Influence of impoundment gravity and pore pressure on reactivation of faults

Abstract

For more than seven decades, the impounding of artificial water reservoirs has been known to trigger earthquakes. Accordingly, reservoir-induced seismicity (RIS) has been studied in terms of its identification and mechanisms. In this paper, we report three laboratory experiments to investigate the effects of water impoundment and pore-pressure diffusion on the reactivation of faults in high tectonic stress areas. Results showed that when the horizontal stress is larger than the vertical stress, according to the Mohr–Coulomb criterion, pore-pressure diffusion might be the main factor inducing seismicity instead of the undrained stress compression. The diffusion and oscillation of pore pressure are the controlling factors in triggering the activation of faults and releasing the elastic energy. The Mohr–Coulomb criterion is consistent in describing the critical damage pore pressure of the pre-existing faults; however, it fails to judge whether a fault is in a stable or unstable state after the pore pressure increases. The hydraulic energy from the injection pressure and volume of water is strongly related to the moment magnitude of the induced seismicity owing to pore-pressure diffusion. Moreover, the fault can be transformed from an unstable state into a stable state after continuous seismic events. The enhancement of fault permeability due to the seismic events can promote the fluid flow through fractures and influence the seepage field around the reservoir.