Abstract

<p>The 2021 Fagradalsfjall eruption on Iceland’s Reykjanes Peninsula was preceded by more than 12 months of elevated activity, beginning around November 2019. This dominantly consisted of episodes of intense seismic swarms, but also featured inflationary episodes in both the Svartsengi and Krísuvík volcanic systems. On 24<sup>th</sup> February 2021, an exceptionally intense episode of seismicity covering the length of the Peninsula marked the initiation of a dyke intrusion, which continued to develop until the 19<sup>th</sup> of March, when melt first erupted at the surface. The fissure eruption lasted 6 months, ending on 18<sup>th</sup> September 2021.</p><p>During the intrusion, melt first propagated northeast towards Mt Keilir, then to the southwest, eventually forming a 10 km-long dyke. This was marked by more than 80,000 microearthquakes, recorded by a dense local seismic network and detected and located using QuakeMigrate[1].</p><p>We present high precision relative relocations of the seismicity, and tightly constrained focal mechanisms of earthquakes which are dominantly located along the base of the dyke. We compare the Fagradalsfjall seismicity to the 2014-2015 Bárðarbunga-Holuhraun intrusion and eruption seismicity [2], in the context of the contrasting tectonic settings, and markedly different precursory activity.</p><p>1: Winder, T., Bacon, C., Smith, J., Hudson, T., Greenfield, T. and White, R., 2020. QuakeMigrate: a Modular, Open-Source Python Package for Automatic Earthquake Detection and Location. https://doi.org/10.1002/essoar.10505850.1</p><p>2: Woods, J., Winder, T., White, R. S., and Brandsdóttir, B., 2019. Evolution of a lateral dike intrusion revealed by relatively-relocated dike-induced earthquakes: The 2014–15 Bárðarbunga–Holuhraun rifting event, Iceland. https://doi.org/10.1016/j.epsl.2018.10.032</p>

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