Study On The Local Stress Field Deflection Of Strike-slip Fault And Its Relationship With The Fault Dislocation Rate

Regional tectonic activity will change the stress state of the crust,and tectonic earthquake is one of the extreme ways of regional tectonic activity.Understanding the changes in the local stress field of seismic faults is of great significance for characterizing fault activity and evaluating crustal stability.This study is based on the particle flow software PFC^2D of discrete element theory.Based on the monitoring data of the YRY-4 borehole strain gauge at the Yushu seismic station near the Ganzi-Yushu fault and the tectonic background of fault activity,a two-dimensional numerical model of the Yushu fault is established to simulate the spatiotemporal deflection characteristics of the maximum principal stress direction in the local stress field of the fault before the Yushu M_S 7.1 earthquake on April 14,2010.Our study were mainly focused on the influence of different parameters on the maximum local stress deflection angle of the fracture,the displacement rate of the two fractured discs under different loading rates,and the changes in the local maximum principal stress direction deflection angle,as well as the relationship between the local stress deflection angle and dislocation rate of the fracture.Based on the above work,the main research results obtained are as follows:（1）The measured data from Yushu Station show that before the earthquake,the local principal stress direction near the fault zone will undergo a significant clockwise deviation,and the absolute value of the deflection angle will gradually increase.After the earthquake,due to the release of stress,the deflection angle will undergo a counterclockwise deviation,and the local principal stress direction will gradually return to its original state.（2）The research results indicate that the deviation angle of the local maximum principal stress direction at the position measurement circle of Yushu Station shows a trend of first decreasing and then increasing.The local maximum principal stress direction first rotates clockwise and recovers with the occurrence of fracture.This is basically consistent with the measured change trend,indicating that the established numerical model can be used to analyze the changes in the direction of the local stress field of the fracture.（3）The variation pattern of local maximum principal stress S_H varies at different locations near the fault,and the closer it is to the fault structure,the more significant the impact on local stress is.Under loading,the spatial distribution of the maximum principal stress in the overall model is relatively uniform,forming concentrated areas of tensile and compressive stress at different locations around the fracture,and the degree of concentration continues to increase.A stress drop phenomenon occurs in the middle of the model.The fault activity before the earthquake caused the maximum principal stress S_HT in the local stress field located in the tensile stress concentration area and the maximum principal stress S_HP in the local stress field located in the compressive stress concentration area to rotate towards the boundary direction of the two regions simultaneously,and the deflection angle gradually increases with increasing steps.The degree of stress accumulation varies in different regions,and it increases with the passage of loading time,and fractures occur at different times,with obvious spatiotemporal asynchronous effects.（4）Sensitivity analysis of different parameters found that before reaching the peak stress,the local maximum principal stress direction located in the compressive stress zone undergoes counterclockwise deflection,and the deflection angle gradually increases.After the peak stress,the deflection angle gradually returns to the initial loading state.The local maximum principal stress direction located in the tensile stress zone first undergoes clockwise deflection and then counterclockwise deflection.In the early stage of loading,different parameters have no significant effect on the stress deflection angle at all measured circles,but the effect is significant after the stress reaches its peak.The significance of the influence of different parameters on the peak strength in different regions varies.（5）The analysis of the displacement,sliding rate,and local maximum principal stress deflection at the fault location shows that the movement directions of the two plates of the fault are significantly different.The NE plate moves in the NW direction,while the SW plate of the fault moves in the SE direction.The fault movement exhibits obvious left-handed characteristics,which is consistent with the left-handed tectonic background of the Ganzi Yushu fault.Under the action of tectonic stress,due to the sinistral strike slip movement,the displacement of different regions on both sides of the fault is different at the same time.The displacement of the leading edge of the two directions of fault movement is smaller than that of the trailing edge due to compression.Under different loading rates,the displacement of the fractured two discs varies.The larger the loading rate,the greater the displacement generated by the fractured two discs.From the profile perpendicular to the fault,it can be seen that the displacement of the right side of the fault is significantly greater than that of the left side of the fault.Under different loading rates,the acoustic emission characteristics during different loading periods are inconsistent.There is a high correlation between the relative dislocation rate of the two fractured discs and the deflection angle.