Implications for existing tremor generation models on volcanoes through newly observed high-frequency tremor on Mt. Etna

Seismic tremor is widely monitored for eruption forecasting, yet its use requires improved understanding of its source processes, which remain debated. Tremor is commonly attributed to magma transport or fluid-induced resonance within volcanic plumbing systems. However, alternative studies suggest that fluids may not be required: weak, unconsolidated edifice materials geomechanically near the brittle–ductile transition can undergo diffusive brittle failure at room temperature, producing numerous low-amplitude, small-stress-drop seismic events that merge into tremor. Minor stress perturbations—caused by magma flow, gas influx, or gravitational loading—may be sufficient to trigger such dry mechanical failure.

Here, we investigate episodic high-frequency tremor (10–20 Hz) recorded at the summit of Mt. Etna during a dense seismo-acoustic deployment in summer 2022. Despite strong attenuation and scattering at these frequencies, we show that variations in the seismo-acoustic energy ratio across tremor episodes reveal differing conditions under which tremor is produced. Using multi-array beamforming and 3D grid-search techniques, we locate tremor sources in multiple regions, including both degassing-related and non-degassing areas. Synthetic tests indicate that some tremor episodes likely comprise multiple simultaneous sources, consistent with diffusive brittle failure. Frequency–magnitude analyses further support a model in which tremor arises from sequences of small-magnitude, very low stress-drop events merging into tremor due to the cumulative scaling observed and comparison with previous numerical work on seismic event population. Together, our results indicate that volcanic tremor does not necessarily require fluid movement and may also be generated by dry brittle failure processes.