  The decay rate of aftershocks is commonly very well described by the modified
Omori law, $n(t) \propto t^{-p}$, where n(t) is the number of aftershocks per
unit time, t is the time after the main shock, and p is a constant in the range
0.9<p<1.5, and usually close to 1. But there are also more complex aftershock
sequences for which the Omori law can be considered only as a first
approximation. One of these complex aftershock sequences took place in the
Eastern Pyrenees on February 18, 1996, and was described in detail by {\it
Correig et al.} [1997]. In this paper, we propose a new model inspired by
dynamic fiber-bundle models to interpret this type of complex aftershock
sequences with sudden increases in the rate of aftershock production not
directly related to the magnitude of the aftershocks (as in the epidemic-type
aftershock sequences). The model is a simple, discrete, stochastic fracture
model where the elements (asperities or barriers) break because of static
fatigue, transfer stress according to a local load-sharing rule and then are
regenerated. We find a very good agreement between the model and the Eastern
Pyrenees aftershock sequence and we propose that the key mechanism for
explaining aftershocks, apart from a time-dependent rock strength, is the
presence of dynamic stress fluctuations which constantly reset the initial
conditions for the next aftershock in the sequence.
