Pre-existing fractures and eruptive vent openings during the 2021 Fagradalsfjall eruption, Iceland

The Fagradalsfjall eruption commenced on 19 March 2021 on a ~180-m-long eruptive fissure, following a 23-day dike intrusion. New eruptive fissures opened northeast of the initial eruption site on 5, 6–7, 10, and 13 April 2021. The northernmost eruption occurred on 5 April, approximately 1 km northeast of the initial fissure, with the other fissure openings between this and the initial eruptive vents. Still images from web cameras and time-lapse cameras are available for five of the fissure openings. These data show that the eruptions were preceded by steam emitted from cracks in the exact locations where the eruptions started. The time between the first steam observations and the visual appearance of glowing lava ranged between 15 s and 1.5 min during night observations and from 9 to 23 min during daytime observations. The difference in observation time is likely explained by the different lighting conditions. The eruptive vents are located where the north-easterly oriented dike intersected pre-existing north-south-oriented fractures, inferred to be strike-slip faults. These fractures could be identified on a high-resolution ICEYE interferogram as well as on pre-existing aerial photographs and digital elevation models. This interferogram spanned the first day of the eruption (19–20 March 2021). It not only displays deformation related to the pre-eruptive dike intrusion but also shows lineations in locations where eruptive vent openings occurred later in April 2021. These findings demonstrate how Interferometric Synthetic Aperture Radar Analysis (InSAR) can be used to forecast likely locations of subsequent eruptive vent openings, which is of great importance for hazard assessment and defining exclusion zones during fissure eruptions.