The long-term Precursory Scale Increase (Y) phenomenon is used to relate minor seismicity to major shallow earthquakes in Zagros. This phenomenon involves an increase in the magnitude and rate of occurrence of minor earthquakes in an area near to the location of a major event. By the modeling of long-term seismogenesis as a three-stage faulting process, the Precursory Scale Increase (Y) phenomenon can be inferred. Scaling characterizes the parameters of space, time and magnitude that relate the precursory seismicity to the main shock and aftershocks. Seismogenesis starts with the formation of a major crack, culminates in the corresponding major fracture and earthquake, and ends with healing. Showing high seismicity in southwest of Iran, Zagros mountain accommodate major part of convergence between Arabia and Eurasia. From 1970 to 2009, 29 earthquakes in this region demonstrated a sudden increase in the scale of seismicity, which can be inferred as a long-term precursor. Range of magnitudes of these earthquakes was from 5.6 to 6.7. Precursory time (TP) interval between onset of the scale increase and occurrence of the earthquake, and the precursory space (AP) that involved space-time is optimized respectively to scale increase. By scaling relations, predictive regressions are found between the magnitude level of the precursory seismicity (MP) versus TP, AP and the main earthquake magnitude separately. As these relations show high goodness of fit for Zagros, the method could be used in long-range forecasting of the place, time and magnitude of major earthquakes in Zagros region.