Impact of water impoundment in medium- and small-sized reservoirs on seismicity in Feidong under poroelastic coupling effects

Feidong County, located in the middle and lower reaches of the Yangtze River Plain in Anhui Province, is characterized by a weakly deformed tectonic setting, with a shear strain rate on the order of 10−9 a−1 and historically low seismic activity. However, in 2024, the China Earthquake Networks Center recorded six earthquakes with magnitudes above 3.0 in this region, the largest being a magnitude 4.7 event on September 18. In this study, we develop a three-dimensional poroelastic coupling model encompassing the Yuejin, Weichongba and Hongmeitang reservoirs. Using the finite element method, we simulate the pore pressure field and Coulomb stress changes induced by reservoir impoundment. The aims of this study are to quantitatively evaluate the effect of fluid diffusion on the Tanlu fault zone and explore how small- to medium-sized reservoirs may influence earthquake triggering and preparation in the Feidong region. Simulation results indicate that the pore pressure change at the hypocenter reaches approximately 2.146 kPa. When pore pressure effects are neglected, the Coulomb failure stress change (∆CFS) under purely elastic conditions is about -67.60 Pa, suggesting that the gravitational load from reservoir impoundment enhances fault stability. However, when the influence of fluid flow is considered, ∆CFS increases to approximately 1.220 kPa, indicating that the combined effect of reservoir impoundment tends to destabilize the fault. Numerical results further show that at a depth of about 12 km, pore pressure diffuses radially outward from the reservoir area and rapidly propagates along the Tanlu fault zone under structural control. Moreover, as pore pressure continues to migrate downward, its pathway is guided by the fault dip direction, leading to localized stress concentration at the intersection between the NWW-trending Feizhong Fault and the NE-trending Tanlu Fault. This localized stress accumulation may form a structurally favorable zone for earthquake nucleation.