Study On The Crustal Deformation And Characteristics Of Main Faults In Central Yunnan Region Constrained By GPS Measurements

The Sichuan-Yunnan region located in the southeastern margin of the Qinghai-Tibet Plateau,is one of the areas with significant seismic activity in mainland China,and the strong earthquakes are concentrated and the seismicity is frequent in the region.The criss-cross faults with different ages and parameters,divide the Sichuan-Yunnan region into several blocks with different sizes and shapes.The most typical and earliest recognized activity block is the Sichuan-Yunnan rhombus block.As an important primary active block in the Sichuan-Yunnan region,the rhombus block plays a dominated role in the understanding of the crustal deformation patterns and active tectonics in the area.At the same time,the northeastern Lijiang-Xiaojinhe fault divides the rhombus block into two portions.In the southern region,the Red River Fault(RRF),Anningriver-Zemuriver-Xiaojiang Fault(refered to Anning River-Xiaojiang fault)constitutes the eastern boundary of the rhombus block.According to the previous studies,the scale of the RRF and the Anning River-Xiaojiang fault are large accompanied by deep structural background and obvious activities since the late Pleistocene.On the basis of research results of Xiang Hongfa,et al,the Quaternary basin and active faults,intermittently distributed along the Dali-Xiangyun-Nanhua-Chuxiong,are possibly the southwest boundary of the re-adjusted rhombus block in Sichuan and Yunnan.The narrow block between the RRF and the Lancangjiang fault is the boundary of the different modern structure stress fields in the western Yunnan area and the inward of the rhombus block in Sichuan and Yunnan.Therefore,the work of this paper is very meaningful.The Anning River-Xiaojiang fault,Chuxiong-Jianshui fault,RRF,Lancangjiang fault,located in the central Yunnan in the southern rhombus block are investigated by obtaining the crustal movement,strain parameters,locking depth and slip deficit distribution characteristic of the secondary blocks constrained by GPS velocity field results and relevant geological geomorphology data.The seismic risk is also discussed based on the seismic activities.The main study results are listed as following:1)Firstly,GPS data from 202 campaign stations between 2009 and 2014 are processed using high-precision GPS data processing software Bernese,in order to obtain the horizontal velocity field relative to the stable Eurasian plate;2)Secondly,the velocities are used to obtain the rigid motion and strain parameters of the blocks,using the motion model of the blocks.Then the singular points in the velocity field results are removed,using the direct removal method and Stepwisedeletion method.The strains and its changes of the active blocks in central Yunnan are computed.Lastly,the profile map of the GPS velocity field and the velocity of main faults in central Yunnan are obtained,using the velocity profile analysis method.It indicate that the rates of strike-slip component of the north,central and south parts of the Lancangjiang fault are 1.9mm/a,0.5mm/a and 1.1mm/a respectively.The rates of the strike-slip component of the north,central and south parts of RRF are 1.2mm/a,0.4mm/a and 0.2mm/a respectively.The rates of the strike-slip component of the north and south parts of Chuxiong-Jianshui fault are 1.1mm/a and 1.4mm/a respectively.These results show the crustal activities on the both side of the three faults are not stronger.Nevertheless,the strike-slip rate of Anning river fault is 4.5mm/a,the strike-slip rate of Zemu river fault is 6.5mm/a,the strike-slip rates of the north and south parts of Xiaojiang fault are 5.9mm/a and 5.1mm/a respectively.As the southeast boundary of the rhombus block in Sichuan and Yunnan,the Anning River-Xiaojiang fault showes obviously more stronger crustal deformation.The results agree with the results of Lu Jiangning;.3)Thirdly,the relevant data are used to inverse the locking coefficient and slip deficit distribution characteristic of the main faults,using the elastic negative dislocation model.The results show that the locking depth of Anning River-Xiaojiang fault decreases at first and then increases from north to south.The locking depth of Chuxiong-Jianshui fault changes little,which ranges from 24 km to 32 km.The locking depth of RRF decreases at first and then increases from north to south,but the main part of the changes are increasing.The locking depth of the Lancangjiang fault increases from north to south,and the locking depth of the central and south parts of the Lancangjiang fault changes little.4)Lastly,the results of the elastic negative dislocation model of the faults and5881 earthquake events(1970.01.01-2016.08.31)in central Yunnan provided by China seismograph network(CSN)are compared and analyzed.The results imply that the locking depths of the north part of RRF and the Lancangjiang fault as well as the adjacent areas of Zemu river fault and Xiaojiang fault,are shallow,and the as small and medium earthquakes are concentrated distribution in this areas.These areas with deep locking depths,mainly distribute the deep earthquakes as well as the medium and strong earthquakes.At the same time,the slip deficit distribution characteristics are related to the historical earthquake distribution characteristics along the faults.The slip deficit amount is large in the concentrated distribution areas of historical earthquakes.The phenomenon confirms the reliability of the invision results to some extent,and it also indicates that the locking coefficient and slip deficit distribution characteristic of the faults,inverted by the elastic negative dislocation model,are related to the depth of hypocenter and earthquake magnitudes.