<p>This paper focuses on the study of the temporal evolution of seismicity and the role of fluids during major earthquake sequences that occurred in the Central Apennines and Southern Walker Lane belt-Eastern California Shear Zone over the last two decades: the 1997 Colfiorito, the 2009 L’Aquila, the 2016 Amatrice-Norcia, and the 2019 Ridgecrest sequences. The availability of high-quality earthquakes catalogs offers the opportunity to evaluate in detail the temporal evolution of the earthquake's size distribution (or b-value) and propose a physical explanation based on the effect of the fluid flow process in triggering seismicity. For all seismic sequences, the b-value time series show a gradual decrease from a few months to one year before mainshocks. The gradual decrease in the b-value is interpreted as a gradual increase in earthquake activity due essentially to the short-term to intermediate-term pore-fluid fluctuations. The temporal variation of the b-value during Amatrice-Norcia and Ridgecrest foreshock sequences is characterized by a double b-value minimum separated by a short-lived b-value increase as observed in laboratory experiments on water-saturated rocks. The observed fluctuation of the b-value is presented here as an accelerating crack growth due essentially to the fluid flow instability. Even though seismic precursors could have been predictable in areas with high dense seismic networks, the different b-value time series reveal the difficulty in establishing a correspondence between the duration of the foreshock activity and the magnitude of the next largest expected earthquake. This may suggest that the fluid migration controls the size of the ruptures.</p>