用背景噪声、天然地震面波、XKS分裂联合反演云南地区上地幔方位各向异性

There are mainly three models that explain the uplifting and deformation mechanism of the Tibetan Plateau. And these three models are disagree on whether crust and mantle are coupled, and how much deformation occurs in the mantle lithosphere, especially in Yunnan. This research attempts to answer the above two questions through imaging the azimuthal anisotropy structure in the upper mantle, by joint inversion of ambient noise, teleseismic surface wave dispersions and Shear wave splitting. This research propose that to assess the extent to coupling of crust and mantle, we can check whether the velocity at the base of the lithosphere is the same with the GPS observations. The velocity at the base of the lithosphere can be constrained by the azimuthal anisotropy structure inside and at the bottom of the asthenosphere. This is due to the fact that differential motion between the base of the lithosphere and the underline convecting mantle produce the deformation inside the asthenosphere. After the assessment of the crust-mantle coupling, and if they are indeed coupled, we can compare the direction of the lithospheric azimuthal anisotropy and that could be produced by surface deformation, in order to evaluate the extent of deformation in the mantle lithosphere. The research on these two questions are helpful to understand the deformation mechanism of the Plateau, to provide scientific insight to the monitoring of the regional seismicity and assessment of seismic hazard, also to a better understanding of the evolution and dynamics of the continental collision.

解释青藏高原变形、隆升机制主要有三个代表性模型。这三个模型在壳幔是否耦合、地幔岩石圈的变形程度上存在分歧，特别是在云南地区。本研究拟通过求解云南下方地幔地震波方位各向异性的手段来试图对这两个问题进行回答。地幔各向异性的求解采用背景噪声、天然地震面波、XKS分裂联合反演的方法，以提高成像的精度。要分析壳幔是否耦合，本研究提出，可以从岩石圈底部的运动方向是否同地表GPS观测一致来判断。而岩石圈底部的运动速度可以由软流圈的各向异性来约束，因为正是岩石圈底部剪切和软流圈底部对流拖曳的差异导致软流圈内部的变形。如果壳幔耦合，可以再对比地震观测到的地幔岩石圈的各向异性方向、地表形变可能导致的各向异性方向，对地幔岩石圈的变形程度做出估计和推断。对这两个问题的探索，有助于厘清变形机制，为该区的地震监测和灾害预测提供科学判据，同时深化对陆－陆碰撞造山带的演化和动力学的认识。

解释青藏高原变形、隆升机制主要有三个代表性模型：岩石圈尺度的连续变形模型、刚性块体挤出模型、地壳流模型。这三个模型在壳幔是否耦合、地幔岩石圈的变形程度上存在分歧，特别是在云南纬度26度以南地区。本研究旨在根据地幔各向异性特征，对云南下方壳幔是否耦合作出判断，从而对物质逃逸的动力来源、变形机制有更好的认识和把握，为该区的地震监测和灾害预测提供科学判据；尝试从各向异性的角度对地幔岩石圈的变形程度作出估计，以深化对陆－陆碰撞造山带的演化和动力学的认识。本研究通过对云南地区地震台站近10年来的波形数据进行XKS分裂测量，得到快波偏振方向、延迟时间随后方位角的变化；采用两层模型对分裂参数随后方位角的变化进行拟合，得到云南下方地震波方位各向异性的分层情况；通过联合区域及全球地震面波频散数据进行有限频率反演，得到相速度方位各向异性。本研究中，26°N以南的研究区域内所得到的两层分裂参数，上层的快波偏振方向近南北向，与前人结果中地壳内的各向异性的方向较为一致；下层的快波偏振方向为近东西向，延迟时间要比上层大得多，因此认为下层的分裂参数指示上地幔的变形，地壳和上地幔之间的变形是解耦的，且地幔变形比地壳变形强烈。