The influence of morphology and the loading-unloading process on discontinuity stress states observed via photoelastic technique and its inspiration to induced seismicity

Stress change in rock mass caused by human activities has the potential to cause the sliding and destruction of faults and joints, resulting in induced seismicity. Laboratory experiments are conducted on a simulated fault with various teeth numbers and undulation angles to uncover the mechanism of stress change-induced seismicity. The potential risk of induced seismicity is explained using three methods: the Mohr-Coulomb failure criterion, localization of stress concentration regions, and visualization of maximum shear stress reduction through photoelasticity. Experimental results indicate that the friction coefficient increases with the undulation angle, and the form of stress change has an unignorable impact on frictional instability. The friction coefficient in the vertical unloading process is slightly lower than that in the loading process and larger than that in the shear unloading process. Loading is the stress change caused by shear displacement under constant normal stiffness conditions and unloading is the process of reducing the stress by controlling the position of the boundary constraints in the corresponding direction. Meanwhile, unloading in the shear direction has both seismic and aseismic features. Although the rapid drop of shear stress at the onset of shear unloading may induce fault instability, the reduction of normal stress and the restoration of displacement prove that unloading in the shear direction may also reduce the risk of fault failure in the subsequent process. In addition, the stress concentration region is mainly distributed perpendicular to the contact surface rather than the entire fault. This research is conducive to promoting the application of photoelasticity in studying induced seismicity and provides a practical method for calculating the energy released during such events. Based on the morphological characteristics and stress states of fault surfaces, the findings can be utilized in engineering practice to assess the risk of induced seismicity under different stress change conditions.