A model for complex aftershock sequences

The decay rate of aftershocks is commonly very well described by the modifi ed Omori law, n(t) proportional to t(-p), where n(t) is the number of after shocks per unit time, t is the time after the main shock, and p is a consta nt in the range 0.9 < P < 1.5 and usually close to 1. However, there are al so more complex aftershock sequences for which the Omori law can be conside red only as a first approximation. One of these complex aftershock sequence s took place in the eastern Pyrenees on February 18, 1996, and was describe d in detail by 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 produc tion not directly related to the magnitude of the aftershocks (as in the ep idemic-type aftershock sequences). The model is a simple, discrete, stochas tic fracture model where the elements (asperities or barriers) break becaus e 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 stre ngth, is the presence of dynamic stress fluctuations which constantly reset the initial conditions for the next aftershock in the sequence.