Tomographic and volcanotectonic control on the 2021-2023 Fagradalsfjall eruptions, Iceland.

Shortly before the 2021 Fagradalsfjall eruption tomographic images indicated a large high V/V anomaly with a top at about 9 km depth directly below the ongoing seismic swarm. Using volcanotectonic principles we interpret this anomaly as part of the roof of a magma reservoir which, furthermore, ruptured on 24 February 2021. The roof rupture resulted in an injected dike-segment propagating vertically at an average rate of about 0.2 m s until it became arrested beneath a stress barrier at ~ 2 km depth, from where the dike-segment propagated laterally. Further magma injections took place, following largely the path of the first dike-segment, resulting in part of the dike reaching outside the stress barrier and propagating vertically as a tiny 'finger' to form a 180-m-long disconnected fissure at the surface on 19 March 2021. The estimated maximum length of the arrested dike is about 9 km, its height 7 km, and its thickness 4 m - in excellent agreement with direct field measurements of basaltic dikes in Iceland. Seismicity also indicates that the same dike path was partly used during the 2022 and 2023 eruptions, in agreement with the location of the resulting volcanic fissures, resulting in the formation of a multiple feeder-dike.