Widespread Anisotropy Research Articles

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

SUMMARYObservations 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% of the spheroidal and 55% of the toroidal modes investigated show significant sensitivity to vs anisotropy, while roughly 57% 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.

Seismic anisotropy in the upper mantle

Abstract Seismic wave propagation in anisotropic media is fundamentally different from isotropic propagation, displaying variation of characteristics with direction, and, perhaps more diagnostic, coupling between motion in the sagittal plane and horizontal transverse motion. Such coupling has been observed for higher mode surface waves indicating anisotropy in the upper mantle beneath much of Eurasia. Synthetic seismograms show that the passage of P or SV body waves through an anisotropic upper mantle will write SH signatures on the wavetrains generated at both isotropic/anisotropic and anisotropic/isotropic interfaces. These signatures have not yet been positively identified on seismograms, but once observed, may be interpretated in terms of depth, thickness, orientation, and degree of anisotropy. The widespread anisotropy already demonstrated implies that the interpretation of every seismic wave which penetrates the upper mantle must be modified for anisotropy if the observations are made with sufficient resolution.