Spatial Distribution Characteristics Of Focal Mechanisms And Stress Field In Tien Shan And The Adjacent Areas In Xinjiang

The Tien Shan orogenic belt is one of the most active intra-continental orogenic belts in the Eurasia.The Tien Shan orogenic belt is bounded by the Kazakh Shield and Junggar Basin to the north,and the Tarim Basin to the south.Those tectonic blocks not only affect the evolution of different geological tectonic units,but also control the activities of the strong earthquakes in the Tien Shan area,forming a special tectonic environment,which leads to strong historical seismic activity in this region,with the characteristics of great intensity,high frequency and uneven distribution.It is significant to investigate the focal mechanism and tectonic stress field for understanding the structural deformation,seismogenic mechanism and seismotectonic background of the Tien Shan.The focal mechanism solutions of 12 earthquakes with M_S?5.0 occurred in Tien Shan and its adjacent areas were inverted by gCAP method,using the broadband waveforms recorded in 2015 to 2018,from the Xinjiang regional digital seismic network.The minimum 3-D rotation angles are calculated respectively with between the focal mechanisms of GCMT,IGP and CENC,and that obtained by gCAP method.The results show that our earthquake focal mechanisms are acceptable with high reliability.There are total of 595 focal mechanism solutions both inversed in this dissertation and collected from previous studies and the GCMT for the earthquakes occurred in Tien Shan and the adjacent areas.With the comprehensive analysis of the types of focal mechanisms and their spatial distribution,it is illustrated that the focal mechanisms of the thrust-type(48.24%)and strike-slip-type(30.92%)are dominant.Thrust-type earthquakes was distributed throughout the Tien Shan and the adjacent areas in Xinjiang,which occurred in the bump region resulted from the collision between the Indian Plate and the Eurasian Plate.The strike-slip earthquakes are mainly clustered in the middle parts of the eastern Tien Shan,the western parts of the southern Tien Shan and the Wuqia intersection area.The pattern and spatial distribution of focal mechanisms indicate that the regional stress fields are more complicated in Tien Shan and adjacent areas in Xinjiang.The Tien Shan and its adjacent areas in Xinjiang are separated into two zones according to orientation of topography,which are the L zone in the western Tien Shan along the SW-NE strike and the R zone in the eastern Tien Shan along the near-EW strike.The stress fields of the L and R zones are inverted using the damped stress tensor inversion method based on the focal mechanisms.The characteristics of the stress fields in these areas have been analyzed by combining with the geological structure.The results suggested that the optimal principal compressive stress axis of the stress field in the L zone is close to horizontal with SN striking,and the optimal tensional stress axis is near-vertical with a high plunge angle,and the R value is large between 0.5 and 0.9,which indicating the tensional intermediate axis.The results of the stress field in the R zone show that the optimal principal compressive stress axis is near horizontal with SN striking,and the extensional stress axis presents nearly vertical.The clockwise rotation of the optimal principal compressive stress axis in the west and east of R zone is directly related to the faults with ENE and WNW striking,and may also be related to the protruding structure in northern tip of the Tarim Basin,or may be resulted from the undergoing bi-directional crustal-mantle subduction in this area.