Space and time characterization of non-tectonic strain changes in the Southern Alps as revealed by the analysis of GPS time-series

Hydrological induced rocks deformation, particularly in karst regions, is a well-known process that has been in the past constrained by terrestrial, mostly underground, geodetic measurements. Recently, continuous GPS stations in the Alpago-Cansiglio sector of the Southern Alps of Italy revealed, with high precisions, the temporal evolution of three-dimensional geodetic displacements that appear correlated with rainfall. Other examples of non-tectonic, hydrological-induced, deformation exists in the European Alps but most of time these processes have been studied at local scales. Here we present the results obtained using a bind-source-separation algorithm, based on a Variational method for Bayesian Independent Component Analysis (vbICA), to the position time-series of 120 continuous GPS stations located in the central (Piemonte, Lombardia and Veneto) and eastern Southern Alps (from Veneto and Friuli to Slovenia, Austria and Croatia). In particular, we analyze de-trended position time-series in the 2005-2016 time window, which have been first filtered for a spatially-correlated Common Mode Error, estimated by means of a Principal Component Analysis performed at the European plate scale. Our analysis provides the spatial response (i.e., the amplitude of the displacements at each station associated to a specific signal/component) and the temporal evolution of several signals (i.e., components), among which the significant ones (in term of description of the observed time-series) are 1) an annual signal with spatially uniform response in the vertical component, 2) an annual signal with spatially uniform response in the horizontal components, and 3) a time-variable, non-cyclic, signal characterized by a quasi-uniform response in the vertical component and a spatially variable response in the horizontal components. In particular, the spatial response in the horizontal components appears larger (and consequently the displacements associated to this third component are larger) in areas of the Southeastern Alps of Italy and Slovenia characterized by karst geology. The largest spatial response is observed in the Alpago-Cansiglio area, in agreement with previous studies, but this signal is also strong in GPS sites located in the Lissini Mountains and in the Friuli and Slovenian Karst. Because the spatial response is not uniform in the horizontal components, and for some sites the direction of motion is in the opposite sense, this time-variable signal is causing significant crustal strains of variable amplitudes thought the investigated time period. We estimate the evolution of strains associated to this signal, and study the orientation of the associated deformation. We find that both extensional and compressional time-variable strains are oriented about normal to the orientation of fractures, as obtained from the analysis of a digital elevation model, which seem to guide the main direction of deformation, rather than its amplitude. We use spatially gridded data to correlate the deformation signal to meteo-climatic observations in the wide study area.