Spectral analysis of background and transient signals at Mount Etna using rectilinear fibre-optic segments

Etna is the largest, most active and closely monitored volcano in Europe,making it a crucial study region for volcanology and geohazard assessment. In earlyJuly 2019, a 1.5 km fibre-optic cable was deployed near the summit of Mount Etnaand interrogated for two months. The cable was divided into four main segments, twoof which point towards different active crater areas. Temporary seismic broadbandstations and infrasound sensors were also deployed along the cable. During theexperiment, three distinct eruptive events were recorded. The first two events arecharacterised by a large number of explosions in the active crater area, together withan increase in background tremor activity. The third event is characterised by a largerincrease in background tremor, but almost no explosions.The continuous recordings are analysed in the frequency-wavenumber domain,which reveals the features of the background tremor activity and the stacked transientsignals, such as explosions. During the first two eruptive events, the stack ofexplosive sources is characterised by a non-dispersive arrival, travelling withdifferent apparent velocities along each segment, and a non-linear ground response upto 25 Hz. These segments can be used as an antenna to estimate an average back-azimuth for the explosions, which come from the same crater area during botheruptive events.Outside of the three eruptive events, the background tremor features two slowdispersion modes, both well resolved on the raw recordings. The slowest mode isaffected by gauge-length attenuation at higher frequencies, due to its shortwavelength, but remains detectable up to 27 Hz, with group velocities as low as 170m/s. These observations showcase the utility of simple, rectilinear geometries indeployments despite their known shortcomings, such as in location procedures. Forknown source regions, such as volcanoes, a well-oriented segment can leveragecontinuous activity to record the incoming wavefield and extract dipersion curveswithout the need to perform cross-correlations, simplifying the workflow.