An Analytical Microcrack-Based Rock Model with Implications for Earthquake Mechanisms Induced by Stress Changes

Shear fracture triggered by subcritical crack extension in intact brittle rocks under long-term compressive loading plays a significant role in the evaluation of earthquake mechanisms. Changes in external loading strongly influence the subcritical crack growth of intact rocks during earthquake nucleation. An important conundrum is how to establish the relationship between shear fracture induced by subcritical crack growth and external loading path in brittle rocks under lithospheric conditions. A novel micromechanical method that introduces shear fracture behavior is proposed to predict the time-dependent shear properties induced by the subcritical cracking of brittle rocks when the initial state of rocks starts from the peak point of the stress-strain curve measured by the conventional triaxial compressive test. This approach is developed on the basis of the wing crack model, subcritical crack growth law, and Mohr-Coulomb strain-softening model. The effect of loading and unloading paths on the evolution of shear properties of rocks under lithospheric conditions is analyzed by drawing a function of historical stress. The corresponding evolution of strain, shear strength, cohesion, and internal friction angle caused by subcritical crack growth under different stress paths is studied. Cohesion and shear strength continuously undergo a weakening process, and the internal friction angle initially undergoes a strengthening and, finally, a weakening process during subcritical crack growth under constant compressive loadings. The effect of the sudden change in axial stress on shear strength is smaller than that of the sudden change in confining pressure. A sudden decrease in confining pressure causes a rapid drop in shear strength, leading to a dramatic rise and drop in the rate of shear strength. Implications for evaluating earthquake mechanisms triggered by stress changes from the evolution of shear properties caused by subcritical crack growth in brittle rocks are also proposed.

Automated summary

The study highlights the influence of external loading paths on shear fracture induced by subcritical crack growth in brittle rocks under lithospheric conditions, as well as the evolution of rock shear properties. By introducing a novel micromechanical method and analyzing the historical stress function, the evolution of cohesion, shear strength, and internal friction angle during shear fracture induced by subcritical crack growth is revealed.