Blanc SIMI 5-6 - Blanc - SIMI 5-6 - Environnement, Terre et Espace

Global S-wave imaging of the Earth mantle – SEISGLOB

Mapping the interior of the Earth with seismology

We study the Earth's mantle (layer between 40 km and 2870 km depths, which is thought to be the engine of plate tectonics) using seismic wave.

Improving our understanding of the mantle through new seismic models.

We build new seismic models, with the aim to improve our knowledge of the mantle. We aim to provide elements of answers to several first order geodynamic questions, concerning the size of convection cells, vertical circulation and exchange of materials between upper and lower mantle or the presence of hidden reservoirs in the mantle. We aim to build a seismic model of the entire mantle using surface waves, normal modes and body waves at long periods. The simultaneous inversion of these three types of data that has so far only be conducted in American universities (Harvard, Caltech, Berkeley). This model will be the first tomographic model of this type built in France.

We build databases that include millions of shear wave travel-times and amplitude measurements from the distribution of earthquakes and seismic stations around the globe. We also add existing and new normal mode observations to obtain additional constraints on the large-scale pattern of S-wave heterogeneities and on the 1D attenuation and density profiles. The whole dataset will then be combined in a tomographic inversion for the three-dimensional structure of the mantle.

Our new model of the upper mantle, DR2012, suggests that the high velocity signature of cratons is shallower (~200 km) than in previous studies. We show that in the transition zone of the mantle, the best current tomographic models only agree at very large wavelengths (about 4000 km).
We show, in an article published in EPSL on the anisotropic part of DR2012 (Debayle and Ricard, 2013), that only fast-moving plates, for which the horizontal motion is greater than 4 cm/year produce sufficient shear at their base to deform and align the mantle minerals. The organization of anisotropy beneath slower plates is complex and often not aligned with current plate motions. This may indicate a return flow of about 4 cm / year in the mantle. The motion of the fastest plates would organize anisotropy at large-scale. This organization would be much more easily modified by secondary convection (hot spots, variations in thickness of the lithosphere) beneath slower plates. Nature Geoscience has published a'' highlight « of our results in August 2013.
We also show that it is possible to extract additional information from body waves by inverting several frequency bands with a finite frequency theory.

Improve elastic and anelastic 3D models of the mantle:
• By improving the quality of automated measurements.
• By using multi-band finite frequency inversions to extract additional information from body waves.
• By including normal modes measurements.

The paper below shows DR2012, our new tomographic model of the upper mantle :
• E. Debayle and Y. Ricard, A global shear velocity model of the upper mantle from fundamental and higher Rayleigh mode measurements, J. Geophys. Res., 117, B10308, doi:10.1029/2012JB009288, 2012.

The paper below presents our results on seismic anisotropy (see also the highlight in Nature Geoscience: www.nature.com/ngeo/journal/v6/n8/full/ngeo1914.html) :
• E. Debayle and Y. Ricard, Seismic observations of large-scale deformation at the bottom of fast-moving plates, Earth. Planet. Sci. Letters, dx.doi.org/10.1016/j.epsl.2013.06.025, 2013.

The article below (under revision) present a preliminary tomographic model of the lower mantle:
• C. Zaroli, Sambridge M., Lévêque J.J., E. Debayle and G. Nolet, An objective rationale for the choice of regularisation parameter with application to global multiple-frequency S-wave tomography, Solid Earth, Solid Earth Discuss., 5, 841-881, doi:10.5194/sed-5-841-2013, 2013

In 2006, the funding of the TOMOGLOB project by the ANR ''young researcher'' program led to the creation of the first global SH-wave tomographic model of the mantle constrained by a finite frequency theory applied to a travel time dataset measured in several frequency bands (Zaroli, 2010). In addition, the TOMOGLOB project allowed us to build one of the best resolved SV-wave tomographic model of the upper mantle and transition zone (Debayle and Ricard, 2010). Finally, we were able to show the global character of the low velocity layer located atop the 410 km discontinuity, a result that has just been published in Nature Geoscience (Tauzin et al., 2010). These results were made possible through the construction of three databases : a first database for the dispersion of the fundamental and first higher modes of Rayleigh waves (sensitive to the SV wave-speed), a second database for long period SH-type body waves (S, ScS and SS) measured in different frequency bands, and a third database for waves reflected and converted at the 410-km and 660-km discontinuities.

The current project aims at exploiting these recent results and the expertise of a young and motivated scientific team in order to improve S-wave imaging of the Earth mantle. This requires to complete the S-wave databases which have been built as part of the TOMOGLOB project. We aim to add a database of normal mode observations, including the existing observations and our own observations using data provided by several very large earthquakes recorded in the last 5 years. These new data will provide new constraints on the long wavelength pattern of SH and SV-wave heterogeneities, as well as on the attenuation and density structure. We also propose to build a comprehensive database for Love wave fundamental and higher mode dispersion curves in order to constrain SH-wave heterogeneities in the upper mantle. Finally, we propose to build a global database of long period SV-type body waves measured in different frequency bands.

These databases will first allow us to build an S-wave tomographic model from the simultaneous inversion of surface waves, normal modes and long period body waves. This model will be the first model of this type built in France, as the simultaneous inversion of these three types of data has so far been performed only by a limited number of US groups (Harvard, Caltech, Berkeley).

We will then built other seismic models by applying more sophisticated modeling to different subsets of our data. We will apply a finite frequency theory to our Rayleigh and Love wave dataset in order to improve resolution in the upper mantle and transition zone. Similarly, we will apply a finite frequency theory to our global dataset of long period SV and SH waves in order to refine seismic imaging in the lower mantle. By inverting data that are sensitive to SV and SH waves, we will extract an information on the 3D distribution of radial anisotropy in the Earth mantle. Finally, we will develop techniques to invert surface wave attenuation and normal mode. The project also incorporates an exploratory part on laboratory measurements of S-wave attenuation at high pressure and temperature which should stimulate discussions on the interpretation of attenuation.

This kind of model is expected to have an important impact on the community. It will contribute to answering geodynamical questions of primordial importance concerning the size of convection cells, vertical circulation within the mantle and material exchange between upper and lower mantle, and the existence of hidden reservoirs in the lower mantle. A detailed knowledge of the geometry and amplitude of seismic anomalies within the mantle is of fundamental importance for constraining the physical parameters and the forms of mantle convection.

Project coordination

Eric Debayle (UNIVERSITE CLAUDE-BERNARD-LYON 1) – Eric.Debayle@ens-lyon.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

GeoAzur - CNRS GeoAzur
LGLTPE - UCBL UNIVERSITE CLAUDE-BERNARD-LYON 1
IPG - CNRS CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ALSACE

Help of the ANR 330,000 euros
Beginning and duration of the scientific project: - 48 Months

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