Evolution of deformation and seismicity on the Reykjanes Peninsula, preceding the 2021 Fagradalsfjall eruption, Iceland

Abstract

<p>The 2021 effusive eruption at Mt. Fagradalsfjall, on the Reykjanes Peninsula oblique rift in Iceland, was preceded by a 14-month long period of volcano-tectonic unrest (comprising both significant ground deformation and intense seismicity). A seismic swarm was initially detected in the Fagradalsfjall region between the 15<sup>th</sup> to 20<sup>th</sup> December 2019. Following a short quiescence, activity re-commenced on the 21<sup>st</sup> January 2020, with a small cluster of earthquakes near Grindavík (~ 10 km west of Fagradalsfjall). Concurrent deformation was detected on two GNSS stations in this area and on Sentinel-1 interferograms. Geodetic modelling of these observations indicated the deformation most likely resulted from the intrusion of a magmatic sill, directly west of Mt. Thorbjörn, at a depth of about 4 km. This was followed by two additional sill-type intrusions in a similar location, between 6<sup>th</sup> March - 17<sup>th</sup> April and 15<sup>th</sup> May - 22<sup>nd</sup> July 2020 respectively. The three intrusions comprised a total volume change of about 9 million cubic meters. In mid-July 2020, inflation was again detected on the Reykjanes Peninsula, this time in the Kýsuvík volcanic system to the east of Fagradalsfjall. This episode of inflation lasted several weeks and geodetic inversions indicated the observed signal was produced by the combination of a deflating sill-like source at a depth of ~16 km and inflation of a body at a depth of ~6 km. The latter, corresponding to a volume change of about 5 million cubic meters. During this period of intrusive activity, seismicity shifted along various regions across the Peninsula, in relation to a combination of processes – magma migration, triggered seismicity and tectonic earthquakes.</p><p> </p><p>Intense seismic swarms commenced on the 24<sup>th</sup> February 2021, concentrated at both Fagradalsfjall and also extending across a 20 km segment along the plate boundary – including triggered strike-slip earthquakes up to Mw5.64. At the same time, deformation was detected on local GNSS stations, and subsequent Interferometric Sythethic Aperture Radar Analysis (InSAR) of Sentinel-1 data confirmed the observed deformation was primarily the result of a dike intrusion and slip along the plate boundary. Geodetic inversions indicated a ~9 km long dike with a total intruded volume of around 34 million cubic meters (Sigmundsson et al., <em>in review</em>). During this period, stored tectonic stress was systematically released, resulting in a decline in deformation and seismicity over several days preceding the eruption onset, on 19<sup>th</sup> March 2021 in Geldingadalir at Mt. Fagradalsfjall. The eruption continued until the 18<sup>th</sup> September 2021 and produced a lava field covering an area of 4.8 km<sup>2 </sup>with an extruded bulk volume of 150 ± 3 × 10<sup>6</sup> m<sup>3</sup> (Pedersen et al., <em>in review</em>).</p><p> </p><p><strong>References</strong></p><p>Sigmundsson et al. (<em>in review</em>). Deformation and seismicity decline preceding a rift zone eruption at Fagradalsfjall, Iceland.</p><p> </p><p>Pedersen et al. (<em>in review</em>). Volume, effusion rate, and lava transport during the 2021 Fagradalsfjall eruption: Results from near real-time photogrammetric monitoring. DOI:10.1002/essoar.10509177.1.</p>