Seismic Monitoring of the 2021 Fagradalsfjall Eruption, SW Iceland

After 800 years of eruptive quiescence on the Reykjanes Peninsula a fissure eruption started within Fagradalsfjall, a volcanic system which has been volcanically inactive for over 6000 years. A plate boundary runs through the peninsula; an oblique rift which is characterized by adjacent earthquake faults and six volcanic systems. In 2020, repeated periods of inflation in two volcanic systems were identified with InSAR and GPS data. They signified repeated magmatic intrusions and coincided with intense earthquake swarms. Earthquakes are common on the peninsula, however, this is the first time in the 30-year history of deformation monitoring that such clear intrusion signals are observed on the Reykjanes Peninsula. On February 24, 2021, an earthquake of M5.7 occurred NW of Fagradalsfjall. It activated seismic faults along the peninsula with repeated main-shock aftershock sequences in addition to intense earthquake activity caused by a dike intrusion in Fagradalsfjall. Modelling of the dike, clearly recorded with deformation data, suggested a roughly 9 km long and narrow dike triggering strike-slip earthquakes and releasing tectonic stress accumulated over decades or centuries. In total, there were 600 earthquakes with M3-M5.7, in a period of 3 weeks in an area of 30x5 km. These earthquakes were felt in the capital area of Reykjavík. At the end of the 3rd week, the seismicity declined, and similar trends were observed in deformation data. It thus came as a bit of a surprise when the fissure eruption started in Fagradalsfjall on March 19, at the center of the dike, without any clear short-time precursors. Here we present the story how the eruption in Fagradalsfjall was monitored over months and in real-time. We show how seismic data was used in different ways to reveal processes at stake both during the dike intrusion as well as during the eruption. We show examples of: Temporal and spatial seismicity trends; How seismic velocity changes using ambient noise (dv/v) was used during the diking process as a proxy for positive and negative strain changes induced by the diking; eruption tremor recorded for the first time with DAS (fiber-optic cable); how the volcanic tremor evolved and could be used to predict new vent openings and identify patterns of eruptive activity.