Stress transfer and poroelastic mechanisms to elucidate seismicity triggered by reservoirs

Induced seismicity has attracted increasing research interest in recent times. This phenomenon is generally associated with fluid injection or extraction wells, in energy industry activities such as hydrocarbon extraction/injection, CO2 sequestration, geothermal energy or underground storage of green hydrogen. However, there are other human activities that can induce or trigger seismic events, such as oscillations in the surface water level due to the construction of hydraulic infrastructures. Surface water level oscillations, whether natural or anthropogenic, can alter the pore pressure regime in the subsurface. Although these alterations usually have a moderate magnitude, they can destabilize faults that were already close to overcoming their slip resistance and eventually trigger an earthquake. On the other hand, the fault slip itself alters the pressure regime due to the undrained response of the porous medium, caused by the sudden deformation of the fault surrounding area. This undrained pressure, which can take up to weeks to dissipate, can alter the stress state of other nearby fractures and trigger new earthquakes or aftershocks. In this work we present numerical simulations of the underground area around the Itoiz dam (Spain). We simulate the subsurface as a saturated poroelastic medium, with a fully coupled scheme between fluid flow and solid deformation in the porous medium. Through a seismological analysis we start from series of recent earthquakes to locate geological faults. With our numerical model we study how the undrained effect produced by this series of events can destabilize other nearby faults. We also add the effects of the oscillations in the reservoir level following the historical series of the last years. Our numerical simulations indicate that in the case of the Itoiz dam, the most recent seismic swarm could have been triggered by the stress transfer from the previous events, which, together with the filling of the reservoir, may have destabilized faults that were critically stressed for failure. Our simulations can contribute to explore how the combined effect of the undrained pressure by the fault slip and the oscillations of the reservoir can trigger faults that, due to the natural state of stress, are already close to slip. Also to clarify if the most relevant mechanism is the oscillation of the reservoir level or the undrained pressure trigger, which according to bibliographic analysis can be of the same magnitude. This could apply to other cases of seismicity induced by hydraulic infrastructures or in general by oscillations in the surface water level. Acknowledgements This research Project has been funded by the Comunidad de Madrid through the call Research Grants for Young Investigators from Universidad Politécnica de Madrid under grant APOYO-JOVENES-21-6YB2DD-127-N6ZTY3, RSIEIH project, research program V PRICIT.