Tilt-strain measurements in the NE Italy seismic area: Precursor analysis and atmospheric noise effects

Tilt-strain measurements have been collected since 1977 by several underground stations in the Friuli seismic area (NE Italy) with the aim of detecting seismotectonic deformations. In the work, meteorological effects are initially considered as being able to obscure possible tectonic signals. Two middle-term anomalous signals (tilt and areal strain), not correlated with atmospheric changes, are successively selected for the analysis. They are the strongest and more evident signals recorded in the area after the 1976 destructive seismic events and also are the only two cases of epicentres very near to the recording stations (M = 4.1 and M = 3.9 at 1.8 and 2.9 km, respectively). Evaluations on the basis of micro-cracking and fault creep are carried out. Estimates based on trivial rheological models furnishes crustal viscosity values in agreement with those obtained in the analysis of the silent earthquakes recorded in the same area before the 1976–1977 seismic events. Our data support the hypothesis that prominent precursory signals can be detected only within a distance few times the dimensions of the source area.