Fault stability under reservoir loading: coupled effects of cyclic loading and cohesion loss in pumped hydro systems

Pumped hydro storage schemes are central to decarbonization strategies, yet their interaction with subsurface faults poses a risk of reservoir-induced seismicity (RIS). This study provides insights on this type of RIS through 3D finite-difference numerical modeling using WAVE3D, investigating how cyclic loading and hydromechanical weakening from reservoir impoundment influence fault stability.

We introduce a set of kinematic geomechanical models that replicate perturbations in a pre-existing vertical strike-slip fault by a hypothetical suggested reservoir (Figure 1). Two weakening processes are investigated: (i) fracture fatigue, representing elastic weakening because of cyclic loading of the reservoir, and (ii) water-weakening, representing cohesion loss because of pore pressure. Each of the processes includes sub-scenarios that differ in the strength and spatial extent of weakening.
Our results show that the RIS potential is mainly controlled by the amplitude of the shear stress as well as the decay rate of cohesion. The water-weakening sub scenario with wide saturation produced the highest number of rupture-prone segments, exhibiting sustained displacement and elevated shear stress at depth. In contrast, fracture fatigue scenarios led to more localized failures with pronounced individual displacements. Seismic moment estimates scaled logarithmically with displacement and excess shear stress, indicating a coupled threshold behavior.
These findings indicate the crucial role that site-specific in-situ stress state, fault dip, and weakening processes play in RIS potential. Notably, we demonstrate that partial drawdown of the reservoir (20%) considerably decreased stress buildup for the near-surface regions, yet left the deep portions of the system highly stressed. These observations can be applied in seismic hazard analysis as well as in operation guidelines of pumped hydro storage facilities in seismically stable, yet faulted areas.