Co-eruptive subsidence during the 2021 Fagradalsfjall eruption: geodetic constraints on magma source depths and stress changes

Geodetic observations during volcanic eruptions are important to constrain from where the eruptive products originate in the sub-surface. Some eruptions are sourced from magma reservoirs shallow in the crust, whereas others may tap magma directly from the mantle. The 2021 Fagradalsfjall eruption took place on the Reykjanes Peninsula, Iceland, during March 19 to September 18, resulting in approximately 0.15 km³ of erupted basaltic lava. A wide-spread crustal subsidence and inward horizontal motion, centered on the eruptive site, was observed during the eruption. Nearest to the emplaced lava flows, additional localized subsidence is observed due to the loading of the lavas. The regional subsidence rate varied during the eruption: it was low in the beginning and then increased, in broad agreement with changes in the bulk effusive rate. In this study we use GNSS and InSAR data to model the deformation source(s) during different periods of the eruption, primarily aiming to resolve the depth and volume change of the magma source. We furthermore calculate crustal stress changes during the eruption and compare to the regional seismicity.