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员组成，一名是地球动力学的，一名是地震学的，还有两名博士生和一名软件工程师，他们正在建立包括底层地幔在内的北美的4-D模型。团队成员正在完善4-D模型，并用USArray地震波形数据测试预测的地震图像。该团队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——结合在一起构建模型，然后使用这些模型整合地震图像（EarthScope项目的主要成果）和大陆的表面演化（地形、构造和岩浆历史）。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）和美国地下地幔的高分辨率层析成像。研究小组通过详细比较预测的地震波形和USArray地震记录，明确地测试了从模型中产生的图像。通过利用地震波形约束地幔结构的清晰度的能力，他们希望更好地理解地幔结构性质的大振幅，但空间上的小尺度变化。该团队正在解决以下问题：(1)从美国东部假定的板块过渡带到下地幔的速度梯度的几何形状和清晰度是什么；(2)法拉龙板块的几何形状如何随时间（地质年代）而变化，以及它是如何进入地幔的；(3)拉腊胺板如何从平板形态演化而来；(4)晚白垩世至今整个大陆的地形演化与北美大陆下地幔的动态演化有何关系；(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？为了更好地理解地质过程，科学家们收集了大量不同的数据，该项目解决了其中一个最重要的模糊性问题。在这个项目中，加州理工学院的团队正在解决这样一个事实：地球是一个多维系统，随着时间的推移而变化，但是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，因为它一方面涉及到与地形和地貌的起源有关的基本科学问题，另一方面涉及到与含有大多数已知不可再生能源的含烃沉积盆地的发展和演化有关的应用问题。模棱两可的是，地震方法给出了一个非常好的“快照”，关于地球内部的力量是什么样子的，即所谓的地幔对流。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的地质观测（如岩石的隆起和侵蚀）在时间域上非常好，超过数百万年，但不能直接描绘力量和过程是如何变化的。将地震图像与地表地质相结合一直是EarthScope项目的主要目标，该项目部署了一个大型地震仪网络来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生，这些新方法在未来可以广泛应用。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