Abstract: The Aketao M_S6.7 earthquake occurred on November 25, 2016, which was located at the intersection of Gongur extensional system and Pamir frontal thrust. This region is characterized by complex fault structure, high altitude, complex terrain conditions, sparsely populated and few observed data, so the conventional geodetic survey technology is difficult to obtain comprehensive surface deformation information, while InSAR can take advantage of its all-weather, all-day, large-area and high-density continuous monitoring of ground motion. Therefore, this study takes M_S6.7 earthquake as the research object to carry out the co-seismic deformation field extraction and fault static slip distribution inversion. Firstly, the co-seismic deformation field was obtained by using ascending and descending data of Sentinel-1A. The results indicate that the interferogram spatial decorrelation is more serious in the north side of fault, which is affected by the steep terrain. The fringes in the south side of fault were distributed as elliptical semi-petal shapes, and the fringes are smooth and clear. The northern and southern part of the fault was asymmetric: The interferogram fringes of the southern part were dense while fewer fringes were formed in the northern part, and the fringes were semi butterfly-shaped on the surface. The horizontal displacements dominated the co-seismic deformation in this event, with magnitude of 12cm in ascending and -21cm in descending. The deformation occurred mainly on the south wall of fault. Based on the right view imaging of Radar, the co-seismic deformation is consistent with the movement features of dextral strike-slip fault and the focal mechanism provided by USGS and GCMT. The cross section of aftershocks after precisely positioning showed that the dip angle of fault is larger above the depth of 15km, which is generally manifested as the shovel-like structure with the dominant tendency of southward dip. By conducting a comprehensive analysis of deformation feature and aftershocks profile, we proposed that the southwest-dipping Muji Fault is the seismogenic fault. Secondly, a large area of continuous deformation images obtained by InSAR technology contains millions of data points and there is a high correlation between them. In order to ensure the calculation efficiency and inversion feasibility in the inversion process, the quadtree sampling method was used to reduce the number of data points and the datasets were finally obtained that can be received by the inversion system on the basis of retaining the original details of the deformation field. The two tracks InSAR datasets which were down-sampled by quadtree method were used to constrain the inversion to retrieve the fault geometry parameters and slip distribution. The single-segment and two-segment static slip distribution on the fault plane based on uniform elastic half space model were constructed during inversion process. The F-test of fitting residuals based on single-segment and double-segment fault model show that the population variance of the two models was significantly different at the confidence level of 95%, and the variance of the double-fault model was smaller. Through the comprehensive analysis of predicted deformation field, residuals and F-test, it is considered that the simulated results of double fault model are better than that of the single, and the observation data can be better interpreted. The result shows that the simulated co-seismic deformation field and its corresponding observed values were consistent in morphology and magnitude, and the correlation between observed and modeled is up to 0.99. In addition, as can be seen from the spatial distribution and frequency histogram of residuals, the overall residual was not large, mainly concentrated in the range of -0.2~0.2cm with the characteristics of normal distribution. However, there were still some larger residuals on the near fault in ascending track, which may be related to the simplified model. There were two patches with significant slip distribution on each segment and the rupture basically reached the surface. The slip was mainly distributed along the downdip range of 0~20km and was about 50km along the fault strike. The rupture reached the surface and the peak slip of 0.7m was at the depth of 9km. The western segment is dominated by the right-lateral strike-slip and the eastern segment is dominated by the right-lateral strike-slip with slightly normal faulting. The seismic moment derived from inversion was 8.81×10¹⁸N·m, which is equivalent to M_W6.57. The average slip angle obtained by inversion is -175° in the west section and -160° in the east section. The synthetic analysis holds that the source characteristics of the M_S6.7 earthquake was characterized by dextral strike-slip with a slightly normal component, which was composed of two sub-seismic events. The western section was basically pure right-lateral strike-slip with a dip angle of 75°, while the eastern was characterized by dextral strike-slip with a small amount of normal component with a dip angle of 55°. The Aketao earthquake occurred on the northern Pamir salient and its tectonic deformation was mainly characterized by crustal shortening, strike-slip and internal extension of the frontal edge observed by GPS. Generally speaking, the Pamir salient was blocked by nearly east-west South Tian Shan in the process of strong northward pushing under the action of NE direction pushing of Indian plate, and “hard and hard collision” occurred between them. The eastern part of Pamir salient extruded eastward along the nearly NS trending Gongur extensional system, and at the same time rotated clockwise, which caused the nearly EW extension since the Late Quaternary. The Aketao earthquake is a tectonic event occurring at Gongur Shan extensional system, which shows that the pushing of the Indian plate in the NE direction is continuously strengthened, and also implies that the internal crustal deformation of the Pamir Plateau is still dominated by extension in EW direction, which is basically consistent with the present observation of GPS.

Key words: Aketao earthquake, InSAR, co-seismic deformation field, slip distribution

摘要： 文中利用Sentinel-1A升、 降轨数据获取了2016年新疆阿克陶地震的同震形变场。 形变场以水平运动为主, 形变主要发生于断层南盘; 升、 降轨最大形变量分别约为12cm和-21cm; 基于雷达影像观测右视成像的特点可知阿克陶地震具有右旋走滑的破裂特征, 结合形变场形态特征与余震剖面推断认为, 阿克陶地震的发震断层为S倾的木吉断裂。 基于均匀弹性半空间模型反演双断层面的静态滑动分布, 结果显示: 2个断层面上各存在1个椭圆状的滑动集中区, 破裂基本到达地表, 滑动主要发生在沿断层面走向长约50km、 沿断层面倾向方向0~20km的范围内, 最大滑动量位于约9km深处, 量级约为0.7m; 西段主要以走滑为主, 东段为走滑兼具少量正断性质, 反演得到的地震矩约为8.81×10¹⁸N·m, 相当于M_W6.57。 综合分析认为, 本次地震的震源特征为右旋走滑兼有少量正断分量, 是发生在公格尔拉张系内拉张环境下的构造地震事件。

关键词: 阿克陶地震, InSAR, 同震形变场, 滑动分布