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- Title
Quantifying Intrinsic and Extrinsic Contributions to Radial Anisotropy in Tomographic Models.
- Authors
Magali, J. K.; Bodin, T.; Hedjazian, N.; Ricard, Y.; Capdeville, Y.; Debayle, E.
- Abstract
Seismic anisotropy in the Earth's mantle inferred from seismic observations is usually interpreted in terms of intrinsic anisotropy due to crystallographic preferred orientation (CPO) of minerals, or extrinsic anisotropy due to shape preferred orientation (SPO). The coexistence of both contributions confuses the origins of seismic anisotropy observed in tomographic models. It is thus essential to discriminate CPO from SPO. Homogenization/upscaling theory provides means to achieve this goal. It enables computing the effective elastic properties of a heterogeneous medium, as seen by long‐period waves. In this work, we investigate the effects of upscaling an intrinsically anisotropic and heterogeneous mantle. We show analytically in 1‐D that the observed radial anisotropy parameter ξ* is approximately the product of the intrinsic ξCPO* and the extrinsic ξSPO* components: ξ*≈ξCPO*×ξSPO*. This law is verified numerically in the case of a homogenized 2‐D marble cake model of the mantle in the presence of CPO obtained from a micro‐mechanical model of olivine deformation. Our numerical findings predict that for wavelengths smaller than the scale of deformation patterns, tomography may overestimate intrinsic anisotropy due to significant extrinsic anisotropy. At longer wavelengths, intrinsic anisotropy is always underestimated due to spatial averaging. Therefore, we show that it is imperative to homogenize a CPO model first before drawing comparisons with tomographic models. As a demonstration, we use our composite law with a homogenized CPO model of a plate‐driven flow underneath a mid‐ocean ridge, to estimate the SPO contribution to an existing tomographic model of radial anisotropy. Plain Language Summary: Small‐scale heterogeneities may generate long‐period seismic observations that are identical to those produced by large‐scale mantle flow and deformation. Because of this, it is difficult to distinguish in the observed seismic anisotropy what is related to the intrinsic crystalline anisotropy and what may be due to the laminated structure of isotropic materials. In this study, we undertook an analytical method and a numerical experiment to identify the separate effects of intrinsic and apparent anisotropy in a long wave‐length tomographic image. We show that the ambiguity depends on the relation between the wavelength of the observed wavefield and the scale of convection patterns in the mantle. This motivated us to develop a simple composite law that can be used to quantify the two separate contributions. Key Points: We propose a theoretical expression that relates the observed radial anisotropy to its intrinsic and extrinsic contributionsAt wavelengths longer than the scale of deformation patterns, tomography underestimates intrinsic anisotropy due to spatial averagingAt shorter wavelengths, tomography overestimates intrinsic anisotropy due to the presence of extrinsic anisotropy
- Subjects
SEISMIC anisotropy; ANISOTROPY; SEISMOLOGY; EARTH movements; EARTH'S mantle
- Publication
Journal of Geophysical Research. Solid Earth, 2021, Vol 126, Issue 10, p1
- ISSN
2169-9313
- Publication type
Article
- DOI
10.1029/2021JB022322