Experimental Study On The Mechanism Of Stick-slip Shear Failure Between Granite And Basalt

Earthquake is a kind of geological movement which has a great impact on human society.Fault instability is the main factor of its occurrence,but there is also a process of development,which is not unexpected.From the perspective of modern mechanics,fault instability has two main forms: one is fault creep,that is,slow and stable sliding;the other is stick slip,that is,fault first stable Interlocking and then suddenly sliding,and then stable interlocking and suddenly sliding such a cycle of behavior.Since Byelee and Brace put forward the term "stick slip" as a formation mechanism of shallow earthquakes in 1966,researchers in this field have been interested in it.Therefore,studying the generation and propagation mechanism of stick slip will have a profound understanding of earthquakes.In this paper,considering that in the actual fault,the geological conditions are usually very complex,and the geological conditions on both sides of the cross-section are obviously different and anisotropic,it is more scientific and practical significance to explore the stick slip of bimaterial fault.According to this idea,the experimental design is carried out.The upper fault and the lower fault respectively use basalt and granite as two different rock materials to simulate the actual heterogeneous contact surface in nature.The specific rock shear rheological testing machine and strain collection system are used to carry out specimen loading and data collection,and the normal stress,shear rate and section roughness are studied The effect of three factors on the stick slip of bimaterials was studied:1)With the increase of the normal stress level,the shear stress drop of the system and the local shear stress drop will show an increasing trend.The increase of the normal stress has no direct influence on the precursory slip,but the critical slip weakening displacement will increase with it,and the surface fracture energy required in the process of slip weakening will increase with the increase of the normal stress.The propagation velocity of fault stick slip fracture will increase with the increase of normal stress level,which shows that the acceleration process in the early stage of fracture is shorter,the acceleration is faster,and the peak velocity in the later stage is larger.With the increase of normal stress level,the fault system will be more likely to produce stick slip behavior.2)Shear rate is one of the main controlling factors of the time interval of stick slip,which is shown as that with the increase of shear rate,the time interval of stick slip decreases gradually,but it is not linear correlation.The relationship between shear rate and time interval of stick slip can be expressed by power exponential function.The influence of shear rate on shear stress drop is mainly used to reflect the influence of time interval of stick slip on shear stress drop,that is,the relationship bet ween earthquake period and magnitude.3)The section roughness is one of the main factors that affect the shear stress drop,which is embodied in the fact that the shear stress drop of fault system is directly proportional to the section roughness on the premise of certain loading normal stress and tangential loading rate,that is to say,the rougher the actual fault section is,the larger the magnitude will be when the earthquake occurs.On the premise of a certain loading normal stress,with the decrease of the roughness,the shorter the acceleration process of fracture propagation,the greater the acceleration,and the greater the peak velocity of the final stable fracture.Cross section roughness affects the accumulation of stick slip energy.The larger the cross section roughness is,the greater the accumulated energy is,the more likely it is to form peak drop phenomenon and stick slip behavior;on the contrary,the smaller the cross section roughness is,the less the energy accumulation is,the more likely it is to have continuous and stable slip behavior.4)There are two types of fracture propagation of fault stick slip.The first corresponds to the fracture growth in the sliding direction of the material with lower wave velocity,which is called the positive direction.The fracture propagation of this type is sub shear propagation,and its velocity is about the generalized Rayleigh wave velocity of the system.The second fracture propagation corresponds to the negative direction of the first propagation direction,and its fracture velocity is super shear,slightly lower than the longitudinal wave velocity of materials with lower wave velocity.