龙门山断裂带深部构造及盆山耦合模式研究

We carried out the three-dimentional (3-D) seismic imaging in the crust and upper mantle beneath the Southeastern Tibetan Plateau (STP) with the supporting of the Natural Science Foundation of China (e.g., NSFC 40872148). We obtained the primarily results of the deep velocity structures in the STP. For instance, we find that anomalously low velocity with low resistivity relative to the Sichuan foreland basin is in sharp contrast to high-velocity anomalies and high resistivity in the Songpan-Ganze block in the upper crust. Our also reveal two crustal bodies with anomalously low velocity and high conductivity underneath the fault zone, which is separated into three contrasting segments by the bodies. However, some important issues are still unclear and were not resolved in our previous study, for example, what the relationship between the Sichuan basin and the Songpang Ganze block colliding? Dose the deep structure controlling the mountain building on the surface across the Longmen-shan fault zone? In this proposal, we attempt to answer these issues basing on the following new studies: 1) To develop a new tomographic methodology of joint nonlinear inversion for the velocity and Poisson's ratio using a large number of arrival times from the same source-receiver pairs. The new inversion scheme was developed to obtain simultaneously earthquake locations and 3-D images of P-wave velocity (Vp) and P- to S-wave velocity ratio (Vp/Vs) or Poisson's ratio. The Vp/Vs ratio (or Poisson's ratio) is an important elastic parameter sensitive to the thermal state of the Earth. The method takes Vp/Vs anomaly directly as a model parameter and thus provides a more robust estimate of Vp/Vs anomaly than the conventional method of dividing the Vp estimate by Vs estimate. The method also takes into account of raypath difference between P- and S-waves and hence yields a less biased Vp/Vs estimate than the method of inverting S-P data for Vp and Vp/Vs anomalies under the assumption of identical P and S ray-paths. 2) To image 3-D high-resoultion multi-parameter structures (such as seismic velocity, Vp/Vs or Poisson's ratio, crack density, Yang's velcoity and so on) in the crust and upper mantle. The mutli-parameter calculation has already been applied in the Southwest Japan for understanding the no-volcano tremors (Wang et al., EPSL, 2006), but the Vp/Vs was inverted by using the conventional tomographic method; 3) To discuss and investigate the connection of mountain-basin coupling and its deep geodynamics across the Longmen-shan fault zone basing on the previous studies of jointed seismic tomography, magnetotelluric and geophysical modeling, which plays a key role for a better understanding of the crustal deformation and seismotectonics along the Longmen-Shan deep suture zone.

在国家自然科学基金（40872148）等项目的资助下，申请人对青藏高原东南部开展了初步研究，取得了一些探索性的研究成果（如 Wang et al., EPSL, 2009; Wang et al., JGR, 2010）。本研究首先开展速度和泊松比结构非线性联合反演新方法研究，从而获得龙门山及其周边区域地壳及岩石圈多参数三维精细结构。然后在此基础上，探索和研究龙门山逆冲断裂带的地壳形变和龙门山-四川盆地的盆山接触关系，建立龙门山断裂带深部地球动力学模型。本项研究的主要目标是采用新的层析成像方法技术，获得龙门山地区的地壳和上地幔的速度、泊松比、速度梯度、岩石密度和杨氏声速的三维精细结构。最后，结合地质、大地电磁和重力学的研究成果，开展综合分析与研究，建立龙门山逆冲断裂带两侧的地壳-地幔的地球物理模型，为探索和研究龙门山断裂带的碰撞造山和盆山耦合模式提供深部地球物理证据。

本研究首先开展速度和泊松比结构非线性联合反演新方法研究，该方法不同于以往联合反演方法，它能直接反演获得地层一致性较好的地震波速度和泊松比。从而为反演获得龙门山及其周边区域地壳及岩石圈多参数三维精细结构提供了方法技术支撑。在此基础上，通过反演大量的纵、横波地震数据，获得了沿龙门山断裂带及周边区域的深部三维精细结构，结合前人二维大地电磁探测研究成果，揭示了2008年汶川地震发生在一高速度、高泊松比和低电导率的区域，2013年庐山地震则位于高速度、低泊松比和低电导率的发震层。在上地壳中，四川前陆盆地的低速、低泊松比和低阻异常与松潘-甘孜地块的高速、高泊松比和高阻异常形成了鲜明的对比。在龙门山断裂带下方的两个低速和低阻块体，将龙门山断裂带分成南、中和北三段。我们的研究表明，这两个异常体可能与来自松潘甘孜地块的下地壳和（或）上地幔的局部熔融或流体侵入到龙门山断裂带的脆弱区有关；2）通过反演大量的纵、横波地震数据组成的综合数据集，获得了南北地震带地壳的多参数三维精细结构，探讨和分析了南北地震带的高地震活动性和强震频发的原因。通过对1976年松潘-平武地震（M7.2）、1955年炉霍地震（M7.4）、1973年康定地震（M7.1）、2008年汶川地震（M8.0）和2013年庐山地震（M7.0）的深部结构及产生机制研究，我们发现在震源区的周边区域兼有高速、高泊松比异常，低速、高泊松比异常以及高速、低泊松比异常，可能分别与含流体的岩石、沿断裂带发育的变质岩以及坚硬的克拉通块体对应。流体的侵入不仅能够改变断层面上的应力情况，还能降低岩石骨架的岩石力学强度，进而触发地震。在此研究的基础上，我们结合深部结构成像、地表应力分布、地表地质、大地电磁和重力学的研究成果，开展综合分析与研究，建立龙门山逆冲断裂带两侧的地壳-地幔的地球物理模型，探索和揭示了龙门山断裂带的碰撞造山和盆山耦合模式的样式。