Weak disturbance-triggered seismic events: an experimental and numerical investigation

Geological masses can be regarded as rock blocks of different scale of structural planes with the ability to store various forms of energy. Propagation of stress waves generated by weak external disturbances in rock blocks may trigger the release of internal potential energies and slip movements along these structural planes, resulting in seismic events, such as residual deformations, fault-slip rock bursts, ground motions, etc. First, based on a simplified rock block system, a novel experimental system, and a numerical model, we investigated weak disturbance-triggered seismic events. We then conducted a theoretical analysis in which we quantitatively characterized the critical energy conditions of seismic events. The experimental and numerical results showed that the tensile stages of the stress waves generated by the disturbance loading reduced the normal stress on the interface of adjacent blocks, leading to an ultra-low friction phenomenon. This phenomenon resulted in the slip movements of the work block. The residual displacements and the critical energy conditions significantly depended on the initial stress states. As the initial shearing force ratio β increased, greater residual displacements were observed and lower disturbance energy was required to trigger a seismic event. When β was close to 1, even an extremely weak disturbance was able to trigger large residual displacements or sustainable slip failures. A dimensionless parameter k was introduced to characterize the critical energy conditions of the seismic events. The critical condition for initiating a slip was that k should exceed a critical value, while the critical conditions for a slip failure were that k should reach a larger critical value and the work block should be in a subcritical stress state. It can be concluded that disturbances, initial shear forces, and friction-weakening mechanisms are the most important factors, with the initial shear forces providing the potential energies, which are locked by the static friction force (the shear strength). The disturbances reduce the shear strength and weaken the restrictions. The friction-weakening mechanisms determine energy conversion coefficient efficiency.