山西断陷带地壳上地幔精细横波速度结构及径向各向异性研究

Shanxi Rift is an earthquake prone area located at the eastern part of Ordos block, which is the boundary and the geotecture decoupling region between eastern China and western China. Understanding the S-wave velocity structure and the radial anisotropy of the crust and uppermost mantle beneath Shanxi Rift is a crucial step in further unraveling the formation and evolution process of the rift as well as the genetic mechanism of earthquakes. Because the permanent station distribution is relatively sparse, additional geophysical data is required for obtaining high-resolution seismic wave velocity and anisotropy distributions of the crust and upper mantle beneath the rift. To address this issue, we propose to integrate and analyze the seismic data recorded by the temporary seismic array, China Earthquake Networks and the regional seismic network. First, with strict data quality control, I will apply joint ambient noise and teleseismic tomography to calculate the Rayleigh and the Love wave phase velocity distributions, respectively; secondly, I will extract the Rayleigh wave horizontal-to-vertical ratio (H/V) which can be highly sensitive to the S-wave velocity of the upper crust and the underlying sediments; finally, I will perform joint inversion of the surface waves phase velocity dispersion curves and the H/V with a Bayesian Monte-Carlo approach to obtain the high-resolution S-wave velocity structure and radial anisotropy of the crust and upper mantle. By focusing on S-wave velocity and radial anisotropy distributions of sediments and Datong Volcanoes, the structural characteristics of the fault depressions and the state of stress should be determined. This is expected to provide key information on the explanation of the formation and the evolution process of rifted basins as well as the magmatism in the crust beneath Datong Volcanoes. As a result, the geodynamic mechanism of formation and evolution of continental rift will be better understood.

山西断陷带是我国东、西部大地构造的分界带和解耦带，也是大地震频次较高的裂谷带。深入研究断陷带地壳上地幔速度结构及各向异性对理解裂谷带形成、演化过程及发震机制具有重要意义。由于固定台站分布较为稀疏，该断陷带地壳上地幔速度和各向异性分布的精细特征还需要新的地球物理证据和制约。本项目利用流动地震台阵和中国地震台网及区域台网数据，基于严格数据质量控制，采用背景噪声面波成像结合地震面波层析成像技术，计算瑞雷波和勒夫波相速度分布；测量对上地壳尤其沉积层横波速度非常敏感的瑞雷波水平垂直振幅比（H/V）；采用贝叶斯-蒙特卡洛方法联合反演面波相速度及H/V，获取地壳上地幔精细横波速度结构及径向各向异性。重点研究沉积层及大同火山群中横波速度和径向各向异性分布特征，以确定区内断陷特征及应力状态，加深对断陷盆地形成与演化、火山群壳内岩浆作用的认识。预期成果对深入认识大陆裂谷形成与演化的动力学机制具有重要科学意义。

山西断陷带是我国东、西部大地构造的分界带和解耦带，也是大地震频次较高的裂谷带。深入研究断陷带的地壳上地幔速度结构及各向异性对理解裂谷带形成、演化过程及发震机制具有重要意义。由于固定台站分布较为稀疏，该断陷带地壳上地幔速度和各向异性分布的精细特征还需要新的地球物理证据和制约。本项目利用流动地震台阵和中国地震台网及区域台网数据，基于严格数据质量控制，采用背景噪声面波成像结合地震面波层析成像技术，计算了瑞雷波和勒夫波相速度分布；测量了对上地壳尤其沉积层横波速度非常敏感的瑞雷波水平垂直振幅比（H/V）；采用贝叶斯-蒙特卡洛方法联合反演面波相速度及H/V，获取了研究区地壳上地幔精细横波速度结构及径向各向异性。.本项目的主要研究成果如下。（1）引入H/V与面波频散曲线进行联合反演显著改善了上地壳尤其沉积层的横波速度和径向各向异性模型。（2）新模型揭示了大同盆地是山西断陷带内沉积层最厚的断陷盆地之一，表明大同盆地具有更大的沉积速率。大同盆地高的沉积速率与被动拉张和大同火山群下方的软流圈上涌有关。（3）在大同火山群的中下地壳观察到了明显的低速异常，将其解释为少量的部分熔融。大同火山群中下地壳的低速异常对应于高温度异常，认为其抬升了大同火山群的脆-韧性转换带的深度，导致大同火山群的更浅的发震深度。（4）山西断陷带的地壳整体呈现正的径向各向异性，对应于明显的拉张变形；而临汾盆地的地壳表现为弱甚至负的径向各向异性，表明临汾盆地主要受到了剪切变形。（5）太行山的地壳整体呈现明显正的径向各向异性，南吕梁山呈现弱的正径向各向异性而北吕梁山呈现明显负的径向各向异性，表明吕梁山的地壳存在显著的差异变形且遭受了比太行山的地壳更强烈的挤压变形。.