Distributed acoustic sensing of very long period strain signals from strombolian explosions

Very long period (VLP; 0.01-0.2 Hz) seismicity is observed at many volcanoes worldwide, and provides key insights into magma and fluid dynamics within volcanic structures. VLPs are typically recorded by sparse networks of seismometers, which limits the ability to resolve the resulting displacement (or deformation) at fine spatial scales. Distributed acoustic sensing (DAS) may help overcome this limitation by densely sampling the projection of the strain tensor along fibre-optic cables with high spatial and temporal resolution, enabling a more complete view of VLP-induced deformation. Here, we analyse VLP strain signals recorded by DAS at Stromboli volcano (Italy) in November 2022 along a 6-km dedicated fibre-optic cable. We designed the cable geometry to provide broad coverage of the craters and to sample the strain at multiple locations and along different directions. We focus on a dataset of approximately 200 VLP events recorded between November 13 and 14, 2022. The VLP strain signals correlate with explosive activity and show consistent features across multiple events, indicating a persistent, non-destructive source. Leveraging the distributed nature of DAS measurements, we recover the principal strain axes of VLPs and estimate both the location and the volumetric change of the source using a quasi-static deformation model. We retrieve the principal horizontal strains for each VLP by inverting strain amplitudes measured along three different fibre directions and at multiple locations along the cable, allowing us to resolve their spatial distribution. The resulting principal VLP strains exhibit radial and tangential orientations with respect to the craters, consistent with observed seismic particle motions and an axisymmetric source. We then model the VLP strain along the fibre using a point-like deformation source (Mogi). The optimal agreement between modeled and observed VLP strain averaged over the 200 events is for a point source located ~500 m beneath the active craters, with an estimated volumetric change of ~30 m³. Under the assumption of a spherical source with a radius of 87 m, the inferred volumetric change corresponds to a pressure change of ~19 kPa. These results are consistent with previous studies and highlight the capability of DAS to investigate volcano deformation at long periods.