Seismic Anisotropy And Microseismic Analysis Of The Upper Crust In The Key Structural Areas Of The South-north Seismic Zone

The South-north seismic zone is located in the tectonic boundary zone between the east and the west in China,where is the important intersection among active Tibetan block,Ordos block,Sichuan basin and South China block.The strong tectonic movements and frequent seismic activities since Late Cenozoic,made this area as the main area occurring strong earthquakes in mainland China.The study area is located in the middle of the South-north seismic zone,influenced by the east excruding of the Tibet Plateau and the blocking of the Southern China block.The internal tectonic stress field and the dynamic environment are very complex in this area,and there are obvious differences between different regions.In the inner rhombic block of Sichuan-Yunnan,the direction of the maximal principal stress field changed from near EW to near SN from north to south,while there are three principal stress directions(NW,NWW and NE)near the Longmenshan fault zone.Generally,the complex tectonic geo-environment leads to strong anisotropy of the crustal medium in this area.The shear-wave is very sensitive to the anisotropic medium,and it can be used to study the seismic anisotropy of the earth's interior and to obtain the distribution of the anisotropic medium.The shear-wave splitting parameters can indicate the direction of the regional principal compressive stress and reveal the regional tectonic information.The relationships between the shear-wave splitting parameters and the regional principal compressive stress can be found by analysing crustal anisotropy,respectively,as well as the strikes of controlling slip faults.The state of regional crust-mantle coupling and deformation mechanism can be analyzed by comparing anisotropy between the crust with the upper mantle anisotropy.However,because of the strict requirements for the data quality,the amount of data used for analysis is very small,which makes it difficult to acquire reliable results.In this paper,the correlation coefficient and significance level in the statistics are introduced into the analysis of seismic anisotropy,so that we can analyze the results of anisotropy more quantitative.Our study zone is located in the central and south of the South-north seismic zone,that is,the Longmenshan fault zone and the Sanjiang crush area.By using small-earthquake waveform data from the permanent seismic stations from January 2010 to October 2017,the Western Sichuan Seismic Array from October 2006 to July 2009 and the Sanjiang array from December 2016 to October 2017,we acquired the spatial distribution and variation of seismic anisotropy in the upper crust in this region.The result shows that the time delay of slow shear-wave ranges from 0.65ms/km-7.39 ms/km in Longmenshan fault zone,which indicates obvious lateral inhomogeneity in study zone and that anisotropic depth in the crust seems to be mainly distributed above 20 km.Dominant polarization direction of fast shear-waves(FPS)are at NW,NWW and NE,showing obvious zoning patterns.Dominant FPS from north to south along the Longmenshan fault zone changes from NWW to NW or NE direction.However,it is at NWW or NE direction in the south part,which indicates tectonically segmentation pattern along the Longmenshan fault zone.Based on effective data in this study,seismic parameters in 19 of 49 stations shows some correlations with the backazimuth,source depth,magnitude and ray path.The Lushan earthquake and the other striking earthquakes in study zone might have changed locally polarization directions of fast shear-waves.The Institute of Earthquake Forecasting,China Earthquake Administration set up a linear seismic array composed of 26 stations in the lateral collision zone of Sanjiang.In this paper,we use the seismic waveform data from the array to study crustal shear-wave splitting and microseismic events identification.Among them,the anisotropy results show that the dominant FPS at the station is near NS direction,and they have a trend of clockwise rotation from west to East.The FPS is in good agreement with the stress field reflected by the local structure.This paper indicates that a large number of events will be beneficial to study quantitatively variation of local tectonic stress field and faults attribution,furthermore beneficial to study fault zone seismology and earthquake forecasting.Besides,we analyzed the data obtained from the linear seismic array with the Match and Locate method to identificate the microseismic events in the Sanjiang collision zone.we tried to use the Match & Locate method to pick up the microseism in the region of the Sanjiang collision zone,and we identified 2328 microseismic events,which are 8.4 times of the number of earthquakes released by network center during the same period.At the same time,the identified microseismic events have the characteristics of high S/N,clear phase and high precision,which could provide the possibility of using the missing earthquake to study the shear-wave splitting.After selection and analysis,we obtained 144 effective events which can be used to do the shear-wave splitting research.By comparing the anisotropic parameters acquired from the lost earthquakes with the template earthquakes,we found that most of the stations' results are in good agreement with each other.The results show that the missing earthquakes we identificated can be used to study the shear-wave splitting of the crust which are not in the catalogue issued by the Seismic Network Center.The addition of the data can provide a good complement to the shear-wave splitting parameters and give more detailed distribution characteristics.