After destructive earthquakes, it is a challenge to estimate magnitude rapidly and accurately for dissemination to emergency responders and the public. Here, we propose criteria to calculate peak ground displacement (PGD) from strong‐motion records, which can be used to calculate unsaturated event magnitude. Using collocated strong‐motion and Global Navigation Satellite Systems observations of five major earthquakes in Japan, we demonstrate the effectiveness and accuracy of our strategy. Our results show that, with the right filtering criteria, PGD estimated from strong‐motion acceleration waveforms is consistent with geodetic estimates. The methodology, however, does not allow for calculation of reliable estimates of coseismic deformation or other ground displacement metrics. We demonstrate a simulated real‐time magnitude estimation that suggests it is feasible to generate an unsaturated magnitude estimate in real time from near‐field strong‐motion records. These findings have important implications for early warning and emergency response in seismically active areas, especially where real‐time strong‐motion data are more widely available than geodetic measurements.