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| Titel |
Full Waveform Tomography for radially anisotropic structure: Theory and application to the Australasian upper mantle |
| VerfasserIn |
Andreas Fichtner, Brian L. N. Kennett, Heiner Igel, Hans-Peter Bunge |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032905
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| Zusammenfassung |
| We combine spectral-element simulations and adjoint techniques in the first non-linear Full
Seismic Waveform Tomography for radially anisotropic upper-mantle structure. Our
approach correctly accounts for the propagation of finite-frequency waves in realistically
heterogeneous Earth models, thus avoiding artifacts arising from approximate solutions of the
wave equation.
The application of our method to the Australasian region allows us to explain 30 s
waveforms in great detail, and it yields tomographic images with unprecedented resolution.
Our final model, obtained after 19 conjugate-gradient iterations, explains data that
were not initially included in the inversion. This provides strong evidence for the
effectiveness of the inversion scheme and the physical consistency of the tomographic
model.
The non-linear nature of the tomographic inverse problem becomes most apparent via the
strong dependence of the inferred anisotropy on the number of iterations. This observation
implies that a sufficiently large number of iterations is necessary to correctly image seismic
anisotropy. Classical linearised inversions are therefore only suitable for the construction of
isotropic Earth models.
Seismic anisotropy in the Australasian region depends strongly on depth, thus reflecting
the various geodynamic and mineralogic mechanisms responsible for its formation. Radial
anisotropy above 150 km depth reveals a clear ocean-continent dichotomy: We find strong
vsh > vsv beneath the Coral and Tasman Seas. The anisotropy is strongest at the top of the
inferred asthenospheric flow channel, where strain is expected to be largest. Radial
anisotropy beneath the continent is weaker and more variable. Localised patches
with vsh < vsv appear, in accord with small-scale intraplate deformation. The
ocean-continent dichotomy disappears gradually between 150-250 km depth, where
the continental lithospheric mantle and the oceanic asthenosphere pass into the
underlying convecting mantle. Significant anisotropy continues to exists below 250 km
depth. |
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