Sources of dynamic and static deformation associated with eruptive and intrusive events on Reykjanes Peninsula, SW Iceland (2023-2026).

Deformation and seismicity often precede and accompany volcanic eruptions. Models of magma emplacement and ground deformation associated with eruptions are obtained from GNSS and InSAR observations and associated seismic source mechanisms from seismometer observations. While satellite sensing techniques benefit from large spatial coverage with coarse temporal resolution and accuracy (mm range), seismometer networks acquire dense temporal data but are sparsely distributed and suffer from spatial aliasing. However, dynamic models of sources prior to the eruptive event are challenging to obtain, because they are in most cases too small or too slow to be observed accurately with conventional instrumentation. Here, we demonstrate that distributed fibre optic sensing with phase optical time domain reflectometry (Φ-OTDR) allows us to retrieve dynamic and static deformation processes associated to magma transfer from the reservoir below Svartsengi in SW Iceland, at depth and through diking events, prior to volcanic eruptions. Since November 2023, we are continuously monitoring an existing telecom fibre optic cable with a commercial iDAS interrogator, set-up on the western Reykjanes Peninsula. Reykjanes Peninsula is the onshore expression of the Mid-Atlantic oceanic ridge, where a series of magmatic intrusions and eruptions have occurred since 2020. Unlike previous studies, the used cable spans across locations from a large inflation/deflation area near dyke outbreaks at its eastern end, to a remote area where little signatures from eruptions are observed at its western end. In-situ down-sampled strain-rate data (1000 Hz to 200 Hz) are transferred continuously via internet to our computing centre at the GFZ in Germany. We further down-sample data to 2 minutes and perform time integration in order to analyse long period strain signals both spatially and temporally. We present resulting distributed dynamic strain (i.e., strain rate) observations and their source inversions associated with a series of eruptions and intrusions. Our inversions comprise a Mogi source and an Okada model, and we test several inversion methods. For each recorded eruption, we invert the distributed spatial strain taken every 2 minutes, allowing us to follow magma progression prior to each eruption with time. We investigate sizes and locations of the deflating reservoir and dykes with observed eruption locations. We also compare faults reactivated during the successive eruptions with the fibre optic cable records. These results show that distributed fibre optic sensing is capable of simultaneous seismological and geodetic observations in a volcanic context, opening the path for a better understanding and potentially improved real-time monitoring of volcanic processes.