Tom Winder, University of Iceland: Repeated dike injections beneath the Sundhn´ukur crater row, Reykjanes Peninsula, Iceland, imaged by relatively relocated seismicity.
Between November 2023 – November 2025 there have been ten dike intrusions and nine fissure eruptions beneath Sundhnúkur, on the Reykjanes Peninsula, Iceland. Geodetic and geochemical analyses show that these have been fed by a common source, located at 3-4 km depth beneath the harnessed Svartsengi geothermal area. This remarkable sequence of magmatic activity has been marked by abundant seismicity.
Relative quiescence on the Peninsula – following the July-August 2023 Fagradalsfjall eruption – was interrupted in late October by elevated seismicity and surface uplift measured at Svartsengi, 8 km further west.
As during inflation episodes at Svartsengi in 2020 and 2022, intense shallow seismicity accompanied the deformation, dominantly consisting of strike-slip faulting above an inferred sill.
From around 15:00 on 10th November 2023, intense migrating seismicity and rapid metre-scale horizontal deformation marked the intrusion of a NNE-SSW oriented dike, which reached approximately 15 km length in just 8 hours, and propagated under the town of Grindavík, which was evacuated. On 18th December, similar (though smaller amplitude) signals marked a second, smaller intrusion, but in contrast this dike quickly breached the surface and culminated in a 4 km long fissure eruption. A similar pattern has repeated in the following 18 months, with cyclical re-inflation beneath Svartsengi, and repeated dike intrusions and fissure eruptions along a common lineament. Through analysis of high-resolution relative relocations of the dike-induced seismicity, we investigate the relative geometry of the repeated dike intrusions, and the relationship between the seismicity and distribution of dike opening and location of eruption onset.
