We utilized the Match&Locate method to characterize the detailed spatial and temporal evolution of earthquakes before the July 2019 Mw 6.4 Ridgecrest, California earthquake. The Mw 6.4 mainshock was preceded by 40 foreshocks within ~2 h (on July 4, 2017 from 15:35:29 to 17:32:52, UTC). The largest foreshock (M 4.0) separates the foreshock activity into two stages with different nucleation mechanisms. A swarm of repeating earthquakes occurred before the M 4.0 event, implying the earthquake sequence initiated from an aseismic slip process. The majority of aftershocks of the M 4.0 event as well as the Mw 6.4 mainshock, occurred within regions of increasing Coulomb stress, indicating that they were triggered by stress transfer. Our observations demonstrate that neither the preslip model nor the cascade model can explain the entire nucleation process of the Mw 6.4 mainshock. Instead, both mechanisms govern the nucleation process, but at different stages.