The Seismic Anisotropy In Yushu And Iceland From Shear-wave Splitting

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.