中南半岛北部三维地壳上地幔结构与中强地震孕震深度关系研究

Indochina peninsula is located to the south of Yunnan, China. Due to the subduction of Indian Plate to Eurasia Plate and the lateral extrusion of Tibetan Plateau, Indochina peninsula has complex tectonic activities and large lithospheric deformation. In northern Indochina peninsula, there are many large faults and moderate earthquakes with magnitude large than 5.0 frequently occur. The crust and upper mantle structure of Indochina peninsula can help us to understand many scientific problems, such as the uplifting mechanism of Tibetan Plateau, the rheology of continental lithosphere and the seismogenic background of strong earthquakes. Meanwhile, the focal depth is very important to the study of seismogenic mechanism, seismogenic structure and hazard assessment. However, due to the sparse seismic stations in the past, there were not too many studies about the crust and upper mantle structure of Indochina peninsula, and the error of the focal depth was large. In this study, we will utilize newly deployed seismic stations in Indochina peninsula and adjacent region. Firstly, we will measure Rayleigh wave phase velocity by jointly using event-based and ambient noise data, and invert for the 3D S-wave and azimuthal anisotropy structure of the crust and upper mantle in northern Indochina Peninsula. Then, we will measure the focal depths of moderate earthquakes by joint inversion of local 3-component seismic waveforms and regional Rayleigh wave amplitude spectra. Lastly, we will study the relationship between 3D crust and upper mantle structure and focal depths of moderate earthquakes.

中南半岛地处中国云南地区南侧，受印度板块俯冲和青藏高原物质侧向挤出作用，形成了复杂的构造运动和强烈的构造变形。在中南半岛北部，众多大断裂纵横交错，5级以上中强地震活动频发。中南半岛地区的地壳上地幔结构研究有助于深入认识青藏高原的隆升演化机制、大陆板块物质流变性质和强震孕育构造背景等科学问题。同时，准确的震源深度对于孕震机理、发震构造和灾害评估等研究具有重要意义。然而，过去由于地震台站的稀疏，研究区的地壳上地幔结构的研究相对较少，震源深度测定误差较大。本项目将综合利用中南半岛及邻区近年来明显增加的地震台站数据，提取背景噪声和天然地震Rayleigh波相速度，反演中南半岛北部的三维地壳上地幔S波速度与方位各向异性结构；采用基于近震三分量波形和区域尺度Rayleigh波振幅谱的反演方法测定中强地震震源深度；结合三维地壳上地幔结构模型和震源深度测定结果，探讨三维结构与中强地震孕震深度的关系。

中南半岛地区的地壳上地幔结构研究有助于深入认识青藏高原的隆升演化机制、大陆板块物质流变性质和强震孕育构造背景等科学问题。同时，准确的震源深度对于孕震机理、发震构造和灾害评估等研究具有重要意义。我们发展了一种模型分辨随空间横向变化的面波层析成像方法，通过背景噪声和天然地震面波频散联合成像，反演了中国南海及邻区的三维壳幔速度结构模型。在呵叻高原内发现一个明显的高速异常，深度范围为80-150km。这一高速异常表明呵叻高原岩石圈厚而稳定，在新生代遭受的破坏很有限。在此基础上，我们通过面波各向异性层析成像，反演了中南半岛北部地区的地壳各向异性结构模型。结果表明中南半岛北部地区各向异性特征存在明显的横向和纵向变化。在地壳浅部，各向异性快波方向与主要断层走向分布比较一致。我们反演了一些典型地震的震源机制和震源深度，结果表明中南半岛北部的地震以走滑地震为主，震源深度较浅，主要位于中上地壳。地震主要发生在地壳中低速区域或高低速交界的地区。呵叻高原地震内部的历史地震非常少，各向异性也比较弱，反映了呵叻高原比较稳定，可能阻挡了青藏高原物质向中南半岛南部挤出。