The rupture process and instability mode of step-like asperity located on sliding plane in granite specimen were studied under triaxial compression at confining pressures of 300-500 MPa. The main experimental results are as follows:
when differential stress (σ₁-σ₃) reaches 0.15-0.17 (σ₁-σ₃)_c, where(σ₁-σ₃)_c is the peak stress level of the first event of stickslip, macroscopic fracture takes place along the base of step-like asperity and the linear relation between stress and displacement is kept; with increasing stress level fracture propagates and cuts through the original asperity at (σ₁-σ₃)≥0.3(σ₁-σ₃)_c, and in the meanwhile, some new lower asperities are formed; as (σ₁-σ₃)≈(σ₁-σ₃)_c, the sliding plane originally with asperity has been flattened relatively, and after that, the sudden instability with large stress drop and violent shock occurs.
The results indicate that the failure of asperity can only cause micro-or fore-shocks and acoustic emission, and the main shock of earthquake is induced by stick-slip along a flattened fault zone. In other words, the mechanism of strong earthquake is not the fracturing of asperities, but the unstable sliding along relatively flatten fault zone.