The self-organized criticality and periodicity of temporal sequences of earthquakes

A new cellular automaton model is developed to examine the nature of temporal sequences of earthquakes. The model takes the space dependence of fault strength into account and assumes that an earthquake produces a continuous area with the stress dropped to zero. In the model, viscous slip is also introduced on the fault and earthquakes are realizable only when the stress accumulates faster than the viscous relaxation. The analysis reveals that the sequences of earthquakes generally satisfy the power law relation between the intensity and frequency of earthquakes so that earthquakes may be in the state of self-organized criticality. On the other hand, periodicity appears in some sequences that consist of the groups of high seismic activity repeated between calm intervals with an almost constant period. Therefore, self-organized criticality and periodicity coexist in these sequences and the claim that earthquakes are unpredictable because of self-organized criticality may be inadequate.