Deformation, seismicity, and monitoring response preceding and during the 2022 Fagradalsfjall eruption, Iceland

Following two periods of dike intrusion in 2021 at Fagradalsfjall, Iceland, one of which led to an eruption, a third dike intrusion commenced on 30 July 2022. A sudden increase in seismicity occurred within the diking area, with approximately 1700 automatically detected earthquakes > M1 within 24 h. Strong earthquakes were felt over several days within a wider area (largest M _W 5.3). The timeline and spatial distribution of seismicity suggested it resulted from diking, together with triggered seismicity in nearby areas releasing stored tectonic stress. Geodetic observations revealed displacements consistent with a dike intrusion, and geodetic modeling on 2 August revealed a best-fit model with a shallow top depth of the dike (~1 km), and high magma inflow rate (~49 m³/s). Also considering a decline in seismicity, a warning was issued that the likelihood of a new eruption in the coming days was high. An effusive eruption started the next day (3 August) on a ~375-m-long fissure, with an initial extrusion rate of 32 m³/s. The projected surface location of the dike (from the optimal model) was within 49–110 m of the eruptive fissure. We present a timeline of the activity and monitoring response in the days both preceding and following the eruption onset. We compare the details of the activity that occurred prior to this diking and eruption to the previous events at Fagradalsfjall to improve understanding of unrest preceding eruptions.