摘要
地震层析成像(Seismic Tomography)是从大量地震观测数据中获取研究区域三维速度结构影像的过程。地震层析成像技术是认识地球内部结构的最新、强有力的工具,它的出现及广泛应用已经并将继续对地球科学产生巨大影响。 首都圈位于燕山构造带与太行山构造带的交汇区,是我国经济、政治和文化的中心地带,也是地震发生频繁,受各类地质灾害严重威胁的地区之一。2001年10月,由107个数字化地震仪构成的首都圈数字遥测台网正式建成并投入运行,为及时、准确地监测首都圈及其附近地区的地震活动提供了全新的保障;同时也提供了一批高质量的透视首都圈地区地下结构的数据资料。 本文将目标定位在首都圈地区Pn和PmP波层析成像研究上。围绕2002年4月在首都圈地区进行的6次人工爆破实验,以最新建成的首都圈遥测台网数据为基础,收集并提取了包括天然地震、人工爆破事件在内的区域Pn到时信息,利用在上地幔顶部滑行的Pn波对首都圈及其邻近地区进行了上地幔顶部速度结构以及各向异性特征的成像研究;同时,还利用布设在首都圈地区三河-平谷一带的三维小台阵,对6次爆破实验中记录到的莫霍面反射PmP波进行了三维透射成像研究,获取了台阵下方地壳浅层不同深度上的P波速度变化图像。 文中对地震层析成像技术的发展历史及地震层析成像研究中几个关键问题进行了论述,并详细分析了上地幔顶部折射波成像技术的理论、方法及其发展过程,反演过程中的正则化、预条件、光滑处理等具体过程以及正演模型的建立、简化和可能引入的误差等。在此基础上获取并整理了包括事件波形数据、震相报告等不同的原始资料,用震中距1.8°-9°之间的3200多条初至Pn走时反演了首都圈地区上地幔顶部P波速度变化图像。还具体讨论了速度横向变化和速度各向异性这一对相互耦合的影响Pn波走时的因素在反演过程中的折衷处理,获取了研究区域P波速度横向变化和各向异性分布特征等。为讨论反演结果在何种程度上再现了首都圈地区上地幔顶部速度结构,采用不同网格尺度的检测板试验对反演结果的空间分辨率进行了分析,并用统计学上的“bootstrap”方法讨论了反演结果的误差。 Pn速度是推断岩石圈地质构造历史、热历史的重要参数之一。尽管Pn波速的横向变化与上地幔温度、物质组成差异,以及压力效应等都有关系,但温度差异通常是起主导作用的因素。我们对比分析了首都圈及其邻近地区岩石圈热流分布与上地幔顶部P波速度变化图像,探讨了大地热流分布与Pn速度分布之间的相互对应关系,对首都圈地区较高的地热值及地热的不均匀分布有了更深的认识。中国地震局地球物理所博士学位论文 首都圈地区是我国进行人工地震测深、深地震测深和宽角反射探测数目最多,获取波形资料最丰富的地区之一。但多数观测都只是在沿剖面的二维空间中进行,对复杂地区地壳结构的空间分辨率不够,从而难以准确提取出有关的地壳动力学信息。利用三维小台阵的布设来研究区域地壳分层构造和速度分布特征是一种有效的解决办法。文中采用基于贝叶斯信息论的“不分块”反演方法,对三维台阵记录到的首都圈地区6次爆破PmP到时共 321个数据进行了反演,获取了90 x 140km2台阵下方浅层地壳三维速度结构,并给出了反演结果的空间分辨核。最后我们结合该地区地质构造情况,对反演结果中表现出来的P波速度不均匀性进行了讨论,并揭示出该地区历史大震及近代仪器记录到的中小震的空间分布与速度异常之间的关系。 地震波是认识地球内部最有利的工具。地震层析成像利用地震波的到时等信息建立起地球内部三维速度结构图像,从中可以提取出有关地球内部温度、压力、介质密度、物质组成等信息,这对理解地震发生机制、地震时震源区附近的变化等都有重要意义。我们对首都圈及其邻近地区上地慢顶部以及台阵下方进行了较高精度的成像研究,对于了解和监测首都圈地区强烈的地震活动,探索大陆强震的发生机理等都会有所帮助。
Seismic tomography is a technique of imaging three-dimensional seismic velocity structure from seismic observations that provide direct coverage of the region of interest with multiple crossing propagation paths. Being the latest and most powerful tool to explore the earth's interior, the appearance and widely application of the technique of Seismic tomography has already and will have far-reaching and profound impacts on earth sciences.The Capital Circle Area (CCA)is Located between Yanshan Mountain and Taihang-shan Mountain. Despite its important role in China's political, economic and cultural development, due to the complex tectonics interaction, it's also one of the regions most severely affected by kinds of geologic hazard and risk, e.g. doubles of earthquakes with magnitude greater than 7 occurred here in the latest 300 years. In Oct. 2001, the modern Capital Circle Digital Network was set up and operated routinely, and abundant seismic data with high quality have been accumulated gradually. The development of observatory network provide a good way to peer into the structure and dynamic process under the earth, and also a useful tool to detect the seismic activity, especially the moderate and small earthquakes happened in this region.Focused on the the investigation of detailed velocity structure under the CCA, an experiment of six man-made explosions was made in the region of CCA during Apr. 2002. The travel times of Pn, which travels directly in the uppermost layer of mantle are used to invert the lateral variation and anisotropy of Pn velocity of the uppermost mantle; and also the reflect wave PmP collected from the six explosions are investigated to map the 3D velocity variation image at different depths in the upper crust.A general review of the developments and findings of seismic tomography and in particular the critical issues in travel-time tomography was given, and the theory and method of refraction tomography at uppermost mantle was made, with detailed studies of the uniqueness, regularization, preconditioned, and smoothing process, as well as analyses of the construction of the forward model and errors introduced. Different kinds of seismic data, like the original seismic wave records and earthquake phases report were collected and dealed with, and more than 3200 first arrival Pn rays between 1.8-9.0?were inverted to map the velocity lateral variation. After a discussion of the trade-off in structure betweenthe velocity image and the anisotropy image, the major patterns of the velocity anisotropy was found. Synthetic checkerboard tests with different grid sizes were investigated and by using the statistical bootstrap method, parameter errors were estimated as the standard error of the parameters.Pn velocity is one of the fundamental parameters that are used to infer the thermal and tectonic history of the lithosphere. Lateral variations in Pn velocity have been associated with variations in upper mantle temperature, compositional differences, and pressure effects, while generally, temperature differences have the dominant effect. Compared with the thermal structure of the lithosphere of North China and Pn velocity structure in CCA, the relationship between the distribution of heat flow and Pn velocity variation was discussed, leading to a deep understanding to the high heat flow and spatial distribution of heat flow in the region of CCA.An interesting inverse method called " inversion without blocks", which is based on the Bayesian Theory was applied to get the 3D velocity disturbance image in the upper crust just beneath the 90 x 140km2 array and the resolution kernal was also calculated, in which totally 321 PmP travel time data of the six explosions recorded by the regional 3D array were involved. Finally considering the geologic tectonic in this region, the inhomogeneous distribution of P velocity and the active tectonics was discussed and also the link between spatial distribution of historic great earthquakes and many modern events with moderate and small sizes and P