Evolution of deformation and seismicity on the Reykjanes Peninsula, preceding the 2021 Fagradalsfjall eruption, Iceland
Authors: Parks Michelle , Vogfjörd Kristín S. , Sigmundsson Freysteinn , Hooper Andrew , Geirsson Halldór , Drouin Vincent , Ófeigsson Benedikt G. , Hreinsdóttir Sigrún , Hjaltadóttir Sigurlaug , Jónsdóttir Kristín , Einarsson Páll , Barsotti Sara , Horálek Josef , Ágústsdóttir Thorbjörg 
Published: March 2022

&lt;p&gt;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&lt;sup&gt;th&lt;/sup&gt; to 20&lt;sup&gt;th&lt;/sup&gt; December 2019. Following a short quiescence, activity re-commenced on the 21&lt;sup&gt;st&lt;/sup&gt; January 2020, with a small cluster of earthquakes near Grindav&amp;#237;k (~ 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. Thorbj&amp;#246;rn, at a depth of about 4 km. This was followed by two additional sill-type intrusions in a similar location, between 6&lt;sup&gt;th&lt;/sup&gt; March - 17&lt;sup&gt;th&lt;/sup&gt; April and 15&lt;sup&gt;th&lt;/sup&gt; May - 22&lt;sup&gt;nd&lt;/sup&gt; 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 K&amp;#253;suv&amp;#237;k 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 &amp;#8211; magma migration, triggered seismicity and tectonic earthquakes.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Intense seismic swarms commenced on the 24&lt;sup&gt;th&lt;/sup&gt; February 2021, concentrated at both Fagradalsfjall and also extending across a 20 km segment along the plate boundary &amp;#8211; 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., &lt;em&gt;in review&lt;/em&gt;). 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&lt;sup&gt;th&lt;/sup&gt; March 2021 in Geldingadalir at Mt. Fagradalsfjall. The eruption continued until the 18&lt;sup&gt;th&lt;/sup&gt; September 2021 and produced a lava field covering an area of 4.8 km&lt;sup&gt;2 &lt;/sup&gt;with an extruded bulk volume of 150 &amp;#177; 3 &amp;#215; 10&lt;sup&gt;6&lt;/sup&gt; m&lt;sup&gt;3&lt;/sup&gt; (Pedersen et al., &lt;em&gt;in review&lt;/em&gt;).&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Sigmundsson et al. (&lt;em&gt;in review&lt;/em&gt;). Deformation and seismicity decline preceding a rift zone eruption at Fagradalsfjall, Iceland.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Pedersen et al. (&lt;em&gt;in review&lt;/em&gt;). Volume, effusion rate, and lava transport during the 2021 Fagradalsfjall eruption: Results from near real-time photogrammetric monitoring. DOI:10.1002/essoar.10509177.1.&lt;/p&gt;