南天山前陆盆地的一次典型逆冲破裂事件——2020年新疆伽师6.4级地震

A TYPICAL THRUST RUPTURE EVENT OCCURRING IN THE FORELAND BASIN OF THE SOUTHERN TIANSHAN:THE 2020 XINJIANG JIASHI M_(S)6.4 EARTHQUAKE

2020年1月19日,新疆维吾尔自治区伽师县发生6.4级地震,震中位于南天山西段的柯坪推覆体附近。文中基于Sentinel-1 SAR卫星升、降轨数据获取了此次地震的InSAR同震形变场,InSAR结果表明,地震形变场的EW向跨度约为40km,SN向跨度约为20km,升轨结果LOS向最大抬升量为5.9cm,最大沉降量为3.7cm;降轨结果LOS向最大抬升量约为6.4cm,最大沉降量约为2cm。以InSAR观测值为约束,首先采用均匀滑动模型对发震断层参数进行反演,为提高计算效率,在反演前利用四叉树分割方法对原始的高分辨率InSAR观测值进行降采样;然后在均匀滑动模型反演结果的基础上,引入基于约束最小二乘的分布式滑动反演方法,计算出此次地震发震断层面的精细滑动分布。结果表明,此次地震的累计地震矩为1.73×10^(18) N·m,合矩震级M W6.1,其发震断层为柯坪断裂。对区域GPS面应变率、构造动力背景和区域深浅部构造特征进行了综合分析,认为此次地震是继1997—2003年伽师6~7级地震震群后发生在南天山强烈逆冲推覆构造环境中的又一次典型的逆冲破裂事件。这些强震是南天山前陆盆地内不同级别、不同性质的断裂相继破裂的结果,共同受控于南天山的逆冲推覆作用。

A M_(S)6.4 earthquake occurred on January 19 th,2020 at Jiashi,Xinjiang,this earthquake is another strong earthquake since the Jiashi M_(S)6~7 earthquake swarm events from 1997 to 2003,and the epicenter was located near the Kalpin nappe in the western part of southern Tianshan.The Kaplin nappe is located in front of southern Tianshan Mountains,which is a thin skinned thrust belt composed of a series of nearly NE-SW thrust nappes under the strong and sustained regenerative orogeny in the Tianshan area.There are some differences in focal positions and fault parameters given by different institutions,therefore in this paper,high resolution InSAR coseismic deformation fields were obtained based on the ascending and descending tracks of Sentinel-1 SAR images to obtain the focal mechanism.The 30m resolution SRTM DEM data is chosen as the external DEM to eliminate the phases caused by topography,the robust Goldstein filtering is applied for phase smoothing,and the Delaunay minimum cost flow method is used for phase unwrapping.The variation range of interference fringes shows that the east-west span of the earthquake deformation field is about 40km,and that of the north-south direction is about 20km,the displacement results show that the maximum uplift displacement is 5.9cm and the maximum subsidence is 3.7cm along the LOS direction of the ascending data,the maximum uplift displacement is 6.4cm and the maximum subsidence is 2cm along the LOS direction of the descending data.And then the InSAR-derived deformation fields are used to obtain the seismogenic mechanism of this earthquake,and to improve the computational efficiency,the quadtree segmentation method is used to desample the original high-resolution InSAR observations before inversion.The coseismic slip distribution of the causative fault was inversed using a uniform sliding inversion method based on a Bayesian approach,and then the fine slip distribution of the fault plane of Jiashi earthquake was inversed using the distributed slip inversion method based on the constrained least squares.It should be noted that the fault plane is set as the shovel shape according to the geometric relationship between the seismogenic fault parameters inverted by uniform sliding and the exposed position of the Kapling Fault on the surface during the distributed slip inversion.According to the difference between the observed and simulated values,it can be seen that the residual error of the inversion model is small,indicating the reliability of the inversion result.The final result shows that the epicenter is located at 39.9°-N,77.28°-E and the strike and dip angle of the seismogenic fault is 276°and 10.7°,respectively,the maximum dip slip and strike slip of fault plane is about 0.29m and 0.03m,respectively,which are located at the depth of about 5km underground.The cumulative coseismic moment is 1.73×1018 N·m from InSAR inversion,which is equal to the moment magnitude of M W6.1 and the Kalpin Fault is supposed to be the causative fault.Then,regional GPS-derived surface strain rate,tectonic dynamic background,and regional deep and shallow structures were comprehensively analyzed.The results show that the Jiashi M_(S)6.4 earthquake is a typical thrust event that occurred in the thrust nappe of the southern Tianshan.The 2020 Jiashi event and the 1997—2003 Jiashi M 6~7 earthquakes swarm are the results of rupture of many faults with different scales and properties.And these events are all controlled by the thrust nappe of southern Tianshan.