Effect of displacement loading rate on dynamic mechanical characteristics of fault stick-slip under constant normal stiffness conditions

The role of the displacement loading rate in fault activation and evolution is of utmost importance, yet most existing studies have primarily focused on its effect under constant normal load (CNL) conditions. However, in reality, the rock mass surrounding the earthquake source at the deep part of the fault provides normal stiffness conditions. Therefore, the influence of the displacement loading rate on the dynamic mechanical characteristics of fault stick-slip under constant normal stiffness (CNS) conditions remains uncertain. To bridge this research gap, we conducted an experimental investigation on simulated faults in granite under constant normal stiffness (CNS) conditions. Our analysis focused on the influence of the displacement loading rate on the spatial and temporal evolution of local stresses along the fault and the size of the fault nucleation zone. The findings revealed the following: Under the CNS condition, the equivalent shear stress is larger compared to the CNL condition, while the shear stress drop is smaller. The recovery of normal stress under CNS conditions depends more on the displacement loading rate. The nucleation location of the fault is influenced by the normal boundary conditions, with a larger nucleation zone observed in the CNS condition than in the CNL condition. In the early stage of friction, the local stress and stress drop of the fault exhibit a positive correlation with the displacement loading rate, whereas in the later stage, they display a negative correlation. Moreover, at the same location, the order of local stress drop rates for the fault is v 3 > v 1 > v 2 , where v 1 represents the local normal stress drop rate, v 2 represents the local shear stress drop rate and v 3 represents the local friction coefficient drop rate. These research results contribute to a further understanding of the deep fault-slip mechanical behavior.