Surface-wave magnitudes from the global Milne seismograph network (1897–1912): methodology and implication for moment magnitude conversion

The global network of Milne seismographs, operated under the coordination of the British Association for the Advancement of Science from 1897 to 1912, was the first globally distributed instrumental seismographic network. Trace amplitude readings from the Shide Circular bulletins, combined with an instrument-specific calibration that corrects for the resonant amplification of undamped Milne seismographs, were later used to derive a set of surface-wave magnitudes Ms estimates for shallow earthquakes of this period. This study presents the first Generalized Orthogonal Regression conversion formula specifically calibrated for the Milne seismograph network period (1897–1912), relating these Ms values to moment magnitude Mw and applies it to produce an updated Mw proxy dataset. The resulting instrument-specific calibration extends Mw proxy information to major earthquakes of the Milne network period, particularly to pre-1904 events for which standardized global instrumental magnitude estimates are unavailable. These results highlight the complementary value of calibrated Milne observations for improving the characterization of major earthquakes during the earliest instrumental period. These findings further highlight that early analog data represent an irreplaceable resource, whose systematic recovery and critical reanalysis are essential for the improved characterization of historical seismicity and assessments of long-term seismic hazard.