Dynamics of the 2021 Fagradalsfjall Eruption (Iceland) Revealed by Volcanic Tremor Patterns

Abstract Co‐eruptive volcanic tremor during the 2021 Fagradalsfjall eruption in Iceland (19 March–18 September 2021) is characterized using seismic and visual data recorded close to the eruption site and across the Reykjanes Peninsula. An automatic seismic network‐based approach reveals several tremor patterns associated with seven phases of the eruption, including (a) continuous tremor located beneath the eruption site and attributed to pressure changes in the shallow vent system(s) in phases I, III, and VII, and (b) two patterns of minute‐ and hour‐long intermittent tremor in May (phase II) and July–August (phases IV–VI), respectively. The first intermittent pattern of minute‐long tremor bursts associated with pulsating lava fountains in May is attributed to magma degassing in a shallow reservoir (top 100 m) connected to a top‐conduit. The progressive enlargement of both the top‐conduit and shallow reservoir with time is estimated quantitatively using a collapsing foam model. Sudden changes of their geometries, as detected from tremor characteristics, are systematically associated with observed crater collapse events. The second intermittent pattern of cyclic hour‐long tremor episodes associated with episodic effusive activity in July–August is attributed to magma flowing and cooling in the feeder dike connected to a sill at 5 km depth. The sill is fed by a constant influx of magma from the deeper plumbing system and stores magma during low discharge periods. The observed cyclicity of both the eruptive activity and the tremor is interpreted quantitatively with a sill‐dike model accounting for magma cooling and induced cyclic viscosity changes in the dike.