EGU22-10219
https://doi.org/10.5194/egusphere-egu22-10219
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

A comprehensive model of the precursors leading to the 2021 Fagradalsfjall eruption 

Ólafur Flóvenz1, Rongjiang Wang2,3, Gylfi Páll Hersir1, Torsten Dahm2,4, Sebastian Hainzl2, Magdalena Vassileva2,5,9, Vincent Drouin1, Sebastian Heimann2, Marius Paul Isken2,4, Egill Árni Gudnason1, Kristján Ágústsson1, Thorbjörg Ágústsdóttir1, Josef Horálek6, Mahdi Motagh2,5, Thomas R Walter2,4, Eleonora Rivalta2,7, Philippe Jousset2, Charlotte M Krawczyk2,8, and Claus Milkereit2
Ólafur Flóvenz et al.
  • 1ÍSOR, Iceland GeoSurvey, Reykjavik, Iceland(ogf@isor.is)
  • 2Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
  • 3Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China
  • 4University of Potsdam, Institute of Geosciences, Potsdam, Germany
  • 5Institute for Photogrammetry and GeoInformation, Leibniz University Hannover, 30167 Hannover, Germany
  • 6Czech Academy of Sciences (CAS), Prague, Czech Republic
  • 7Department of Physics and Astronomy, University of Bologna, Italy
  • 8TU Berlin, Institute of Applied Geosciences, Germany
  • 9Leibniz University Hannover, Institute of Photogrammetry and GeoInformation, Germany

A period of intense seismicity started more than a year prior to the 2021 Fagradalsfjall eruption in Iceland. During the same period, repeated cycles of surface uplift and subsidence were observed in the Svartsengi and Krýsuvík high-temperature (HT) fields, about 8-10 km west and east of the eruption site in Fagradalsfjall, respectively. Such an uplift has never been observed during 40 years of surface deformation monitoring of the exploited Svartsengi HT field. However, cycles of uplift followed by subsidence have been observed earlier at the unexploited Krýsuvík HT field.

Shortly after the start of the unrest, a group of scientists from GFZ-Potsdam and ÍSOR installed additional seismometers, used an optical telecommunication cable to monitor the seismicity and performed gravity measurements in the unrest area.

The data was used for multidisciplinary modelling of the pre-eruption processes (see Flóvenz et al, 2022. Cyclical geothermal unrest as a precursor to Iceland's 2021 Fagradalsfjall eruption. Nature Geoscience (in revision)). It included a poroelastic model that explains the repeated uplift and subsidence cycles at the Svartsengi HT field, by cyclic fluid intrusions into a permeable aquifer at 4 km depth at the observed brittle-ductile transition (BDT). The model gives a total injected volume of 0.11±0.05 km3. Constraining the intruded material jointly by the deformation and gravity data results in a density of 850±350 kg/m3. A high-resolution seismic catalogue of 39,500 events using the optical cable recordings was created, and the poroelastic model explains very well the observed spatiotemporal seismicity.

The geodetic, gravity, and seismic data are explained by ingression of magmatic CO2 into the aquifer. To explain the behaviour of cyclic fluid injections, a physical feeder-channel model is proposed.

The poroelastic model and the feeder-channel model are combined into a conceptual model that is consistent with the geochemical signature of the erupted magma. It explains the pre-eruption processes and gives estimates of the amount of magma involved.

The conceptual model incorporates a magmatic reservoir at 15-20 km depth, fed by slowly upwelling currents of mantle derived magma. Volatiles released from inflowing enriched magma into the sub-Moho reservoir migrated upwards. The volatiles were possibly trapped for weeks or months at the BDT at ~7 km depth beneath Fagradalsfjall, generating overpressure, but not high enough to lift the overburden (~220 MPa) and cause surface deformation. After reaching a certain limiting overpressure, or when a certain volume had accumulated, the magmatic volatiles were diverted upwards, just below the BDT towards the hydrostatic pressurized aquifer (~ 40 MPa) at 4 km depth at the bottom of the convective HT fields. They passed through the BDT and increased the pressure sufficiently (>110 MPa) to cause the uplift.

The lessons learned enlighten the most important factors to help detect precursory volcanic processes on the Reykjanes Peninsula; including detailed monitoring of seismicity, surface deformation, gravity changes and gas content in geothermal fluids. Furthermore, geophysical exploration of the deeper crust by seismic and resistivity measurements are crucial to map possible melt and possible pathways towards the surface.

How to cite: Flóvenz, Ó., Wang, R., Hersir, G. P., Dahm, T., Hainzl, S., Vassileva, M., Drouin, V., Heimann, S., Isken, M. P., Gudnason, E. Á., Ágústsson, K., Ágústsdóttir, T., Horálek, J., Motagh, M., Walter, T. R., Rivalta, E., Jousset, P., Krawczyk, C. M., and Milkereit, C.: A comprehensive model of the precursors leading to the 2021 Fagradalsfjall eruption , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10219, https://doi.org/10.5194/egusphere-egu22-10219, 2022.