| 龙门山断裂带中段深地震宽角反射/折射剖面研究 |
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| 引用本文： | 李佳欣,洪启宇,郑需要.龙门山断裂带中段深地震宽角反射/折射剖面研究[J].地震学报,2017,39(2):188-206. |
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| 作者姓名： | 李佳欣 洪启宇 郑需要 |
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| 作者单位： | 中国北京 100081 中国地震局地球物理研究所 |
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| 基金项目： | 国家自然科学基金40974033国家自然科学基金 (41090292，40974033) 资助国家自然科学基金41090292 |
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| 摘 要： | 射线追踪方法是研究地震波在横向非均匀地壳介质中传播的重要方法.本文推导了理论走时对网格化节点速度的偏导数公式，提出了针对深地震宽角反射/折射剖面数据反演的联合迭代法，并使用该方法对横跨龙门山断裂带中段的一条深地震宽角反射/折射剖面进行了反演和解释.首先，对每一炮的观测数据进行一维反演，在此基础上插值出一个粗略的二维速度模型；然后，使用射线追踪方法计算理论走时，再根据理论走时与观测走时的拟合程度对二维模型进行调整，以获得更加接近实际的二维速度模型；最后，利用联合迭代法对观测走时进行反演，经反复迭代使所有接收点理论走时与实测走时的残差平方和最小，最终获取该剖面的二维地壳速度结构.反演结果表明:测线东段的沉积盖层明显厚于中段褶皱带和西部高原，中部褶皱带部分地区出现基岩裸露；构造转换带两侧的地层分界面近于水平层状分布，其西侧的中、下地壳内各存在一个层间速度间断面；构造转换带内存在薄厚不等的低速层，自西向东有增厚趋势.此外，龙门山断裂带的3条主断裂向下深切结晶基底，这是由于西部松潘—甘孜地块自西向东运动，受到刚性扬子地块的阻挡，沿铲式断裂向上爬升所致；而在断层上盘距地表约15 km深处出现的最大剪应力极值区，正是发生汶川MS8.0地震的震源位置.
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| 关 键 词： | 龙门山断裂带 深地震测深 联合迭代法 速度结构 走时反演 |
| 收稿时间： | 2016-05-31 |
Study on the deep seismic wide-angle reflection/refraction profile in the middle of Longmenshan fault zone |
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| Affiliation: | Institute of Geophysics, China Earthquake Administration, Beijing 100081, China |
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| Abstract: | Ray tracing is an important method to study seismic wave propagating in the lateral inhomgeneous media of the crust. Partial derivatives of theoretical trival time with respect to the velocity at the meshing point of the model are derived. Simultaneous iterative reconstruction tomography (SIRT for short) method is used to invert the data of a deep seismic wide-angle reflection/refraction profle across the middle of Longmenshan fault zone, which can cover the shortage of time consuming and non-quantitative error analysis for trial-and-error method. Observed data for each shot are firstly inverted to get a one-dimensional velocity model, and then a simple two-dimensional velocity model is obtained by interpolating method. The two-dimensional model is adjusted by fitting observed and theoretical travel times, which makes the new model more close to the real one. SIRT method is further used to travel time inversion, and thus reaches final two-dimensional crustal velocity structure. The inverted result shows that the deposit in the east part of the profile is thicker than that in the middle folded zone and the western plateau, and there is basement rock appearing in the part area of the middle folded zone. The interfaces of the strata in the two sides of the transitional zone are almost horizontal, and in the western side of the transitional zone, there is a discontinuity interface in both the middle and lower crust. In addition, there are also lower velocity layers with different thickness in the transitional zone, and their thickness shows an increase tendency from east to west. It should be mentioned, that the three main faults in Longmenshan fault zone cut deeply the bottom of basement, which was resulted from the upward climbing movement of Songpan-Garze block to the eastern direction along a shovel-like fault and prevented from Yangtze block. There is a maximum shear stress region in the footwall of the fault inside the upper crust at the depth of 15 km, which is just the source location of the MS8.0 Wenchuan earthquake. |
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