  We evaluated the efficiency of various models in describing the time decay of
aftershock rate of 47 simple sequences occurred in California (37) from 1933 to
2004 and in Italy (10) from 1976 to 2004. We compared the models by the
corrected Akaike Information Criterion (AICc) and the Bayesian Information
Criterion (BIC), both based on the log-likelihood function but also including a
penalty term that takes into account the number of independent observations and
of free parameters of each model. We compared the performance of different
models by varying the starting time Ts and the minimum magnitude threshold Mmin
for each sequence. We found that Omori-type models including parameter c are
preferable to those not including it, only for short Ts and low Mmin while the
latters generally perform better than the formers for Ts longer than a few
hours and Mmin larger than the main shock magnitude Mm minus 3 units. This
clearly indicates that a value of parameter c different from zero does not
represent a general property of aftershock sequences in California and Italy
but it is very likely induced in most cases by catalog incompleteness in the
first times after the main shock. We also considered other models of aftershock
decay proposed in the literature: the Stretched Exponential Law in two forms
(including and not including a time shift) and the band Limited Power Law
(LPL). We found that such models perform worse than the Modified Omori Model
(MOM) and other Omori-type models for the large majority of sequences, although
for LPL, the relatively short duration of the analyzed sequences (one year)
might also contribute to its poor performance.
