青藏高原东南部区域下地壳流以及上地幔形变的地震学研究

The large-scale surface deformation, uplifting and faulting occurring at the SE margin of the Tibetan plateau are generally believed to be caused by the continuous collision between the India and Eurasia since approximately 50 millions years ago. However, the style and amount of the subsurface deformation induced by the collision, especially those inside the lower crust and upper mantle, are still debated, and are one of the fundamental problems in solid earth science. In this proposal, we plan to take advantage of the high quality waveform data recorded by the regional seismic networks of the China Earthquake Administration. We will apply our recently developed receiver function analysis techniques and other available seismic imaging techniques to the data to develop a seismic image of the mantle beneath Southeast China from the crust to depths on the order of 660 km. The seismic image of the crust and mantle will include a three-dimensional P and S velocity model with scale lengths of 30 km or less, and a map of crustal and upper mantle anisotropy. The seismic results will be used to answer a number of questions including:.1).Is there a large-scale lower crustal flow beneath the SE margin of the Tibetan plateau?.2).Is the deformation within the crust and upper mantle coherent? Is the crustal and mantle motion coupled or decoupled..3).What is the major process responsible for the large-scale surface deformation and uplifting? Is it due to a channelized lower crustal flow, or caused by a whole-crust shortening?.4).Has the entire lithosphere beneath the margin been thickened? If not, is part of the lithosphere delaminated? Is the upper mantle deformation beneath the area predominantly vertical and associated with the delamination

青藏高原东南部边缘大规模的地表隆起、形变和断层运动一般认为与印度板块和欧亚板块碰撞有关。但目前对于这种大规模地表运动的具体成因以及由碰撞引起的地壳和岩石圈内的形变仍未有定论，是地震学和地球动力学研究领域中的热点问题之一。本项目将充分利用中国地震局在四川、云南、广西和贵州等区域的数字台网和其它台站所观测到的地震波形资料，利用最新的地震学分析技术，获取青藏高原东南边缘和周边区域的莫霍界面的深度，地壳内波速比，地壳和上地幔三维P波和S波速度结构，地壳和上地幔的各向异性分布等资料。利用这些资料，探讨以下重要的科学问题：1）下地壳流是否存在；2）地壳与上地幔形变是否一致？地壳和上地幔的运动是否是耦合或解耦；3）该地壳形变和地表隆起的主要机理是什么？是主要由下地壳流，还是由地壳整体短缩所引起的；4）是否存在岩石圈的增厚或岩石圈的拆沉？是否存在与拆沉相对应的地幔垂直对流？

本项目采用青藏高原东南部边缘地区既存地震记录，获取该地区的地壳及上地幔的速度和各向异性结构，从而分析研究该地区的地表隆起，地壳形变，断层运动的动力学机制。系统分析了国家地震局在青藏高原东南缘的 80多个固定台站的接受函数与SKS波形资料，获取了中下地壳及地幔的各向异性参数，发现该地区的中下地壳存在较强的各向异性，相比而言，上地幔的各向异性较弱。并且地壳与地幔的快波方向存在明显不同，表明地壳和上地幔的形变相对独立，存在解耦现象。同时，该结果与下地壳流相对应的隆起与形变机制比较吻合。利用有限频层析成像技术反演了该地区地壳和上地幔的三维P波和S波的速度构造。发现龙门山地区下地壳物质存在明显的向下挤压，与下地壳流模型比较吻合。.在研究过程中，开发了两项新的地震成像技术：1）为更有效地对下地壳及上地幔进行地震成像，开发了利用背景噪声的三维伴随层析成像方法与技术，利用该噪声伴随层析成像技术分析了宽频数据，发现在青藏高原东南部边缘地区的中下地壳中存在速度低于周边6-12%的强低速区，低速区的形状复杂，与地表地形存在较好的相关性，说明中下地壳流的形态比较复杂。2）在进行接收函数研究中发现盆地台站的远震宽频记录相对复杂，沉积层内的多次反射波比较发达，直接影响接收函数等成像技术的应用。开发了利用波场延拓和分解理论，滤去沉积层多次波的新方法与处理技术。