Seismic Anisotropy across the USArray using Surface-wave Arrival Angles

美国阵列可移动阵列(TA)的部署使地震学家能够以前所未有的细节汇编地震数据集和图像。到目前为止，对约2000个台站的极其丰富的数据集进行的地震分析都是基于旅行时间等标准地震数据。然而，其他类型的数据，如表面波的粒子运动，提供了补充观测来模拟浅层地球结构中的较小细节。TA台站之间的短距离使地震学家能够详细追踪远距离地震的面波在穿越TA时如何弯曲。反过来，这可以用来微调北美下面的地球结构模型，并提高科学家对北美大陆是如何构成的理解。某些地幔岩石表现出地震各向异性，其中地震波的传播速度取决于它们接近的方向。当板块，如北美板块，对沿其边缘的拉力做出反应时，也会形成各向异性。TA站的小子集可以像天线一样用来测量这种各向异性的特性。这反过来又允许科学家探索形成、移动并最终摧毁地球板块的地幔过程。这个项目将收集这两种类型的数据。在全球层析研究中，面波到达角已被证明对增强短波结构的成像非常有用。由于全球地震台网的稀疏性，对这类数据的解释采用了大圆法的摄动理论。密集的美国阵列移动式阵列(TA)使地震学家能够以前所未有的详细程度观测大型网络中由结构引起的远震波场的演变。Helmholtz层析成像利用了单分量面波波形的幅度和相位，而面波粒子运动的演化对北美以下的小尺度非均质性提供了补充约束。虽然到达角支持基于相位的地幔弥散分析，但倾角和ZH比等观测值对于约束地壳浅层特别有用。该项目将提供一个新的面波粒子运动观测数据库。该项目的第二个方面关注瑞雷波的方位各向异性。TA的密集站间距允许使用小的子阵的跨越式“波束形成”类型的方法。与针对各向异性和非均质性的包罗万象的层析联合反演方案相比，这种方法可以消除两者之间的一些权衡。这里的基本假设是子阵中的结构是均匀的，但考虑到TA的密集站间距，这应该是一个有效的假设。作为回报，获得的观测数据将是“原地观测”。

The deployment of the USArray transportable array (TA) has allowed seismologists to compile seismic datasets and images with unprecedented detail. Seismic analyses of an extremely rich dataset of some 2000 stations have so far been based on 'standard' seismic data such as travel times. However, other kinds of data such as the particle motion of surface waves provide complementary observations to model smaller details in shallow Earth structure. The short distance between TA stations allows seismologists to trace in detail how surface waves from distant earthquakes bend around as they traverse the TA. This, in turn, can be used to fine-tune models of Earth structure beneath North America and improve scientists' understanding how the continent is made up.Certain mantle rocks exhibit seismic anisotropy where the speed at which seismic waves travel depend on the direction from which they approach. Anisotropy also forms when a plate, such as the North American plate, responds to forces tugging along its edges. Small subsets of TA stations can be used like antennas to measure the properties of this anisotropy. This, in turn, allows scientists to explore mantle processes that form, move and ultimately destroy Earth's plates. This project will collect both types of data.In global tomographic studies, surface-wave arrival angles have proven extremely useful to enhance the imaging of short-wavelength structure. Due to the sparseness of the global seismic networks the interpretation of such data were interpreted using perturbation theory to a great-circle approach. The dense USArray Transportable Array (TA) allows seismologists to observe the structure-induced evolution of the teleseismic wavefield across a large network in unprecedented detail. While Helmholtz tomography utilizes the amplitude and phase of one-component surface wave waveforms, the evolution of surface wave particle motion provides complementary constraints on small-scale heterogeneity beneath North America. While arrival angles support phase-based dispersion analyses of the mantle, observables such as the dip and the ZH ratio are particularly useful to constrain shallow crustal layers. This project will provide a new database of surface-wave particle motion observable.A second aspect of this project focuses on Rayleigh wave azimuthal anisotropy. The dense station spacing of the TA allows a leap-frogging "beam forming" type approach using small subarrays. In comparison to an all-inclusive tomographic joint inversion scheme for anisotropy and heterogeneity, this approach can remove some of the trade-offs between the two. The underlying assumption here is that structure within a subarray is homogeneous, but given the dense station spacing of the TA, this should be a valid assumption. In return, obtained observables will be "in-situ".