Swarm seismicity illuminates stress transfer prior to the 2021 Fagradalsfjall eruption, Iceland 

<p><strong>&#160;</strong></p><p>The 6 months long effusive volcanic eruption of 19 March 2021 at Fagradalsfjall, Reykjanes Peninsula, Iceland was preceded by an intensive earthquake swarm lasting one month, with several earthquakes exceeding M_L 5. We analyse seismic data recorded by the Reykjanet local seismic network to trace the processes leading up to the eruption in order to understand the relation between seismic activity and magma accumulation.</p><p>&#160;</p><p>The precise relocations show that the seismicity is located in two clusters in the depth range of 1-6 km. A NE-SW trending cluster maps the dyke propagation; a WSW-ENE trending cluster follows the plate boundary. In comparison, we relocated the preceding earthquake swarms of 2017, 2019 and 2020 and found that they form two branches along the plate boundary, coinciding with the 2021 WSW-ENE trending cluster. These branches form a stepover of about 1 km offset, forming a pull-apart basin structure at the intersection with the dyke. This is the exact location of the eruption site, which shows that magma erupted at the place of crustal weakening.</p><p>&#160;</p><p>The 2021 earthquake swarm initiated by a M_L 5.3 earthquake on 24 February, which triggered the aftershocks along the plate boundary and in the dyke segment, both occurring in an area of elevated Coulomb stress. The swarm seismicity shows complex propagation of the dyke, which started at its northern end, migrated south-westward and then jumped back to the central part where the effusive eruption eventually took place. The strike-slip focal mechanisms of the larger magnitude events, with N-S striking fault planes, are interpreted as right-lateral antithetic Riedel shears that accommodate the left lateral slip along the plate boundary. The fact that both seismic and magmatic activities occur at the same location shows that the past seismic activity weakened the crust in the area of the eruption site. We show that the M_L 5.3 earthquake on 24 February 2021 triggered the whole seismic swarm and perturbed the magma pocket which eventually led to the 19 March Fagradalsfjall eruption.</p><p>&#160;</p>