Shear Wave Splitting and Mantle Anisotropy: Measurements, Interpretations, and New Directions

Abstract

Measurements of the splitting or birefringence of seismic shear waves that have passed through the Earth's mantle yield constraints on the strength and geometry of elastic anisotropy in various regions, including the upper mantle, the transition zone, and the D 00 layer. In turn, information about the occurrence and character of seismic anisotropy allows us to make inferences about the style and geometry of mantle flow because anisotropy is a direct consequence of deformational processes. While shear wave splitting is an unambiguous indicator of anisotropy, the fact that it is typically a near-vertical path-integrated measurement means that splitting measurements generally lack depth resolution. Because shear wave splitting yields some of the most direct constraints we have on mantle flow, however, understanding how to make and interpret splitting measurements correctly and how to relate them properly to mantle flow is of paramount importance to the study of mantle dynamics. In this paper, we review the state of the art and recent developments in the measurement and interpretation of shear wave splitting-including new measurement methodologies and forward and inverse modeling techniques,-provide an overview of data sets from different tectonic settings, show how they help us relate mantle flow to surface tectonics, and discuss new directions that should help to advance the shear wave splitting field.

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

“…In addition, interpreting trench-parallel direction of SKS data close to the trench is not trivial (e.g. Long and Silver, 2009 Jolivet et al, 2013) showing middle Miocene to present-day evolution of slab geometry, mantle flow and overriding plate deformation in the Aegean-Anatolian regions and speculative interpretation of their evolution in the near future on the basis of outputs from the three-dimensional dynamic models.…”

“…10a, Paul et al, 2010;Hatzfeld et al, 2001;Evangelidis et al, 2011) possibly advocates for alongarc mantle flow that could be interpreted as originating from the Kefalonia window, but the not-so-clear nature of alongarc anisotropy (e.g. Russo and Silver, 1994;Long and Silver, 2009;Faccenna and Capitanio, 2013) prevents from being conclusive. Overall the trend of seismic tomography in the sublithospheric mantle does not accurately match the motion of Anatolia.…”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…Significant azimuthal anisotropy has been observed for Pn waves beneath oceans (Hess, 1964) and continents (Smith & Ekström, 1999) as well as for travel time residuals of teleseismic P waves (e.g., Babuška et al, 1998). There are clear observations of polarization anomalies of long period P-waves which have been used to infer upper mantle anisotropy (Schulte-Pelkum et al, 2001), but the biggest wealth of observations comes from SKS-splitting measurements (for a review see e.g., Long & Silver, 2009). Surface waves also exhibit a clear anisotropic behaviour.…”

“…They implicitly or explicitly employ Occam's razor. In splitting measurements, the razor is implicit as the interpretation assumes a one-or multi-layered medium a priori (e.g., Long & Silver, 2009). A similar situation holds when normal-mode splitting functions are used as the medium is parametrized with a small number of unknowns (e.g., Beghein & Trampert, 2003;Deuss et al, 2010).…”

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

“…elliptical, cruciform) with different shapes that depend on the anisotropy along the ray path. If the anisotropy is the only cause of splitting, then the observed shear wave (fast or slow) can be rotated in such a way that two very similar phases are seen, apart from scaling and a time delay between them (Silver and Chan, 1991;Savage, 1999;Long and Silver, 2009). Resultant splitting parameters, φ and δt, indicate the rotation angle in relation to the flow direction and shearing or extension along the ray path under the assumption of LPO in the upper mantle (a.k.a.…”