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 early
July 2019, a 1.5 km fibre-optic cable was deployed near the summit of Mount Etna
and interrogated for two months. The cable was divided into four main segments, two
of which point towards different active crater areas. Temporary seismic broadband
stations and infrasound sensors were also deployed along the cable. During the
experiment, three distinct eruptive events were recorded. The first two events are
characterised by a large number of explosions in the active crater area, together with
an increase in background tremor activity. The third event is characterised by a larger
increase 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 transient
signals, such as explosions. During the first two eruptive events, the stack of
explosive sources is characterised by a non-dispersive arrival, travelling with
different apparent velocities along each segment, and a non-linear ground response up
to 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 both
eruptive events.

Outside of the three eruptive events, the background tremor features two slow
dispersion modes, both well resolved on the raw recordings. The slowest mode is
affected by gauge-length attenuation at higher frequencies, due to its short
wavelength, but remains detectable up to 27 Hz, with group velocities as low as 170
m/s. These observations showcase the utility of simple, rectilinear geometries in
deployments despite their known shortcomings, such as in location procedures. For
known source regions, such as volcanoes, a well-oriented segment can leverage
continuous activity to record the incoming wavefield and extract dipersion curves
without the need to perform cross-correlations, simplifying the workflow.