Multi-volcano observations of coupled tremor spectra and subsurface velocity changes

Episodes of volcanic tremor provide valuable insights into subsurface processes at active volcanoes, yet the physical origin of temporal variations in tremor spectra remains debated. Previous work at Mt. Etna (Italy) demonstrated a strong correlation between relative frequency changes (df/f) during broadband volcanic tremor and seismic velocity changes (dv/v) derived from passive seismic interferometry. Such correspondence suggests that tremor spectra are responding to changes in medium properties rather than variations in the tremor source.Here, we extend this observation beyond Etna to include Tungurahua volcano (Ecuador) and Rincón de la Vieja volcano (Costa Rica). At both volcanoes, we observe consistent correlations between df/f extracted from broadband tremor and dv/v. At Tungurahua, these changes are linked to earthquake-induced damage and meteorological processes, once again suggesting that their modulation reflects changes in the phase velocity within near-surface layers.The persistent relationship between dv/v and df/f at both Tungurahua and Rincón de la Vieja not only supports previous interpretations at Etna, but shows that such a relationship is present across varied volcanic systems. This strengthens the case for using df/f during broadband tremor as a proxy for tracking subsurface changes within volcanic systems, particularly where using traditional methods may be challenging. Furthermore, our results highlight the need to clarify the respective roles of source, path, and site effects in shaping the recorded seismic wavefield in volcanic environments. Doing so avoids misattributing spectral changes as source-driven, and opens the door to exploiting tremor spectra for monitoring purposes.