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

波动梯度测量是利用EarthScope USAray移动台站采集的地震波场形状来确定基波传播特征的一种新技术。地震学中的大多数技术应用平均方法，该方法消除可能反映真实的结构或复杂性的波场差异。波梯度测量方法被设计成利用这些波场变化来解析不能严格从平均方法导出的结构。梯度学的奇妙之处在于，如果已知单个地震台站的波场空间梯度，那么在该点上也可以确定波慢度、波方位角、几何扩展和辐射的变化。因此，EarthScope USAray的美妙之处在于，它提供了一个以前所未有的规模量化波场空间梯度的机会。这项拟议的工作将涉及在整个USAray的基于发现的研究中进一步开发和使用波动梯度测量方法，这将为解决岩石圈和最上部地幔软流层内的三维结构带来新的重要约束。我们现在有一个重要的机会来利用USAray采样的波场内迄今为止尚未利用的信息。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3-D结构复杂性的新的重要发现。我们在这里提出了新的适应梯度法，包括治疗连续位移梯度场内的子阵列，并使用多个时间序列的空间梯度获得每个地震台站使用互易原理。我们研究的另一个重要组成部分是合成数据的工作（1）用于校准和“地面真相”的方法和（2）更好地理解波场参数估计（慢度，波方位角，几何传播的变化和辐射模式）如何与结构复杂性联系在一起，包括各向异性的横向变化的影响。迄今为止生成的相速度图显示，在一些主要的地幔边界附近的速度有趣的跳跃，表明这些功能中的一些延伸到地幔岩石圈深处。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在低速/高速区传播时的聚焦和散焦一致。使用这样的信息，从多个不同的事件，并从多个不同的背方位角，有可能是转型的领域，如3-D层析反演。这项工作的更广泛的影响将包括培养一名博士。该项目的主要内容包括：对大学生刘媛媛（Yuanyuan Liu）的培训、在资助期间在实验室工作的本科生和高中生的培训，以及涉及波场动画的教育产品的生成。私家侦探在与本科生和高中生合作方面有很好的记录。在拟议期间，将继续开展这一大量辅导工作。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USAray的波场相关的扭曲、剪切和旋转（参见图1中的快照）。这些将有助于可视化很少有研究人员或学生习惯于思考的领域的组成部分，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USAray地震图确定的位移梯度张量场将使用NSF支持的极端科学和工程发现环境（XSEDE）设施存档。将建立一个综合模型网站，我们将在其中提供已发布模型中使用的数据集、已发布的模型结果、基准测试练习和文件、所有梯度测量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沿着教程。

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.