Numerical Simulation Of Anisotropic And Medium Anisotropic Features In The Southern Part Of South - North Seismic Zone

Seismic anisotropy in the crust and upper mantle can be used to address a wide range of geological and geophysical problems, like the plate movement, seismicity, geodynamics, and so on. Crustal anisotropy is often attributed to local principle compressive stress and the polarization of fast shear wave related to the orientation of the local maximum horizontal compressive stress. Moreover, crustal anisotropy can be complicated by shallow crustal structure. Seismic anisotropy in the upper mantle is one of best ways to image deformation in Earth’s interior and study the continental dynamics.North-South seismic belt is located in the east of Tibetan Plateau, which has a complex tectonics and strong seismicity. According the seismic anisotropy beneath North-South seismic belt, we discuss the relationship between the distribution of crustal local stress and geological structure under the effect of crustal shortening and uplift of Tibetan Plateau. We also study the deformation of lithosphere in the southeast of the Tibet Plateau under effect of Indian and Eurasian Plates collisionIn this study, shear wave splitting observations were reported from local and teleseismic earthquake data, which is recorded by Sichuan and Yunnan Regional Seismic Networks. Shear wave splitting obtained from near-field records by77seismic stations from January2000to April2010mainly uses direct seismic shear waves in the crust, and is indicative of anisotropic characteristics of the crust. Teleseismic events recorded by92seismic stations during the period of August2007and October2010are measured to extract the splitting parameters by measuring teleseismic phases XKS (SKS, SKKS, PKS).The polarization of fast shear waves (PFS) and time delay of slow shear-wave beneath the77stations are computed using shear-wave splitting analysis method (SAM). Due to the impact of the regional principal compressive stress and local geological structure, the spatial distributions of polarization of the fast shear-wave show the localized characteristic. The principle PFS beneath the Songpan-Ganzi block is NE direction. The principle PFS beneath the northwest and southeast part of the Sichuan-Yunnan block shows nearly EW and North direction, respectively. However, the localized distribution of shear-wave splitting parameter could be caused by complex geological structure and the geometry of active faults. The principle PFSs beneath the northeast and southwest segment of LongmenShan fault belt exhibit the NE and NW direction. The dominant direction of PFS beneath the north and south side of Qingchuan fault are nearly north and south direction, respectively. The time delay beneath north side is larger than that in the south side of Qingchuan fault.The mechanisms of crustal anisotropy can be divided into two major categories: stress-induced and structural anisotropy. External stresses generate micro-cracks and cause them to be the stress-aligned anisotropy. The local principal compressive stress dominates the orientations of the micro-cracks. However, the aligned macroscopic fractures associated with regional tectonics, like sedimentary bedding planes, the alignment of minerals and so on, are essential components of the structural anisotropy. Seismic observations show that crustal anisotropy is mainly related to the structural anisotropy. This study suggests that the distribution of shear-wave splitting parameter from the station close to or at fault, could be consistent with the geometry of active faults.In order to discuss the influence of the complicated structure on the characteristics of seismic wave propagation, seismic wave propagation are simulated in orientated heterogeneous medium with an appropriate smooth disturbances autocorrelation function in this study. Our modeling suggests that the transverse isotropy medium with a horizontal axis can be equivalent of heterogeneous medium with oriented arrangement. This method does not need any constraint on the geometry and scale of micro-cracks. We can obtain the effect of the different scale, strike of geological structure on the observation seismic anisotropy.Moreover, the splitting parameters from teleseismic events are obtained by the method of Silver and Chan, which consists of minimizing the energy on the transverse component by rotating and time shifting the traces. In this study, the anisotropy of upper mantle is complicated beneath North-South seismic belt. displaying a feature of "Strong South Weak North, Strong West Weak East". The directions of fast XKS are nearly the NE direction beneath Songpan-Ganzi block. The directions of fast XKS are nearly EW direction beneath Sichuan basin. In the Sichuan-Yunnan block, taken the Xiaojinghe as the boundary, the directions of fast XKS are nearly NNW and EW direction beneath north and south part of Sichuan-Yunnan block, respectively. The average time delay of slow XKS wave is about1s with a large spatial variation in the North-South seismic belt. The average delay time is larger in the south than it in the north of North-South seismic belt, and the minimum of delay time is in the Sichuan basin.Our results suggest that the eastward extension of lithosphere and asthenosphere of Tibeten Plateau subsequently turn into the nearly NS direction beneath southeastern Tibetan Plateau, due to the resistance of Sichuan Basin. Xiaojinhe fault, which locates in the middle Sichuan-yunnann block, play an important role for the NS extrusion of the Tibeten Plateau.