Deformation patterns of the Reykjanes Peninsula unrest 2020-2024, Iceland: evidence for interconnected neighboring volcanic systems

Neighboring volcanic systems sometimes show evidence of some form of interconnection, for example by inflating or deflating either in phase or in anti-phase. We review here the course of events on the Reykjanes Peninsula (RP) in the ongoing unrest since approximately 2020, using volcano geodesy.

There are several volcanic systems on the RP, from west to east: Reykjanes, Svartsengi, Fagradalsfjall, Krýsuvík, Brennisteinsfjöll, and Hengill, with Fagradalsfjall being the least developed. All these volcanic systems, except Brennisteinsfjöll, have shown signs of unrest in the past years and decades: Three uplift episodes occurred at Svartsengi during 2020; one or two subtle deformation events further west on Reykjanes in 2020, and further uplift episodes at Svartsengi in May 2022, October 2023, November 2023, and December 2023 - January 2024. Inflation was observed at Krýsuvík during the summer of 2020; and a M5.6 earthquake occurred in Krýsuvík in October 2020.

As of beginning of 2024, three eruptions have occurred at Fagradalsfjall (in 2021, 2022, and 2023) and one eruption at Svartsengi in December 2023. Each eruption has been preceded by a dike intrusion, often intertwined with complex patterns of faulting, near-surface fracturing over wide areas, and creep along segments of the plate boundary. Additional dike intrusions in December 2021 in Fagradalsfjall and in Svartsengi in November 2023 did not breach the surface. The dike growth has spanned timescales of just over an hour to several weeks; furthermore, small dikelets accompanied new vent openings during the 2021 eruption. The dikes were emplaced in the brittle crust, above ~8 km depth, spanned several decimeters to meters in thickness, and released locally a great amount of plate-tectonic stresses. Re-inflation following each eruption or dike intrusion is usually observed, however, the temporal style of uplift rates varies considerably from time to time. Co-eruptive deflation was observed during the 2021 Fagradalsfjall eruption and the 2023 Svartsengi eruption.

The detailed deformation observations and modeling for the unrest periods reveal complex interactions of tectonics and magmatism across several volcanic systems on the RP. During 2020-2024, localized deformation and seismicity have alternated between different volcanic systems on the RP, such that only one system is inflating or erupting at a time. This observation may be interpreted in terms of deep pressure coupling between the systems. Furthermore, the deformation events cause significant stress changes at neighboring volcanic systems, affecting the probability of dike propagation and seismicity as well as conditions for magma accumulation.