Hydrogeological Response of a Karst Aquifer to Extreme Recharge: Insights from the 2026 Grazalema–Líbar Hydro-Seismic Crisis (Southern Spain)

Abstract

Extreme precipitation can profoundly modify groundwater dynamics in karst aquifers, although the processes governing their response to exceptional recharge remain poorly understood. In particular, the evolution of hydraulic connectivity and confinement during aquifer filling may strongly influence groundwater pressure propagation and associated geological processes. This study investigates the hydrogeological response of the partially confined Grazalema–Líbar karst aquifer (southern Spain) during an exceptional recharge event in January–February 2026, when cumulative rainfall exceeded 2800 mm. Meteorological observations, spring discharge records, hydrogeological information, and seismic data were integrated to analyze groundwater recharge, hydraulic behavior, and seismic activity. The aquifer responded rapidly, with large groundwater level rises, major increases in spring discharge, progressive hydraulic connection between previously disconnected sectors, and expansion of confined conditions. These changes increased hydraulic diffusivity and promoted rapid pressure transmission through the karst conduit network. The strongest seismic response occurred only after this hydraulic reorganization developed, with earthquakes concentrating in confined sectors and progressively migrating towards shallow depths. The results demonstrate that evolving hydraulic connectivity, rather than recharge alone, controls pressure transmission and seismic activation. They provide a conceptual framework for understanding hydro-seismic responses in partially confined karst aquifers during increasingly frequent extreme rainfall events.