FEM Simulations Of Static Coulomb Failure Stress Changes Based On Hydro-mechanical Coupling: Applications To Earthquake Triggering

Earthquake stress triggering has been one of the hotspots in earthquake science research in recent years.The stress triggering model that stress transfer induced by an earthquake may trigger the subsequent events has been supported by the studies of a large amount of earthquakes.The key of the stress triggering theory is how to calculate the Coulomb stress changes.In the past,the co-seismic static Coulomb stress changes were calculated by the analytical solution of Okada,which barely considered the impact of fluid on the mechanical behavior of the solid skeleton.And in the calculation,the co-seismic dislocation is usually the only factor,without the effect of other post-seismic factors,such as afterslip and pore fluid migration.Actually,the fluid is widely available in underground rock,and has a very important influence on the solid deformation.After the mainshock,the post-seismic effect such as the afterslip and pore pressure changes gradually becomes prominent,and the resulting static Coulomb stress changes also gradually come into play.Thus,based on the fully-coupled poro-elastic theory,I build three typical fault-slip models(Strike-slip fault,thrust fault and normal fault),by means of the finite element method,to study the spatial distribution of co-seismic static Coulomb stress changes and compared with the computed results of the traditional method to investigate the effect of hydro-mechanical coupling.On that basis,I calculate the temporal-spatial distributions of Coulomb stress changes generated by the afterslip and pore fluid migration,then quantitative analyze the effect on the earthquake triggering.The main results are as follows:1.Three different earthquakes exhibit completely different pore-pressure change distributions in 3D space.The transient pore-pressure change after a strike-slip faulting event is axis symmetrical with increased and decreased areas evenly distributed in a juxtaposed pattern.And the static Coulomb stress decreases significantly in the near field due to the fluid-solid coupling effect.The spatial distributions of the pore-pressure generated by the thrust shock and the normal shock have the similar patterns,but opposite positive and negative areas.The pore-pressure rises in the epicenter of the thrust model but drops in the normal fault.Compared with the traditional method of Coulomb stress change calculation,the pore-pressure changes of the thrust faulting earthquake can reduce the stress shadow,which may trigger more aftershocks.Conversely,the normal fault event increases the region of stress shadow near the epicenter and lowers the probability of aftershocks.2.Three different earthquakes also exhibit different post-seismic effect.In the medium with the same permeability,the pore-pressure change of a strike-slip earthquake is attenuated slowly in 20 days,while it drops shapely in 10 days for the thrust and normal shock.Finally,pore-pressure changes caused by all three different earthquakes vanish in 60 days.Although the pore pressure change continues to evolve after the quake,it has little effect on the static Coulomb stress changes due to the relatively small contribution of pore-pressure change.The distributions of co-seismic and post-seismic Coulomb stress changes are almost the same.Therefore,the ability of post-seismic pore-pressure changes to the seismic triggering is weak,and the physical mechanism of post-seismic fluid migration triggering earthquakes should be studied further.3.Afterslip can cause different impact on the three different earthquakes.The Coulomb stress changes of the strike-slip earthquake will be raised and more aftershocks will be triggered.The increasing area of Coulomb stress changes generated by the thrust shock will expand in space and trigger aftershocks that are farther away from the mainshock.Finally,the patterns of the normal fault event are barely affected,with only slight expansion along the fault.4.Besides,the impact of the Lushan Ms7.0 earthquake on the aftershocks is calculated by the traditional method.The computed results show that most of the aftershock hypocenters did not occur on the region with the positive Coulomb stress changes caused by the Lushan mainshock.In conclusion,the calculation of Coulomb stress changes caused by earthquakes is not a simple question.It involves many functions,such as the earthquake occurrence,post-seismic response,tectonic stress field and the change of medium environment.Only making full use of the numerical simulation,considering hydro-mechanical coupling effect and other physical factors,is likely to get close to the real coulomb stress changes,thus make the theory of earthquake triggering more perfectly,and analyze the seismic trend and earthquake disasters more scientifically and reasonably.