Transcript 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
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
Upper mantle anisotropy
12%
7%
4%
4%
Correlation with ridges
Inconsistency of radial anisotropy
models
From Becker et al., 2008
Correlation
of ξ models
above 350
km
Correlation
of VS 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 model
SAW642AN
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 –
less regularization
New corrections –
more regularization
Same dataset with
linear corrections and
longer wavelengths
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!