【作者】 张风雪；

【导师】 吴庆举；

【作者基本信息】 中国地震局地球物理研究所 ， 固体地球物理学， 2011， 博士

【摘要】 自层析成像被引入到研究地球内部结构以来,地震层析成像发展迅速,现已成为研究地球深部结构的有效方法之一。按层析成像的理论依据,可以把地震层析成像分为射线层析成像和有限频层析成像。本文旨在掌握一种有限频灵敏度算核的计算方法,进而构建有限频层析成像的反演计算,并用有限频层析成像的方法研究华北克拉通上地幔的速度结构及其动力学意义。本文的主要研究内容及结论如下：在射线层析成像的方面,本文改进了FMM (Fast Marching Method)射线追踪方法,这种方法可以得到整个空间的完整走时场,并且求得的射线路径是最速路径。本文在算法上对FMM方法求取射线路径的过程进行了改进,使其求解速度更快。有限频层析成像是随着宽频带数字地震仪的发展而兴起的一种新方法,该方法克服了射线理论中“无限高频”假设所带来的弊端,考虑更接近于真实情况的地震波频率衰减、波散射及波前复原等情况。本文使用改进的FMM射线追踪方法构建了走时表,并在此走时表的基础上采用查表算法计算一维速度模型下的灵敏度算核,结果表明：查表算法可以获得较高的计算效率,为下一步开展有限频层析成像提供了有力的支持。本文讲述了波形的多道互相关法(VanDecar和Crosson,1990)和自适应叠加法(Rawlinson和Kennett,2004)拾取相对走时残差的基本原理,实现了两种方法的图形界面操作,简化了操作过程,方便结果的对比,剔除差别较大的数据,从而提高了相对走时残差的可信度。在实际地震资料方面,本文从华北地震科学台阵中的190套宽频带地震仪在2006年10月～2009年3月期间所记录的震中距在30°～90°的地震事件中选取多频段的相对走时残差进行P波和S波的速度结构的成像反演,得到如下结论：(1)研究区域西部的低速异常延伸至约400km,此低速异常反映了深部存在的热物质,本文的成像结果发现此低速异常没有贯穿地幔转换带到达下地幔,所以本文认为此低速异常并不是来源下地幔,至于这个低速异常的形成原因不能单从速度结构一方面去推断,应结合其它参数(热值、密度、Q值、物质成份等)进行综合的分析。(2)区域地震层析成像研究发现太平洋俯冲板块前缘已经抵达约120°E处(Huang和Zhao,2006)。本文研究发现在地幔转换带中也有一个高速异常体,推断其为太平洋俯冲板块的前缘。(3)华北盆地为裂谷型盆地,其速度异常的分布较为复杂,在100km-250km深度的范围内存在岩石圈拆沉的高速异常残留体。(4)与古老的克拉通岩石圈厚度相比,华北地区的岩石圈正在减薄,华北平原地区表现为高速异常与低速异常交互分布的复杂现象,而燕山地区则为高速异常,表明华北平原地区发生了明显的侵蚀与破坏,燕山地区则没有发生侵蚀与破坏过程或此过程发生的现象不明显。更多还原

【Abstract】 Since tomography is introduced to the area of the internal earth structure study, Seismic tomography, with a noticeable progress, has become one of the most effective methods to explore the deep structure of the earth. Based on different theories seismic tomography can be sorted as ray theory tomography and finite-frequency tomography (FFT). The purpose of this thesis is to calculate the finite-frequency sensitive kernel, write the FFT codes and explore the upper mantle velocity structure of North China Craton (NCC) by FFT.The major studies and conclusions are as follow:We improved Fast Marching Method (FMM) ray tracing method. FMM can get all the traveltime in the study media space, and the ray path is the steepest path. We improved the FMM method in algorithm to make it more fast and efficient.FFT is a newly developing method in the age of broadband digital seismic observation. This method overcomes the abuses of the infinite frequency approximation, considers the actual cases such as wave frequency attenuation, wave scattering and wave front healing. We used the improved FMM method to construct a traveltime table, and based on this table we got a 1-D velocity model sensitive kernel by table look-up method. The results show that, table look-up method can enhance the computation speed, which give powerful support to FFT.In this thesis we depicted two methods to pick arrive time, multi-channel cross correlation (VanDecar & Crosson,1990) and adaptive stacking (Rawlinson & Kennett, 2004). The author simplified the operational procedures of picking arrive time by graphical interface. Compare the two results from these two method, we rejected those among which the difference is much big, which increased the credibility of relative traveltime residual.The following part of the thesis is an application on real seismic data. There were 190 broadband stations from North China Seismic Array, and all of the teleseismic occurred during Oct.2006 and Mar.2009, with the epicenter 30°～90°. We chose the relative traveltime residual from these data to invert the P- and S-wave velocity structure, the results reveal that: (1) An obvious low-velocity anomaly, extends to～400km, is located at the west region of study area. This low-velocity anomaly reflects the deep thermal material. Because this low-velocity anomaly doesn’t penetrate the mantle transition zone, we deduced this low-velocity anomaly is not come from lower mantle. We can’t conclude how this low-velocity formed just from only one aspect (velocity anomaly structure). We should consider the other elements such as thermal, density, quality factor Q, components.(2) The results from regional tomography show that the western edge of Pacific slab reaches to 120(?) longitude (Huang & Zhao,2006). In this thesis we also found a high-velocity anomaly in the mantle transition zone. This high-velocity anomaly is the front edge of Pacific slab.(3) The North China basin is a rift style basin. In the basin the velocity anomaly distribution is more complicated. At the depth range of 100km～250km, there is a high-velocity anomaly block. This is the residue existing in the lithosphere delamination.(4) Compare to the lithospheric thickness of old craton, the lithosphere of NCC is thinning. There is alternate distribution between low- and high-velocity anomalies. Beneath the Yanshan Uplift region, there is a high-velocity zone. These show that the North China basin has obviously experienced strong erosion and tectonic alteration, but the destruction region doesn’t include the Yanshan Uplift or the destruction of North China basin is more apparent than that of Yanshan Uplift.更多还原