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| Titel |
Synthetic seismograms for a synthetic Earth: long-period P- and S-wave traveltime variations can be explained by temperature alone |
| VerfasserIn |
B. S. A. Schuberth, C. Zaroli, G. Nolet |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070172
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| Zusammenfassung |
| Current interpretations of seismic observations typically argue for significant chemical
heterogeneity being present in the two large low shear velocity provinces under Africa and
the Pacific. Recently, however, it has been suggested that large lateral temperature variations
in the lowermost mantle resulting from a strong thermal gradient across D″ may provide an
alternative explanation. In case of a high heat flux from the core into the mantle, the
magnitude of shear wave velocity variations in tomographic models can be reconciled with
isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong
core heating has been tested in a consistent manner only against tomographic S-wave velocity
models, but not against P-wave velocity models. Here, we explore a new approach to assess
geodynamic models and test the assumption of isochemical whole mantle flow with
strong core heating directly against the statistics of observed traveltime variations of
both P- and S-waves. Using a spectral element method, we simulate 3-D global
wave propagation for periods down to 10 seconds in synthetic 3-D elastic structures
derived from a geodynamic model. Seismic heterogeneity is predicted by converting
the temperature field of a high-resolution mantle circulation model into seismic
velocities using thermodynamic models of mantle mineralogy. Being based on
forward modelling only, this approach avoids the problems of limited resolution
and non-uniqueness inherent in tomographic inversions while taking all possible
finite-frequency effects into account. Capturing the correct physics of wave propagation
allows for a consistent test of the assumption of high core heat flow against seismic
data.
The statistics of long-period body wave traveltime observations show a markedly
different behaviour for P- and S-waves: the standard deviation of P-wave delay
times stays almost constant with turning depth, while that of the S-wave delay times
increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations
computed for the isochemical mantle circulation model reproduce these different
trends. This is not expected from a ray-theoretical point of view and highlights the
importance of finite-frequency effects. Most important, the large lateral temperature
variations in the lower mantle related to strong core heating are able to explain
most of the standard deviation of observed P- and S-wave delay times. This is a
strong indication that seismic heterogeneity in the lower mantle is likely dominated
by thermal variations on the length-scales relevant for long-period body waves. |
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