GROUND TILT ANOMALIES ACCOMPANYING THE MAIN EARTHQUAKES OCCURRED IN THE CENTRAL APENNINES (ITALY) DURING THE PERIOD 1986-1989

A four-year set of tilt data (1988-1989) recorded at three sites of the Central Apennines (Italy) and which in the past pointed out quite good raw tilt responses to seismicity, only with moderate thermoelastic effects, have been investigated to search for the possible existence of tilt anomalies accompanying the main earthquakes of the region. It has been observed that, even if fairly good tilt responses to principal events seem to appear, correlations of similar or better quality are obtained when comparing the same tilts with pressure and mainly with temperature. However, when taking into account earthquakes with magnitudes M greater than or equal to 3.0 and more strict E (strain) and L (source dimension) threshold values (the last two parameters being used to evaluate tilt field at a distance R), only one event (M = 3.5; R = 5.1 km) has been selected and tectonic from non-tectonic contributions have been pointed out clearly at one tilt site. The quoted earthquake showed the best E and R/L values (E = 10(-6); R/L = 2.4) in the whole period and it was preceded by a clear tilt anomaly with amplitude and duration of about 5 mu rad and 2 months, respectively. Since the tilt signal in this period is not affected by any non-tectonic effect (maximum correlation coefficient with temperature r = 0.2) the quoted anomaly has been considered as a probable precursor of the above-mentioned earthquake it preceds. Residual, tilt vector oscillations along the direction-including the epicentral area of this event, were also observed. On the contrary, all the remaining anomalous tilts proved to be well justified with pressure and/or temperature variations (r values up to 0.8).