InSAR Monitoring And Mechanism Inversion Of Post-seismic Deformation Of The 2017 Mw7.3 Earthquake In Iran

The Zagros Mountains are formed by the collision of the Eurasian and Arabian plates over a long period of time.Strong plate activity results in frequent seismic activity in this region.On November 12,2017,an Mw7.3 earthquake occurred in the town of Sarpol Zahab on the Iranian border,which is the strongest earthquake occurred in Zagros Mountains since this century.It has a large influence area and significant post-seismic effect.Monitoring the post-seismic deformation of this earthquake by means of Geodesy can provide constraints for the simulation and inversion of post-seismic deformation mechanism,which is of great significance for the study of post-seismic deformation mechanism and geodynamic environment in this region.In this paper,the post-seismic deformation field of Iran earthquake is obtained by using the SBAS-InSAR technique.The viscoelastic relaxation deformation is simulated based on the regional subsurface stratification structure,and the influence of post-seismic afterslip is studied.The post-seismic deformation characteristics of Iran earthquake are discussed in depth.Firstly,in order to investigate the characteristics of post-seismic surface deformation of Iran earthquake,we collect 45 ascending and 45 descending sentinel-1A Satellite SAR images covering the study area,and use the SBAS-InSAR technology to acquire the post-seismic annual average deformation rate field along the LOS direction of the region during the period from November 2017 to September 2020.The results show that there are two deformation centers in both ascending and descending orbits,and the distribution of deformation is similar to that of Iran Mw7.3 co-seismic.The cumulative deformation of observation points continues to increase,and the deformation increases rapidly in the early stage after the earthquake.With the delay of time,the deformation growth rate gradually decreases.In some observation points with obvious deformation,the difference of deformation rate between the early and late stages is about 10 cm/yr.Secondly,based on the crustal layered structure and InSAR observed deformation,we use E-M and E-M-M models to simulate the deformation caused by the post-seismic viscoelastic relaxation effect,and use the grid search method to obtain the viscosity coefficients of the lower crust and upper mantle.The results show that the lower limit of the viscosity coefficient of the lower crust is 1×10¹⁹Pa·s,and the optimal viscosity coefficient of the upper mantle obtained by the E-M-M model is 7.5×10¹⁸Pa·s,and the viscoelastic relaxation is main controlled by upper mantle.The higher viscosity coefficient leads to no obvious relaxation deformation in the early post earthquake period.We further use the kinematic inversion method to inverse the distribution of post-seismic slip model with the constraint of early post-seismic deformation.The afterslip fault plane presents a similar thrust movement with the co-seismic fault,but its dip angle is smaller,which indicates that the location of afterslip is not in the same plane as the co-seismic fault.216 days after the earthquake,the cumulative slip amount of afterslip is 0.35m,and the released seismic moment is 1.25×10¹⁹N·m,equivalent to an earthquake with Mw6.67.The simulated LOS deformation and residual value show that the afterslip model fits well.Finally,the comparative analysis of afterslip and co-seismic fault shows that the post-seismic deformation in the first 7 months is due to the afterslip on the west side of the rupture of the co-seismic fault,and the energy released by afterslip is about 11%of the co-seismic fault;viscoelastic relaxation effect cannot fully explain the post-seismic deformation in the later period,which indicates that the later deformation is the comprehensive effect of viscoelastic relaxation and afterslip.However,the high viscosity coefficient obtained by using the late deformation as the constraint can explain the high-speed structural characteristics of the upper mantle in the Zagros area.Based on the time series InSAR deformation,this paper further analyzes the post earthquake deformation mechanism of the Iran earthquake,which is helpful to understand the nature of its co-seismic fault movement,and also provides a scientific reference for the interpretation of the underground structure and rheological properties of the Zagros fold belt.