Rainfall modulated deformation of a mountain front: example from the European Southern Alps

Changes in groundwater levels cause measurable deformation of the Earth’s surface and may affect the stress state at seismogenic depths. We highlight a time-dependent deformation signal interesting a large portion of the Euro-pean Southern Alps and Dinarides by applying a blind-source-separation algorithm to the position time-series of hundreds of continuous GPS stations. This signal is characterized by spatially variable amplitudes and directions, implying a time-dependent horizontal deformation that is larger in areas of karst geology, with directions normal to the orientation of fractures detected from a digital elevation model analysis and about parallel to the direction of tectonic strains. The temporal evolution of this signal is correlated with cumulated rainfall values over periods >200 days. The deformation can be explained by pressure changes associated with variable water level within ver- tical fractures, and water level changes required to open or close these fractures are consistent with the fluctuations of precipitation and with the dynamics of mature karst systems.