Crustal and Uppermost Mantle Anisotropy Across Tibet and East China

在美国(例如，美国阵列)和中国(例如，中国地震阵列，中国阵列)同时部署在大陆上的大型地震仪阵列的安装推动了地震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关于地震各向异性的定性新型信息。地震各向异性是地震波速的方向依赖性，它提供了有关组成地球的矿物的组成和取向(组构)的信息。新的分析方法的初步应用揭示了西藏地壳的各向异性分层，其中上下地壳各向异性组构的取向和倾角有很大的不同。这导致了一种假设，即这些观察到的特征是地壳浅层和深层不同变形模式的结果：上地壳的脆性变形和中、下地壳可能部分熔化的韧性流动。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结果同时进行。地震学和地壳各向异性岩石学之间日益增长的协同作用，让人想起了早期对地幔各向异性的研究。从这种协同效应中，出现了一套统一的跨学科研究目标，其中两个目标从根本上推动了这项工作。首先是绘制大陆地壳的各向异性图，以阐明正在进行的地壳形变和古地壳形变。二是通过研究大地测量数据推导出的持续地表形变与用应变解释的地壳和地幔各向异性之间的关系，阐明活动形变的垂直一致性。这项工作是基于环境噪声和地震数据，这些数据提供了关于Rayleigh和Love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，对倾斜六角对称介质的随深度变化的地壳弹性张量进行反演。建议的工作包括：(1)增加新的测量，以提高估计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，以便能够在不同的地质区域(包括西藏东部和西部及其周围地区)之间进行比较；(3)引入新类型的数据(特别是接收函数)，以帮助解决当前方法中的模糊性；以及(4)进行数值模拟，以测试当前的正演计算方法，并探索面波幅度测量中各向异性的特征。这项工作的一个关键组成部分是使用蒙特卡洛方法和相关的贝叶斯统计方法估计可能与所得到的地震学模型相关联的初级测量的不确定性。

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.