Transient deformation of karst aquifers due to seasonal and multiyear groundwater variations observed by GPS in southern Apennines (Italy)

Abstract

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.

Hydrologically Induced Karst Deformation: Insights From GPS Measurements in the Adria‐Eurasia Plate Boundary Zone

“…However, we can approximate their overall behavior as the one due to a single, equivalent tensile fracture (e.g., Jade & Sitharam, ; Sitharam et al, ), to gain a first‐order estimation of the pressure changes required to explain the observed horizontal displacement patterns. A similar approach has been used by Silverii et al () to model displacements associated with karst aquifers in the southern Apennines with elastic dislocation.…”

“…Devoti et al (), in particular, described a localized active slope deformation signal, highlighted by GPS and tiltmeter stations, in a karstic limestone plateau in the southeastern Alps (the Cansiglio Plateau, CP in Figure ), which correlates with rainfall data. Silverii et al () discussed seasonal and multiyear deformation in GPS time series affecting a wider region, the southern Apennines of Italy, which have been associated with groundwater variations in karst aquifers. Karst systems appear thus ideal settings for investigating the effects of rainfall, rainwater infiltration, and changes in the water table on crustal deformation, potentially affecting the earthquake rates.…”

Crustal Deformation and Seismicity Modulated by Groundwater Recharge of Karst Aquifers

“…Separation of seismicity in two different depth ranges shows that peaks of seismicity at 0-6 km are essentially in phase with spring discharge whereas delayed 0.1-0.2 years at 6-12 km (supporting information Figure S3). Time series of crustal displacements from permanent Global Positioning System (GPS) stations show the occurrence of seasonal and multiannual modulation of extensional strain transients near to karst aquifers with a distinct temporal correlation with phases of groundwater recharge (Silverii et al, 2016). A description of GPS data and analysis can be found in the supporting information (Avallone et al, 2010;Bertiger et al, 2010;Blewitt et al, 2016;Métois et al, 2015;Rebischung et al, 2012).…”

“…The hypothesis of seismicity modulation associated to the elastic response of the lithosphere to hydrologic loading conflicts with Gravity Recovery and Climate Experiment observations indicating maximum total hydrological loading in December-January, 5-6 months earlier than the increase in seismicity rate (Silverii et al, 2016). The positive correlation between horizontal strain transients with phases of maximum groundwater recharge appears, instead, more consistent with a poroelastic process.…”

Poroelasticity and Fluid Flow Modeling for the 2012 Emilia-Romagna Earthquakes: Hints from GPS and InSAR Data

“…As the second IC exhibits a large time scale, one hypothesis could be that it represents a multiannual signal. Multiannual signals in the GPS displacement time series, in fact, have been recently highlighted in the Apennines and in the Alps [29,30] and associated with variations of hydrological processes acting at different spatial scales. The temporal function of the third IC shows a rapid increase in the early postseismic stage (first 10 days after the May 20 mainshock).…”