Rainfall Induced Horizontal Deformation in the European Eastern Alps Measured by GPS

We analyzed the time-series of continuous GPS stations operating in the Italian, Austrian, Slovenian and Croatian Alps and Dinarides with a blind-source-separation algorithm based on a variational Bayesian Independent Component Analysis method, characterizing the spatiotemporal evolution of ground displacements and crustal deformation associated to hydrological processes over a vast area of ~70.000 km2. The study area is part of the broad zone of deformation where the N-S Adria-Eurasia convergence and the E-ward escape of the Eastern Alps toward the Pannonian basin are accommodated. We characterized the spatial response and the temporal evolution of several signals, among which the most significant ones are two annual signals with spatially uniform response in the vertical and horizontal components, respectively, and a time-variable, non-cyclic, signal characterized by a spatially variable response in the horizontal components. Because of its non-uniform spatial response, this latter signal induces a succession of extensional/compressional deformation, which is larger in areas characterized by karst geology, varying in amplitudes during the study time-span (2007-2016). The orientation of the principal time-variable strain-axes is normal to the orientation of lineaments and fractures detected from the analysis of a digital elevation model and parallel to the direction of tectonic stress. We have compared the time evolution of this signal with hydrological observations by exploiting the availability of gridded datasets for the European area. The detected deformation signal is highly correlated with cumulated rainfall over a period of 180 days, suggesting that the opening of fractures in karst rocks caused by cumulated rainfall is likely the primary mechanism of this deformation signal, whose kinematics is guided by the orientation of rock's fractures. We discuss the implication that this time-variable non-tectonic deformation may have for the estimate of the long-term interseismic strain accumulation and the seismic potential of active faults in the study area.