南北地震带深部构造与强震活动

The North-South Seismic Zone of China has a complex tectonics and strong seismicity. A good image of the complex 3-D velocity heterogeneity and seismic anisotropy and deformation could be a critical step to understand the tectonic evolution of eastern Tibet and the cause of large earthquakes and the hazard they pose in the North-South Seismic Zone. In our study, we present well-resolved tomographic images of 3-D shear wave velocity and seismic anisotropy (azimuthal and radial anisotropy) in the crust and upper mantle beneath the North-South Seismic Zone obtained from ambient noise and two-plane-wave earthquake tomography with data from the permanent Chinese Seismograph Network and a much denser temporary array deployed by China Earthquake Administration. The azimuthal anisotropy from surface wave is quantitatively compared with that from shear wave splitting analysis of local and teleseismic earthquake through numerical simulation. The deformation of crust and upper mantle is obtained through the 3D distribution of azimuthal and radial anisotropy. Based on the anisotropic S wave velocity structure with high-resolution, seismic anisotropy and inferred deformation patterns, we hope to provide some constraints on the mechanism of earthquake occurrence.

南北地震带主要位于青藏高原东缘及周围地区，构造变形复杂，地震活动强烈。区域内深部结构及物质运移变形的研究对理解青藏高原东缘地区的深部动力环境和认识南北地震带强震危险性具有重要意义。本项目使用区域固定地震台网和中国地震科学台阵的密集流动观测台网的波形资料，采用背景噪音及远震面波双平面波层析成像获取Rayleigh和Love波相速度，并通过反演Rayleigh波相速度，获取S波方位各向异性；联合Rayleigh和Love波相速度进行各向异性反演，获取各向异性条件下的S波速度及径向各向异性。采用近场和远震剪切波分裂分析技术提取S波方位各向异性，通过数值模拟方法定量比较由体波和面波获取的方位各向异性结果。综合S波方位各向异性及径向各向异性结果，获取地壳及上地幔变形模型。依据高精度的速度结构、各向异性及深部变形模式分析结果，探讨南北地震带地区的强震发生的构造含义及深部物质运移对地震发生的影响，探索强震分布的规律性。

作为中国大陆东西部大地构造的主要分界，南北地震带构造作用强烈、强震活动频繁。了解南北地震带精细结构及变形，对于认识南北带强震构造成因具有重要意义。本项目利用中国地震科学探测在南北地震带架设的两期密集流动台阵记录的连续波形和远震面波数据，采用背景噪音和远震双平面波成像方法，得到Rayleigh和Love波6-100 s相速度和Rayleigh波方位各向异性。联合反演Rayleigh和Love波相速度，我们获得地下10-130 km范围内高精度三维S波速度和径向各向异性。.结果表明，南北地震带南段7级及以上地震一般发生于S波高速区或高低速过渡带。高速区有利于应变积累，高低速过渡带应变积累不均匀，外部应力作用时，均容易发生地震。南段地区地壳浅部和中下地壳变形不一致，以深大断裂为边界的块体旋转主导地壳浅部变形，应变主要集中于深大断裂；中下地壳则以地壳流模式主导变形。地壳变形模式随深度而变。地震监测需重点关注深大断裂。腾冲火山可能与东构造节下印度板片北东向俯冲撕裂引起的软流圈上涌有关。扬子克拉通东西部S波速度和地震各向异性（方位各向异性和径向各向异性）明显不同，表明可能受到的构造运动存在差异。西扬子克拉通保留了二叠纪地幔柱的构造和变形；东扬子克拉通则反映了现今海南地幔柱的影响。.南北地震带北段结果表明祁连造山带中下地壳存在低速层，地壳增厚可能通过中下地壳低黏性软弱加厚。中下地壳S至上地幔的低速揭示祁连造山带的高海拔由增厚的地壳和上地幔的热浮力共同引起。西秦岭造山带岩石圈的S波高速不支持其作为青藏高原物质逃逸的东向通道。