Observation of pluri-decadal movement at the NE-border of the Adria-Plate by subsurface long-base geodetic instrumentation

The movement of the Adria plate is important in the understanding of the seismicity of Italy and the Dalmatian region. Its northern margin delineates the most seismic area of the Alps and has been hit by destructive earthquakes. The deformation of the margin has been observed over four decades in a natural cave with long-base highly stable and sensitive instruments. We discuss the observed deformation on the decadal time scale. We show that the movement is the superposition of a steady state deformation rate, and a time varying deformation with half-period of about twenty years. The station is near to the Adriatic Sea, wherefore a sea loading effect is present. The change of sea-level in time involves also a steady state as well as decadal scale variations. We model the geodetic observations in terms of the deformation due to the plate-tectonic movement and discuss the deformation due to the sea-loading.