Laboratory Studies Of Mechanics Of Stick-slip Motion In Granular Simulated Fault

Stick-slip phenomenon is widespread in nature,it is often considered as an analog for naturally occurring earthquake.Related studies have shown that the fault gouge inside the core of fault plays a key role in controlling the frictional properties of the fault.Therefore,it would help us to understand the underlying mechanism of earthquake to investigate how the characteristics of stick-slip cycle vary in granular simulated fault.However,the existing researches are not sufficient enough for the microscopic mechanism in stick-slip cycle and the effects of various physical quantities and fault properties.In this paper,we carried a few series of laboratory double direct shear test,focusing on investigating how the fault zone properties and characteristics of gouge particles such as normal stress,fault thickness,environmental humidity impact characteristics of stick-slip cycle and then improving the understanding of the micromechanical model for force chain formation and breakage.Some major achievements are listed as below:(1)A series of experiments of different thickness faults under variable normal stress is systematically carried out.The influence of normal stress and fault thickness on the characteristics of stick-slip cycle is discussed.We propose that the friction drop in the process of stick-slip dynamic failure is mainly controlled by normal strain.Based on the experimental data of the layer thickness change and the shear modulus of the fault controlled by normal stress,we modified the micromechanical model for force chain during stick-slip cycle.(2)By conducting the shearing test of granular fault under the relatively low normal stress,we studied the normal stress influence(when lower than 8MPa)of friction drop,stick-slip recurrence time(including elastic and inelastic loading time),pre-seismic creep,b-value of acoustic emission,system stiffness.We conclude that the influence of the shear strain history on the stickslip cycle is negligible compared with the normal stress step.(3)The double direct shear test under different humidity conditions was carried out.Based on the analysis of the results,the influence of humidity on the stick-slip event was studied.The proportional amplification effect of humidity on the stick-slip cycle was proposed.By water injection into faults undergoing stable sliding we find out the stimulation of stick-slip in laboratory.(4)By assembling multiple layers of different materials into the same fault and changing the thickness of these two material layers,the frictional effect of coexisting of two different types of gouge material is studied.The dominant effect of the existence of glass beads layer on the fault sliding mode is revealed.It is proved that only a small thickness of the glass beads layer can make the fault exhibit a stick-slip mode.(5)The transition from stick-slip motion to stable sliding of granular simulated fault during shearing under low normal stress is introduced.Multiple velocity-step test is used to determine the rate and state friction parameters of the glass beads simulated fault.By conducting a few sets of experiments studying various parameter influences,we proposed that the shear strain is the key parameter controlling the occurrence of the transition and suggested that localized shear strain may be responsible for the transition by observing the behavior of a strain marker in the fault.