Extreme recharge triggers seismicity in a confined karst aquifer in southern Spain

Antonio Pedrera , Jesús García- Senz, Antonio González-Ramón, Gómez-Fontalva José Manuel, Ana Ruíz-Constán, Sergio Martos-Rosillo

Hydrological forcing during extreme recharge events can perturb crustal stress and trigger seismicity in a critically stressed crust, yet the coupling between aquifer dynamics and the spatio-temporal distribution of earthquake swarms remains poorly constrained. Here we document such a response during an exceptional rainfall episode in early 2026 that affected a confined karst aquifer in southern Spain. Seismicity displays a coherent spatio-temporal evolution within the aquifer, from localized nucleation to distributed activation and outward migration, consistent with diffusion-driven pressure propagation through a connected fracture network. Stress modelling indicates that modest pore-pressure perturbations of ~0.4–0.7 MPa, comparable to decades of tectonic loading in the region, are sufficient to trigger slip on optimally oriented faults. In addition to this dominant shallow seismicity, a subsidiary population of deeper earthquakes occurs within the underlying basement, exhibiting a systematic delay of ~4 days relative to shallow events. The delayed occurrence of basement seismicity is consistent with stress transfer from hydrologically driven shallow activity, indicating mechanical coupling between the two depth domains. These results demonstrate that confined karst systems can act as transient stress modulators, controlling the timing and spatial organization of seismicity. As the frequency of extreme hydrological events increases, such systems may play a broader role in regulating shallow earthquake activity in tectonically active regions.