Accurately mapping the seismic structure of the deep crust of the continental United States

大陆地壳是固体地球的最外层，在了解地球系统方面起着至关重要的作用：它承载着关于地球自然资源的重要信息，例如关键矿物；其厚度控制着高程和景观演化的一级变化；它是浅层地壳和上覆地幔之间物质交换的通道，岩浆从这里升起；它的组成和温度具有大陆早期历史的特征，控制着地热的分布。最后，它的强度和动力学决定了地震等自然灾害的分布。然而，由于缺乏通过钻井直接获取的方法和间接采样的不足，准确推断其物理性质，特别是深层部分的物理性质一直具有挑战性。另一方面，利用穿过地壳深处的地震能量，可以为地壳深处提供系统的采样，并允许对其物理特性（如厚度）进行大陆尺度的测量。这项研究通过改进传统的地震方法解决了测量深部地壳性质的挑战，从而减少了对深部地壳条件的假设。此外，研究人员将结合新获得的测量数据，以提供有关地壳深层的进一步信息，这将有助于推断对成分和强度敏感的弹性特性。最后，这些新技术将应用于通过EarthScope USArray在整个美国大陆收集的数据，以阐明美国邻近的大陆尺度深部地壳结构。这项研究将资助一名研究生。研究机会也将向来自长岛和纽约地区社区学院的学生开放。额外的K-12外展将通过与nsf资助的EarthBUS项目合作进行。研究对象大陆地壳，特别是中下地壳，在地学研究中具有重要的作用：1)其下边界（Moho）控制着地形变化及其演化的一级，2)它是浅层壳幔物质交换的通道，是岩浆上升的通道；3)其组成具有大陆早期历史的特征；4)其温度控制着地温和地表热通量的分布。该项目旨在更好地绘制出美国大陆地下的地壳深层结构。深部地壳的弹性特性（如莫霍深度、与深度相关的地震速度和泊松比）表明了地壳的热学和组成特性，但往往难以准确测量。特别令人感兴趣的是深部地壳的泊松比，因为它表明石英含量的丰度，因此在确定地壳的化学成分和强度方面起着至关重要的作用。在该项目中，将分三个阶段进行研究：首先，将开发和测试一种结合两种传统地震成像技术优势的新方法，以全面解决速度-深度权衡问题；其次，将进一步纳入新的地震观测数据，即瑞利波局部放大，以提供泊松比的深度相关信息；最后，这些新技术将应用于EarthScope USArray的数据，以阐明美国大陆的大陆尺度深部地壳结构。用合成数据对新方法进行的初步测试显示出解决这些挑战的希望。该研究将解决由于不同地震观测值的权衡和缺乏数据敏感性而在测量深部地壳特性方面面临的挑战。这项工作将在大陆尺度上准确地绘制出邻近美国的深部地壳结构。这项研究工作的成果是建立一个新的美国大陆地壳和上地幔的三维模型，将加深对深部地质过程的认识；它还提供了泊松比如何随深度变化的见解，增加了一个重要的地震约束，可以潜在地解决大陆的化学成分和强度。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。

Continental crust, the outermost layer of the solid Earth, plays a vital role in understanding the Earth system: it bears important information about the Earth’s natural resources, such as critical minerals; its thickness controls the first order changes in elevation and landscape evolution; it serves as a pathway for material exchange between the shallow crust and overlying mantle, and where magma rises through; its composition and temperature bear the signature of the early history of continents and controls the distributions of the geotherms. Finally, its strength and dynamics dominate the distributions of natural hazards such as earthquakes. However, accurately inferring its physical properties, especially for its deep part, has been challenging due to the lack of direct access through drilling and insufficient indirect sampling. Using seismic energies that travel through the deep crust, on the other hand, can provide systematic sampling to the deep crust and allow a continental-scale measurement of its physical properties such as thickness. This research addresses challenges in measuring the deep crustal properties by improving traditional seismic methods so they rely less on assumptions about deep crustal conditions. Additionally, the researcher will incorporate newly obtained measurements to provide further information about the deeper part of the crust, which will help infer the elastic properties that are sensitive to the composition and strength. Finally, these novel techniques will be applied to data collected throughout the continental United States through the EarthScope USArray to illuminate the continental-scale deep crustal structure of the contiguous US. The research will support a graduate student. Research opportunities will also open to students from community colleges in the Long Island and NYC area. Additional K-12 outreach will be performed by collaborating with the NSF-funded EarthBUS project.The research target, the continental crust, especially the mid- and lower parts, plays a crucial role in Earth Sciences as 1) its lower boundary (Moho) controls the 1st order of topographic variation and its evolution, 2) it serves as a pathway for material exchange between the shallow crust and mantle, and where magma rises through; 3) its composition bears the signature of the early history of continents; and 4) its temperature controls the distributions of the geotherm and surface heat flux. The project aims to better map out the deep crustal structures beneath the continental US. Elastic properties of the deep crust (e.g., Moho depth, depth-dependent seismic velocity, and Poisson’s ratio) indicate the thermal and compositional properties but are often challenging to measure accurately. Of particular interest is the Poisson’s ratio of the deep crust, as it is indicative of the abundance of quartz content, and thus plays a crucial role in determining the chemical composition and strength of the crust. In this project, a phased, 3-stage research will be conducted: First, a novel method that combines the strengths of two traditional seismic imaging techniques to solve the velocity-depth trade-offs comprehensively will be developed and tested; Secondly, a new seismic observable, Rayleigh wave local amplification, will be further incorporated to provide depth-dependent information of the Poisson’s ratio; Finally, these novel techniques will be applied to data from the EarthScope USArray, to illuminate the continental-scale deep crustal structure of the continental US. Preliminary tests of the new methods with synthetic data present a promising sign of solving the challenges. The research will address challenges in measuring the deep crustal properties due to the trade-offs in different seismic observables and a lack of data sensitivity. This work will accurately map out the contiguous US's deep crustal structure at a continental scale. The product of the research work, a new three dimensional (3-D) model of the crust and uppermost mantle beneath the continental US, will deepen the understanding of deep geological processes; It also provides insights into how Poisson’s ratio varies with depth, adding an important seismic constraint that can potentially solve for the chemical composition and strength of the continents.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.