Study On Current Activities Of The Middle Segment Of The Altyn Tagh Fault By InSAR Technology

With the India-Eurasian plates collision and continuous migration to the northeast since Cenozoic,the Altyn Tagh fault,as the northern boundary of the Tibetan Plateau,continues to move left-laterally,which not only controls the tectonicgeomorphic framework and evolution of the northern Tibetan Plateau,but also leads to strong seismic activities in the Altyn area.Due to the large-scale and long-term activity of the Altyn Tagh fault,the activity characteristics of different sections in different periods are obviously different.Therefore,it is of great significance to study the wide-area continuous surface activity characteristics of the Altyn Tagh fault to understand the dynamic transmission of plate collision,the tectonic-geomorphic effects and the seismic activities.In this paper,the middle segment of the Altyn Tagh fault(about 88°E-93°E)with strong tectonic and seismic activities is selected as the study area.Based on InSAR(Interferometric Synthetic Aperture Radar)technology,the wide-area and high-precision surface deformation field is obtained,and the coseismic deformation characteristics of four moderate and strong earthquakes occurred in this segment in recent years are analyzed to reveal the activity laws of different tectonic and geomorphologic units in the fault zone and its surrounding areas,and to identify the response of the focal mechanism to regional tectonic stress.This study can provide a reference for understanding the tectonic evolution of western China and the uplift and expansion process of northern Qinghai-Tibet Plateau,studying the seismic geological characteristics of Altyn area,and carrying out seismic risk assessment in strain accumulation area.The main conclusions and understandings are as follows:1.Based on the Sentinel-1A ascending orbit images(from April 2018 to April2020),the temporal surface deformation information of the middle segment of the Altyn Tagh fault is obtained by SBAS-InSAR technology.Based on the fluctuation characteristics of time series cumulative deformation results,it is concluded that the surface deformation is not only driven by regional tectonic activities,but also affected by non-structural factors caused by mass load changes or other external factors.The fluctuation analysis of surface displacement caused by land water storage load,atmospheric pressure load and non-tidal ocean load shows that the surface deformation of mass load is mainly in the vertical direction,and the main influencing factor is land water storage load.2.Using the inversion of two-dimensional deep-buried dislocation model,it is obtained that the strike-slip rate of the fault in the middle part of the Altyn Tagh fault(89.3°E-92.2°E)decreases from about 12.3 mm/a to 4.4 mm/a from west to east,and the locking depth is about 5.0-10.2 km.The strike-slip rate is similar to that obtained by previous inversion results,but the locking depth is smaller,which is related to the the detection accuracy of different SAR sensors and the vertical motion component of the crust.3.According to the results of InSAR surface cumulative deformation analysis and GPS surface deformation measurement,the study area is divided into four main deformation blocks,including the northern strike-slip fault basin area(uplift is about4-8 mm(cumulative deformation along the radar line of sight,the same below)),the western double fault clamping area(consists of south and north parts,uplift are about7-12 mm and 6-10 mm,respectively),the southern intermountain valley area(subsidence is about 4-9 mm)and the eastern fold active area(maximum uplift and subsidence are about 4 mm and 7 mm,respectively).It is pointed out that the strikeslip pull-off effect of the Sorkuri Basin,the extrusion uplift of parallel faults,the uplift movement of the Jinyan Mountain,the wind erosion and deposition of alluvial fans and fold activities have important effects on surface deformation.4.The coseismic surface deformation analysis based on D-InSAR technology is carried out for four moderate and strong earthquakes(27 September 2019 Ruoqiang Ms 4.8 Earthquake in Xinjiang,28 March 2019 Mangya Ms 5.0 Earthquake in Qinghai,16 June 2021 Mangya Ms 5.8 Earthquake in Qinghai,26 August 2021 Akesai Ms 5.5 Earthquake in Gansu)in the study area in recent three years.Combined with the focal mechanism solution of CMT,it is found that the seismogenic mechanism of three earthquakes with M ≥ 5 is thrust type,and the seismogenic faults are distributed along NW-NWW directions,which proves that the continuous NE compression of the Tibetan Plateau is the main inducing factor of earthquakes in this area.5.Using SBAS time series deformation results and the reference DEM data with different resolutions,the error analysis of the D-InSAR results is carried out.The results show that the D-InSAR technology has a high identification accuracy for weak signals of moderate and strong earthquakes in the Altyn area,and the atmospheric delay phase error has relatively little influence on the extraction of coseismic deformation calculation,while the residual terrain is the main factor affecting the solution accuracy of D-InSAR.