Tracking Subsurface Changes via Frequency Shifts in Volcanic Tremor Spectral Lines: Observations From Mt Etna

Abstract Episodes of volcanic tremor provide valuable insights into the dynamics of subsurface processes at active volcanoes. Previous studies have suggested that evolving tremor properties may relate to changes in the stress conditions of the plumbing system. However, a strong causative link has remained elusive. At Etna volcano, we observe subtle changes in the frequency of stable spectral lines recorded during broadband tremor over 8 months in 2020. Comparison with seismic velocity changes computed using passive seismic interferometry reveals a remarkable correlation between relative frequency and seismic velocity changes. The most significant of these changes are interpreted to result from a shear‐wave velocity decrease in response to local seismicity. We argue that these observations are best explained by resonance within near‐surface structures. These findings demonstrate that monitoring frequency shifts of stable spectral lines produced during tremor can provide meaningful insights into subsurface properties.