Crustal and Uppermost Mantle Anisotropy Across Tibet and East China

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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.