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) th rough the employment of geodetic instrumentation are analysed to evaluate o bserved signals considered as preseismic. Once the disturbing action of atm ospherical/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 eve nts. An effort is also performed in their interpretation and modelling. Eva luations on the basis of micro-cracking of the source region and aseismic f ault creep are done. Modelling based on trivial rheological models furnishe s crustal viscosity values in agreement with the values obtained in the ana lysis of the silent earthquakes recorded in the same area before the 1976-7 7 destructive seismic events. We put forward the hypothesis that for both t he presumably preseismic signals the viscosity plays an important role, alt hough in one case slow dislocations could also have occurred. The two stron g possible preseismic signals here considered support the hypothesis propos ed by some authors that evident and reliable precursory signals can be dete cted only within a distance of 2-3 times the dimensions of the source area. (C) 1999 Elsevier Science B.V. All rights reserved.