Quasi-dynamic Simulation Of The 2017 M_w7.3 Iran Earthquake Based On The Rate And State Friction Law

Nowadays Earthquake and its secondary disasters(such as tsunami,landslides,etc.)have become a major hidden danger for the development of human society.We must face and address this problem.Modern seismology believes that tectonic earth-quakes are that rocks on both sides of the fault are locked by friction and the stress near the fault gradually accumulates under the drive of tectonic movement until the shear stress is greater than the static friction,which leads to the sudden instability of rock friction and the phenomenon of rupture to release the accumulated strain energy.Therefore,the study of rock friction properties has become an important part of mod-ern seismic research.A large number of rock friction experiments have made scientists realize that the friction coefficient observed in classical friction theory is only its low-order approximation.The rate-state friction criterion proposed on the basis of a large number of experimental data can more accurately describe the rock friction properties.Based on the single spring slider model,this paper analyzes the conditions of stick-slip instability of the spring slider system under the rate state friction criterion.The results show that:when the spring stiffness of the spring slider system is less than the critical stiffness(k_cr=?(b-a)/D_c),the system will be unstable under small disturbances.In order to have a more intuitive understanding of the stability characteristics of the spring slider system,this paper uses the spring slider model and the slowness criterion to conduct a numerical simulation to show the stable state and instability of the spring slider.It is worth noting that the current rate state friction criterion is based on the normal temperature and pressure and low speed rock friction experiment.However,the friction properties of rock are also affected by factors such as temperature,pressure,and rock friction rate.Therefore,the rock friction properties under high temperature,high pressure,high speed,and other conditions need to be further studied.In this paper,the fault quasi-dynamic model established on the basis of the rate-state friction criterion and the continuum model successfully simulates periodic seismic events on the fault.The simulated interseismic creep,co-seismic rupture,and afterslip characteristics are consistent with the actual seismic characteristics.However,observa-tions by broadband digital seismographs and digital seismograph networks,as well as strain gauges,gravimeters,tide gauges,and GPS instruments show that earthquakes that occur during the periodic stick-slip motion of the fault only release part of the tectonic stress on the fault.Another part of the energy is released in the form of vari-ous other fault slips,such as non-volcanic tremors,low-frequency earthquakes and slow slip events.Scientists have proposed a variety of theoretical models to simulate seis-mic activity on faults,but they all have defects.Therefore,in this paper,the fault quasi-dynamic model is used to simulate various slip forms such as interseismic creep,co-seismic rupture,afterslip,and slow slip,and the control factors are discussed.The results show that the slip on the fault is affected by the velocity weakening zone,ve-locity strengthening zone and the ratio of W/h^*.When the fault plane is the velocity strengthening zone,the fault plane will creep under the tectonic movement or the stress changes caused by coseismic rupture.Under the tectonic movement,velocity weakening zone will be locked due to the friction.When the shear stress in the locked area accu-mulates to be greater than the static friction,the rupture slip will occur in the velocity weakening zone.Whether it is the earthquake rupture slip or slow slip depends on the ratio of W/h^*.Next,this paper selects the M_W7.3 earthquake that occurred on the border between Iran and Iraq on December 11,2017 as the research object.Because the fault geometric parameters,co-seismic slip distribution and afterslip distribution of seismogenic faults are the basis for further study of the earthquake seismogenic mechanism,this paper first uses geodetic inversion methods to estimate the kinematic characteristics.First,the paper uses Interferomtric Synthetic Aperture Radar(InSAR)and InSAR time series analysis technology to obtain the coseismic deformation and post-seismic deformation time series of 222 days.The results show that the earthquake and its afterslip have a large-scale and long-term transformation effect on the topography,which indicates that the earthquake and its post-seismic deformation play an important role in the evolution of the terrain of the foreland system.Subsequently,in this paper,the multi-peak particle swarm global optimization algorithm is used to estimate the fault geometric parameters of co-seismic rupture and afterslip.The inversion results show that this is a low-dip thrust earthquake;its co-seismic slip mainly occurs on the fault plane with strike angle of 351.75°and dip angle of 14.47°,and the slip distribution mainly concentrated between the depth of 12 km to 17 km,and composed of two sliding areas,the centers of the two are separated by about 16 km;but after the earthquake,the afterslip occurs at a lower inclination that is nearly parallel to the ground,the strike angle is 355.72°,the dip angle is 6.71°,and the afterslip is mainly concentrated between the depth of10 km to 14 km.The inversion results show that the earthquake occurred on a blind fault in the crystalline basement of the Zagros Fold Belt,and the rupture propagated to the top of the crystalline basement layer,which shows that the seismic activity will complete the crustal in the basement of the earthquake zone.The stress changes caused by coseismic rupture will drive the slippage structure around the seismogenic zone to produce fault creep.In the seismic risk analysis,this paper believes that the Zagros Fold Belt is undergoing rapid crustal shortening movement,which means that the area has a potentially high seismic risk.Based on the inversion results,this paper uses quasi-dynamic simulation to analyze the effect of double dip angles faults on co-seismic rupture,afterslip,and earthquake recurrence cycle.The results show that,compared with the single dip angle fault,the smaller the dip angle of the upper half of the double dip angles fault,the greater the coseismic deformation produced during coseismic rupture,and conversely,the smaller the coseismic deformation produced;When the dip angle of the upper half is less than the dip angle of the lower half,bending of double dip angles faults outside the velocity weakening zone will not affect coseismic rupture.For the afterslip,the model with the smallest co-seismic deformation has the smallest maximum total afterslip,and the model with the largest co-seismic deformation has the largest maximum afterslip.From the perspective of earthquake recurrence period,it has a similar law to afterslip,mainly because the coseismic rupture releases the strain energy accumulated between the earth-quakes,and the small amount of coseismic sliding means that the accumulation time is short.Subsequently,this paper uses quasi-dynamic model to simulate the co-seismic slip and afterslip of the Iranian earthquake.The results show that the co-seismic and post-seismic deformations simulated by the quasi-dynamic double dip angles fault model can fit the observation well.The two parameters of the quasi-dynamic fault model searched during the fitting process:effective normal stress(?)_nand the characteristic slip distance D_care consistent with the actual effective normal stress(?)_nin the crust and the results of other quasi-dynamic researchers.In the analysis of the simulation results,this paper believes that the quasi-dynamic fault model can more accurately reflect the coseismic surface deformation,and will not be affected by the signal-to-noise ratio of the observed signal;the quasi-dynamic fault model reflects that this earthquake not only triggered afterslip above seismic rupture but also below seismic rupture,which was not revealed by the geodetic inversion.Therefore,this paper believes that fault quasi-dynamics can better reproduce the seismic dynamic process.