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. - PowerPoint PPT PresentationTRANSCRIPT
Anisotropic seismic tomography: Potentials and
pitfallsMark 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
But large regions of the Earth appear nearly isotropic to seismic waves!
Origins of mantle anisotropy
A random mix of orientations makes seismic waves see an isotropic average
Origins of mantle anisotropy
Deformation can lead to preferential orientation (LPO) and seismic anisotropy
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
from Karato et al, 2008
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
• 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
from Becker et al, 2003
“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
Sensitivity to strength and orientation of anisotropy depends on the starting model
Potential?
Chevrot and Monteiller, 2009 synthetic tests with non-linear inversion of body wave splitting data
Matching models
from Gaboret et al, 2003
Matching models
from Becker, 2008
12% 7%
4% 4%
Upper mantle anisotropy
Correlation with ridges
Inconsistency of radial anisotropy models
From Becker et al., 2008
Correlation of VS models above 350 km
Correlation of ξ models above 350
km
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
• 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
• 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.
VS modelSAW642AN SAW642ANb
Changing ξ model
What remains
General pattern of radial anisotropy beneath oceanic and continental lithosphere remains. Ridge signature also remains.
Troublesome details – D” structure
SAW642AN
New corrections – more regularization
Same dataset with linear corrections and longer wavelengths
New corrections – less regularization
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!