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Triggered seismicity in karst regions has been explained assuming the existence of a hydraulically connected fracture system and downward diffusion of surface pore pressures. Karst systems are, in fact, able to swiftly channel large amount of rainfall through networks of conduits increasing the hydraulic head loading upon the fluid‐saturated, poroelastic crust. Here we use Global Positioning System and hydrological and seismicity data to show that poroelastic strain in the shallow crust (0–3.5 km) controls seasonal and multiannual modulation of seismicity along the Irpinia Fault Zone (Southern Italy) without requiring a hydraulically connected fracture system from the surface to hypocentral depths. We suggest that groundwater recharge of karst aquifers along the Irpinia Fault Zone produces stress perturbations large enough to modulate strain accumulation and seismicity and temporarily modify the probability of nucleation of seismic events such as the 1980 Irpinia, MS 6.9, earthquake.

We present GPS, hydrological, and GRACE (Gravity Recovery and Climate Experiment) observations in southern Apennines (Italy) pointing to a previously unnoticed response of the solid Earth to hydrological processes. Transient patterns in GPS horizontal time series near to large karst aquifers are controlled by seasonal and interannual phases of groundwater recharge/discharge of karst aquifers, modulating the extensional ∼3 mm/yr strain within the tectonically active Apennines. We suggest that transient signals are produced, below the saturation level of the aquifers and above a poorly constrained depth in the shallow crust, by time‐dependent opening of subvertical, fluid‐filled, conductive fractures. We ascribe this process to the immature karstification and intense tectonic fracturing, favoring slow groundwater circulation, and to multiyear variations of the water table elevation, influenced by variable seasonal recharge. The vertical component displays seasonal and multiyear signals more homogeneously distributed in space and closely correlated with estimates of total water storage from GRACE, reflecting the elastic response of the lithosphere to variations of surface water loads. The different sensitivities of vertical and horizontal components to the hydrologically induced deformation processes allow us to spatially and temporally resolve the different phases of the water cycle, from maximum hydrological loading at the surface to maximum hydrostatic pressure beneath karst aquifers. Finally, we suggest that transient deformation signals in the geodetic series of the Apennines are correlated to large‐scale climatic patterns (Northern Atlantic Oscillation) through their influence on precipitation variability and trends at the regional scale.

Based on the long hydrological time series, the correlation between karst spring discharge series and rainfall has been analysed, using the Standard Precipitation Index (SPI). Analysis has been focused on the drought periods. Data come from a large karst system (Campania, Southern Italy), in an area characterised by a distribution of the precipitation prevalently during autumn-winter period. Insufficient recharge due to poor rainfall results in flat spring hydrographs (with no peak during spring season) that indicate a continuously decreasing discharge. Specifically, it has been found that 12 months cumulative rainfall, expressed by SPI 12 , and spring discharge have similar trend. When SPI 12 will be equal or less that −1, springs reduce the discharge, and a flat spring hydrograph will be produced when SPI reaches value less than −1.5. In these cases, the prolonged shortage of accumulated rainfall causes a reduction in spring discharge also during the following year as well, pointing out a memory effect of the karst aquifer, and more complex rainfalldischarge relationship is observed.