Common initiation point of repeated dyke intrusions during the 2021-2023 Fagradalsfjall volcano-tectonic rifting event, Reykjanes Peninsula, Iceland.

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Magmatic unrest within the Reykjanes Peninsula oblique rift zone, SW Iceland, ongoing since December 2019, has been closely monitored by a dense network of seismic and geodetic stations. A total of 12 dyke intrusions and 10 fissure eruptions have occurred near Fagradalsfjall and Svartsengi-Grindavík. The 2021-2023 Fagradalsfjall volcano-tectonic event consisted of 4 dyke intrusions, 3 of which surfaced in fissure eruptions. On 24 February 2021, intense seismicity along a 10 km long dyke path, fed a 6-months long eruption, the first in around 780 years on the Peninsula. The three subsequent dyke intrusions were shorter in time and space, propagating for around 5 days, along 5-7 km long paths, each illuminating a section of the February-March 2021 dyke’s path. Out of the subsequent dykes, the December 2021 dyke was most intense seismically, propagating to the SW, but not breaching the surface. The July-August 2022 dyke seismicity was more diffuse, illuminating the central to NE part, whereas the July 2023 dyke intrusion almost exclusively propagated NE. The 2022 and 2023 dyke intrusions both fed short lived eruptions. Our data show that all the Fagradalsfjall dyke intrusions were governed by N-S strike-slip faulting. Using high-resolution relative relocations of the dyke-induced seismicity, we find that the December 2021, the 2022 and 2023 intrusions all initiated at 6-8 km depth within an area of about 1 km2. All three dykes then propagated laterally at depths of 2 - 6 km. The December 2021 dyke was associated with seismicity at 4-6 km, the 2022 dyke at 1.5 - 3 km and the 2023 dyke at 2.5 - 5.5 km depth. The dykes initiated directly above a zone of deep long-period events (DLPs) at 8-14 km depth (Greenfield et al., 2022), between the 2022 and 2023 eruption sites, suggesting that the dykes were fed from near Moho magma levels. Greenfield, T., Winder, T., Rawlinson, N., Maclennan, J., White, R.S., Ágústsdóttir, T., Bacon, C.A., Brandsdóttir, B., Eibl, E.P.S., Glastonbury-Southern, E., Gudnason, E.Á., Hersir, G.P. and Horálek, J. (2022). Deep long period seismicity preceding and during the 2021 Fagradalsfjall Eruption, Iceland. Bulletin of Volcanology, 84,101. https://doi.org/10.1007/s00445-022-01603-2.

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