青藏高原东南缘及邻区Lg波衰减成像及动力学启示

The southeastern margin of Tibet plateau is characterized by intense crustal deformation, faults developed and active strong earthquake activities, which is a critical area for studying the southeastward motion of crustal material. Several geodynamic evolution mechanisms have been proposed to explain the evolution characteristics in the southeastern margin of Tibet plateau, such as, rigid block extrusion, crustal flow and vertical continous deformation/crust-mantle decoupling. The crustal low-velocity zone or its distribution in the southeastern margin of Tibet plateau is crucial to distinguish the geodynamic evolution mechanisms, and understand the orogenic mechanisms and earthquake pregnancy. Lg wave attenuation model is an important parameter for indicating the presence of a partially melted crust/low-velocity layer or not, and its distribution. This proposal aims to study how the focal depth affects the measurement of Lg wave Q value, explore the characteristics of Lg wave group velocity dispersion, and utilize the Reverse Two-Station method to obtain the high-resolution Lg wave attenuation tomographic model in the southeastern margin of Tibet plateau by using the seismic data of China Earthquake Science Array with 350 sets of broadband seismic stations, and regional permanent stations including 105 sets of broadband seismic stations. By combining the information of seismic velocity, electrical conductivity, heat flow and petrological data, the seismic attenuation results are used to study the distribution of partially melted crust or low-velocity layer, and are expected to provide deep structure information for understanding the eastward movement of plateau materials and the evolution mechanism of Tibet Plateau.

青藏高原东南缘地壳变形强烈、断层发育、强震频发，是研究高原物质侧向运移的关键区域。目前对该区域的演化特征提出了侧向逃逸、地壳流、连贯变形与解耦等诸多的构造演化模型。该区域的地壳低速层分布特征对甄别上述模型、认识造山机制与地震孕育环境至关重要。Lg波的衰减模型是指示地壳介质低速层/部分熔融存在与否及分布范围的重要参数。本研究拟利用中国科学台阵在青藏高原东南缘布设的350套宽频带流动地震台站以及中国地震台网105个固定台记录的波形数据，开展Lg波群速度频散分析，研究震源深度对Lg波Q值测定的影响，采用反双台法获取研究区Lg波的衰减特征，结合地壳介质的地震波速度、电阻率、热状态及岩石学等信息，揭示研究区地壳低速层/部分熔融可能的分布范围，为探讨青藏高原物质东向运移和高原演化机制提供深部结构约束。

本项目按照研究计划围绕地震Lg波传播特征和青藏高原东南缘地区衰减成像开展了相关的工作。我们优化了Lg波衰减成像计算程序，解决了衰减算法软件的运行稳定性问题，提高了运行效率。利用青藏高原东南缘和中亚造山带地区的地震波形数据，开展了地震Lg波传播特征和地壳衰减Q值研究。结果显示震源深度对地震Lg波衰减成像的影响不大，不同震源深度获得的二维衰减Q值成像结果基本一致。青藏高原东南地区地壳Lg波Q值低约为200，表现为强衰减的特征。在小江断裂和川滇菱形块体北部地区呈现显著的低Q值强衰减特征，暗示着可能存在部分熔融。在中亚造山带中部地区Lg波强衰减可能与新生代的火山以及中新生代的裂谷盆地等相关。阿巴嘎火山西部660km间断面出现明显的下沉，可能是古太平洋俯冲板块的残留或造山带的岛弧拆沉有关，基本上排除了中亚造山带中部地区源于下地幔或地幔转换带的地幔热物质上涌的可能性。