Long-term strain oscillations related to the hydrological interaction between aquifers in intra-mountain basins: A case study from Apennines chain (Italy)

We analyze the multiyear strain cycle of a network of continuous GPS stations in the L'Aquila intra-mountain basin in Central Apennines, Italy. The basin is bounded by two aquifers (Nuria-Velino and Gran Sasso) hosted in two tectonically active mountain ranges. All stations show coherent displacements, mostly in the horizontal plane and orthogonal to the normal fault zones of the Apennine mountain range. The displacement repeats back and forth, roughly every 4-5 years with elongation up to 6 mm. The deformation occurs in the region bounded by the two aquifers and the displacement amplitude decreases with distance as predicted by two tensile sources acting in an elastic medium. We interpret this type of deformation as induced by the opening of pre-existing fractures related to two different fault systems as a response to hydrostatic pressure variations. The strain cycle follows the average multiyear rainfall cycle and foresees a dilatation phase during high rainy seasons and a contraction during drier periods. The estimated depth of the equivalent tensile sources ranges from 1.2 to 1.5 km affecting stations at distances up to 20 km away. GPS data show the cumulated movement induced by the two nearby aquifers and suggest a mutual interaction in the elastic space related to the different average annual rainfall and effective infiltration. We fit two tensile sources located along the main faults affecting the aquifers. The eastern source corresponds to the major active fault system affecting the Gran Sasso Range, while the western one includes the sub-parallel active faults of the Mt. Ocre in the Nuria-Velino Range, where carbonate rocks show a higher structural permeability and effective infiltration. These fault systems affect Jurassic-Miocene limestones and dolomites inducing parallel fracturation style of the rock matrix and favoring a strong anisotropy allowing a prevalent lateral expansion as a reaction to pore pressure variations. The recorded hydrological signal may hamper other geophysical signals in GPS time series, thus the measure of tectonic strains requires a careful mapping and modeling of major aquifers.