Hydrology Drives Crustal Deformation and Modulates Seismicity in the Matese Massif (Italy)

Abstract

We analyze the interplay between hydrology, deformation and seismicity in the Matese massif, located in the Italian Southern Apennines. We find that this area is characterized by the concurrent action of two hydrologically-driven processes: the first is the deformation detected by GNSS data in the shallowest part (above the elevation of the major springs) of the Earth crust, in phase with the hydrological forcing; the second is the triggering of seismicity at depth with a delay suggesting a downward diffusive process. We study the first process by applying a Principal Component Analysis to the GNSS displacements time series, aiming to identify a common signal describing the largest data variance. We find that the maximum horizontal displacements associated with the first principal component (PC1) are larger than 1 cm in two GNSS sites and the PC1 temporal evolution is well correlated and in phase with the flow of the largest spring of the region, which we consider as proxy of the water content of the massif. This suggests that the main source of horizontal deformation is the water content fluctuations in the shallow portion of the Matese aquifer, in particular within fractures located in correspondence of the main mapped faults. The deformation rates caused by this process are one order of magnitude larger than the tectonic ones. Finally, we infer the second process by observing the correlation between the background seismicity and the spring discharge with a time lag of 121 days. In our interpretation, downward diffusive processes, driven by aquifer water content variations, propagate pore pressure waves that affect the faults strength favoring the occurrence of micro-earthquakes. This is supported by the values of hydraulic diffusivity (1.5 m^2/s) and rock permeability (3.2-3.8⋅10^−13 m^2), which are compatible with what is observed in karstified limestones.