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| Titel |
Seismic velocity and anisotropy of the upper mantle and transition zone at global and regional scales imaged using Surface and S waveform tomography |
| VerfasserIn |
Andrew Schaeffer, Sergei Lebedev |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250055957
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| Zusammenfassung |
| The continued development and deployment of large-scale high-resolution seismic arrays
(e.g., the Earthscope US Array) are producing massive new datasets that sample the Earth at
scales from tectonic units to continent-wide domains and enable resolution of structures and
deformation of the lithosphere previously possible only at regional scales. With this resolving
power comes new challenges relating to efficient management and processing of such large
data volumes. In this study, we have assembled a massive global dataset (with focus on North
America) of three-component broadband seismic waveforms collected from more than
2000 stations, and have carried out full waveform inversions resulting in more than
700,000 successfully fit source-receiver paths for vertically polarized shear waves. We
augmented available US Array stations with additional stations of the GSN and
affiliates, Canadian National Seismograph Network, regional arrays, past PASSCAL
experiments, and other stations from Iceland, Greenland, Central and South America, the
Caribbean, and several Mid-Atlantic Islands. We exploit the resolving power of this
unprecedentedly large dataset using Automated Multimode Inversion of surface-
and S-wave forms. Vertically polarized shear waves (Rayleigh waves) are inverted
for path-averaged linear constraints on elastic structure along the source-receiver
paths. Of these almost three-quarters of a million waveform fits, many provide
constraints not only on the fundamental mode, but higher modes as well. These
linear equations are then simultaneously solved for a high-resolution 3D upper
mantle shear velocity and azimuthal anisotropy model. Embedded multi-resolution
grids afford higher model resolution in regions where data sampling (i.e., station
distribution) justifies, and permit the exploration of features at both global and regional
scales.
We present a global model of upper mantle shear velocity and azimuthally anisotropic
structure down to the 660 km discontinuity. In continental domains, clearly identifiable
boundaries between different tectonic features such as basins and relic mountain ranges are
readily observable, as well as the signature of deep cratonic roots versus juvenile accretionary
margins. Both active and fossil subduction zones are marked by clearly discernible slab
signatures deep in the upper mantle and extending through the transition zone. In
oceanic regions, spreading ridges are clearly visible down to depths of 100-120 km,
and the evolution (cooling and thickening) of lithosphere away from the spreading
ridges clearly matches with the expected signature from geodynamic and thermal
modeling.
The pattern of azimuthal anisotropy in the ocean basins, in particular the Pacific Ocean,
aligns with the paleo-spreading orientations at shallow depths within the lithosphere and
modern plate motions at greater depths within the asthenosphere. However, there are
exceptions, where the observed azimuthal anisotropy does not necessarily follow such a
pattern, such as parts of the Atlantic Ocean. Through the computation and ongoing
integration of full waveform fits from horizontally polarized Love waves, we are
assembling high-resolution constraints on the global distribution of radial anisotropy
throughout the upper mantle and transition zone. We discuss preliminary results from this
model, including comparisons with the global distribution of azimuthal anisotropy. |
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