Séminaire IPGS le 30 juin à 13h45.
intervenant : Bernhard Schuberth, Ludwig Maximilians Universität München
Titre : Synthetic seismograms for a synthetic Earth : Computing 3-D wavefields in mantle circulation models to test geodynamic hypotheses directly against seismic observations
Lieu : EOST, 5 rue Descartes, amphi. Rothé
Résumé :
Understanding the dynamic behaviour of Earth mantle is of fundamental importance as mantle flow drives plate tectonics and controls the way the Earth loses its heat. However, the origin of seismic heterogeneity and the nature of flow in the mantle still remain elusive. 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 high core heat flow and associated strong thermal gradient across D$''$ may provide an alternative explanation.
Recently, we developed a new joint forward modeling approach to test geodynamic hypotheses directly against seismic data: Seismic heterogeneity is predicted by converting the temperature field of a high-resolution 3-D mantle circulation model into seismic velocities using thermodynamic models of mantle mineralogy. 3-D global wave propagation in the synthetic elastic structures is then simulated using a spectral element method. Being based on forward modelling only, this approach allows us to generate synthetic wavefields and seismograms independently of seismicobservations. Another key advantage is that our approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account.
We concentrate on the statistics of long-period body wave traveltime data, which 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, our synthetictraveltime variations from an isochemical mantle circulation model with strong core heating reproduce these different trends. Moreover, the related strong lateral temperature variations in the lower mantle are able to explain most of the standard deviation of observed delay times, when contributions from errors in the real data and uncertainties in the mineralogical parameters are taken into account. This is a strong indication that seismic heterogeneity in the lowermost mantle is dominated by thermal variations on the length-scales relevant for long-period body waves. Finally, we study wavefield effects of direct P- and S-waves in the elastic and isotropic 3-D seismic structures derived from our geodynamic model. More
specifically, we quantify the dispersion of traveltime residuals caused by diffraction in structures with dynamically constrained length-scales and magnitudes of the lateral variations in seismic velocities and density. To this end, we have created a synthetic dataset of finite-frequency traveltime residuals measured in four different frequency bands. One question that we wish to adress is whether a comparison of predicted and observed traveltime disperson can be used to further constrain the relative contributions of thermal and chemical variations to the seismic heterogeneity in Earth's deep interior.