Widespread fracture movements during the 2019–2021 volcano-tectonic unrest on the Reykjanes Peninsula from TerraSAR-X interferometry

Fractures and tectonic structures have been related to dyke emplacements, eruption location or dynamics in several volcanic areas around the world. Mapping of active faults is therefore key for assessing the potential tectonic and volcanic hazard within a region. The 2021 eruption in the Fagradalsfjall volcanic area (Reykjanes Peninsula, SW Iceland) was preceded by two years of volcanic unrest, including four non-eruptive unrests in the Svartsengi and Krýsuvík volcanic areas and a dyke intrusion in the Fagradalsfjall volcanic segment. Nine earthquakes of magnitudes M 5–5.6 were recorded during this time period and were widely felt by the surrounding population. Using interferometric synthetic aperture radar (InSAR) applied to TerraSAR-X data collected over 2019–2021, we mapped fracture movements over the Reykjanes Peninsula. We identified ~1250 active structures across 54 interferograms during this time period, complementing previously mapped structures. Our study reveals extensive fracture movements across most of the Peninsula, extending from Reykjanes to NE Krýsuvík volcanic areas. We particularly highlight previously undetected structures beneath the town of Grindavík as well as a N45°E striking structure in the Fagradalsfjall volcanic area, active during summer-autumn 2020, prior to the 2021 dyke intrusion. We propose that this structure influenced the location of the longest lasting vent of the 2021 eruption. The observations presented in this study have important implications for improving our understanding of volcano-tectonic interactions and hazard assessments in Iceland and worldwide.