According to the seismic gap hypothesis, the next big earthquake will occur with a higher probability on segments of an active fault that have experienced little or no seismic activity for a long period. It can be considered a complementary view of the hypothesis of alternation, formulated by Gilbert at the beginning of the previous century (Gilbert, 1909), and stating that "When a large amount of stored energy has been discharged in the production of a great earthquake and its after-shocks, it would seem theoretically that the next great seismic event in the same seismic district was more likely to occur at some other place, and that successive great events would be distributed with a sort of alternation through the districts …". According to this hypothesis one could be able to identify the fault section which is far away from failure whereas the seismic gap should allow one to identify the one closer to it. Both hypotheses are based on the reasonable idea that big earthquakes occur with higher probabilities in sections of the faults where stress has mostly accumulated but none of the two provides an estimate on how close or how far the region is to failure. After the development of the elastic rebound theory, they have been gradually replaced by a stronger hypothesis which is usually termed seismic cycle model (Fedotov, 1965), which provides also an estimate of the timing until the next big earthquake. This model, indeed, assumes that