Abstract
The authors compute the maximum Lyapunov exponent lambda of an earthquake model which exhibits deterministic chaos and they discuss its relation with the predictability time of the system. A method is proposed to estimate lambda by the calculation of the entropy of Markov processes which mimic (i) a Poincare map of the model and (ii) a random map related to the seismic signal. The latter map can be obtained using experimental records generated by low-dimensional chaotic system where Poincare maps are not feasible.