Experimental Study On The Mechanism For Stick-Slip Instability Of Basalt And Granite Restraint System

There are various fault zones distributed in the earth’s crust,and the relative dislocation of the faults in the fault zone will induce earthquakes.As the main cause of destructive shallow-focus earthquakes,stick-slip dislocation between faults has become the key research object of scholars in related fields.Considering the inhomogeneity of the two sides of the actual fault contact interface,this paper selects the fault composed of basalt and granite as the background,and carries out the laboratory friction experiment on the contact surface of two kinds of rock mass to simulate the mechanism for stick-slip instability.In the experiment,a rock shear rheometer is adopted to simulate the stress conditions of the fault boundary,and the multichannel acquisition instrument is used to obtain the stress and strain states for different parts of the upper and lower rock masses during the stick-slip process of the fault,and the mechanism for stick-slip instability of the bi-material interface is studied and analyzed.It is expected to play a certain guiding significance for the research on the seismic conditions of shallow-focus earthquakes and for earthquake prevention.In this paper,the evolution law for stress field and displacement field when stickslip instability occurs in the upper and lower samples is obtained by calculation and analysis,and the mechanism for stick-slip instability is explained from the three aspects of stress,displacement and rupture propagation,that is,the nucleation and rupture propagation from points to surfaces leads to local-to-global slip instability.The experimental results show that,before the stick-slip instability occurs,the uneven stress distribution at each point of the section is the inducement for the formation of the nucleation zone.In the process of stick-slip instability,this region is also the place where the system pre-slips from "sticky" to "slippery".When this unstable state is transmitted from the nucleation region to other regions,and finally penetrates the entire section,the system will experience overall slip instability.This process is accompanied by successive stress drops at various points along the section.As for the rupture propagation during stick-slip instability,when stick-slip instability occurs at the non-homogeneous interface composed of two materials in this experiment,although the rupture propagation of the materials on both sides of the section is consistent in the overall direction,there is an obvious sequence and the propagation speed and terminal speed are also different.At the same time,the rupture propagation of the non-homogeneous interface during stick-slip instability has a certain directionality.According to whether the rupture direction is consistent with the sliding direction of the softer material,the rupture propagation can be divided into positive direction rupture and negative direction rupture.Among them,the rupture in the positive direction is mostly sub-shear rupture,while the rupture in the negative direction is mostly super-shear rupture.In addition,this paper also calculates a series of characteristic parameters of stickslip instability when external conditions(vertical normal stress,lateral loading rate)change,such as stress drop,premonitory slip,slip weakening displacement,rupture propagation velocity etc.The influence of the two on stick-slip instability is explored.The experimental results show that the influence of normal stress on stick-slip instability is mainly reflected in three aspects: stress,displacement and rupture propagation,and finally affects the slip mode of the fault.In terms of stress,the stress drop of the fault system has a nonlinear positive correlation with the normal stress,and the stress drop at each local point of the sample above and below the fault will also increase with the normal stress;in terms of displacement,there is no direct relationship between the normal stress and the premonitory slip,the normal stress only affects the slip weakening displacement,and the slip weakening displacement will increase with the normal stress;in terms of rupture propagation,normal stress will aggravate the propagation process of the rupture,and the acceleration will increase with its magnitude,so the acceleration phase will be shorter,and the final peak velocity that can be reached will also increase.As the normal stress increases,the fault system is more prone to stickslip instability.The analysis finds that there is a critical normal stress that controls when the sliding mode transitions from stable sliding to stick-slip.The influence of lateral loading rate on stick-slip instability is mainly reflected in two aspects: time and stress,thus affecting the slip mode of the fault.In terms of time,the stick-slip time interval is a power-exponential function of the lateral loading rate,and the two are nonlinearly negatively correlated;in terms of stress,the level of lateral loading rate indirectly affects the magnitude of the stress drop during the stick-slip process by affecting the stick-slip recurrence interval,and the greater the stick-slip recurrence interval,the greater the magnitude of the stress drop;in terms of displacement,it is found that there is no direct correlation between lateral loading rate and stick-slip displacement.With the increase of the loading rate,the system is more prone to stable sliding,and the analysis finds that there is a critical loading rate that controls when the sliding mode transitions from stick-slip to stable sliding.