GROUND TILT VARIATIONS DETECTED IN THE CENTRAL APENNINES (ITALY) IN THE PERIOD 1986-1989 AND THEIR CORRELATION WITH SEISMICITY

Ground tilt measurements carried out at three tilt sites of tite Central Apennines (Italy) revealed a quite good correspondence with seismicity (M less-than-or-equal-to 4.9) which took place within a distance of 100 lan from the sites and over a period of four years (1986-1989). The best correlation is obtained when the tilt signals are delayed by about 1.5 months with respect to the seismic one (number of earthquakes per month), suggesting that seismicity is predictable to a certain extent from the local tilt response. Correlations of similar quality showed also that tiltmeters located at different sites seem to react in a different way to events distributed along the two main active fault zones (namely, the Apennine and anti-Apennine ones), indicating an anisotropic response of the medium. To explain how the seismicity, selected along the Apennine and anti-Apennine directions, give rise to different tilt responses at tilt sites located at a relative small distance, the behaviour of the geological structures and of the geodynamic processes of the investigation area have been taken into account.