Anisotropic seismic tomography potentials and pitfalls
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Anisotropic seismic tomography: Potentials and pitfalls. Mark Panning University of Florida CIDER Research Talk 7/5/2010. Cartoon land motivation: tomography of scientists. What is seismic anisotropy?. ?. Origins of mantle anisotropy. Single crystal has anisotropic elastic properties.

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Anisotropic seismic tomography: Potentials and pitfalls

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Anisotropic seismic tomography potentials and pitfalls

Anisotropic seismic tomography: Potentials and pitfalls

Mark Panning

University of Florida

CIDER Research Talk 7/5/2010


Cartoon land motivation tomography of scientists

Cartoon land motivation: tomography of scientists


What is seismic anisotropy

What is seismic anisotropy?

?


Origins of mantle anisotropy

Origins of mantle anisotropy

Single crystal has anisotropic elastic properties

But large regions of the Earth appear nearly isotropic to seismic waves!


Origins of mantle anisotropy1

Origins of mantle anisotropy

A random mix of orientations makes seismic waves see an isotropic average


Origins of mantle anisotropy2

Origins of mantle anisotropy

Deformation can lead to preferential orientation (LPO) and seismic anisotropy


Complications

Complications

  • Anisotropy depends on deformation mechanism

    • Varies by stress state and grain size

    • Varies by volatile content

  • Depends on integrated strain history

  • Requires many model parameters to describe


Fabric development

Fabric development

from Karato et al, 2008


Not all gloom and doom

Not all gloom and doom

  • Natural samples (e.g. Montagner and Anderson, 1989) and numerical modeling (e.g. Becker et al, 2006) suggest hexagonal symmetry is dominant


Why we like hexagonal symmetry

Why we like hexagonal symmetry

  • Reduces number of elastic coefficients from 21 to 5 (2 isotropic properties, 3 anisotropic ones) plus 2 orientation angles

  • With scaling, we can reduce the number of parameters even further (scale Vp to Vs, and the various anisotropic parameters to each other)


Why we like finite strain ellipses

Why we like finite strain ellipses

from Becker et al, 2003


Vectorial tomography

“Vectorial tomography”

  • Arbitrarily oriented hexagonal medium

  • Can be linearized – with assumptions to reduce number of parameters

  • Also can invert directly for anisotropic strength and orientation angles

symmetry axis


Nonlinearity

Nonlinearity

Sensitivity to strength and orientation of anisotropy depends on the starting model


Potential

Potential?

Chevrot and Monteiller, 2009 synthetic tests with non-linear inversion of body wave splitting data


Matching models

Matching models

from Gaboret et al, 2003


Matching models1

Matching models

from Becker, 2008


Upper mantle anisotropy

7%

12%

4%

4%

Upper mantle anisotropy


Correlation with ridges

Correlation with ridges


Inconsistency of radial anisotropy models

From Becker et al., 2008

Inconsistency of radial anisotropy models

Correlation of ξ models above 350 km

Correlation of VS models above 350 km


Poor crustal corrections source of some inconsistency

Poor crustal corrections - source of some inconsistency?

  • Inversions of synthetic data using Crust2.0 but no mantle anisotropy show anisotropy

From Bozdağ and Trampert, 2008

From Lekic et al, 2010


The crust and anisotropic models

The crust and anisotropic models

  • All seismic data is influenced by crustal structure

  • Varying crustal models has similar effect on data fit as mantle radial anisotropy (Ferreira et al, 2010)

  • Corrections based on linear perturbations from 1D crustal models are inadequate for long-period data


Testing the impact of crustal corrections

Testing the impact of crustal corrections

  • SAW642AN (as well as S362WMANI) incorporated non-linear crustal corrections based on regionalized mode calculations

  • Other methods of non-linear crustal corrections exist

  • We can compare models using different corrections and look at stability of model parameters.


V s model

VS model

SAW642AN

SAW642ANb


Changing model

Changing ξ model


What remains

What remains

General pattern of radial anisotropy beneath oceanic and continental lithosphere remains. Ridge signature also remains.


Troublesome details d structure

Troublesome details – D” structure

New corrections – more regularization

SAW642AN

New corrections – less regularization

Same dataset with linear corrections and longer wavelengths


Takeaway message

Takeaway message

  • Anisotropic modeling has great potential for constraining flow patterns (and therefore mantle rheology, etc.)

  • Inverse approach and crustal correction matter and can strongly affect anisotropic models

  • In order to resolve anisotropic structure (and other secondary effects like attenuation), we need to figure out the crust!


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