InSAR observations and modeling of volcano-tectonic fracture movements on the Reykjanes Peninsula, SW Iceland

Since 2019, unrest on the Reykjanes Peninsula, SW Iceland, has demonstrated that fracture movements are a significant component of volcano-tectonic deformation and pose major hazards to infrastructure. TerraSAR-X data covering the Reykjanes Peninsula were processed to produce 57 interferograms for fracture mapping for September 2021 to July 2024. The most extensive fracture movements during this period are associated with the 2022 Meradalir eruption, 2023 Litli-Hrútur eruption, and 2023 Nov. 10-11 Grindavík dike intrusion. Extensive activation of N-S trending strike-slip faults and NE-SW trending normal faults is observed during periods of shallow dike propagation and associated seismicity. Simple elastic modeling suggests that most of the observed displacements are a result of shallow slip within the upper tens to hundreds of meters of the crust. Many have existing topographic expressions and have been activated multiple times since 2020, highlighting the role preexisting weaknesses in the upper crust play in accommodating volcano-tectonic deformation. While some fracture movements can be explained by co-diking stress transfer, e.g. normal faulting directly above dikes or bookshelf faulting along the North American-Eurasian plate boundary, subtle (<10 mm) movements within neighboring fissure swarms occur where Coulomb stress transfer modeling indicates co-diking normal stress changes should suppress fracture movements. As such, other processes like shallow strain localization along preexisting weaknesses may be occurring. InSAR data also reveal repeated fracture movements within the Búrfell graben, SE of Reykjavík and within the Krýsuvík Fissure Swarm, which was surveyed with high-precision geodetic leveling in the summer of 2024. Since 2012, portions of the profile have subsided up to 34 ± 0.3 mm, corresponding to a rate of -2.8 mm/yr, substantially greater than that observed in previous decades. Fracture movements within the graben are only seen in interferograms spanning bursts of shallow microseismicity in October 2018, March 2021, and September 2023. Line of sight deformation of up to 5 cm is also observed during earthquake swarms (M_max= 3.1) in January and June 2024 within the adjacent portion of the Krýsuvík Fissure Swarm. In both cases, observed deformation is larger than expected for the seismic moment released, implying that this deformation is both episodic and has a significant aseismic component. These observations offer insight into the mechanisms of fracture movements and may be applied to locations where similar processes occur such as Iceland’s Northern Volcanic Zone or the East African Rift.