Sources of Ultra-Slow Dynamic and Static Deformation of dyke eruptive events revealed by telecom fibre optic cable sensing on western Reykjanes Peninsula, SW Iceland.

Understanding volcanic processes is fundamental for anticipating impacts of eruptive activity on human activities and environment. 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 seismic observations. While satellite sensing techniques benefit from a large spatial coverage but coarse temporal resolution and accuracy (mm range), seismometer networks acquire dense temporal data but are sparsely distributed and suffer from spatial aliasing. Dynamic models of sources during the event at the minute scale 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 using phase optical time domain reflectometry (Φ-OTDR) allows us to retrieve dynamic and static deformation processes associated to diking events prior to volcanic eruptions, at the minute time scale. Since November 2023, we are continuously monitoring an existing telecom fibre optic cable with a conventional iDAS interrogator, set-up on western Reykjanes Peninsula, SW Iceland. Reykjanes Peninsula is the onshore expression of the Atlantic mid-oceanic ridge, where a series of magmatic intrusions and eruptions have occurred since 2020. 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 from 1000 Hz to 200 Hz is transferred continuously via internet to our computing center 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. Here we present resulting distributed dynamic strain observations and their source inversions associated with a series of several eruptions and intrusions (18.12.2023 22:17; 14.01.2024 07:57; 08.02.2024 06:03; 16.03.2024 23:14; 22.08.2024 21:26; 20.11.2024 23:14 – all times UTM). 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. Inverted dimensions and dyke locations match rather well with observed eruption locations. We also analyse links between seismic velocity changes from distributed dynamic strain ambient noise records and the eruptions. These results show that fibre optic distributed 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.