Source Scaling of Shallow Volcano‐Tectonic Earthquakes at Piton de la Fournaise Volcano (La Réunion, France) Estimated From Codas of Seismograms

Abstract Despite their importance for understanding faulting processes in volcanic environments, the source scaling of volcano‐tectonic earthquakes remains poorly constrained, mainly due to the difficulty of measuring their source parameters. We analyze source parameters such as seismic moments () and corner frequencies () of several thousand volcano‐tectonic earthquakes that occurred below the summit of the Piton de la Fournaise (PdF) volcano between 2014 and 2023. This swarm seismicity, preceding frequent PdF eruptions, occurs on a ring‐fault system, regularly forced by pressurizations of the shallow magma reservoir and stresses caused by the pre‐eruptive dike propagation. We use 3‐component broadband stations located within short epicentral distances (). Coda amplitude envelopes estimated using narrow‐band filters are inverted to simultaneously extract the frequency‐dependent coda decay factors, site effects, and source terms. The latter are used to compute the source spectral ratios between event pairs, resulting in statistically estimated and relative , calibrated using well‐constrained magnitudes. For very small earthquakes values are extrapolated beyond the Nyquist frequency, extending the reach of conventional spectral analyses. We find an average stress‐drop of the order of for the majority of shallow PdF earthquakes and a general trend , consistent with the scaling observed for tectonic earthquakes, suggesting similar faulting physics in volcanic and tectonic environments. Given that the coda shape remained unchanged over 10 years of eruptions and stress drops varied insignificantly in space and time, this seismicity is associated mostly with a pre‐existing fault system without the formation of new cracks.