Repeated dike injections beneath the Sundhnúkur crater row, Reykjanes Peninsula, Iceland, imaged by relatively relocated seismicity

Between November 2023 – July 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 10^th 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 18^th 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 2 years, 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.

We find that most dikes initiate from a common point, likely marking a repeatedly used connection to the shallow magma storage region beneath Svartsengi. The dikes vary in propagation direction, forming a complementary pattern of seismicity and inferred opening, and occupy at least two sub-parallel planes, which closely match the geometry of eruptive fissures at the surface.