  In this work the distribution of inter-occurrence times between earthquakes
in aftershock sequences is analyzed and a model based on a non-homogeneous
Poisson (NHP) process is proposed to quantify the observed scaling. In this
model the generalized Omori's law for the decay of aftershocks is used as a
time-dependent rate in the NHP process. The analytically derived distribution
of inter-occurrence times is applied to several major aftershock sequences in
California to confirm the validity of the proposed hypothesis.
