Characterisation and locations of volcanic high frequency tremor above 10 Hz on Mount Etna

When it comes to volcanic tremor, low frequency signals (below 5 Hz) are well investigated. Such tremor signals can usually be linked to magma movement or gas fluctuations. However, little is known about seismic tremor signals on Mount Etna above 10 Hz. Hence, a large field campaign targeting high frequencies was undertaken in the summer of 2022. It consisted of the deployment of six dense circular arrays ranging from 30 to 200 m apertures of seismic nodes installed around the summit craters. It led to the detection of tremor bands between 10 and around 20 Hz as well as the typical tremor signals below 5 Hz.

The tremor is detected with good coherency at stations within one array (despite an extreme level of scattering) in good agreement with the energy distribution in the average amplitude spectra of the array. The high frequency tremor varies strongly in intensity over time periods of one hour and re-occurs several times throughout the deployment period of almost a week. In contrast the tremor below 5 Hz is relatively constant. This suggests that the high frequency tremor could be a separate signal due to a process that may not yet be fully understood.

Localisations of these tremor episodes point to or near the Bocca Nuova Summit Crater which was actively degassing at the time. Interestingly, high frequency seismic tremor is matched in time very well by a narrow 3.5-5 Hz acoustic band. While the match in time clearly suggests a connection between the two signals, the different frequencies indicate two different but linked processes happening simultaneously. The acoustic signal implies degassing processes. Later during the deployment tremor episodes are found which are accompanied by much weaker acoustic signals (if at all present) suggesting gases might not necessarily be involved in generating the detected seismic tremor at all.

In summer 2023 we undertook a complementary second deployment of seismic, acoustic and optical camera data in the Bocca Nuova summit area. Once again, we find tremor below 5 Hz, however high frequency characteristics are different to the previous year with tremor bands less dominant than before and much more constant over time. More than one acoustic band is found as well, also constant over time. In this second data set we use camera recordings of the crater activity as a proxy for degassing activity to try and understand the precise origin of these seismic and acoustic volcanic signals.