【作者】 吴晶；

【导师】 陈运泰； 高原；

【作者基本信息】 中国地震局地球物理研究所 ， 固体地球物理， 2007， 博士

【摘要】 地震各向异性是地球的基本特征之一，而剪切波分裂则是分析地震各向异性的一个有效方法。剪切波分裂是指，当剪切波在各向异性介质中传播时，会分裂成两列波，速度较快的为快剪切波，速度较慢的为慢剪切波，两列剪切波的偏振方向近似相互垂直。快剪切波偏振方向与慢剪切波时间延迟是描述地震各向异性的两个主要参数。本研究主要是从两个主要方面研究了剪切波分裂的特性。1)首都圈地区与福建地区地壳介质各向异性特征研究剪切波分裂计算对观测资料质量要求较高，要求选择剪切波窗口内、高信噪比的波形资料。因此，在我国大陆区域数字化地震台网建成并运行前，有关我国大陆不同区域地壳介质各向异性的研究并不多见。随着20世纪末，大量区域地震台网的建成与运行，现已积累了大量地震波形资料，这为研究中国大陆区域地壳介质各向异性特征提供了条件。首都圈地区位于我国华北块体北部，包括太行隆起、燕山隆起与华北盆地三大地质构造单元，区内的张家口—蓬莱断裂带为该区重要的活动构造。福建地处欧亚板块东南缘，临近欧亚板块、太平洋板块、菲律宾海板块的交接部位，是地质活动较强烈的构造活动区。首都圈地区与福建地区应力环境均比较复杂，是学者们比较关注的热点地区。本研究分别根据首都圈地区地震台网资料(2002年1月～2005年8月)与福建地区区域地震台网资料(1999年1月-2003年12月)，采用SAM分析方法(高原等，2004)，分析区域地壳介质各向异性的空间分布特征，得到了一些有意义的结论。(1)首都圈地区各台站有效记录的平均快剪切波偏振方向为79.9°±44.3°，福建地区各台站有效记录的平均快剪切波偏振方向为109.4°±42.6°，前者与华北地区最大主压应力场方向一致，后者与福建地区最大主压应力场方向一致，这表明，快剪切波平均偏振方向与区域最大主压应力方向一致，是反应区域构造应力环境的有效方法之一。(2)位于不同构造分区内的台站，台站下方地壳介质各向异性特征有所不同。(3)局部应力环境对快剪切波优势偏振方向有明显作用：位于活动断层上或附近的台站，快剪切波优势偏振方向通常与活动断层走向一致；位于两组断裂交汇位置的台站，快剪切波偏振方向比较复杂，呈现出两个优势偏振方向。复杂的地质构造环境，会造成地壳介质地震各向异性特征的复杂化。2)冰岛2000年强震前剪切波分裂参数特征研究有关地壳介质各向异性随时间的分布特征，是地壳介质各向异性特征研究的一个重要方向。地壳介质各向异性随时间分布特征的研究，对观测资料要求更为苛刻，需要剪切波窗口内较为连续的小震资料。冰岛位于欧洲最西部，大洋中脊在此穿过，分别于冰岛的北部与南部出露。冰岛境内活山活动、小震活动比较频繁，这为研究地壳介质各向异性随时间的分布特征提供了大量资料。2000年6月发生在冰岛西南部转换断层上的一组M＞6.0强震，是冰岛地区30年来最强的一组地震。这次强震前，记录到了大量小震资料。本研究根据SWAS方法，重新分析了冰岛2000年6月强震序列前近半年的小震资料，研究结果表明在强震序列发生前，台站SAU与BJA均能够观测到慢剪切波延迟时间随应力积累而增大的现象，以及在临震前慢剪切波延迟时间随裂隙闭合而突然下降的现象。综合冰岛地区其它地震、美国的几个震例、中国大陆的震例与中国台湾地区的集集地震的剪切波分裂分析，通过数据拟合，进一步证实了震级与慢剪切波时间延迟在地震前增加的持续时间、临震前下降的持续时间的对数，存在一个正比线性关系。根据对首都圈地区和福建地区地壳各向异性的研究结果，本研究进行了多种观测结果的综合讨论。综合区域地区地壳介质各向异性与地震活动性、小震精定位、小震震源机制解、上地幔介质各向异性、地震层析成像、GPS观测结果等其它地球物理现象，得到以下初步认识。震源机制解最大主压应力轴方向通常与地壳介质各向异性方向一致；快剪切波偏振方向分布较为复杂的地区，似乎与速度结构有一定的关联性；上地幔介质各向异性与地壳介质各向异性的关系能够反应区域浅部与深部介质动力学特征，并暗含区域动力机制信息；GPS观测结果显示地表速度运动特征，同地壳介质各向异性方向一样，可以验证区域介质应力环境的复杂空间分布特征；地震活动与小震精定位能够反应区域构造信息，与地壳介质各向异性方向的区域应力环境相对应。本研究比较全面地分析了地壳介质各向异性空间与时间分布特征，讨论了地壳介质各向异性特征与其它地球物理现象之间的关系，得到的一些结论对地震各向异性和应力特性等研究具有重要的参考意义。更多还原

【Abstract】 Seismic anisotropy is one of the main specialties of the Earth, which can be well understoodby shear-wave splitting. In anisotropic rocks, shear-wave splits into two approximately orthogonalpolarizations that travel at different velocities and write characteristic easily-identifiable signaturesinto three component seismic wave trains. The wave at faster velocity is fast shear-wave, while thewave at slower velocity is slow shear-wave. The main parameters of shear-wave splitting arerespectively polarization of fast shear-wave, and time delay of slow shear-wave.The following two aspects are the main work on seismic anisotropy in crust in the thesis.A Seismic anisotropy in crust in Capital and Fujian areasThe requirements of shear-wave splitting analysis are very strict, which means that the waveshould be within shear-wave window with high signal-noise ratio. Thus, seismic anisotropyresearch in regional areas is not so abundant before regional seismic networks are widely built inChina. Since the end of 20st century, many regional seismic networks are built, and a lot ofwaveform data are available, which are quite good for the development of research about seismicanisotropy.Capital area is in the north of North China, including three main geologic structures (TaihangUplift, Yanshan Uplift, and North China Basin). Furthermore, Zhangjiakou—Penglai fault is themain seismic structure in the area. As for Fujian area, it is in the southeast border area of Eurasiaplate, and is near the crossing area of Eurasia plate, Pacific plate, Philippine plate. The geologicstructures in the Capital area and the Fujian area are complicated, which are always the interestedresearch areas in China.With SAM technology (Gao et al., 2004), the seismic waveform data in Capital area (Jan.2002-Aug. 2005) and in Fujian area (Jan. 1999～Dec. 2003) are analyzed. Seismic anisotropy incrust in the Capital and Fujian areas are obtained, and some meaningful conclusions are available.(1) The average polarization of all the records in Capital area is 79.9°±44.3°, whichcorresponds to the direction of maximum horizontal principal compressive stress in North China.Similarly, the average polarization of all the records in Fujian area is 109.4°±42.6°, which isrelated to the direction of maximum horizontal principal compressive stress in South China. Theresults show that the average of polarization of fast shear-wave parallels to the maximumhorizontal principal compressive stress, and is an effective tool to depict the regional stress field.(2) At different local structure areas, the seismic anisotropy specialties in crust are different.(3) Locally complicated stress structures influence on the polarizations of fast shear-waves.For example, the predominant polarization is usually related to the strike of the fault, if the stationlocates on (or near) an active fault; Polarizations are very complicated, or multi-predominant polarizations are shown, if the station locates in the crossing area between two faults.B The specialties of shear-wave splitting before M＞6.0 in 2000 in IcelandThe temporal change of seismic anisotropy in crust is an important research item. However,the requirement of the research is much stricter, which needs the successive small events recordswithin shear-wave window.Iceland, in the west border of the Europe, is passed through by Mid-Atlantic Ridge runningonshore in Southwest and North Central Iceland. Both the seismic and volcanic activities arefrequent in Iceland, which provides a lot of waveform data.In Jun. 2000, a series of M＞6.0 earthquakes occurred in the transform fault in SouthwestIceland, which is the strongest event since recent 30 years. With SWAS technology, theshear-wave splitting parameters of nearly 6 months’ small earthquakes before the main shock areobtained. The results show that the time delays of station SAU and BJA show some interestingphenomenon before the main shock. The time delays of slow shear-wave increase with the stressaccumulation before the main shock, while they decrease quickly with the stress relaxationimminently before the main shock.Integrating other events in Iceland, several events in USA, events in Chinese mainland, andChi-Chi event in Taiwan, the relation between seismic magnitude and the duration of time delayincreasing, and the relation between seismic magnitude and the duration of time delay decreasingare calculated. It is found that the relations all show linear direct proportion.Considering seismic anisotropy in crust, seismic activity, small earthquakes relocation, smallearthquakes focal mechanism, seismic anisotropy in upper mantle, seismic tomography, GPS andso on, some preliminary conclusions are available.The principal stress direction is related to the polarization of seismic anisotropy in crust;Multi-predominant polarizations (or complicated polarizations) seem to be connected to thevelocity structure; The relation between seismic anisotropy in crust and in upper mantle can showthe geodynamic specialties of shallow and deep structure, and can hint information aboutgeodynamic mechanism; GPS results show the movement velocity of surface, which can testifythe complicated stress field, just like seismic anisotropy in crust can do; Seismic activity andseismic relocation can show the regional structure information, which just correspond to thestructure information shown by seismic anisotropy in crust.Both the spatial and temporal changes in seismic anisotropy in crust are discussed in thethesis. In addition, the relation between seismic anisotropy in crust and other geophysicalphenomenon are discussed too. Some meaningful conclusions are obtained for the research aboutseismic anisotropy and stress specialties.更多还原