Technical Description: The goal of the proposed research is to determine the three-dimensional variations of azimuthal anisotropy in the upper 1000 km of the mantle, at the global scale. Seismic anisotropy, that is the dependence of seismic wave velocity on the direction of propagation or polarization, offers a more complete description of Earth's elastic structure than isotropic velocities alone, and may be a signal of mantle deformation. Therefore, it constitutes a unique way of understanding and constraining Earth?s interior. However, aside from the base of the mantle and the top of the upper mantle, little is known about mantle seismic anisotropy, mostly because of the reduced resolution of commonly used seismic data below ~250 km. The proposed work will take advantage of a new global surface wave dataset to model the three-dimensional changes in azimuthal anisotropy in the upper 1000 km of the mantle. These data are azimuthally anisotropic fundamental mode and overtone surface wave phase velocity maps for Love and Rayleigh waves, and because higher mode measurements are included these data have high sensitivity to the targeted deep structure. In order to constrain anisotropy, we will combine a traditional least-squares inverse technique with a model space search approach. This forward modeling method will enable us to determine the common properties of all the models that satisfy the data. It will therefore allow us to ascertain which model features are robust and which parameters trade-off with others, a key-element in making meaningful interpretation of the results.
This project will enable us to determine the likelihood of presence of azimuthal anisotropy in the transition zone and the top of the lower mantle, how azimuthal anisotropy changes with depth, and how it differs beneath oceans and continents. To the extent we can relate seismic anisotropy to deformation, these results will be used to shed new light on (1) the geometry of mantle deformation, (2) whether and where sub-lithospheric deformation couples with plate motion, (3) the contribution to shear-wave splitting of present-day asthenospheric deformation versus fossil lithospheric deformation, and (4) the passive vs. active nature of mid-ocean ridges.
Non-Technical Description: Our current knowledge of Earth?s deep mantle is poor, but essential to comprehend surface plate tectonics and how they relate to deformation at greater depths. The goal of this project is to improve our understanding of mantle deformation at large depths by mapping the three-dimensional directional dependence of seismic wave velocities, i.e. seismic anisotropy, down to depths of 1000 km. Seismic anisotropy is probably a signal of mantle deformation and therefore constitutes a unique tool to constraint its mineralogy, composition, and dynamics. Combined with mineral physics data and geodynamic modeling, it can help us understand the evolution of our planet.
To reach our goal we will combine a new global seismic dataset with an innovative forward modeling method, which will help us explore many different types of models and select those that explain the data the best. With this computationally intensive approach we will be able to estimate model uncertainties, a key-element to make sensible interpretation of the results in terms of composition, mineralogy, and deformation. The models obtained will help constraining geodynamic models, and will guide mineral physics experiments trying to understand mantle deformation. Funding of this project will thus benefit a large part of the Earth science community. The results, methods and models will be shared with the broader audience and the seismological research community through scientific publications, conference presentations, and a website. In addition, the proposed research will provide support for a graduate student who will be trained and acquire knowledge in the fields of global seismic tomography, modeling techniques, and parallel computer programming.
In this project, the PI and her students used a method called seismic tomography to map changes in the speed of the seismic waves generated by earthquakes and traveling through the upper 800 km of Earth's mantle. Seismic tomography is similar to medical imaging techniques like CT scans, but instead of using X-rays, it employs recordings of the seismic waves generated by earthquakes. Variations in seismic wavespeed can reveal layering within Earth's interior, and can help scientists determine the temperature and chemistry of the mantle rocks, and the general flow patterns in the warmer parts of the mantle. The Earth's surface is divided into about 12 large tectonic plates (and a few smaller ones) that move slowly (a few inches per year). These motions at the surface are responsible for most earthquakes, tsunamis, volcanic eruptions, and mountain building, and they are connected to slow movement of rocks inside Earth's mantle, a thick layer of several thousands of kilometers thickness that lies below the thin crust and above the iron core. With the technique used in this project, the PI's research group was able to map in 3-D the direction along which seismic waves travel the fastest inside the mantle, which can be an indication of the direction of mantle flow. The dataset they used allowed them to find significant mantle deformation down to greater depths than previously mapped, which challenges preconceived ideas of how mantle rocks deform at large depths. Their results also indicated possible changes in mantle flow direction between 410 km and 670 km depths, which has important consequences for our understanding of Earth's cooling history and heat flow between the core and the surface. Another important outcome of this project concerned the formation and evolution of oceanic plates. They form at ocean ridges where hot material rises to the surface and cools down while being transported away from the ridge until the rocks forming the oceanic plate meet another plate underneath which they plunge or subduct to disappear inside the mantle. The process of formation and evolution of such oceanic plate is not well understood and there was a vigorous debate in the scientific community regarding the nature of the boundary that separates the bottom of the plate from the rest of the mantle. The PI's research helped map different layers in the upper mantle beneath the Pacific plate, which led the researchers to discover a new layer inside the plate that they attributed to a change in chemical composition due to release of water during the formation of the plate at the ridge. This change in composition formed a boundary inside the plate that seismic waves are able to "see" and that is transported horizontally away from the ridge as the plate becomes older, cools down, and thickens. Funding for this project helped a junior female faculty member develop a strong research program and further establish herself in the scientific community. It also provided funding for one graduate student who was trained in geophysics, seismic tomography, seismic anisotropy, inverse problems, forward modeling, parallel computing, and programming. The student gained oral and written presentation skills through the writing of scientific articles, seminar presentations, and poster and oral presentations at various conferences. The results have been disseminated to the scientific community and the broader public through 3 peer-reviewed scientific journals, including the high profile Science Magazine, several press releases, social media, 2 seminars available on Youtube, and webpages where the results were made freely available.
