Collaborative Research: Seismic Investigation of the Sub-ice Environment and Crustal Composition of Antarctica

南极冰盖下的地质为其移动的冰川系统提供了一个关键条件：一个脆弱的、多孔的、可能含水的沉积层提供了一个光滑的床，而接触处坚硬的基底岩石可能会减少冰的流动。然而，了解地质情况是极其困难的，因为这块大陆地处偏远，冰盖又很厚。利用部署在冰盖顶部的地震台站提供了另一种透过冰层观察大陆的工具，因为它们记录的地震信号包含了关于冰盖底部和下面地壳地质的敏感信息。例如，厚而多孔的沉积层会使地震波传播得更慢。在这个项目中，研究人员将1)利用过去18年来在南极洲200多个地点收集的地震数据，2)执行一套现代地震数据分析工具来产生各种地震信号，3)将它们转换成关于下地壳物理性质（如地震波速度）和化学成分（如二氧化硅含量）的地图集。这些地震速度图通过不确定度测量量化了精度，提供了对亚冰沉积层分布的一级评估。其他信息，如其他地壳性质和推断出的这些地点下面的化学成分，将进一步阐明被厚冰盖掩埋的大陆的地质演化。值得注意的是，确定地壳的二氧化硅含量将有助于更好地估计冰下地壳的热剖面。第二部分：技术描述从冰冻圈和岩石圈的角度来看，南极冰盖下地壳具有很强的重要性：在局部和区域尺度上，南极冰盖底部的浅层物理性质为快速移动的冰盖动力学设定了重要的边界条件；在更大的尺度上，地壳的结构（厚度和泊松比）在整个大陆上的变化保留了过去地质演化的特征，其化学成分对地热结构有重要贡献。然而，由于大陆的偏远和厚厚的冰层覆盖，这些特性尚未得到系统的研究。在这项拟议的研究中，研究人员将分析来自200多个永久和便携式地震台站的18年地震记录，以限制冰下地震结构，地壳结构，并利用多种地震技术对体地壳的化学成分产生影响。他们的分析将包括：1)系统地测量来自环境噪声和地震的宽带瑞利波水平-垂直比（H/V）；2)高频接收函数波形分析和两层h-k叠加，提供泊松比和亚冰不连续面深度信息；3) Love波频散测量及径向各向异性反演；4)环境噪声和地震作用下瑞利波方位各向异性的测量；5)联合解释亚冰各向同性结构的瑞利波频散、H/V比和接收函数波形；6)所有地震各向异性测量的蒙特卡罗横向各向同性（TTI）模型。这些分析将被结合和综合，以产生一套地震成果，包括确定冰下沉积物的厚度图与层次分辨率，更准确的地壳厚度和泊松比的确定，以及地壳成分的量化。拟议的工作将满足两个主要的科学目标。首先，它将提供有关南极冰盖底部接触和地震台站所在地沉积物厚度的详细信息，改善冰盖模拟的边界条件，并确定南极洲主要沉积盆地的位置。其次，对南极地壳厚度和组成的限制将提高对该大陆地质历史的理解。值得注意的是，地壳结构和组成将进一步促进未来对形成南极洲的各种构造过程的地质调查，如裂谷、岩石圈移动、造山运动和造山运动。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。

Part I: NontechnicalThe geology beneath the Antarctic Ice Sheet sets up a key condition to its moving glacial systems: a weak, porous, and possibly water bearing sedimentary layer provides a slippery bed, whereas hard basement rock at the contact may reduce the ice flow. Knowing the geology, however, has been extremely difficult since the continent is remote and the ice cover is thick. Using seismic stations deployed on top of the ice sheet provides an alternative tool to see the continent through the ice, as seismic signal they record contains sensitive information about the base of the ice-sheet and the geology of the crust below. For example, a thick and porous sedimentary layer will cause the seismic waves to travel more slowly. In this project, the investigators will 1) utilize the seismic data collected in the past 18 years at more than 200 sites in Antarctica, 2) perform a suite of modern seismic data analysis tools to produce a variety of seismic signals, and 3) translate them into sets of maps about the physical properties (such as the speed of the seismic wave) and chemical composition (such as silica content) of the underlying crust. These seismic speed maps, with accuracy quantified by uncertainty measurements, provide a first-order assessment to the distribution of the sub-ice sedimentary layer. Additional information such as other crustal properties and inferred chemical composition beneath these sites will further shed light on the geological evolution of the continent buried by the thick ice sheet. Notably, determining the silica content of the crust will help produce better estimates of the thermal profiles of the sub-ice crust. Part II: Technical DescriptionThe underlying crust of Antarctica Ice Sheet holds strong importance from perspectives of both cryosphere and lithosphere: at local and regional scale, the shallow physical properties at the base of the Antarctic Ice Sheet sets an important boundary condition for the dynamics of the fast moving ice sheet; at a greater scale, the architecture (thickness and Poissons ratio) of the crust that varies across the continent retains the signature of the past geological evolution and its chemical composition contributes significantly to the geothermal structure. However, these properties have not been systematically studied due to both the remoteness of the continent and its thick ice-coverage. In this proposed study, the investigators will analyze 18 years of seismic records from more than 200 permanent and portable seismic stations, to constrain the sub-ice seismic structure, crustal architecture, and draw implications on the chemical composition of the bulk crust using multiple seismic techniques. Their analysis will include: 1) systematic measurement of the broad-band Rayleigh wave horizontal-vertical ratios (H/V) from both ambient noise and earthquakes; 2) analysis of high frequency receiver function waveforms and 2-layer h-k stacking to provide information on Poissons ratio and the depth of sub-ice discontinuities; 3) measurement of Love wave dispersion and inversion for radial anisotropy; 4) measurement of Rayleigh wave azimuthal anisotropy from both ambient noise and earthquakes; 5) Joint interpretation of Rayleigh wave dispersion, H/V ratios, and receiver function waveforms for detailed sub-ice isotropic structure; 6) a Monte Carlo transversely isotropic (TTI) modeling of all seismic anisotropic measurements. These analyses will be combined and synthesized to produce a set of seismic deliverables including the determination of a map of thickness of the sub-ice sediments with hierarchical-resolution, more accurate crustal thickness and Poissons ratio determination, and quantification of crustal composition. The proposed work will meet two major scientific objectives. First, it will provide detailed information about the bottom contact of the Antarctic ice-sheet and sediment thickness at localities of seismic stations, improving the boundary conditions for the ice-sheet modeling and determining the locations of major sedimentary basins within Antarctica. Second, constraints on the thickness and composition of the Antarctic crust will improve the understanding of the geology history of the continent. Notably, the crustal architecture and composition will further facilitate future geological investigation to various tectonic processes that have shaped Antarctica, such as rifting, lithospheric removal, orogeny, and mountain building.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.