九寨沟M_S7.0地震的InSAR观测及断层破裂研究

InSAR Observation and Fault Rupture Study of the Jiuzhaigou M_S7.0 Earthquake

为提高九寨沟2017-08-08 M_S7.0地震的InSAR同震形变场精度,厘清发震断层的构造形态及形成机制,对形变区相干系数进行统计分析并确定相干性阈值,利用GACOS对形变场进行大气校正,再根据余震分布和地质背景确定发震断层的基本形状,最后基于Okada弹性半空间位错模型反演发震断层的滑动分布。改正后的InSAR同震形变场显示,视线向最大下沉为25 cm,最大抬升为10 cm,分别位于震中西北和东南,形变长轴为北西向,主要形变区位于发震断层西部。改正后的InSAR形变场残差均方根较改正前小,最大滑动量为0.9 m,平均滑动角为-0.5°,破裂主要集中在地下1～20 km范围内,矩震级为6.5,与USGS和GCMT等机构的结果一致。研究表明,利用GACOS改正九寨沟InSAR同震形变场对提高形变场精度具有一定的作用,反演断层滑动分布的结果较改正前差别不明显,发震断层的属性与虎牙断裂北段的性质基本一致。结合余震重定位结果可以推断,发震断层为虎牙断裂的北向延伸部分,此次地震事件为巴颜喀拉地块南东向扭转与华南地块碰撞的结果,中下地壳粘性流体的差异分布是导致虎牙断裂倾角变化的主要原因。

A Ms 7.0 earthquake occurred in Jiuzhaigou,Sichuan,on August 8,2017.In order to improve the precision of the InSAR coseismic deformation field of the earthquake and to clarify the structural morphology and formation mechanism of the seismogenic fault,the coherence coefficient of the deformation zone is statistically analyzed to determine the coherence threshold.Atmospheric correction of the deformation field is performed by GACOS(general atmospheric correction on-line service).Then,the basic shape of the seismogenic fault is confirmed according to aftershock distribution and geological background.Finally,the slip distribution of the seismogenic fault is inverted based on the Okada elastic half-space dislocation model.The corrected InSAR coseismic deformation field shows that the maximum sink is 25 cm and the maximum lift is 10 cm in LOS,which are respectively located in the northwest and the southeast of the epicenter.The long axis of deformation is in NE direction and the main deformation zone is located on the west of the seismogenic fault.The inversion results show that the RMSE of the corrected InSAR deformation field is smaller than that before correction.The maximum slip amount is 0.9 m,the average slip angle is-0.5°and the moment magnitude is M W6.5.The rupture is mainly concentrated in the range of 1~20 km underground.The results are consistent with the results of USGS and GCMT.The consequences indicate that using GACOS to correct the InSAR coseismic deformation field of Jiuzhaigou has some effect on improving the accuracy of the deformation field,but the inversion result of fault slip distribution is not significantly different than before correction.The attributes of the seismogenic fault are basically consistent with the properties of the northern segment of the Huya fault.Combined with the results of the aftershock repositioning,it can be inferred that the seismogenic fault is the northern extension of the Huya fault.The earthquake event is the result of the collision between the southeast torsion of Bayankala block and the South China block.The difference distribution of viscous lower crust is the main reason for the change of the slope angle of the Huya fault.