Strain sensitivity of seismic velocity variation induced by hydrological forcing of karst aquifers in the Apennines, Italy

Non-linear response of the elastic properties of the crust has been studied using the analysis of seismic velocity variations induced by various natural forcing agents (earthquakes, tides, volcanic processes, and others). Here we study 1) the variations of seismic velocities in response non-tectonic deformations associated to phases of groundwater recharge/discharge in large karstic aquifers in the Southern Apennines of Italy and discuss 2) the implications in terms of non-elastic behavior of the crust. Karst systems are complex aquifers common within the carbonate rocks of the Apennines. They store large amount of groundwater producing significant horizontal dilatational strains that modulate the secular, tectonic deformation (~3 mm/yr extension across the Apennines) and background seismicity (Silverii et al., 2019; D’Agostino et al., 2018) with seasonal and multi-seasonal signatures. The availability of accurate and temporally-long hydrological measurements (rainfall and karst spring discharge) in addition to dense seismic and geodetic networks provide the opportunity to assess the elastic response of the crust to strain forcing at various periods and the sensitivity of relative velocity variations to non-tectonic, hydrological strain variations. We performed velocity variation measurements on seismic noise autocorrelation signals recorded at seismic stations for different coda waves time lapse and compared them with strain measured by the GPS network. We observe that seismic velocities decrease during dilatation episodes (high hydraulic head) and increase during contraction (low hydraulic head). We find that the retrieved strain sensitivity of seismic velocity changes is of the order of ~10³ and discuss such sensitivity with previous natural and laboratory results.