Quantifying Intrinsic and Extrinsic Contributions to Radial Anisotropy in Tomographic Models

Abstract

AbstractSeismic 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 is approximately the product of the intrinsic and the extrinsic components: 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.