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 distribution of the earthquakes observed in a seismic active area of Southern Apennine Chain (Italy) by means of quantitatively fractal tools (Fano Factor and Allan Factor). Data consist in a sequence of seismic events instrumentally recorded during the period 1983-1995 in the Irpinia-Basilicata Region (Southern Italy), in which in past and recent years, many destructive events occurred. 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 gradually increasing the threshold magnitude up to Mth = 3.1, the value of the scaling coefficient monotonically decreases, pointing out a falling-off in the correlation strength. Although the increasing of the threshold magnitude seems to act as a randomizing filter which removes clustered structures, no firm sign of Poissonian, memoryless behavior is detectable in our analysis.