Previous experiments and modeling on the nucleation process of an earthquake reveal accelerating slip and development of a final patch of fixed or expanding length, but whether the nucleation phase is spatially large enough to be detected in real-Earth conditions with durations long enough to be helpful is unknown. This study performed a new round of simulation of the nucleation process based on the rate-and-state friction law. Our results reveal the development of a weakening-zone core where stress releases continuously, which expands first to a dimension less than the half fault length then shrinks to a final length before re-expanding, consistent with seismicity migration before large subduction earthquakes. Onset times of the weakening-zone core are weeks before large subduction earthquakes under high loading rate but years for continental faults with loading rate below 10mm/yr. The onset of weakening-zone core is marked by fault-length-dependent slip rates>3.6-4.8 times the loading rate.