青藏高原东北缘岩石圈三维结构的重力与全波形反演研究

The distribution of low velocity layer within the crust in the NE margin of Tibetan Plateau and the deep contract relationship between Tibetan Plateau and Asian lithosphere are two important scientific questions. In this research, aiming at the above two questions, we conduct 3-D constrained gravity inversion and full-wave 3-D tomography respectively on high-resolution gravity data and broad-band seismic data (including national network and dense portable array from the ChinArray-II project) to obtain 3-D Vp, Vs and density structures of lithosphere in the NE margin of Tibetan Plateau. Firstly, we perform joint inversion of gravity, geoid and elevation data with the constraints of previous seismic results, and obtain the thicknesses of crust and lithosphere in the study area. Secondly, we construct the initial 3-D Vp, Vs and density structures of lithosphere based on the above gravity inversion and previous seismic results. And then using these initial models, we performed full-wave 3-D tomography based on the Scattering integral algorithm on the seismic data to obtain the 3-D Vp, Vs and density structures of lithosphere in the study area with considerable resolution and accuracy. Thirdly, with the constraints of the result of full-wave 3-D tomography, we operated 3-D constrained gravity inversion on the Bouguer gravity anomalies to improve the 3-D density structure of lithosphere in the study area. Based on the above results, we comprehensively analyze the distribution of low velocity layer within the crust of the study area, and discuss the deep contract relationship between Tibetan Plateau and Asian lithosphere. This research will provide important constraints for promoting the studies of tectonic deformation and dynamic process in the NE margin of Tibetan Plateau.

针对青藏高原东北缘壳内低速层分布、青藏高原与亚洲板块岩石圈深部接触关系的科学问题，本项目利用宽频地震固定台网和密集流动台阵观测数据及高分辨率重力异常数据，开展重力与全波形反演研究，获取东北缘岩石圈三维P波、S波速度和密度结构。首先，以前人地震成果为约束，应用重力、大地水准面和地形数据联合反演获取地壳厚度和岩石圈厚度；其次，综合上述重力界面反演结果和前人地震速度结果构建岩石圈三维速度与密度初始模型，进而应用基于散射积分法的全波形反演技术获取岩石圈三维P波、S波速度和密度结构，提高成像分辨率与精度；之后，以全波形反演的密度模型为约束，应用重力三维约束反演技术完善三维密度结构；最后，基于上述研究内容，追踪东北缘壳内低速层展布，探讨青藏高原与亚洲板块岩石圈深部接触关系，为促进青藏高原东北缘构造变形与动力学过程研究提供重要的深部制约。

本项目研发了重力资料处理新方法，并将这些新方法应用于青藏高原东北缘。同时，还开展了近震体波层析成像、背景噪声全波形成像和重力约束成像研究，获得了地壳厚度、地壳平均波速比、岩石圈底界面深度、地壳速度与密度结构，揭示了青藏高原东北缘深部结构特征与孕震环境。. 研究结果表明，青藏高原东北缘地壳变形不支持下地壳流模式。松潘-甘孜地块和西秦岭构造带均衡补偿不仅有地壳还有岩石圈地幔的贡献。研究区岩石圈底界面深度介于130~260km之间，亚洲板块未向南俯冲到青藏高原下方。松潘-甘孜地块和祁连造山带中下地壳（20~40km）表现为显著的P、S波低速和低密度异常，推测松潘-甘孜地块壳内低速、低密度异常可能与软流圈上涌导致的部分熔融有关，而祁连造山带的壳内低速、低密度体则可能与地壳增厚有关。孔兹岩带下方的S波高速异常从上地壳延伸至上地幔顶部，推测其与古元古代晚期构造热事件导致的地壳深熔作用及碰撞后伸展和抬升过程有关。青藏高原东北缘中强地震多发生在上地壳，推测震源区的壳内低速、低密度层可能处于部分熔融或易于蠕变的状态，脆性的上地壳更容易积累应变能，从而导致地震的发生。