Analysis Of Shear Wave Splitting In The Northern Xiaojiang Fault Zone

Xiaojiang fault zone is located on the southeastern margin of the Qinghai-Tibet Plateau and on the western boundary of the South China block.Due to the interaction between the Indian plate and the Eurasian plate,the Xiaojiang fault zone has become one of the most active deep faults in China.And it has been many destructive earthquakes in history.However,the calm of the major earthquakes for more than 100 years has made the fault become the focus of seismologists.In 2012,the Institute of Geophysics,China Earthquake Administration installed a wide-band seismic station and a continuous GNSS dense array(Qiaojia Array)in the northern of the Xiaojiang fault zone for detecting the movement of the fault zone and seismic activity.in order to further study the destructive earthquake formation mechanism.An Ms6.5 earthquake occurred in Ludian County,Yunnan Province on August 3,2014.The epicenter was located about 20 km northeast of the Qiaojia Array.It is the largest local earthquake recorded since the Array was deployed.A large number of studies have shown that seismic anisotropy generally exists in the upper crust media,which causes the propagating of shear-wave split into fast and slow waves with different propagation speeds,and the shear-wave splitting parameters are closely related to the environmental stress field.The polarization direction of the fast wave is consistent with the direction of the main axis of the stress field and the arrangement direction of the cracks under stress.The time delay of the slow wave depends on the aspect ratio of the crack.The larger the aspect ratio,the greater the delay time.Changes usually change the aspect ratio of the crack.In this study,we use the seismic data recorded from January 2013 to December 2016 of Qiaojia Array by using shear-wave system analysis method(SAM),5227 records of 1402 seismic events meeting the conditions were analyzed.351 shear-wave splitting results from 25 stations were obtained,and the partition characteristics of shear wave splitting were further discussed.At the same time,the slow wave time delay characteristics of the study area before and after the Ludian Ms6.5 earthquake were also analyzed.Shear wave splitting analysis reveals the spatial complexity of the polarization direction of the fast wave.The polarization direction differs depending on the observation point.However,the different zones analysis of the polarization direction shows that the distribution direction of the fault seems to play an important role,so that all zones have two dominant directions,one at N10°E and another at N90°E,both of which are different from the main direction of compressive stress.The average value of the slow shear-wave time delay in the entire observation area is 4.56ms/km,which is close to the upper limit of the delay time range recognized globally,but it is higher than the average value measured in most parts of mainland China,which may mean that the Xiaojiang fault zone The segment has more micro-crack arrangement.The analysis of the slow shear-wave time delay before and after the 2014 Ludian Ms6.5 earthquake shows that the time delay of the entire observation area has declined to a certain extent after the earthquake,and it has remained for about 4 months.It is another example of changes in shear wave splitting parameters caused by earthquakes or seismic activity found within in worldwide.