The Deep Structure And Seismogenic Environment Of Potential Risk Areas Of Strong Earthquakes In Sichuan-Yunnan Border Area

Sichuan-Yunnan region is located in the southeastern margin of the Qinghai-Tibet Plateau, and its special seismic tectonic environment and frequent earthquake activity characteristics show that the area is an ideal place to study the present tectonic movement, the background of strong earthquakes in mainland China and the predict the future strong earthquake danger zone. Study on the structural environment and physical characteristics of the deep structure in this area will be helpful to explore the deep tectonic environment and deformation field characteristics of the active blocks in Sichuan and Yunnan, strengthen the understanding of the role of each anisotropy and tectonic deformation, and study the deep tectonic background of the seismic origin of the block interior. In recent years, the earthquake frequencyand seismic intensity are increasing in the Sichuan and Yunnan border area, the medium and long-term risk background and potential of seismogenic ability have aroused strong concern by geo-scientific research workers at home and abroad. But due to lack of reliable data on deep geophysical field for a long time, when study seismogenic potential danger zone, the deep medium environment and earthquake tectonic background, the relationship between the seismic activity exist great difficulties. Because of various geophysical inversion methods tend to have different degree of non-uniqueness, so, on the same research area, using different geophysical observation data source and a variety of data processing and inversion methods to study the deep structural background has become an effective way to solve this problem.In this paper, the three-dimensional P wave velocity structure in Sichuan and Yunnan area is obtained through selecting the observational data of 224 fixed seismic stations in the regional digital seismic network of Yunnan and Sichuan and the observational data of 356 mobile seismic arrays of "Chinese seismic array detection--the southern section of North-South seismic belt ("Himalaya" project phase 1),35 arrays of Scientific investigation of Lushan earthquake, and 19 mobile stations in Xichang, collected and handpicked the number of P wave arrival times is 249316, and the number of relative travel time residuals is 103902. By using the method of joint inversion of regional and teleseismic; and also, the distribution characteristics of gravity and Aeromagnetics anomalies in Sichuan and Yunnan region are also achieved by using the method of field separation and anomaly feature extraction of three dimensional gravity inversion and Aeromagnetics data, and on this basis, such problems as ductility flow boundary of the middle and lower crustal materials in the southeastern margin of the Tibetan Plateau, and the response on the basis of gravity and Aeromagnetics for the flow direction shift of middle and lower crust material are comprehensively analyzed and discussed in the paper, and, the deep structure and seismogenic environment of three potential strong earthquake risk areas (Zhaotong-Liangfeng fault zone, Muli-Yanyuan arc tectonic belt, Panxi tectonic zone) in Sichuan Yunnan border area are also analyzed and studied, the main achieved results are as follows:P wave imaging results show that the middle and lower crust of 20km depth below the Western Sichuan Plateau shows obvious low velocity anomaly distribution characteristics, and there are a large range of low velocity layers in the Songpan-Ganzi block and the sub-block in the northwest of Sichuan, meanwhile, there are continuous low velocity layer distributions in the middle and lower crust of the Daliangshan block, and the distribution direction of the low velocity anomaly is near SN, which is basically consistent with the trend of Daliangshan, and this low-velocity layer crosses over the Daliangshan fault from the west to the East, and finally stops near Yingjing-Mabian fault zonein the western margin of the Sichuan Basin. The existence of the low velocity layer in the crust is also a deep source of deep dynamic deformation and seismic activity of Daliangshan block and its boundary faults.The results of characteristics of gravity and Aeromagnetics anomalies show that the ductility flow of the middle and lower crustal materials in the eastern margin of the Tibet Plateau is forced to turn to the lower strength Daliangshan sub-block interior because of being strongly resisted by the high-density, strong magnetic western margin of the Sichuan basin and the area of Yaan-Luding-Shimian and its rigid basement on the east side. Therefore, the existence of rigid basement in the area of Yaan-Luding-Shimian is the deep constraint factor which results in the middle and lower crustal materials in the eastern margin of the Tibet Plateau moves from the east to the SE-SSE in the process of ductility flow.P wave velocity structure combined with the apparent density inversion and Aero magnetic regularization filter together reveal the Ludian Ms6.5 earthquake and its aftershock locates in the intersection zone of high upper crust and low velocity, while aftershocks aggregated distributes in the direction of NW along with the boundary of high velocity abnormal body, which is agreed with the results of the regularization filter that reveal the epicenter of the Ms6.5 earthquake in Ludian is located in the boundary between positive and negative magnetic anomalies. Apparent density inversion results and P wave velocity structure all show that the existence of low velocity and low density anomalies below the source body of Ludian earthquake, and then they demonstrate that the Ludian earthquake source body is inside the rigid and brittle middle upper crustal medium. Considering the northern segment of Zhaotong fault section (LuDian-YiLiang section) along the high velocity and positive magnetic anomaly zone towards NE distribution, show that the media within crust depth of the northern segment of Zhaotong fault is harder than southern segment, is beneficial to the accumulation of stress, The northern segment of Zhaotong fault of the eastern Sichuan-Yunnan border has had mid-to long-term risk for a strong or major earthquake occurrence.P wave imaging results show that the low velocity layer exists in the vicinity of the depth of 10-30km inside the crust of Muli-Yanyuan arc tectonic belt, We think this low velocity layer should be a reflection of the deep and important tectonic slip interface in the crust, which constitutes the important source of the deep dynamics in this area, on the one hand, the crust low velocity layer plays a important role in crust tectonic movement, is a ductility weak layer, which is difficult to accumulate strain energy, and easy to transfer the stress to the brittle medium on the upper crust, and makes it produce a series of faults in the low velocity layer in the crust. On the other hand, the flow process of the low velocity layer is resisted by Yangtze block, so the ductility deformation is produced, and the stress is transmitted to the upper part of the brittle crust, when the stress continues to accumulate, the fault block of the upper crust is made to slide along the low velocity layer, so that a series of nappe tectonics are formed in Muli-Yanyuan area.The results of P wave velocity structure, apparent density inversion and the characteristics of Aeromagnetics anomaly comprehensive show that the anomalous distribution of high density, strong magnetic and high wave velocity exists inside the crust of Panxi tectonic zone, It is concluded that this may be related to the late Paleozoic mantle plume activity which leads to a large number of mafic and ultra mafic intrusions into the crust. In crustal doming process, massive intrusion of mantle derived material enhances the mechanical strength of the crustal medium, thus, the hard block inside shell which taking Panzhihua as the core is formed. A series of mafic and ultra mafic intrusive rock of Hercynian in the axis region is a good evidence. The existence of hard material in Panxi area has played a certain blocking role for Materials in the Tibetan Plateau escaping to the south and east direction, resulting the neotectonics in Panxi-Dianzhong area mainly belongs to intermittent uplift movement, and there is a lifting range about 2000m since Quaternary period, because of the strong uplift movement in the area, the deep valley is formed and characterized by bedrock exposed mountain canyon topography, and meanwhile, more second level differential fault depression exists in the are, thus, such graben basins as Yuanmou basin, Kunming basin, etc.P wave velocity structure also reveals upper mantle exist low velocity layer in 80 km to 120 km depth of the panxi tectonic belt, Due to the upper mantle uplift and the existence of deep faults in Panxi area, Provided the condition for the mantle thermal material into crust, become the deep tectonic background of strong earthquake activity.