  The distribution of inter-occurrence time between seismic events is a
quantity of great interest in seismic risk assessment. We evaluate this
distribution for different models of earthquakes occurrence and follow two
distinct approaches: The non homogeneous Poissonian and the non Poissonian one.
In all cases we obtain either a power law or a power law damped by an
exponential factor behaviour. This feature of the distribution makes impossible
any prediction of earthquakes occurrence. Nevertheless it suggests the
interpretation of the earthquake occurrence phenomenon as due to some
non-linear dynamics to be further investigated.
