy models (for both the lowermost and upper mantle) at moderate computational cost. Our eventual goal is to reproduce observational results from our previous work beneath Iceland and elsewhere using synthetic waveforms based on our suggested anisotropic scenarios, taking into account finite frequency effects. Specifically, we compare a series of ray theoretical predictions with full waveform measurements from waves sampling the lowermost mantle close to edges of different anisotropy. Further, we systematically vary the dimensions of anisotropic blocks in the lowermost mantle to find the minimum size of a block that can be reliably detected in the waveforms. The modelling leads to our eventual goal to understand whether a more accurate representation of wave propagation in anisotropic media changes the interpretation of shear wave splitting measurements due to lowermost mantle anisotropy.