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Abstract:
The 2021 Fagradalsfjall volcanic eruption in the Reykjanes Peninsula, Iceland, was preceded by a ML 5.3 earthquake of 24 February 2021, which was followed by an intensive earthquake swarm lasting one month. Precise relocations show two hypocenter clusters in the depth range of 1-6 km. The swarm and later eruption occurred in the area which was the place of several swarms in the period of 2017-2020. We analyze the spatiotemporal characteristics of the activity, the Coulomb stress change caused by the ML 5.3 earthquake and by the magmatic dyke and the moment tensors of previous swarms to better understand the stress transfer between seismic and magmatic activity. Our analysis shows a coincidence of several phenomena which could have contributed to driving the seismic and volcanic activity. The stress change analysis suggests that the forming magmatic dyke has increased the Coulomb stress on the ML 5.3 earthquake fault. The following earthquake rupture resulted in unclamping of the fissure where the dyke formed and in Coulomb stress increase in the area of aftershocks and the following earthquake swarm. The WSW-ENE trending cluster of the 2021 and previous swarms show a stepover, forming an extension structure at the intersection with the dyke. And the moment tensors of the 2017 swarm events show that volumetric components vary along the cluster. Interestingly, the 2021 eruption occurred in the extension structure, suggesting that magma erupted at the place of crustal weakening.
