Crustal and Uppermost Mantle Anisotropy Across Tibet and East China

在美国（例如，USArray）和中国（例如，中国地震阵列，中国阵列）同时部署的大型大陆地震仪阵列的安装推动了地震表面波层析成像新方法的发展，这些方法以前所未有的分辨率和可靠性提供了有关地震各向异性的定性新类型信息。地震各向异性是地震波速度的方向依赖性，它提供了有关组成地球的矿物的组成和方向（结构）的信息。新分析方法的初步应用揭示了西藏地壳各向异性的分层，其中上下地壳各向异性结构的取向和倾角存在较大差异。这导致了一种假设，即这些观察到的特征是由地壳较浅和较深的不同变形模式造成的：上部地壳的脆性变形和中下部地壳可能部分熔融的韧性流动。新的地壳各向异性地震估计与改进的地壳矿物组合各向异性实验室测量同时进行。地壳各向异性的地震学和岩石学之间日益增长的协同作用使人想起早期对地幔各向异性的研究。从这种协同作用中，一套统一的跨学科研究目标正在出现，其中两个从根本上激励了这项工作。首先是绘制大陆地壳的各向异性，以阐明正在发生的和化石的地壳变形。二是通过研究从大地测量数据推断的持续地表变形与用应变解释的地壳和地幔各向异性之间的关系，阐明活动变形的垂直相干性。这项工作是基于环境噪声和地震数据，这些数据提供了有关瑞利波和洛夫波群和相速度的各向同性和方位各向异性信息，现在可以通过贝叶斯蒙特卡罗公式，对倾斜六边形对称介质的深度相关地壳弹性张量进行反演。建议的工作进行如下：(1)增加新的测量方法，以提高估算地壳深部和上地幔各向异性的能力；(2)扩大研究的空间范围，以便在地质上不同的区域（包括西藏东部和西部及其周边地区）之间进行比较；(3)引入新的数据类型（特别是接收函数），以帮助解决当前方法中的歧义。(4)进行数值模拟，验证现有正演计算方法，探索面波振幅测量的各向异性特征。这项工作的一个关键组成部分是利用蒙特卡罗和相关贝叶斯统计的不确定性估计，这些不确定性来自于可以与所得到的地震模型相关联的初步测量。

The installation of large simultaneously deployed on-continent arrays of seismometers in the US (e.g., USArray) and China (e.g., Chinese Earthquake Array, China Array) has motivated the development of new methods of seismic surface wave tomography that provide qualitatively new types of information about seismic anisotropy at unprecedented resolution and reliability. Seismic anisotropy is the directional dependence of seismic wave speeds, which provides information about the composition and orientation (fabric) of the minerals that compose the Earth. Preliminary applications of the new analysis methods reveal the stratification of anisotropy in the Tibetan crust in which the orientation and inclination of the anisotropic fabrics of the upper and lower crust differ substantially. This has led to the hypothesis that these observed features result from different modes of deformation at work at shallower and deeper depths in the crust: brittle deformation in the upper crust and ductile flow with possible partial melt in the middle to lower crust. The new seismic estimates of crustal anisotropy are occurring in parallel with improved laboratory measurements of the anisotropy of crustal mineral assemblages. This growing synergy between the seismology and the petrology of crustal anisotropy is reminiscent of earlier studies of mantle anisotropy. From this synergy a set of unifying cross-disciplinary research objectives is emerging, two of which fundamentally motivate this work. The first is to map anisotropy in the continental crust to illuminate on-going and fossil crustal deformation. The second is to illuminate the vertical coherence of active deformation by investigating the relationship between ongoing surface deformation, deduced from geodetic data, with crustal and mantle anisotropy interpreted in terms of strain. The work is based on ambient noise and earthquake data which provide both isotropic and azimuthally anisotropic information about Rayleigh and Love wave group and phase speeds that now can be inverted, via a Bayesian Monte Carlo formalism, for the depth-dependent crustal elastic tensor for a tilted hexagonally symmetric medium. The proposed work proceeds by: (1) adding new measurements to improve the ability to estimate anisotropy in the deep crust and uppermost mantle, (2) extending the spatial extent of the study to allow comparisons to be made between geologically distinct regions (including East and West Tibet and its surrounding regions), (3) introducing new types of data (notably receiver functions) to help to resolve ambiguities in the current method, and (4) performing numerical simulations to test the current forward computation method and explore the signature of anisotropy in surface wave amplitude measurements. A key component of this work is the estimation of uncertainties using Monte Carlo and related Bayesian statistics from primary measurements that can be associated with the resulting seismological models.