【作者】 刘莎；

【导师】 杨建思； Stuart Crampin；

【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， 2013， 博士

【摘要】 地震波各向异性反映了地球内部的结构、构造变形、应力状态等方面的信息,在岩石圈演化、壳幔耦合变形以及地幔对流等研究中发挥着重要的作用。剪切波分裂现象是地震各向异性最直观的表现形式,在各向异性的研究中扮演着重要的角色。本文回顾了地震各向异性的研究进展和研究意义,特别介绍了剪切波分裂的原理、方法,以及近年来剪切波分裂研究的进展和现状。利用剪切波分裂分析方法,可以获得地壳和上地幔的各向异性空间分布特征,分析各向异性分布与构造背景之间的关系,进而探讨壳幔变形机制、地幔物质流动等地球动力学过程。此外,通过对剪切波分裂参数在时间上的变化分析,可以获得地震、火山等自然灾害发生前后地壳应力及地壳中微裂隙的变化特征等。本论文围绕剪切波分裂方法在地震各向异性研究中的应用,开展了以下工作。一,利用玉树地震震源区密集流动观测台站数据,研究了该地区地壳各向异性分布特征。研究结果表明玉树地区的快波偏振方向为南东向,与该区域的水平主压应力方向及断裂带的走向一致,揭示了此次玉树地震断裂走滑性质的特征。研究区域内平均慢波延迟时间是5.68ms/km。通过对比其他地区的慢波延迟时间结果,玉树地区各向异性强度高于首都圈西北部地区和四川紫平铺水库地区。慢波延迟时间的空间分布显示慢波延迟时间在玉树台周围最大,而且也揭示了研究区域各向异性强度的空间分布特征。二,针对青藏高原东部以往各向异性研究结果的稀缺,我们利用青藏高原东部固定台站、玉树地震应急流动观测台站及玉树科考流动台站的资料,研究了青藏高原东部地区上地幔各向异性特征。研究结果表明,快波偏振方向在金沙江缝合带及羌塘地块为南东向,在巴颜喀拉地块表现为北东向和南东向。结合研究区域地壳各向异性的结果,我们推断青藏高原东部地区壳幔变形类型属于垂直连贯变形。研究区域内上地幔慢波延迟时间普遍较高,而且变化范围较大。我们推断上地幔各向异性多源于上地幔矿物的晶格优选方位和熔体的定向分布,进一步表明青藏高原东部是地幔流活动较强烈的地区。三,冰岛作为天然的地球物理实验室,为火山喷发的剪切波分裂研究提供了先决条件。基于冰岛Eyj afj allaj okull火山周围的地震事件数据,通过剪切波分裂研究获得了火山喷发前后剪切波分裂参数在时间上的变化,从各向异性的角度分析并讨论了冰岛火山喷发前后地壳应力的变化。该研究区域快波偏振方向为北东向,这与台站周围断裂带的走向以及区域主压应力的方向是一致的。在火山喷发前后快剪切波偏振方向并没有发生明显变化。慢波延迟时间在Eyjafjallajokull火山喷发前表现出长期的增大和突然降低。我们推断在火山喷发之前地壳应力长期的积累和突然地释放,使得地壳中微裂隙的几何形态发生变化。Eyjafjallajokull火山喷发之后,位于邻区的Katla火山周围地震活动性增强,但是慢波延迟时间的结果表明地壳中并没有积累足够的应力引起下一次可能的喷发。然而,我们也并不能排除慢波延迟时间突然增大这一可能性。最后,基于剪切波分裂方法在玉树震源区、青藏高原东部及冰岛南部火山地区开展的研究,本论文讨论并总结了剪切波分裂在地壳和地幔各向异性研究中的意义以及在火山地区地壳应力变化监测的应用。通过以上的分析讨论,一方面使得我们对研究区域的各向异性特征有了更为全面、深刻的认识,另一方面我们的结果也为之前的各向异性结果进行补充,同时为后续的各向异性研究提供相关的参考。更多还原

【Abstract】 Seismic anisotropy reflects the internal structure of Earth, tectonic deformation, stress status and other information, and plays a significant role in the evolution of the lithosphere, crust-mantle coupling deformation and mantle convection and other research. Shear-wave splitting is the most direct manifestation of seismic anisotropy, and plays an important role in the anisotropic study. This thesis reviews the research progress and significance of seismic anisotropy, especially introduces the principles, methods, and the recent studies of the shear-wave splitting.Seismic anisotropic spatial distribution characteristics of the crust and mantle can be obtained by using shear-wave splitting analysis method, which can be used to discuss the relationship between anisotropy and tectonic background, crust-mantle deformation mechanism, mantle flow and other geodynamic processes. In addition, the change of the stress and micro-cracks in the crust before and after the earthquakes, volcanoes and other natural disasters can be obtained by analysis on the parameters changes of shear-wave splitting. In this thesis, we carry out the following work around the applications of the shear-wave splitting in the seismic anisotropic research.Firstly, we use the seismic data from the dense seismic stations array in Yushu, and analyze the crustal anisotropic distribution. The results show that the polarization direction of the fast shear-wave in the crust is southeast, which is consistent with the direction of the horizontal principal compressive stress and the strike of the faults. The patterns of the polarization directions of fast shear-wave reveal this fault on which the earthquake happened is a strike-slip fault. The average value of the delay time of slow shear-wave is5.68ms/km. By comparing the delay times of slow shear-wave in other study areas, the anisotropic strength in Yushu area is bigger than that in northwestern capital area and Sichuan Zipingpu reservoir region. The spatial distribution of the delay time of slow shear-wave shows the value is high around the YUS station, and also reveals the characteristics of the spatial distribution of the anisotropic strength in the study area.Secondly, on account of the scarcity of anisotropic results in eastern Tibetan Plateau, we study the upper mantle anisotropic characteristics in the eastern Tibean Plateau using the data recorded by the permenant seismic staions, temporary staions and the staions of scientific investigation. The results show the polarization direction of fast shear-wave is southeast in Jinshajiang Suture zone and Qiangtang block. However, in Bayan Har block, there are two directions which are northeast, southeast, respectively. Consolidating the crustal and mantle anisotropic results, we conclude that the crust and upper mantle in this area have vertically coherent deformation. The delay time of slow shear-wave in the upper mantle is generally high, and varies greatly. We infer the lithospheric mantle melting and the melt pocket preferred orientation strengthen and change the anisotropic characteristics in our study area, which suggests the mantle flow activity is strong in eastern Tibetan Plateau.Thirdly, Iceland, as a natural geophysical laborary, provides the prerequisite for the shear-wave splitting of volcanic eruptions. We obtain the change of the shear-wave splitting parameters, and discuss the stress change in the crust before and after the eruptions of Eyjafjallajokull volcano from the anisotropic perspective based on the micro-earthquakes data around Eyjafjallajokull volcano. The polarization direction of the fast shear-wave is northeast in our study area, which matches well the strike of the surface faults and the regional principal stress direction. There is no obvious change in the polarization direction before and after eruptions. The delay time of the slow shear-wave increases before the eruption until there is an abrupt change to decreasing delay time which terminates when the eruption begins. This is due to the stress-accumulation increases, and stress-relaxation decreases as microcracks coalesce onto the eventual fault-plane, which causes the changes on the parameters of shear-wave splitting. After Eyjafjallajokull volcanic eruptions, the seismic activity increases around the adjacent Katla volcano. But the results of shear-wave splitting indicate that Katla is not preparing for enough stress to cause the next possible immediate eruption. However, we cannot exclude the possibility of the delay time of slow-wave increasing abruptly.Finally, based on the applications of shear-wave splitting in the source area of Yushu earthquake, eastern Tibet Plateau and volcanic area in southern Iceland, we dicusse and summarize the research significance of the shear-wave splitting method on the dynamics research of the crustal and mantle structure, and the applications prospects of the stress monitoring before and after earthquakes or volcanoes. Through the above analysis and discussion, on the one hand, we have more comprehensive and profound understanding on the anisotropic characteristics in the study area. On the other hand, our results supplement the previous anisotropic results, and also provide the relevant reference for the successive anisotropic study.更多还原