  We propose a new version of the ETAS model, which we also analyze
theoretically. As for the standard ETAS model, we assume the Gutenberg-Richter
law as a probability density function for background events' magnitude.
Instead, the magnitude of triggered shocks is assumed to be probabilistically
dependent on the triggering events' magnitude. To this aim, we propose a
suitable probability density function. This function is such that, averaging on
all triggering events' magnitudes, we obtain again the Gutenberg-Richter law.
This ensures the validity of this law at any event's generation when ignoring
past seismicity. The probabilistic dependence between the magnitude of
triggered events' and the one of the corresponding triggering shock is
motivated by some results of a statistical analysis of some Italian catalogues.
The proposed model has been also theoretically analyzed here. In particular, we
focus on the inter-event time which plays a very important role in the
assessment of seismic hazard. Using the tool of the probability generating
function and the Palm theory, we derive the density of inter-event time for
small values.
