Post-seismic Deformation Mechanism Study Based On InSAR Timeseries Technology

In recent years,earthquake disasters have occurred frequently around the world,causing serious economic losses and casualties.Thus,it is of practical significance to carry out researches on earthquakes.Based on the long-term studies,it is well known that there is always aseismic surface deformation after the earthquake and this process is named as post-seismic deformation.Post-seismic deformation is useful for geologists to determine the distribution of frictional behavior along fault,explore the lithospheric rheology,analyze the loading or unloading stress state of nearby faults and estimate the seismic hazard.Therefore,post-seismic deformation research is the key point in the earthquake study.Geodetic technique is the only observation method that can provide accurate surface deformation data.Interferometric synthetic aperture radar(InSAR),which is a relatively new geodetic technique and is irreplaceable in the geodetic techniques,has the characteristics of high precision and large range.In this study,Sentinel-1 data and InSAR time seires technique is employed to study the post-seismic deformation of large earthquakes occurred in Zagros orogenic belt,Makran subduction and Tibet plateau,where the GPS observation states are very sparse.The main research contents include:(1)We use 2.3 years(November 2017-February 2020)InSAR time series data to study post-seismic deformation processes following the 13 November 2017 Mw 7.3 Kermanshah,Iraq earthquake.Based on the temporal characteristics of post-seismic displacement,we consider two time periods.Time period 1 is the 0-0.6 years after the earthquake and time period 2 is the 0.6-2.3 years after the earthquake.Then,the postseismic deformation mechanisms of these two time periods are inverted.For time period 1,the post-seismic displacement was due to the afterslip along the up-dip extension of the rupture zone.This means that the velocity-strengthening area is above the velocity-weakening area along the buried coseismic fault.The moment released by afterslip was 12%of that released by the coseismic slip,which is lower than other earthquakes.The main explanations for short afterslip duration time and low released moment were the relatively low heat flow and pore-fluid pressures in the Zagros region.For the time period 2,the post-seismic displacement field was not consistent with afterslip but more consistent with viscoelastic relaxation in the lower crust.The best-fitting lower crustal viscosity is ηlc=1-0.4+0.8×1019Pa·s and the best-fitting upper mantel viscosity is ηum=1-0.4+∞ ×1019 Pa·s.which means a relatively strong lower crust and upper mantle.This was consistent with the indirect measurement which are the relatively lower heat flow,low average Vp/Vs value and the existence of the upper mantle high-velocity anomalies in the Zagros region.(2)We use 1-7.5 years(December 2014-April 2021)InSAR time series data to study post-seismic deformation processes following the 24 September 2013 Mw 7.7 Balochistan,Pakistan earthquake.Based on the temporal characteristics of postseismic displacement,we consider two time periods.Time period 1 is the 1-4 years after the earthquake and time period 2 is the 4-7.5 years after the earthquake.Then,the post-seismic deformation mechanisms of these two time periods are inverted.For both time periods,the post-seismic deformation is caused by the widespread aseismic slip along the megathrust.The aseismic slip is dominated by the left-lateral slip,the area of aseismic slip is 5500 km2,the magnitude aseismic slip is 80 cm,and the corresponding moment is Mw7.3.This widespread aseismic slip is caused by the mainshock-induced stress changes according to the Coulomb stress calculation.Aseismic slip along the subduction fault provides quantitative data for accurately constraining the lower boundary of velocity-weakening area of the seismogenic area along the megathrust.The left-lateral aseismic slip appears at the distance of 220 km from the deformation front,that is,the lower boundary of the velocity-weakening area of the subduction fault is at most 220 km from the deformation front,and the region north of 220 km from the deformation front in the subduction fault is the velocitystrengthening zone,stable slip occurs,and no earthquake will occur.This result not only accords with the historical record of few large subduction earthquakes in the Maklan subduction zone,but also means that the probability of large subduction earthquakes larger than Mw 8.8 in the Makran subduction zone is very low in the future.(3)We use 13-20 years(November 2014-July 2021)InSAR time series velocity data to study the late post-seismic deformation velocity field of the 14 November 2001 Mw 7.8 Kokoxili earthquake.First,the deformation velocity caused by the interseismic slip along the major active faults in Tibet was simulated.Comparing the simulated deformation velocity with the observed deformation velocity,it is found that the maximum ratio of the simulated deformation velocity to the observed velocity is 0.42,indicating that the viscoelastic relaxation caused by the 2001 Kokoxili earthquake is still continuing.Then the rheological structure of the Kokoxili region was explored by using a mixed model that combines the viscoelastic relaxation mechanism with the buried elastic dislocation model.The best estimate viscosities areηlc=1-0.44+0.78 ×1019 Pa·s and ηum=1+0+0.78 ×1020Pa·s for the lower crust and upper mantle,respectively.The inverted lower curst viscosity was compared with the steady-state viscosity estimated by the Burgers theory model and found they are almost the same.The the results obtained in this study were compared with those previous studies that used the early post-seismic displacement following the earthquake,the effective lower crustal viscosity of the Kokoxili region shows a logarithmic trend with time.Finally,the estimated viscosity of the lower crust is higher than that indicated by the crust flow model(1017Pa·s),which means that the crust flow model cannot explain the uplift of the North Tibet.The main innovation points are as follows:(1)Based on the InSAR post-seismic deformation field from 0-2.3 years after the 2017 Kermanshah earthquake,a viscoelastic relaxation model was constructed to invert the viscosities of the lower crust and upper mantle in the northwestern zagros fold thrust fault zone.It is found that the rheological strength model of the lithosphere in this area is "jelly sandwich" type.(2)Based on the InSAR post-seismic deformation field during 1-7.5 years after the 2013 Balochistan earthquake,a aseismic slip model was established.It was found that the post-seismic deformation was caused by the left-lateral aseismic slip along the megathrust fault in the subduction zone,and the quantitative constraint of the lower boundary of the velocity weakening zone in the seismogenic zone of the megathrust fault in the Maklan subduction zone was obtained:The lower boundary of the velocity weakening zone in the seismogenic area of the megathrust fault in the subduction zone is at most 220 km from the deformation front.(3)Based on the InSAR post-seismic deformation field during 13-20 years after the 2001 Kokoxili earthquake,for the first time,the viscoelastic relaxation caused by the earthquake has been confirmed quantitatively until At least July 2021,and the viscoelastic relaxation model has been established to invert the effective viscosity coefficient of the late post-seismic period in the lower crust and upper mantle,which proves that the rheological structure in the earthquake region has almost reached a stable state.