Experimental And Numerical Simulation Study Of The Effects Of Stress Perturbations On Fault Sliding

Natural fault zones are continuously subject to variations in normal and shear stress,ranging from slowly and steadily changing tectonic stress caused by plate movement and other tectonic processes to the transient and periodic stresses caused by solid tides,surrounding strong earthquakes,reservoir water storage,and industrial injection.These stresses act on faults to cause fault stress adjustment,change seismicity on faults,and lead to the advance or delay of strong earthquakes,the so-called “stress triggering.”To improve our understanding of the mechanism and conditions of stress-triggered earthquakes,this thesis first performed a series of frictional experiments using a servo-controlled loading machine to study the effects of stress perturbation on stick-slip instability,the acoustic emission activity,and shear strain evolution by modulating the stress perturbation with different periods and amplitude to normal and shear loading.The feasibility of predicting fault instability by the acoustic emission activity is investigated.Then,the water-induced fault slip experiments were carried out on saw-cut granite,sandstone,and limestone samples to investigate the effects of water injection on the fault slip behaviors using a triaxial rock friction experimental device.Moreover,the mechanism and influence factors of the injection-induced earthquake are discussed based on the influence of water injection on fault sliding behavior and the velocity dependence of faults.Finally,numerical models using the rate-and state-dependent friction law were applied to investigate the role of localized stress perturbation on fault reactivation by simplifying the pore pressure as a normal perturbation stress distribution over an isosceles triangle centered at given positions with a fixed size.Furthermore,the effect of locally applied fluid overpressure on the nucleation and onset of reactivation of critically or sub-critically stressed faults is discussed.The main results are as follows.1)In stress perturbation experiments,the fault strain shows a significant difference between normal and shear stress perturbation,indicating that the mechanism of the normal and shear stress perturbation triggering fault failure is different.The normal stress perturbation is applied on the whole fault,altering the contact status of the asperity and the fault’s friction strength but not the spatial pattern of fault stress.In contrast,the shear stress perturbation is applied at the loading point of the fault.It cannot change the fault frictional strength but increases the heterogeneity of the fault stress.The acoustic emission(AE)activity can reflect the fault stress and usually increases before the fault failure.The normal stress perturbation may weaken the increase of AE activity before the fault failure.However,the shear stress perturbation can strengthen the increase of AE activity.The distribution of AE events caused by normal stress perturbation differs from the shear stress perturbation,but neither spread along the fault as the stress increases.The amplitude and period of stress perturbation,fault normal stress,and loading rate play little effect on the spatiotemporal evolution of the AEs activity.2)In the injection experiment,15 MPa pressurized water injected into the vicinity of the fault can decrease the effective normal stress and induces fault slip on high permeability sandstone fault but slightly changes the stress of granite and limestone faults due to the low permeability,suggesting that the rock permeability determines whether the high-pressure water injected into the vicinity of the fault affects the fault stress.When the pressurized water is injected into the fault,1 MPa pressurized water cannot significantly affect fault stress,but the 15 MPa pressurized water causes a significant reduction in frictional strength and induced fault slip.When the 15 MPa pressurized water is injected,the actual pore pressure on three faults shows a significant difference,resulting in apparent differences in stress drop,slip duration,and displacement.Injection does not alter the fault’s velocity-strengthening properties at room temperature,suggesting that faults are not conducive for unstable slip and earthquake nucleation.Due to the direct response of fault strength to the reduction of effective stress,the 15 MPa pressurized water induces a fault slip.The experiment results suggest that when the pressurized can reach the fault quickly and decrease the fault’s effective stress,the high-pressure water injection may cause unstable slips and induce earthquake activities at the shallow crustal faults usually characterized by velocity-strengthening.3)In the numerical simulation,the failure stress of localized stress perturbation exceeds that predicted by the Coulomb failure criterion,inversely proportional to the size of the perturbation zone and the criticality of the tectonic stress on the fault but proportional to stressing rate.The size and spatial distribution of perturbation relative to the velocity weakening zone strongly impact rupture timing,location,and size.Broadly distributed pressurization can lead to rupture well before reaching the reference failure stress without perturbation.When the perturbation stress is applied at a critically stressed fault or segment,the rupture runs away beyond the perturbation zone.Conversely,the rupture is self-arrested and limited within the perturbation zone.If the fault contains multiple segments,diversified rupture patterns,in addition to normal earthquakes,doublet events,and slow slips,are sometimes observed,even in the velocity-weakening zones.