ORCID Profile
0000-0002-9438-7060
Current Organisation
Université Claude Bernard Lyon 1
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Publisher: American Geophysical Union (AGU)
Date: 10-2021
DOI: 10.1029/2021JB022322
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 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.
Publisher: Elsevier BV
Date: 12-2000
Publisher: Wiley
Date: 07-12-2021
Publisher: Springer Science and Business Media LLC
Date: 02-2005
DOI: 10.1038/NATURE03247
Abstract: Differences in the thickness of the high-velocity lid underlying continents as imaged by seismic tomography, have fuelled a long debate on the origin of the 'roots' of continents. Some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth, with horizontally polarized shear waves travelling faster than vertically polarized ones. This azimuthally averaged anisotropy could arise from present-day deformation at the base of the plate, as has been found for shallower depths beneath ocean basins. Such deformation would also produce significant azimuthal variation, owing to the preferred alignment of highly anisotropic minerals. Here we report global observations of surface-wave azimuthal anisotropy, which indicate that only the continental portion of the Australian plate displays significant azimuthal anisotropy and strong correlation with present-day plate motion in the depth range 175-300 km. Beneath other continents, azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans. We infer that the fast-moving Australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy. Simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents.
Publisher: American Geophysical Union (AGU)
Date: 02-2004
DOI: 10.1029/2003JB002652
Publisher: Springer Science and Business Media LLC
Date: 21-10-2020
Publisher: Oxford University Press (OUP)
Date: 16-06-2010
Publisher: Springer Science and Business Media LLC
Date: 19-01-2021
Publisher: Copernicus GmbH
Date: 15-10-2013
Abstract: Abstract. In a linear ill-posed inverse problem, the regularisation parameter (d ing) controls the balance between minimising both the residual data misfit and the model norm. Poor knowledge of data uncertainties often makes the selection of d ing rather arbitrary. To go beyond that subjectivity, an objective rationale for the choice of d ing is presented, which is based on the coherency of delay-time estimates in different frequency bands. Our method is tailored to the problem of global multiple-frequency tomography (MFT), using a data set of 287 078 S-wave delay times measured in five frequency bands (10, 15, 22, 34, and 51 s central periods). Whereas for each ray path the delay-time estimates should vary coherently from one period to the other, the noise most likely is not coherent. Thus, the lack of coherency of the information in different frequency bands is exploited, using an analogy with the cross-validation method, to identify models dominated by noise. In addition, a sharp change of behaviour of the model & ell ∞-norm, as the d ing becomes lower than a threshold value, is interpreted as the signature of data noise starting to significantly pollute at least one model component. Models with d ing larger than this threshold are diagnosed as being constructed with poor data exploitation. Finally, a preferred model is selected from the remaining range of permitted model solutions. This choice is quasi-objective in terms of model interpretation, as the selected model shows a high degree of similarity with almost all other permitted models (correlation superior to 98% up to spherical harmonic degree 80). The obtained tomographic model is displayed in the mid lower-mantle (660–1910 km depth), and is shown to be compatible with three other recent global shear-velocity models. A wider application of the presented rationale should permit us to converge towards more objective seismic imaging of Earth's mantle.
No related grants have been discovered for Eric Debayle.