| The fault zone on the northern edge of the West Qinling Mountains is located in the northern section of the South-North strong earthquake structural belt in China.It is one of the most important active boundary faults in the Qinghai-Tibet block,and it is also an important dividing line of geography,landform,geological structure and crustal structure in western China.Therefore,it is of great significance to study the current tectonic activity characteristics of the fault zone in the northern margin of the west Qinling Mountains for a deeper understanding of the current crustal tectonic activity characteristics in the border area between the Qinling orogenic belt and the northeastern margin of the Qinghai-Tibet block.In history,there have been many strong earthquakes in the fault zone on the northern edge of the western Qinling Mountains,such as the west Gangu earthquake of magnitude 7.3 in 143,the Tianshui-Longxian earthquake of magnitude 6-7 in 600,the Tianshui earthquake of magnitude 7.5 in 734 and the Kangle earthquake of magnitude 6.8 in 1936.At present,relevant scholars have carried out relevant research work on the crustal movement and deformation in the northern margin of West Qinling and its surrounding areas,mostly focusing on using seismic geological data to study,such as the geometric structure segmentation of the fault zone,the horizontal and vertical movement characteristics of the fault zone,or using GNSS monitoring results in a single time period,but there is still a lack of in-depth research on the long-term and phased activity characteristics of the northern margin of West Qinling by using GNSS data in longer and different time periods.In addition,what about the locking degree,movement characteristics and strong earthquake risk section of the fault zone on the northern edge of the West Qinling after the Wenchuan earthquake?These problems need further in-depth study.Based on this,this paper,based on the results of multi-period high-precision GNSS monitoring,inverses and analyzes the present locking degree,slip loss rate and strain parameters of the fault zone in the northern margin of West Qinling by constructing various crustal deformation-strain models,such as block movement and strain model,negative dislocation model,fracture spiral dislocation model and crustal strain rate calculation model,and on this basis,discusses the seismic risk of each secondary fault in the northern margin of West Qinling.The main work and conclusions of this paper are as follows:1.Based on the data of GNSS monitoring station of"crustal movement observation network of China"located in the northern margin of West Qinling and its surroundings,GAMIT/GLOBK software was used to carry out high-precision data processing,and 144 in ITRF2008 framework,187 in 2009-2013,243 in 2013-2017 and 2017-2021 were obtained.The results show that the current crustal movement in the west Qinling region is characterized by clockwise rotation as a whole,and an obvious velocity gradient zone is formed at the fault on the northern edge of the west Qinling,and the velocity field of crustal movement on the north side of this velocity gradient zone is significantly larger than that on the south side.2.The fault slip rate is calculated by REHSM（rotation in the entire block and homogeneous strain model）,and the differential motion characteristics of each secondary fault in the northern margin of West Qinling are analyzed by GNSS velocity profile.The results show that the secondary faults in the northern margin of West Qinling are mainly left-handed strike-slip movement and characterized by compression movement.The sinistral strike-slip rate calculated in this paper is less than that obtained from seismic geological data（2.1-2.8mm/a）,and the vertical fault movement rate is basically consistent with the existing research results（0.2-0.7 mm/a）.Considering the velocity profiles of GNSS stations parallel to and perpendicular to the secondary faults in the northern margin of the western Qinling Mountains,it is considered that the activity of Zhangxian section and Tianshui-Baoji section is lower than that of Yuanfeng section and Guomatan section,and they have higher locking degree and strain accumulation background.3.By constructing the negative dislocation model of regional faults and active blocks in West Qinling,the characteristic parameters such as the present locking degree and sliding loss rate of each secondary fault in the northern margin of West Qinling are obtained by inversion calculation.The results show that from 1999 to 2007,Guomatan section and Tianshui-Baoji section were in a state of strong atresia,with the depth of atresia reaching about 30km,while Zhangxian section and Yuanfeng section were only partially occluded,and the degree of atresia was low and the depth of atresia was shallow.After the Wenchuan earthquake in 2009-2013,the blocking degree of the middle section of the fault in the northern margin of the West Qinling Mountains was enhanced,and the eastern section of Tianshui-Baoji section showed a creeping state,while the middle sections of Guomatan section and Tianshui-Baoji section remained in a strong blocking state.From 2013 to 2017,the fault in the northern margin of the West Qinling Mountains was in a strong locking state as a whole;From 2017 to 2021,Zhangxian section was in a state of creep,and only some sections remained in a state of strong locking.To sum up,at present,the whole fault in the northern margin of West Qinling is in a strong locking state and has a strong strain accumulation background.4.By constructing the spherical wavelet strain model of the west qinling region,the characteristic parameters of expansion rate,principal strain rate and maximum shear strain rate under different scales in different time periods are obtained.The results show that the principal strain rates along the zhangxian section of the fault in the northern margin of the west qinling increased significantly during 2009-2013 compared with 1999-2007,while the principal strain rates of the other three secondary faults were still small.From 2013 to 2017,the activities of the faults in the northern margin of west qinling and their adjacent areas weakened.From 2017 to 2021,the activities of the faults in the northern margin of western qinling and their adjacent areas were enhanced,and the maximum shear strain was located at the yuanfeng section of the fault in the northern margin of western qinling,with a value of about 118×10-9/yr.The above characteristics indicate that the high degree of atresia in the guomatan and tianshui-baoji segments has a strong background of strain accumulation,while the degree of atresia in the zhangxian segment is relatively low. |
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