Structural basin

Geology

Volcanic hazards

Poromechanics

Geodetic datum

Porosity

Aquifer

Groundwater

Politics

Volcanology

Geomorphology

Volcano

Porous medium

Geodesy

Oceanography

Geothermal gradient

Political science

Seismology

Subsidence

Magma

Geotechnical engineering

Phreatic eruption

Deformation (meteorology)

Unrest

Geophysics

Induced seismicity

Nature Geoscience

Understanding and constraining the source of geodetic deformation in volcanic areas is an important component of hazard assessment. Here, we analyse deformation and seismicity for one year before the March 2021 Fagradalsfjall eruption in Iceland. We generate a high-resolution catalogue of 39,500 earthquakes using optical cable recordings and develop a poroelastic model to describe three pre-eruptional uplift and subsidence cycles at the Svartsengi geothermal field, 8 km west of the eruption site. We find the observed deformation is best explained by cyclic intrusions into a permeable aquifer by a fluid injected at 4 km depth below the geothermal field, with a total volume of 0.11 ± 0.05 km3 and a density of 850 ± 350 kg m–3. We therefore suggest that ingression of magmatic CO2 can explain the geodetic, gravity and seismic data, although some contribution of magma cannot be excluded. Ingression of magmatic gas into a geothermal aquifer generated cyclical deformation and primed the system for the 2021 Fagradalsfjall eruption, Iceland, according to a catalogue of 39,500 precursory earthquakes combined with a poroelastic model.

Cyclical geothermal unrest as a precursor to Iceland’s 2021 Fagradalsfjall eruption