Deep excavations in rock masses have the potential to break the frictional equilibrium of nearby faults, resulting in induced seismicity. We conducted experimental and numerical studies on a simulated granular fault to investigate the mechanism of excavation-induced seismicity. A series of laboratory experiments was used to investigate the effects of initial shear stress, initial normal stress, and its unloading rate on the frictional instability of the fault, and a numerical simulation was carried out to interpret stress variation and particle evolution during the unloading process. Our results show that both normal and shear stresses drop sharply when the fault is approaching a critical stress state. The stress reduction is due to a decrease in interparticle force and particle contact breakage. The evolution of the state of the fault depends on the initial stress condition and excavation process. A greater initial normal stress and a lower initial shear stress provide a favourable environment for accumulation of higher strain energy in adjacent rock, leading to larger slip displacement. A larger normal stress unloading rate can also cause higher strain energy and a larger slip displacement. Understanding unloading-induced instability of a simulated fault allows us to interpret the seismic events occurred during the excavation of the Gotthard Base Tunnel in Switzerland. The reduction of normal and shear stresses associated with the excavation work decreased the differential stress applied to a natural fault zone, and subsequently resulted in induced earthquakes.
Induced seismicity occurs when human activities perturb the frictional equilibrium of pre-existing faults in the Earth's crust. Underground excavations create usable space, but cause nearby faults to reach or approach a critical stress state. For example, during the excavation of the Gotthard Base Tunnel in Switzerland, a seismic network recorded a series of seismic events, among which a magnitude 2.4 earthquake with a focal depth of 1–1.5 km drew extensive attention of nearby residents and caused severe damage in the excavated tunnel (Husen, Kissling, and Deschwanden, 2013). The induced seismicity was due to stress redistribution in hard rock in the vicinity of the tunnel and a fault zone striking at a small angle to the tunnel axis (Hagedorn and Stadelmann, 2010). Nevertheless, the mechanism driving unloading-induced fault instability still eludes explanation. Particularly, the process of fault instability related to stress redistribution remains unclear.