Transient ground deformation observed by GNSS and InSAR during and following the 2021 Fagradalsfjall eruption, Iceland

Abstract Geodetic observations, coupled with modelling of the detected signals, can help discriminate between different processes contributing to measured surface deformation during a volcanic eruption, providing insight into its evolution, the associated magma transport, and processes occurring in the subsurface. Global Navigation Satellite System geodesy and Interferometric analysis of Synthetic Aperture Radar satellite images reveal gradual deflation during the six-month-long 2021 eruption in Geldingadalir at Mt. Fagradalsfjall, in SW-Iceland. The co-eruptive deflation shows three temporal phases: T1, 19 March–10 May; T2, 11 May–31 July; T3, 1 August–18 September, correlating with changes in the effusion rate, eruptive style, and geochemistry of the erupted basalt. Effects of lava loading are evident in the geodetic observations. We remove this signal with a Finite Element Method model and infer geodetic sources driving the observed ground deformation, by testing point-pressure, sill, and ellipsoid models. Model fit and parameter evaluation indicate an ellipsoidal source (centered at ~ 7–8 km depth, and deflating volume change of 21–25 Mm 3 ) fits the data marginally better, though geochemical evidence supports a sill source (~ 12–14 km depth and deflating volume change of 21–27 Mm 3 ). Inflation was detected after the eruption and can be modelled at a similar depth as the co-eruptive source. Understanding co- and post-eruptive ground deformation patterns and their correlation with other observables at volcanoes, e.g., effusion rate and geochemistry, is essential to unveil the architecture of the underlying magmatic plumbing system and hazard assessment.