Deep seismicity preceding and during the 2021 Fagradalsfjall eruption, Reykjanes Peninsula, Iceland

Using a dense network of seismometers located on the Reykjanes Peninsula of Iceland we image a cluster of earthquakes located at a depth of 10-15 km, beneath the brittle-ductile transition and active before and during the Fagradalsfjall eruption. The deep seismicity has markedly different properties to those earthquakes located in the upper, brittle crust with a lower frequency content and a high b-value suggesting that fluids and/or high temperature gradients could be involved in their initiation. Detailed relocation of the deep seismicity reveals that the locus of the activity shifts southwest after the onset of the eruption, suggesting that although the location of the deep seismicity is unlikely to be the source for the magma which erupted, nevertheless the eruption and the deep earthquakes are linked. We interpret the deep earthquakes to be induced by the intrusion of magma into the lower crust. In such an interpretation, the intruded region could be offset from the conduit that transports the magma from the source region near the base of the crust to the surface.