Abstract Seismic radiated energy is fundamental to earthquake physics, yet its scaling remains debated. Two conflicting views—magnitude‐invariant apparent stress and systematically increasing apparent stress—have persisted partly because source spectral estimates are poorly constrained. Poorly constrained source spectra also create a long‐standing problem for seismic hazard assessment: many estimates imply ground‐motion variability substantially larger than observed. We show that addressing this inconsistency for peak ground velocity (PGV) yields an independent constraint on radiated‐energy scaling. For crustal earthquakes in Japan (Mw 3.4–7.5), apparent stress increases with magnitude for small‐to‐moderate earthquakes but becomes nearly constant above Mw 5–6, reconciling the previously conflicting views. Our source spectral estimates reproduce both the scaling and variability of observed PGV, supporting the inferred apparent‐stress scaling. Regional comparisons further support a similar scaling transition. This transition may reflect enhanced dynamic weakening due to thermal pressurization. PGV consistency provides a stringent quality‐control criterion for radiated‐energy estimates.