Collaborative Research: Shear-wave Splitting and Mantle Dynamics of the North American Plate

在地球表面观察到的许多地质特征和现象，如巨大的山带、深海盆地、地震和火山，都是地壳、地幔和地核中动态过程的结果。因此，更好地了解这些过程对于了解地球的起源和发展以及减轻自然灾害至关重要。然而，目前只有地球最浅的部分可以用钻孔的岩心直接观测到；因此，一系列计算密集型的地球物理技术被应用于对这颗活跃行星的成像。这些技术类似于医院里使用的人体成像技术。技术描述横波分裂是一项基本的地球物理观测，它独特地指示了沿射线路径存在的方位各向异性。这种各向异性是地球软流圈和岩石圈动态过程的结果。然而，剪切波分裂的动态解释需要均匀、高质量的测量和定量正演模型。该项目将测量美国东部所有USArray可移动阵列和其他宽带地震台站的剪切波分裂参数，并更新整个北美板块的测量结果。数据产品包括约10,000个剪切波分裂测量值，我们正在合并约16,000个测量值，这些测量值是我们在美国西部和中部由美国国家科学基金会通过EarthScope预先资助的。我们正在利用一套程序来可靠地测量和排序分裂参数，以产生一个统一的数据库，量化剪切波分裂的复杂性和各向异性的可能深度。为了补充这个参考剪切波分裂数据库，我们正在通过计算地幔流和简化构造模型的结构，计算大量用于表面波研究的合成剪切波分裂和各向异性结构的地球动力学正演模型。通过这种方式，我们可以测试哪些关于各向异性起源的假设可以可靠地辨别出来，哪些构造情景最能反映观测结果，以及如何指导我们对大陆和上地幔结构和动力学的区域改进理解。利用这种联合方法，我们正在解决一些关于地震各向异性起源的重要问题，包括岩石圈继承和最近的软流圈流动，以及北美板块下地幔对流的性质。此外，我们所有的测量结果以及合成波形都是公开共享的，以便其他人可以完善他们的参考模型；这确保了我们的数据产品将对广泛的地球科学问题产生重大影响。

Non-technical DescriptionNumerous geological features and phenomena observed on the surface of the Earth -- such as giant mountain belts, deep ocean basins, earthquakes, and volcanoes -- are the results of dynamic processes operating in the Earth's crust, mantle, and core. Therefore, an improved understanding of these processes are essential for the understanding of the origin and development of the Earth and for mitigation of natural hazards. At the present time, however, only the shallowest part of the Earth can be observed directly using cores from drill holes; consequently, an array of computing-intensive geophysical techniques are applied to image the active planet. Those techniques are similar to those used in the hospital to image the human body.Technical DescriptionShear-wave splitting is a fundamental geophysical observation that uniquely indicates the presence of azimuthal anisotropy along the ray path. Such anisotropy is the result of dynamic processes in the Earth's asthenosphere and lithosphere. However, dynamic interpretation of shear-wave splitting requires homogeneous, high quality measurements, and quantitative forward models. This project will measure shear-wave splitting parameters at all of the USArray Transportable Array and other broadband seismic stations in the eastern U.S. and update measurements throughout the North American plate. The data product includes ~10,000 shear-wave splitting measurements that we are merging with ~16,000 measurements that we produced for the western and central U.S. with NSF prior funding via EarthScope. We are utilizing a set of procedures for reliably measuring and ranking the splitting parameters for producing a uniform database, quantifying shear-wave splitting complexity and likely depth of anisotropy along the way. To complement this reference shear-wave splitting database, we are computing a large number of geodynamic forward models of synthetic shear-wave splitting and anisotropic structure for surface wave studies by computing fabrics from mantle flow and simplified tectonic models. This way, we can test which hypotheses for the origin of anisotropy can be reliably discerned, which tectonic scenarios best reflect observations, and how to guide regional refinement of our understanding of continental and upper mantle structure and dynamics. Using this joint approach, we are addressing a number of significant questions regarding the origin of seismic anisotropy, including lithospheric inheritance and recent asthenospheric flow, and the nature of mantle convection underneath the North American plate. In addition, all our measurements, as well as synthetic waveforms, are being openly shared so as to allow others to refine their resulting reference models; this ensures our data product will have a significant impact on a broad range of Earth science questions.