  We investigate the spatial distribution of aftershocks and we find that
aftershock linear density exhibits a maximum, that depends on the mainshock
magnitude, followed by a power law decay. The exponent controlling the
asymptotic decay and the fractal dimensionality of epicenters clearly indicate
triggering by static stress. The non monotonic behavior of the linear density
and its dependence on the mainshock magnitude can be interpreted in terms of
diffusion of static stress. This is supported by the power law growth with
exponent $H\simeq 0.5$ of the average main-aftershock distance. Implementing
static stress diffusion within a stochastic model for aftershock occurrence we
are able to reproduce aftershock linear density spatial decay, its dependence
on the mainshock magnitude and its evolution in time.
