# Global seismology

The global seismology team in Lyon has broad interests ranging from developing new methods for imaging Earth's interior, to understanding the history and evolution of crustal and mantle structures at different scales. We cover different topics of theoretical seismology and data inference. We are particulartly interested in mapping seismic anisotropy, which may provide important constrains on the geometry of mantle deformation.

We are interested in the multitude of ways of translating the data extracted from the seismogram into a representation of Earth structure. Different parts of the seismic record may be used, including body waves, surface waves, or ambient noise. Different components of the signal can be exploited such as travel-times, amplitudes, deconvolved components, full waveforms or the entire wave-field. We have access to extensive computational facilities through the in-house Transcale cluster.

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Researchers

**Eric DEBAYLE** (DR)

I study the deep Earth interior with the aim of understanding its structure and dynamics. With my colleagues, students and post-doc, I have produced a number of regional and global tomographic models of the shear wave velocity, anisotropy and attenuation of the mantle.

Our latest achievement is a global shear wave velocity model of the entire mantle, SEISGLOB2, which suggests a global change of the shear wave structure near 1000 km depth (Durand et al., 2017). Shear wave anisotropy can also be used to map mantle flow and we have recently shown that only plates moving faster than 4 cm/yr can produce sufficient shearing at their base to align anisotropic crystals at the scale of the entire tectonic plates (Debayle et al., 2005; Debayle and Ricard, 2013). Our recent global attenuation models suggest that several thermal plumes of the Pacific oceans are deeply rooted down to

the transition zone and pond in the asthenosphere at the base of tectonic plates (Adenis et al., 2017). Comparison of velocity and attenuation models argue for compositional heterogeneities at the base of cratons and in a number of active regions (Adenis et al., 2017). I'm also interested in mapping seismic discontinuities and we have recently shown the existence of a global low velocity layer located just above the 410 km discontinuity (Tauzin et al., 2010). More detail can be found at http://perso.ens-lyon.fr/eric.debayle/ with a number of datasets, seismic models and tomographic codes which are made available to the community.

**Benoit TAUZIN** (MCF)

My research activity consists in imaging seismically the deep Earth interior, from the lithosphere to the lower mantle, relating observed seismic heterogeneities to what we know about mineral physics, geodynamics and geochemistry. I mainly focus on seismic discontinuities in the transition zone (i.e. 400-700 km depth) and upper mantle (0-400 km). My seismological tools are body-waves reflected (SS-precursors in my PhD) and converted (P-S and S-P receiver functions in Tauzin et al. 2008, 2013) at discontinuities recorded at permanent seismic networks and temporary arrays.

The recent impact of the 15 Feb 2013 meteor in the region of Chelyabinsk, Ural, also provided an interesting exercise of classical seismology. Due to the rarity of such an event (1 per century), its observation provided unusual constraints on the physics of meteoroid collision (Tauzin et al., 2013). I plan to further develop the study of such events on Earth, as they might be interesting analogue experiments for meteor impact surveys on Mars.

**Thomas BODIN** (CR)

I am interested in quantifying the state of knowledge we have about the Earth, given the measurements that we make at the surface. This includes solving an inverse problem and finding a model of the Earth that explains our observations. In particular, I am interested in quantifying uncertainties and trade-offs, and exploring the level of resolution associated with different data types and inverse schemes. I have been mainly working on Bayesian (i.e. probabilistic) inverse methods where the solution is a probability density function describing the information we have about the Earth. The goal is to fully take into account observational and theoretical errors, and to propagate them towards model uncertainties. I am also

interested in the structure and evolution of the Earth. For example, transdimensional inversion has enable to detect a mid-lithospheric low velocity zone under the Indian craton, and has the potential to better image mantle discontinuities.

**Fabien DUBUFFET** (IR)

I'm in charge of the "Pôle de calcul" (HPC) at the laboratory (4 clusters, 2240 cores, Linux OS, Torque and Slurm batch scheduler). I'm developing/optimizing numerical codes and shell scripts, and automating data processing for Geodynamics and Seismology. I'm in charge of the "Numerical Methods in Earth Sciences" unit at Ecole Normale Supérieure de Lyon (master first year).

I put a link to the module numerical methods:

http://sciencesdelaterre.ens-lyon.fr/enseignements/master/master-1/m1-semestre-1/methodes-numeriques-1/methodes-numeriques-1?set_language=en&cl=en

**Stéphanie DURAND** (Researcher)

My recent research focuses on imaging the deep Earth using seismic data but also on understanding deep Earth processes from a theoretical point of view. At LGL-TPE I did work on seismic attenuation and anisotropy and global tomography, and more recently, I moved to Muenster (Germany) to work at the Institut fuer Geophysik on seismic array methods and seismic noise. In a long term perspective, my aim is to combine all these approaches to provide a global image of the Earth's mantle for all seismic parameters (velocity, anisotropy, attenuation), and include for the first time noise data for deep Earth imaging.

Website: http://earth.uni-muenster.de/~durand

## Post Doctorates

**Suzanne ATKINS** (Post doc)

I am using machine learning to study mantle anisotropy in a variety of different ways. At a crystallographic scale, I am investigating using neural networks to include anisotropy in mantle dynamic calculations. At much larger scales, I am investigating the effects of geodynamic heterogeneity on seismic anisotropy.

**Navid HEDJAZIAN** (Post doc)

I work on the development of new methods to invert seismic data in a Bayesian formulation. Particularly, I will try to incorporate constrains from geodynamic modelling in the inversion process to reduce the number of possible Earth's models. Previously, I have been interested in multi-scale modelling of the development of seismic anisotropy in the Earth's upper mantle.

**Rhys HAWKINS** (Post doc)

Modelling of physical phenomena with a focus on seismic wave propagation and heat flow. Quantification of uncertainty in geophysical inverse problems at various scales through the use of adaptive parameterisations and Bayesian techniques.

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Student

**Florian MILLET** (PhD Student Univ Lyon1, Univ de Bergen)

I am doing my research in a joint collaboration between the University Lyon 1 (France) and the University of Bergen (Norway). I am developping a new method to migrate Receiver Functions in a fully 3D scheme combined with a multi-mode approach making use of efficient 3D Eikonal solvers and elastic scattering theory. These new developpements will let us image lateral variations in strong seismic reflectors such as the Moho, subducting plates and the transition zone at a higher resolution than ever before. This method is expected to perform especially well in complex subduction settings such as Cascadia, the Egean Sea or Japan.

**Chloé ALDER** (PhD Student, ENS de Lyon, Univ. Lyon 1)

As a PhD student, I study the link between seismic anisotropy and small-scale heterogeneities in the Earth’s mantle, i.e. heterogeneities thatcannot be resolved by long-period seismic waves.

During the first part of my PhD, I showed that in plausible isotropic mantlemodels,the level of extrinsic radial anisotropy increases as the square of chemical small-scale heterogeneities.

This work implies the use of the Fast Fourier Homogenization code, developed by Yann Capdeville in Nantes. It is a tool which allows to find the long-wavelength equivalent of a small-scale isotropic medium. This equivalent medium is smooth, exhibits anisotropy and represents what would be observed in tomography.

One aspect of the future work is to study how much extrinsic azimuthal anisotropy is induced in 3D structures such as oceanic ridges.

**John Keith MAGALI **(Phd Student Univ Lyon 1)

Traditionally, the inversion of long period surface waves producing models of azimuthal anisotropy within the mantle has been interpreted by seismologists as horizontal mantle flow. Since mantle dynamics is rather a complex process, the flow may not be horizontal. This boils down to the question: Could these waves be used to constrain the full patterns of mantle deformation?

As such, I develop methods for geodynamic tomography, a reconciliation of seismology and geodynamics. In this work, I apply nonlinear inversion algorithms with Bayesian inference of surface waves constrained by geodynamics. Such methods reduce the number of model parameters, and will try to capture intrinsically the effects of thermal(and later, compositional) structure onto long wavelength anisotropy.

## Former member

**Alice ADENIS **(Member)

We aim to build a 3-D attenuation model of Earth's upper-mantle from a unique dataset built by Debayle & Ricard (2012).The dataset consists of 375,000 Rayleigh-wave attenuation and velocity measurements for the fondamental mode and the five first harmonics between 40 and 240 s periods. First, attenuation measurements are processed to extract the effects of geometrical attenuation and of focusing and defocusing, in order to minimize the influence of errors on the seismic source, to avoid potentially incorrect data, and to cluster redondant measurements. Then, measurements are regionalized to obtain Rayleigh-wave maps for each mode and each period. The last step is the inversion of these maps to obtain the depth dependent attenuation. Eventually, we obtain QsADR17, a 3-D model of S-wave attenuation in the upper mantle.

QsADR17 is correlated with surface tectonics down to 200 km depth, with low attenuation under the continents and high attenuation under the oceans. High-attenuation anomalies are found under oceanic ridges down to 150 km depth, and under most of the hotspots at larger depth down to the transition zone. A large high-attenuation anomaly at 150 km depth under the Pacific ocean suggest that thermal plumes pound into the asthenosphere.

We also detect compositional heterogeneities at the base of the cratons and in active areas.