Detection of deep volcanic tremor sources during the 2021 Tajogaite eruption (La Palma, Canary Islands)

In recent years, the use of Distributed Acoustic Sensing (DAS) in seismology has gained extensive usage in different applications. A High-Fidelity DAS system (HDAS) was deployed during the 2021 Tajogaite eruption on Cumbre Volcano (La Palma, Canary Islands), allowing the recording of most of the syn-eruptive and post-eruptive seismicity. The eruption lasted from Sep. 19th until Dec. 13th of 2021. The HDAS was installed on Oct. 19th and is still operating. The HDAS was installed around 10 km from the eruptive vent and was connected to a submarine fibre optic cable directed toward Tenerife Island. Since then, the HDAS has been recording seismic with a temporal sampling rate of 100 Hz and a spatial sampling rate of 10m for a total length of 30 (first phase) and 50 km using Raman Amplification (last period). The HDAS recorded thousands of local earthquakes as well as regional and teleseism events. It was revealed to be an excellent tool for volcanic monitoring, allowing a better location of deeper events, whose location was made difficult by the small aperture of the seismic network of La Palma. The HDAS was also able to record the low-frequency (<1 Hz) component of the volcanic tremor up to a distance of tens of kilometres from the volcano. We show how using array-like techniques (MUSIC and Beamforming), it is possible to identify and separate the volcanic tremor signals from the oceanic ambient noise and characterize its source. In particular, this analysis revealed a complex wavefield consisting mostly of surface waves. The array analysis shows that, apart from the ballistic arrivals of surface waves radiated by the eruptive vents, the wavefield contains arrivals related to the scattering from topographic features of the island and its surroundings. Furthermore, it revealed that, apart from surface waves, the wavefield contains arrivals compatible with body waves radiated by deeper sources. We interpret these sources as the effect of the resonance of the volcanic tremor caused by the flow of magma within the system of feeder dikes.  This work demonstrates the effectiveness of using DAS as a real-time volcano monitoring tool.