Seismological Constraints on Global Mantle Anisotropy

技术说明：拟议的研究的目标是在全球范围内确定上1000公里的地幔方位各向异性的三维变化。地震各向异性，即地震波速度对传播方向或偏振的依赖性，比各向同性速度单独提供了对地球弹性结构的更完整描述，并且可能是地幔变形的信号。因此，它构成了一种独特的方式来理解和约束地球？的内部。然而，除了地幔底部和上地幔顶部之外，对地幔地震各向异性知之甚少，主要是因为常用的地震数据分辨率低于~250 km。拟议中的工作将利用一个新的全球面波数据集的优势，在上1000公里的地幔方位各向异性的三维变化建模。这些数据是方位各向异性的基本模式和泛音表面波相速度地图的爱和瑞利波，因为更高的模式测量包括这些数据具有高灵敏度的目标深层结构。为了抑制各向异性，我们将结合联合收割机的模型空间搜索方法与传统的最小二乘反演技术。这种正演建模方法将使我们能够确定满足数据的所有模型的共同属性。因此，它将使我们能够确定哪些模型功能是强大的，哪些参数权衡与他人，一个关键因素，在作出有意义的解释的结果。该项目将使我们能够确定的可能性存在的方位角各向异性的过渡带和下地幔顶部，方位角各向异性如何随深度变化，以及它如何在海洋和大陆下的不同。在我们能将地震各向异性与变形联系起来的范围内，这些结果将用于阐明（1）地幔变形的几何形状，（2）岩石圈下变形是否与板块运动耦合以及在何处耦合，（3）现今软流圈变形与化石岩石圈变形对剪切波分裂的贡献，以及（4）大洋中脊的被动与主动性质。非技术性描述：我们目前对地球的认识？的深部地幔是贫穷的，但必不可少的理解表面板块构造，以及它们如何与变形在更大的深度。该项目的目标是通过绘制下至1 000公里深度的地震波速度的三维方向依赖性，即地震各向异性，提高我们对大深度地幔变形的理解。地震各向异性可能是地幔变形的信号，因此构成了限制其矿物学、成分和动力学的独特工具。结合矿物物理数据和地球动力学模型，它可以帮助我们了解我们星球的演化。为了达到我们的目标，我们将联合收割机与创新的正演模拟方法相结合，这将有助于我们探索许多不同类型的模型，并选择那些最好地解释数据的模型。通过这种计算密集型方法，我们将能够估计模型的不确定性，这是对成分，矿物学和变形结果进行合理解释的关键因素。所获得的模型将有助于约束地球动力学模型，并将指导矿物物理实验试图了解地幔变形。因此，为该项目提供资金将使地球科学界的很大一部分人受益。研究结果、方法和模型将通过科学出版物、会议报告和网站与更广泛的受众和地震研究界分享。此外，拟议的研究将提供支持的研究生谁将接受培训，并获得知识的全球地震层析成像，建模技术和并行计算机编程领域。

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.