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| Titel |
Heterogeneity of the Earth's upper mantle, from waveform inversion of over one million broadband seismograms |
| VerfasserIn |
Andrew Schaeffer, Sergei Lebedev |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079376
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| Zusammenfassung |
| We present a new global, vertically-polarized shear speed model of the upper mantle. The
model, SL2013sv, is constrained by a massive new dataset selected from more than
one million successful waveform fits generated using the Automated Multimode
Inversion of surface- and S-waveforms. The waveform inversion of each seismogram
produces a set of linear equations describing perturbations in elastic structure within
approximate sensitivity volumes along the source-receiver path, with respect to a
three-dimensional (3D) reference model; structural information is extracted from
fundamental modes in addition to overtones. These equations are then simultaneously
inverted for a high-resolution, 3D model of shear and compressional speeds and
azimuthal anisotropy within the crust and upper mantle. From our total dataset, more
than half-a-million seismograms were selected to constrain the final model, using
elaborate outlier analysis. The selection of only the most mutually consistent equations
strongly reduced the effect of errors in the data, enabling improvement upon past
global model resolutions, resulting from significantly increased data sampling and
redundancy.
In continental regions, lateral resolution approaches that of regional-scale studies, and
good correlation can be observed with the surface expression of tectonics. In the oceans,
spreading ridges are very well resolved, with the main anomalies closely confined near the
ridge axis. Additionally, clear images of major subduction zones world-wide are captured,
extending from shallow depths (in some cases, 150km depth) through much of the transition
zone. We also briefly discuss the distribution of azimuthal anisotropy, which is
well resolved globally, although over longer length-scales compared to isotropic
heterogeneity.
Finally, the large size of our waveform fit dataset provides a strong statistical foundation
to re-examine the validity field of the JWKB approximation (surface-wave ray theory), in
addition to the quantification of the success rate of fitting using AMI. We conclude that, when
selecting the correct portions of the seismogram on a case-by-case basis, the validity of the
approximations is warranted in many cases, often at shorter periods than previously assumed.
As a by-product of the waveform inversions, numerous phase and group velocity curves of
the fundamental and first 15-18 higher modes of Rayleigh and Love waves have been
measured. This large, new dispersion dataset and our new upper-mantle model
offer complementary perspectives on the bulk heterogeneity of the Earth’s upper
mantle. |
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