Aftershocks hazard in Italy part I: Estimation of time-magnitude distribution model parameters and computation of probabilities of occurrence

We analyzed the available instrumental data on Italian earthquakes from 1960 to 1996 to compute the parameters of the time-magnitude distribution model proposed by Reasenberg and Jones (1989) and currently used to make aftershock forecasting in California. From 1981 to 1996 we used the recently released Catalogo Strumentale dei Terremoti 'Italiani' (CSTI) (Instrumental Catalog Working Group, 2001) joining the data of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and of the Italian major local seismic network, with magnitude revalued according to Gasperini (2001). From 1960 to 1980 we used instead the Progetto Finalizzato Geodinamica (PFG) catalog (Postpischl, 1985) with magnitude corrected to be homogeneous with the following period. About 40 sequences are detected using two different algorithms and the results of the modeling for the corresponding ones are compared. The average values of distribution parameters (p = 0.93±0.21, Log10(c) = 1.53±0.54, b = 0.96±0.18 and a = -1.66±0.72) are in fair agreement with similar computations performed in other regions of the World. We also analyzed the spatial variation of model parameters that can be used to predict the sequence behavior in the first days of future Italian seismic crisis, before a reliable modeling of the ongoing sequence is available. Moreover some nomograms to expeditiously estimate probabilities and rates of aftershock in Italy are also computed.