Waveform cross-correlation analysis and magnitude estimation for the 1887 Ligurian earthquake determined from magnetogram recordings

The Ligurian earthquake of 23 February 1887, one of the most significant historical earthquakes in southwestern Europe, is re-evaluated in this study through the analysis of digitized records from 19th-century magnetometers. Traces from the magnetic observatories in Greenwich, Kew, Falmouth (UK) and Paris Saint-Maur (France) were retrieved and digitized to identify disturbances caused by the earthquake ground motion. Synthetic seismograms were generated using full-wavefield modeling and then convolved with the transfer function of classical magnetometers to simulate the expected responses. A cross-correlation analysis between observed and synthetic traces was performed to assess the consistency of different fault-plane scenarios. The results support the plausibility of a low-angle, north-dipping reverse fault as the source mechanism, which is consistent with tsunami modeling results and regional tectonic constraints. A comparative approach with a recent, well-recorded reference earthquake was used to estimate the moment magnitude of the 1887 event based on magnetogram amplitudes. This highlights the potential of historical magnetograms to contribute to the quantitative reassessment of major pre-instrumental events and provides valuable insights for seismic hazard assessment.