Mapping steps of the crust-mantle boundary over the contiguous United States

地壳-地幔边界，或莫霍，是地球上普遍存在的结构不连续。莫霍几何的横向变化，特别是莫霍偏移量，对于理解地壳的起源和演化非常重要，并且已经成为许多区域和全球研究的目标。然而，以前的方法只能提供地壳的斑点照明或平滑的莫霍地形。在这里，我们提出了一种基于散射的方法来系统地绘制莫霍偏移。利用USArray的密集数据覆盖，这项工作将提供美国相邻地区莫霍偏移的完整地图。这些结果将揭示北美大陆的构造演化，并提供有关下地壳物质特性的信息。该项目还将支持培养一名博士后。提出的新方法依赖于入射远震体波散射的表面波。当远震体波碰到莫霍偏移时，后者就成为一个强的次级震源。结果，表面波在远离莫霍偏移的水平方向上辐射。利用适当的速度模型对散射表面波进行反向传播，可以检测和定位莫霍偏移量。这项工作的结果可能会解决莫霍补偿是否普遍存在的争论。莫霍偏移分布的区域差异（或不差异）将提供关于厚皮与薄皮变形机制以及下地壳韧性流动对莫霍偏移平滑的影响的有价值的信息。新方法将应用于USArray数据，绘制美国地下的莫霍地形图。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The crust-mantle boundary, or the Moho, is an ubiquitous structural discontinuity of the Earth. Lateral variations in Moho geometry, especially Moho offsets, are important to understand the origin and evolution of the crust, and have been the target of many regional and global studies. However, previous methods offer only either spot illumination of the crust or a smoothed Moho topography. Here we propose a scattering-based method to systematically map Moho offsets. Taking advantage of the dense data coverage from the USArray, this work will provide a complete map of Moho offsets over the contiguous United States. The results will shed light on the tectonic evolution of the North American Continent and provide information on the material properties of the lower crust. This project will also support the training of a postdoctoral scholar.The proposed new method relies on surface waves scattered from incident teleseismic body waves. When teleseismic body waves hit a Moho offset, the latter serves as a strong secondary source. As a result, the surface waves are radiated horizontally away from the Moho offset. By back propagating the scattered surface waves with a proper velocity model, the Moho offset can be detected and located. The result of this work can potentially resolve the debate of whether Moho offsets are widespread or not. Regional differences (or not) in the distribution of Moho offsets will provide valuable information on the thick-skinned vs. thin-skinned deformation mechanisms and the effects of lower crust ductile flow in smoothing out Moho offsets. The new method will be applied to USArray data to map Moho topography beneath the United States.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.