A comprehensive model of the precursors leading to the 2021 Fagradalsfjall eruption&#160;
Authors: Flóvenz Ólafur , Wang Rongjiang , Hersir Gylfi Páll , Dahm Torsten , Hainzl Sebastian , Vassileva Magdalena , Drouin Vincent , Heimann Sebastian , Isken Marius Paul , Gudnason Egill Árni , Ágústsson Kristján , Ágústsdóttir Thorbjörg , Horálek Josef , Motagh Mahdi , Walter Thomas R , Rivalta Eleonora , Jousset Philippe , Krawczyk Charlotte M , Milkereit Claus 
Published: March 2022

&lt;p&gt;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 Kr&amp;#253;suv&amp;#237;k 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 Kr&amp;#253;suv&amp;#237;k HT field.&lt;/p&gt;&lt;p&gt;Shortly after the start of the unrest, a group of scientists from GFZ-Potsdam and &amp;#205;SOR installed additional seismometers, used an optical telecommunication cable to monitor the seismicity and performed gravity measurements in the unrest area.&lt;/p&gt;&lt;p&gt;The data was used for multidisciplinary modelling of the pre-eruption processes (see Fl&amp;#243;venz 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&amp;#177;0.05 km&lt;sup&gt;3&lt;/sup&gt;. Constraining the intruded material jointly by the deformation and gravity data results in a density of 850&amp;#177;350 kg/m&lt;sup&gt;3&lt;/sup&gt;. 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.&lt;/p&gt;&lt;p&gt;The geodetic, gravity, and seismic data are explained by ingression of magmatic CO&lt;sub&gt;2&lt;/sub&gt; into the aquifer. To explain the behaviour of cyclic fluid injections, a physical feeder-channel model is proposed.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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 (&gt;110 MPa) to cause the uplift.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;