POSSIBLE CREEP-RELATED TILT PRECURSORS OBTAINED IN THE CENTRAL APENNINES (ITALY) AND IN THE SOUTHERN CAUCASUS (GEORGIA)

Daily averaged tilt component data from two sites of the Central Apenn ines (Italy) and of the Southern Caucasus (Georgia), respectively, rev ealed intermediate-term tilts as possible precursors to earthquakes (M = 3.0 divided by 4.7) which occurred in the above-mentioned seismic a reas within a distance of 50 km from the sires. A good temporal correl ation as well as a fair spatial correspondence between these residual tilts (with amplitude and duration of some microradians and months, re spectively) and main shocks were pointed out, by removing both secular trends and seasonal thermoelastic effects from the raw tilts. An atte mpt was made to justify the above-mentioned results, based on the assu mption that the observed intermediate-term preseismic tilts are the ma nifestation of aseismic creep episodes of comparable duration in the f ault materials of thrust faults close to the tilt sites. The mechanism refers to a strain field slowly propagating from the preparation foca l area to the tilt site, through crustal blocks separated by weak tran sition zones. This propagation is thought to be the cause of the local aseismic fault slip recorded by the tiltmeters. Previously, both disc rete structures and strain propagation effects were revealed in the Ce ntral Apennines and are thought also to exist in the Southern Caucasus . As in the past, the rheological properties of fault materials are re vealed as viscoelastic ones. In fact, creep equations obtained by appl ying several viscoelastic models on our data, proved to fit quite well some of the observed tilt precursors, producing viscosity and rigidit y values very similar to those reported in literature.