  Aftershock occurrence is characterized by scaling behaviors with quite
universal exponents. At the same time, deviations from universality have been
proposed as a tool to discriminate aftershocks from foreshocks. Here we show
that the change in rheological behavior of the crust, from velocity weakening
to velocity strengthening, represents a viable mechanism to explain statistical
features of both aftershocks and foreshocks. More precisely, we present a model
of the seismic fault described as a velocity weakening elastic layer coupled to
a velocity strengthening visco-elastic layer. We show that the statistical
properties of aftershocks in instrumental catalogs are recovered at a
quantitative level, quite independently of the value of model parameters. We
also find that large earthquakes are often anticipated by a preparatory phase
characterized by the occurrence of foreshocks. Their magnitude distribution is
significantly flatter than the aftershock one, in agreement with recent results
for forecasting tools based on foreshocks.
