Long-term crustal deformations in NE Italy revealed by tilt-strain gauges

Tilt and strain gauge records are analysed to determine the long-term variations in space and time of the strain field in the Friuli area (NE Italy), located at the eastern boundary of the Adria plate. For such long-term variations a deep crustal origin is supposed. Spectral and cross-correlation analyses show that the strain energy involving a large area is polarized in two main directions, N20W and N70E, the first one characterized by aperiodic (or very long period, undetectable by the present analysis) variations, the second one by a period of about 8 years. The direction of the aperiodic motion (N20W) is normal to the Alpine chain, and parallel to the maximum horizontal compression acting in the Alpine area, whereas the direction of the 8 year period motion (N70E) is normal to the Dinaric Alps chain, and coincident with the maximum horizontal compression acting on the eastern boundary of the Adria plate, in the Balkans region. To explain such results, and particularly those related to the N70E direction, slow-travelling stress-strain waves are hypothesized.