Numerical Simulation Study On Major Influence Factors Of The Aftershock Activity Decay

Evolution of postseismic seismicity and prediction of strong aftershock are major concerns of the government and the public during the process of earthquake rescue and relief,prevention of secondary disaster and post-disaster reconstruction.The fundamental here is deep understanding of statistical characteristics of earthquake sequence and the mechanism for aftershock activities.In this thesis,a physical conceptual model of aftershock activities is constructed by considering inhomogeneity of the main shock rupture,loading and aftershock stress interaction.The complex aftershock sequence is simulated by a numerical simulation method.The agreement between simulated output and the actual statistical characteristics of aftershock activities is examined by the G-R relationship and modified Omori formula as a basis for optimization of the model parameters.While checking the stability of the numerical results,the major factors that influence aftershock activities are examined,and the possible factors controlling different characteristics of the temporal evolution of aftershocks for several events are also discussed.Assuming the rupture of residual asperities on the main shock rupture plane leads to the aftershock,a data set with N residual asperities distributed on a fault plane is generated.According to the statistical relationship between magnitude and rupture scale,the assumption that the aftershock distribution scale may be roughly equal to a main shock rupture scale,the rupture scale is set.The size of residual asperities follows fractal distribution,the fractal dimension is adjustable,and the asperities are randomly distributed in space.The initial perturbation strength of residual asperities follows the lognormal distribution.I assume that the stress from the postseismic stress relaxation of viscoelastic medium and the afterslip in the upper or lower section of seismogenic layer directly load the residual asperities which in turn rupture.The Coulomb failure criterion is considered as the instability criterion.The instability of each asperity is therefore considered as an earthquake event.Considering the stress loading to the surrounding area due to the asperity instabilities,and combining the mechanical interactions between the residual asperities,the artificial aftershock sequences?occurrence time,location and magnitude?are simulated under different medium conditions,mechanical loading conditions and fracture criteria.The aftershock events randomly following 100 magnitude 7.0 main shocks are simulated under different stress loading conditions of stress relaxation,afterslip,the combination of stress relaxation and afterslip,respectively.The results show that:?1?considering stress relaxation alone,the p value of the aftershock sequence is slightly lower than that of the natural earthquake sequence,however,the statistical characteristics are generally consistent with those for the natural aftershock activities,indicating that the stress relaxation is one of the most important factors controlling the aftershock activities.?2?Considering afterslip alone,the b value of G-R relationship and the p value of the Omori formula of the aftershock activities are consistent with the statistical characteristics of the natural earthquake activities,but the aftershock activity duration is short,therefore the contribution from afterslip to the aftershock activities may be important in the early stage after a main shock.To simulate a longer duration of aftershock activities,it is necessary that more evident stress loading should be in the later stage.?3?Under a combination of stress relaxation and afterslip condition,the b value of G-R relationship,the p value of the Omori formula,the characteristic of aftershock time-series attenuation and clustering are consistent with the associated patterns of the natural aftershocks.Therefore,in the early stage of the earthquake attenuation,the afterslip loading may play the leading role.As the elapsed time increase,stress relaxation may play a more important role.According to the parameters selected in this thesis,it is inferred that stress loading from a combination of stress relaxation and afterslip control the aftershock activities.Based on three different stress loading patterns as mentioned before,the major influence factors are systematically discussed,including the initial stress,the elastic modulus of viscoelastic medium,and the viscous coefficient in the stress relaxation process,and the thickness,elastic modulus,regional fault sliding velocity,rheological parameter,and stress perturbation and so on during the afterslip process.The simulation results show that,except for the fractal dimension,the other factors all affect the duration and decay rate of the aftershock activities.The duration of the aftershock activities is obviously influenced by the viscous coefficient,the thickness,and the sliding velocity of the rate-strengthening zone.The decay rate of the aftershock activities is mainly controlled by viscous coefficient,thickness,sliding velocity,rheological parameter,and stress perturbation.The contributions from remaining factors are relatively weak.The fractal dimension determines the ratio of small and large earthquakes,the initial stress of the stress relaxation process and stress perturbation of rate-strengthening zone from the main shock have a slightly effect on the temporal evolution of aftershock activities,and the contributions from other factors are subtle.Three typical events,namely the Tangshan?Kunlun and Wenchuan earthquakes,are similar in magnitude;However,the duration and decay rate of the aftershocks activities are different.The sequence of Tangshan earthquake has a long duration and a slow decay,and the sequence of West Kunlun Mountain earthquake has a short duration and a fast decay.Based on the previous studies about the source rupture process,source parameters,aftershock relocation,tectonic loading rate,and viscoelastic properties of the medium and so on,the effects of the main controlling factors on aftershock activities are discussed from both the qualitative analysis and numerical simulations.First,the qualitative study shows that the reason why Tangshan aftershock sequence has a long duration and a slow decay is the increase of the thickness of the rate-strengthening zone,the slower long-term sliding velocity?0.12⁴mm/a?,the better viscosity(higher viscosity coefficient,1.0×10¹⁹Pa·s),and the lower stress drop?1.2⁴MPa?.On the contrary,the rate-strengthening zone of the Kunlun earthquake is thinner,which results from the upwelling of low density materials from low crust.The factors,including faster loading?10¹2mm/a?,weaker viscosity(lower viscosity coefficient,5.00×10¹⁷Pa·s),higher stress drop and so on,may control the time evolution of the Kunlun sequence.In addition,based on the physical conceptual model of aftershock activities,which is similar with the"medium environment"and"stress variation"of the Tangshan-Kunlun earthquakes,the artificial aftershock sequence,including occurring time,location and magnitude,are simulated and the possible effects are discussed.The simulation results show that there are some similarities in the temporal attenuation between the simulated aftershock sequences and the natural earthquakes.The parameters and attenuation characteristics of simulated earthquakes are similar to those of the typical earthquakes,which further confirm the effects on the duration and that decay rate are controlled by the major influence factors of aftershock activities.The magnitude of the Wenchuan earthquake?M8.0?is similar to that of Tangshan and Kunlun,but there is a great difference between the aftershocks.According to the influence factors of aftershocks,the relatively slow slip rate?1²mm/a?of Longmenshan fault zone and the obviously increased crustal thickness?10²0km?in the focal zone are the main factors that control the long duration and slow attenuation of the Wenchuan aftershocks.The relatively high stress drop?6¹8MPa?is one of the main factors of the rapid decay after a short time of the Main earthquake.Taking the actual research results of Wenchuan earthquake as the model parameters and the statistical relationship as a constraint condition,the aftershock activities are simulated.The result shows that the viscous coefficient of the lower lithosphere in Wenchuan area between 6.0×10¹⁸⁷.0×10¹⁹Pa·s and the rheological parameters between 0.58⁰.64 are more reasonable.