Investigating clustering structures in time-occurrence sequences of seismic events observed in the Irpinia-Basilicata Region (Southern Italy)

We reveal the existence of clustering properties in the temporal distributi on of the earthquakes observed in a seismic active area of Southern Apennin e Chain (Italy) by means of quantitatively fractal tools (Fano Factor and A llan Factor). Data consist in a sequence of seismic events instrumentally r ecorded during the period 1983-1995 in the Irpinia-Basilicata Region (South ern Italy), in which in past and recent years, many destructive events occu rred. The analysis of the Fano and Allan Factors shows that the sequence of the occurrence times of events with magnitude Mth greater than or equal to 2.5 is characterized by a scale-invariant behavior from the time scale tau similar to 5.10(3) s with a scaling coefficient alpha similar to 0.3. By g radually increasing the threshold magnitude up to Mth = 3.1, the value of t he scaling coefficient monotonically decreases, pointing out a falling-off in the correlation strength. Although the increasing of the threshold magni tude seems to act as a randomizing filter which removes clustered structure s, no firm sign of Poissonian, memoryless behavior is detectable in our ana lysis.