Collaborative Research: Mapping Crustal Tectonic Structure using Seismic Anisotropy

地震各向异性已被观察到与浅层地壳的排列裂缝有关，并被推断与地幔流导致的橄榄石晶格优先取向（LPO）有关。然而，尽管变质作用和变形是板块汇聚的基本表现形式，并有可能在区域尺度上普遍构造地壳并影响波的传播，但由于地壳物质的结构分层或固有叶理作用而引起的各向异性行为却没有得到很好的研究。区域面理和成分排列是由与构造过程相关的变形和变质作用产生的，并应导致地震方法可探测到的“各向异性地体”。因此，地壳各向异性的识别和制图可以作为构造变形存在和空间程度的代表。为了验证我们的假设，即区域变形和变质作用导致地壳各向异性，这种各向异性可以在记录的地震波场中观察到，并用作绘制地下岩石结构的代理，我们研究了位于巴基斯坦Nanga Parbat的三维区域片理反形构造。在我们的研究中，我们使用了由NSF-大陆动力学Nanga Parbat项目（NSF EAR-9418849）收集的现有数据集。我们已经确定了365次高质量的局部地震，这些地震是由56个临时短周期/宽带阵列记录的。当地地震/台站的组合提供了Nanga Parbat反形式的内部照明。使用这个数据集，我们的研究目标是：(1)量化地震数据中地震各向异性的观测结果；(2)开发系统分析地震波场中记录的各向异性特征的技术（解释层析成像）；(3)使用直接从Nanga Parbat岩石样品中测量的岩石物理性质来校准这些观测结果；(4)使用Christoffel各向异性理论和三维各向异性波传播代码计算的合成地震图来验证我们的分析。(5)开发一套工具，可用于帮助设计通过复杂三维结构测量各向异性波场的现场实验；(6)确定Vp、Vs和Vp/Vs比值，以更好地约束地壳变形的地球动力学建模。这些目标使我们能够理解地震各向异性与地壳构造变形之间的关系。

MAPPING CRUSTAL TECTONIC STRUCTURE USING SEISMIC ANISOTROPY Seismic anisotropy has been observed to correlate with aligned cracks in the shallow crust and inferred to be related to lattice-preferred orientation (LPO) of olivine due to mantle flow. However, anisotropic behavior due to structural layering or inherent foliation of crustal material is not as well studied even though metamorphism and deformation are fundamental expressions of plate convergence and have the potential to pervasively structure the crust and influence wave propagation at regional scales. Regional foliations and compositional alignments are produced by deformation and metamorphism associated with tectonic processes and should result in "anisotropic terranes" detectable by seismic methods. Thus the identification and mapping of anisotropy within the crust can serve as a proxy for the presence and spatial extent of tectonic deformation. We examine a three-dimensional exhumed regionally-foliated antiformal structure located at Nanga Parbat, Pakistan, to test our hypothesis that regional deformation and metamorphism result in crustal anisotropy and that this anisotropy can be observed in the recorded seismic wavefield and used as a proxy to map subsurface rock fabric. For our study we use an existing data set collected by the NSF-Continental Dynamics Nanga Parbat project (NSF EAR-9418849). We have identified 365 high-quality local earthquakes which were recorded within a temporary 56 short-period/broadband array. The combination of local earthquakes/stations provides internal illumination of the Nanga Parbat antiform. Using this data set, our research objectives are to: (1) quantify the observations of seismic anisotropy in the seismic data, (2) develop techniques to systematically analyze the anisotropic signatures recorded in the seismic wavefields (interpretive tomography), (3) calibrate these observations using petrophysical properties directly measured from Nanga Parbat rock samples, (4) use Christoffel anisotropy theory and synthetic seismograms computed by 3D anisotropy wave propagation code for verification of our analyses, (5) develop a set of tools that can be used to help design field experiments to measure the anisotropic wave field through complex 3-D structures, and (6) determine Vp, Vs, and Vp/Vs ratios to better constrain geodynamic modeling of crustal deformation. These objectives allow us to understand the relationship between seismic anisotropy and crustal tectonic deformation.