Testing observables for teleseismic shear-wave splitting inversions: ambiguities of intensities, parameters, and waveforms

Abstract

We assess the capabilities of different observables for the inversion of core-refracted shear waves (XKS phases) to uniquely resolve the anisotropic structure of the upper mantle. For this purpose, we perform full-waveform calculations for relatively simple, canonical models of upper-mantle anisotropy. The models are characterized by two and four domains of different anisotropic properties. Specifically, we assume hexagonal symmetry with arbitrarily chosen strength of the anisotropy and orientation of the horizontal fast axis. XKS waveforms, generated from plane-wave initial conditions, traverse through anisotropic models and are recorded at the surface by a single station (in case of vertical variations) and by a dense station profile across the laterally and vertically varying structure. In addition to waveforms, we consider the effects of anisotropic variations on apparent splitting parameters and splitting intensity. The results show that, generally, it is not possible to fully resolve the anisotropic parameters of a given model, even if complete waveforms (under noisefree conditions and for the complete azimuthal range) are considered. This is because waveforms for significantly different anisotropic models can be indistinguishable. However, inversions of both waveforms and apparent splitting parameters lead to similar models that exhibit systematic variations of anisotropic parameters. These characteristics may be exploited to better constrain the inversions. The results also show that splitting intensity holds some significant drawbacks: First, even from measurements over a wide range of back-azimuth, there is no characteristic signature that would indicate depth variations of anisotropy. Secondly, identical azimuthal variations of splitting intensity for different anisotropic structures do not imply that the corresponding split waveforms are also similar. Thus, fitting of observed and calculated splitting intensities could lead to anisotropic models that are incompatible with the observed waveforms. We conclude that (bandlimited) XKS-splitting inversions and related tomographic schemes, even if based on complete waveforms, are not sufficient to fully resolve the heterogeneous anisotropic structures of the upper mantle and that combinations with alternative methods, based on e.g., receiver-function splitting, P-wave travel-time deviations, or surface waves, are required.

Shear‐Wave Splitting Reveals Layered‐Anisotropy Beneath the European Alps in Response to Mediterranean Subduction

“…We ultimately obtain 189 stations that show improvement in data fit with two‐layer splitting parameters. Analyses for layered anisotropy often suffer from ambiguities in the solution (see Rümpker et al., 2023), which we also observe for parts of our data set. Therefore, the final distribution of the two‐layer parameters after bootstrapping is analyzed for spatial clustering (by comparison with neighboring station) to better constrain these ambiguities and their respective uncertainties.…”

Discriminating Lithospheric and Asthenospheric Anisotropy Beneath Northern Oman: Sharp Contrast Observed at the Semail Gap Fault Zone

“…The required mathematical equations to correct the waveforms for a certain φ and δt are explained in detail in Rümpker et al. (2023). After correcting the waveforms for the crustal anisotropy parameters, we applied the joint splitting method for a two‐layer anisotropy.…”

“…In scheme II, in the first step, the XKS waveforms are corrected according to the splitting parameters obtained by the Ps-splitting analysis of receiver functions. The required mathematical equations to correct the waveforms for a certain φ and δt are explained in detail in Rümpker et al (2023). After correcting the waveforms for the crustal anisotropy parameters, we applied the joint splitting method for a two-layer anisotropy.…”

“…This underscores the necessity for conducting additional tests to validate the assumption of systematic backazimuthal variations. Moreover, Latifi et al (2018) and Rümpker et al (2023) argued that the results of layered anisotropy modeling could be highly non-unique and emphasized the need for prior information from the region to validate these findings. In our study, we conducted several tests by incorporating prior information from the study area to fix the FPDs in the upper layer (in the direction of mafic intrusions) or lower layer (in the direction of the absolute plate motion (APM)) (Figures S8 and S9 in Supporting Information S1).…”

Shear Wave Splitting Analysis using Deep Learning (SWSNet)

“…The equations to describe shear-wave splitting in layered structures have recently been summarized by Rümpker et al (2023). In the frequency domain, the radial and transverse displacement components, after passing through the layer, can be expressed as…”