Structure of the North American Continent Using EarthScope USArray Data and Applied Wave Gradiometry Methods

波浪梯度测量是一种利用EarthScope USArray可移动台站捕获的地震波场形状来确定基本波传播特征的新技术。地震学中的大多数技术都采用平均方法，这种方法可以消除可能反映真实结构或复杂性的波场差异。波浪梯度法的设计是为了利用这些波场变化来解析那些不能从平均方法严格推导出来的结构。关于梯度的神奇之处在于，如果在一个地震台知道了波场的空间梯度，那么波的慢度，波的方位角，几何传播和辐射的变化也可以在那一点确定。因此，EarthScope USArray的美妙之处在于，它提供了一个机会，可以在前所未有的尺度上量化波场的空间梯度。这项拟议的工作将涉及在整个USArray的基于发现的研究中进一步发展和使用波浪梯度法，这将为解决岩石圈和最上层地幔软流圈内的三维结构提供新的重要约束。我们现在有一个重要的机会来利用USArray采样的波场中的信息，这些信息到目前为止还没有被利用。这种观测方法，涉及波场内位移梯度的量化，是地震学的下一个重要步骤，当与理论相结合时，有可能导致关于三维结构复杂性的新的重要发现。本文提出了对梯度法的新适应，包括处理子阵列内的连续位移梯度场，以及使用互易原理为每个地震台站获得的空间梯度的多个时间序列。我们研究的另一个重要组成部分是使用合成数据(1)进行校准和“地面真相”方法，(2)更好地理解波场参数估计（慢度，波方位角，几何传播变化和辐射模式）如何与结构复杂性联系在一起，包括各向异性横向变化的影响。迄今为止生成的相速度图显示，在一些主要地体边界附近，速度出现了有趣的跳跃，表明其中一些特征延伸到了地幔岩石圈深处。此外，瓦努阿图两个事件的结果表明，几何扩散和辐射的变化与能量通过低/高速带传播时的聚焦和散焦一致。利用来自多个不同事件和多个不同后方位角的信息，有可能在3d层析反演等领域实现转型。这项工作的更广泛影响将包括培养一名博士生刘媛媛，培养在资助期间将在实验室工作的本科生和高中生，以及制作涉及波场动画的教育产品。私家侦探在与本科生和高中生合作方面有着良好的记录。在建议的期间内，这种指导方面的重大努力将继续进行。在资助期间，我们将制作并提供波场动画（应变动画）作为教育产品。这些动画显示了与穿过USArray的波场相关的膨胀、剪切和旋转（参见图1中的快照）。这将有助于将很少有研究人员或学生习惯于思考的领域的组成部分可视化，从而为意想不到的洞察力和发现提供机会。所有代码将被共享，从USArray地震图确定的位移梯度张量场将使用nsf支持的极端科学与工程发现环境（XSEDE）设施进行存档。我们将建立一个综合模型网站，在那里我们将提供发布模型中使用的数据集，发布的模型结果，基准练习和文件，所有梯度建模的代码和教程，以及波场动画。预计结果将及时公布，他们计划在网站上提供所有数据集和代码，以及教程。

Wave gradiometry is a new technique utilizing the shape of seismic wave fields captured by EarthScope USArray transportable stations to determine fundamental wave propagation characteristics. Most techniques in seismology apply averaging methods, which smooth out wave field differences that may reflect real structure or complexity. Wave gradiometry methods are designed to take advantage of these wave field variations to resolve structure that cannot be derived strictly from averaging methods. The amazing thing about gradiomery is that if the spatial gradients of the wave field are known at a single seismic station, then the wave slowness, wave azimuth, change in geometrical spreading and radiation can also be determined at that point. The beauty of the EarthScope USArray, therefore, is that it provides an opportunity to quantify the spatial gradients of wave fields on a scale never before possible. This proposed work will involve further development and use of the wave gradiometry method in discovery-based research across the entire USArray, which will lead to new vital constraints for resolving 3-D structure within the lithosphere and upper-most mantle asthenosphere.We now have an important opportunity to take advantage of information within the wave fields, sampled by USArray, that have not been exploited to date. This observational approach, involving quantification of the displacement gradients within the wave fields, is the next important step in seismology that, when combined with theory, has the potential to lead to new important discoveries regarding 3-D structural complexity. We present here new adaptations to gradiometry method, including treatment of continuous displacement gradient fields within subarrays, and the use of multiple time series of spatial gradients obtained for each seismic station using reciprocity principle. Another important component of our research is the work with synthetic data (1) for calibration and to 'ground truth' the method and (2) for a better understanding of how wave field parameters estimates (slowness, wave azimuth, changes in geometric spreading, and radiation pattern) are linked with structural complexity, including the influence of lateral variations in anisotropy. Phase velocity maps generated to date show interesting jumps in velocity near some major terrane boundaries, indicating that some of these features extend deep into the mantle lithosphere. Furthermore, results from two events in Vanuatu show that changes in geometrical spreading and radiation are consistent with focusing and defocusing of energy as it propagates through low/high velocity zones. Use of such information from multiple different events, and from multiple different back azimuths, has the potential to be transformational in areas such as 3-D tomographic inversions. The Broader Impact of this work will consist of the training of a Ph.D. student, Yuanyuan Liu, the training of Undergraduate and High School students who will work in the lab during the funding period, and the generation of educational products involving wave field animations. The P.I. has a strong track record in working with both undergraduates and High School students. This substantial effort in mentoring will continue during the proposed period. During the funding period we will produce, and make available, wave field animations (animations of strain) as an educational product. These animations show the dilatations, shear, and rotations (see snap shot in Figure 1) associated with the wave fields that cross USArray. These will be useful for visualizing a component of the field that few researchers or students are used to thinking about, thereby providing the opportunity for unexpected insight and discovery. All codes will be shared, and displacement gradient tensor fields determined from the USArray seismograms will be archived using the NSF-supported Extreme Science and Engineering Discovery Environment (XSEDE) facility. An integrated model website will be set up where we will provide data sets that are used in the published models, published model results, benchmarking exercises and files, codes and tutorials for all gradiometry modeling, and wave field animations. Results are expected to be published in a timely way and they plan to make all data sets and codes, along with tutorials, available on the web site.