The role of magma intrusions on faulting mechanisms during pre-eruptive seismic swarms in the Reykjanes Peninsula

In March 2021, a fissure eruption in Fagradalsfjall marked the onset of the first volcanic activity in over 800 years on the Reykjanes Peninsula, Iceland. Since then, three more fissure eruptions occurred in August 2022, July 2023, and December 2023. Intense seismic swarms that included Mw 4.0 earthquakes preceded all eruptions. Concurrently with swarm activity, large surface deformations related to magma intrusions were observed by Interferometric Synthetic Aperture Radar (InSAR), and Global Navigation Satellite System (GNSS) data. Ground displacements exceeded the deformation expected from earthquakes by far, suggesting the intrusion process was primarily aseismic. However, the intrusions may have influenced the prevalent earthquake faulting style in the pre-eruptive swarms. For instance, seismic moment tensors before and during the early 2021 swarm indicate that right-lateral bookshelf faulting along roughly north-south trending strike-slip faults dominated seismic deformation. This changed to more northeast-southwest oriented oblique-normal to normal faulting mechanisms for the later swarms consistent with graben formation after a dyke intrusion. Seismic moment release during the 2021 seismic swarm, which included ten relatively large Mw 5+ earthquakes, was larger than for subsequent swarms, where only a few events reached Mw 5. This may indicate that the 2021 intrusion, which marked a reawakening of volcanic activity, may have triggered bookshelf faults close to failure that might have otherwise ruptured in the near term due to the underlying oblique rifting. The transition of source mechanisms toward oblique and normal faulting during the later swarms, though, may reflect an active role of the intrusion processes on fault orientation of triggered seismicity rather than simply inducing seismicity on pre-existing faults.