Ultra-small volcano deformation recorded at Etna by fiber optic sensing

Etna volcano (Italy) is one of the most active volcanoes in the world with a great variety of events leading to effusive and/or explosive eruptions. The eruptive events are usually preceded and accompanied by ground deformation, that is often so tiny that only high-precision borehole strainmeters are capable to capture. So far, the analysis of more than 10 years of records from the Etna strainmeter network has shown the importance of continuous strain monitoring both for surveillance purposes and research advancements. Despite their valuable contribution, the number of deployable borehole high-precision strainmeters is limited by costs, logistics and challenges in the installation. Here, we demonstrate that fiber optic sensing is a valid alternative for measuring ultra-small slow volcano deformation.

In 2024 an innovative Distributed Fiber Optic Sensing prototype has been set up to interrogate a fiber optic cable installed behind casing in a 200-m deep borehole in Serra La Nave (on the southern Etna flank at ca. 5 km away from the summit craters). The new interrogator uses a reference-based Rayleigh backscatter correlation method which allows for precise long-term strain measurement. The quality of the fiber optic data is assessed by comparing the signals against well-known rock deformation responses and against strain time series recorded by the Etna strainmeter network. Thanks to the accuracy better than few nanostrain in the low frequency range, we clearly observe Earth tidal components in the fiber optic strain data. Peaks in the M2 (period 12.42 h) and O1 (period 25.82 h) tidal constituents emerge well above the background noise. The reliable detection and extraction of tidal components provide the opportunity to characterize and quantify the coupling between the fiber and the rock formation. Moreover, strong correlations with atmospheric pressure changes and rainfall events are observed. These evidences demonstrate a good coupling between the fiber optic cable and the surrounding rocks, although the degree of coupling is highly variable along the cable. The analyses show a long-term stability of the interrogator capable to record volcano deformation.

On the morning of 10^th November 2024 Etna experienced a weak lava fountain preceded by a short and small seismic swarm. Despite the tiny deformation induced by the volcano activity, the fiber optic interrogator was able to discern strain variations on the order of 125 nanostrain over 2 h (0.02 nanostrain/s). The first analyses and interpretation of the signal related to the volcano activity will be presented.