Umbria-Marche sequence (central Italy): a study on its aftershock sequence

We studied the aftershock sequence following the mainshock (M s 6.4) of September 26, 1997 with its epicenter at Colfiorito (Italy). This work studies a methodological aspect finalized to observe possible anomalies in the temporal decay before a large aftershock with magnitude M>5.5. Such anomalies are thought to be variations from the mean temporal trend. In fact, modeling the dacay as a non-stationary Poissonian process, the number of aftershocks in a small time interval ∆t is the mean value n(t)⋅∆t, with standard deviation . In this work, besides the analyzed parameters of other sequences, we have extended the spectrum of parameters by using fractal properties of the temporal sequence. Earthquakes belong to a class of phenomena known as multifractals. In general, it is important to define the fractal dimension D, but sometimes is not useful if we are describing a natural phenomenon; so it is neces- sary to define the box-counting dimension D0, and the correlation dimension D2. Usually D 0 ≥D 2 . The data concerning the temporal series, checked according to complete- ness criteria, come from the NEIC-USGS data bank (http://neic.usgs.gov/neis/epic/).