Influence of horizontal stress ratio on frictional stability of fault under true triaxial stress conditions

Abstract

To understand the fault friction stability and seismic mechanisms under in-situ stress conditions of the Earth's crust, the rectangular prismatic rock sample with a sawcut fault inclined at an angle of 29° to the axis is introduced to withstand the three stress components of true triaxial stress conditions (vertical stress σ_v, horizontal stress σ_h, and stress parallel to the strike of the fault plane σ_p). Velocity-stepping experiments are performed on the sawcut faults to investigate fault frictional behavior and slip stability under different σ_h/σ_p ratios and stress σ_p within the rate-and-state framework. Results indicate that increasing σ_h/σ_p ratios decrease the frictional velocity-dependent parameter (a-b), exhibiting a transition from velocity-strengthening to velocity-weakening behaviors. Shallow grooves develop along the fault surface and act as stress barriers at the low σ_h/σ_p ratio. With increasing σ_h/σ_p ratios, the fault surface gradually converges to uniform smoothness due to asperity abrasion, suggesting that the weakening effect of increasing σ_h/σ_p ratios on fault stability is related to stress redistribution. The enhanced fault critical stiffness (K_c) with increasing σ_h/σ_p ratios promotes fault instability nucleation. Our results reveal that true triaxial in-situ stress states in seismogenic zones exert significant control on frictional behavior and fault stability.