Formulating and Testing 4-D Dynamic Models for the North American Continent and Mantle

在这个项目中，加州理工学院的一个地球物理学家团队由两名教师组成，一名在地球动力学领域，一名在地震学领域，沿着两名博士生和一名软件工程师，他们正在建立北美的4-D模型，包括底层地幔。该小组成员正在完善4-D模型，并使用USAray地震波形数据测试预测的地震图像。该团队正在通过将地球物理学中的两种核心方法-板块构造重建和地幔对流-合并在一起来构建模型，然后使用这些模型来整合地震图像（EarthScope项目的主要成果）和大陆的表面演变（地形，结构和岩浆历史）。这些模型包括最近的发展（如板块重建中的变形板块和大大改进的地球动力学建模方法）和美国地幔的高分辨率层析成像，该团队正在通过详细比较预测的地震波形与USAray地震图来明确测试模型中出现的图像。 通过利用地震波形限制地幔结构锐度的能力，他们希望更好地了解地幔结构性质的大幅度但空间小尺度的变化。该团队正在解决以下问题：（1）从美国东部下方假定板块的过渡区到下地幔的速度梯度的几何形状和锐度是什么; (2)法拉隆板块的几何形状是如何随着时间而变化的（3）Laramide板片是如何从平坦板片构造演化出来的;（4）从晚白垩世到现在整个大陆的地形演化与北美大陆下地幔的动力学演化有何关系;（5）大陆东侧的地形演化与北美在Mezcalera/Farallon板块上的运动有何关系？该项目解决了与整合科学家为更好地了解地质过程而获得的大量数据相关的最重要的模糊性之一。在这个项目中，加州理工学院的团队正在解决地球是一个随时间变化的多维系统的事实，但我们所做的不同测量只限制了整个系统的一部分。这一点很重要，因为它一方面涉及与地形和地貌的起源有关的基本科学问题，另一方面涉及与含碳氢化合物沉积盆地的发展和演变有关的应用问题，这些盆地含有大多数已知的不可再生能源。不确定性在于地震方法对地球内部的力量，即所谓的地幔对流，给出了一个非常好的“快照”。但这些力量总是在变化，就像天气模式每天都在变化一样。另一方面，我们用来感知地球表面演变（如岩石的隆起和侵蚀）的地质观测在时间域中非常好，在数百万年的时间里，但不能直接成像力和过程如何变化。将地震图像与地表地质相结合一直是EarthScope项目的一个主要目标，该项目部署了一个大型地震仪网络，以绘制大陆下方的地幔对流图。加州理工学院的项目旨在使用最先进的方法将所有数据和信息与计算方法结合在一起。此外，该项目旨在培养两名博士。这些新方法在未来可以广泛使用。学生们在复杂的数值方法、超级计算技术以及数据与数值模型的联系方面获得了丰富的经验

In this project, a team of geophysicists at Caltech comprised of a two faculty members, one in geodynamics and one in seismology, along with two PhD students and a software engineer, are building 4-D models of North America that includes the underlying mantle. The team members are refining the 4-D model and testing the predicted seismic images with USArray seismic waveform data. The team is building the models by merging together two core methods in geophysics -- plate tectonic reconstructions and mantle convection -- and then using the models to integrate seismic images (a primary outcome of the EarthScope project) and surface evolution of the continent (topographic, structural, and magmatic history). The models include recent developments (such as deforming plates in plate reconstructions and greatly improved geodynamic modeling approaches) and high-resolution tomographic images of the mantle beneath the U.S. The team is explicitly testing the images emerging from the models through detailed comparison of predicted seismic waveforms with USArray seismograms. By capitalizing on the ability to constrain the sharpness of mantle structures with seismic waveforms, they are hoping to better understand the large amplitude, but spatially small-scale changes in the properties of mantle structures. The team is addressing the following questions: (1) what is the geometry and sharpness of velocity gradients from the transition zone to the lower mantle of the putative slab below the eastern U.S.; (2) how has the geometry of the Farallon slab changed as a function of time (geological age) and how has it descended into the mantle; (3) how did the Laramide slab evolve out of the flat slab configuration; (4) how is the topographic evolution of the entire continent from the Late Cretaceous to the present related to the dynamic evolution of the mantle below the North American continent; (5) how is the topographic evolution of the eastern side of the continent related to the motion of North America over the Mezcalera/Farallon slab? This project address one of the most significant ambiguities associated with integrating the large variety of data that scientists have made to better understand geological processes. In this project, the team at Caltech is addressing the fact that the earth is a multi-dimensional system that changes over time, but that different measurements we make only constrain part of the overall system. This is important because it relates to basic scientific questions associated with the origin of topography and landforms on the one hand and applied questions associated with the development and evolution of hydrocarbon bearing sedimentary basins that contain most of the known non-renewable energy sources, on the other hand. The ambiguity is that seismic methods give a very good "snap shot" on what the forces inside earth look like, the so-called mantle convection. But those forces are always changing, akin to the way weather patterns change on a daily basis. On the other hand, the geological observations we use to sense the evolution of the earth's surface (like the uplift and erosion of rocks) are very good in the time-domain, over millions of years, but don't directly image how the forces and processes change. The integration of the seismic images with the surface geology has been a major goal of the EarthScope project, which has deployed a large network of seismometers to map mantle convection below a continent. The Caltech project is aimed at using state-of-the-art methods to bring all of the data and information together with computational methods. In addition, the project aims at the training of two Ph.D. students in these new methods that can be used widely in the future. The students are gaining enormous experience with sophisticated numerical methods, supercomputing technologies, and the linkage of data with numerical models