Remarkable tilt¯strain anomalies preceding two seismic events in Friuli (NE Italy): their interpretation as precursors

Crustal deformation data collected in the Friuli seismic area (NE Italy) through the employment of geodetic instrumentation are analysed to evaluate observed signals considered as preseismic. Once the disturbing action of atmospherical/hydrological factors on local crustal deformation data has been considered in previously published studies, two middle-term precursors (one tilt and one areal strain) are presented, as being the strongest and more evident signals recorded in the area after the 1976 destructive seismic events. An effort is also performed in their interpretation and modelling. Evaluations on the basis of micro-cracking of the source region and aseismic fault creep are done. Modelling based on trivial rheological models furnishes crustal viscosity values in agreement with the values obtained in the analysis of the silent earthquakes recorded in the same area before the 1976¯77 destructive seismic events. We put forward the hypothesis that for both the presumably preseismic signals the viscosity plays an important role, although in one case slow dislocations could also have occurred. The two strong possible preseismic signals here considered support the hypothesis proposed by some authors that evident and reliable precursory signals can be detected only within a distance of 2¯3 times the dimensions of the source area.