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

PUBBLICATO: 30 Gennaio 2026

Geophysical Research Letters,Yates et al. 2026 - Figure 1 — **Figure 1** Overview of seismic network at Etna volcano and recorded tremor in 2020. (a) Seismic
stations (triangles) on Etna, with used stations highlighted pink. Broader tectonic context shown in
adjacent map (red box corresponding to region shown). Local earthquakes (≥ML2.5) are shown as
small circles, colored by date (see legend). Lateral sensitivity kernels are shown around EBDA
station (further detailed in Section 4.2, Section S7 in Supporting Information S1). (b) Spectrogram
for ECPN station. Red‐dashed vertical line marks the day 2020‐10‐10, with its frequency content
shown in panel (c). (c) Amplitude spectrum comparing daily measurement with long term average
(reference). (d) Cross‐correlation between the daily and reference spectra in panel (c). (e)
Normalized spectrogram at each station between 2020‐06‐20 and 2020‐07‐01, with approximate
distance to summit craters.

Yates A.S., S. Heuninck, A. Barajas, H. Bektas, C. Caudron, S. De Angelis, L. Zuccarello (2026).
Geophysical Research Letters, 53(2). https://doi.org/10.1029/2025GL117819

Abstract

pisodes 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 (df/f) and seismic velocity (dv/v) 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.

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