Rapid drone and ground magnetic mapping of subsurface fractures during a volcano-tectonic crisis in Grindavík, on the Reykjanes Peninsula SW Iceland

We applied an integrated drone- and ground-based magnetometry workflow to map shallow subsurface fractures and cavities inside the town of Grindavík, during the 2023-2025 volcano-tectonic crisis on the Reykjanes Peninsula SW Iceland. Since10 November 2023 a total of 12 dike intrusions have occurred under the Sundhnúkur crater row from a shallow magma reservoir at Svartsengi, 9 of which resulted in fissure eruptions. Three of these dikes propagated underneath the town of Grindavík triggering widespread fault reactivation, fracturing, and surface deformation. Two main grabens formed above intrusions in the west and east of the town, with a maximum measured vertical displacement of 1.5 m. Short-wavelength linear and elliptical magnetic lows delineated open or partially open fractures and localized cavities hidden beneath the surface. These open fractures and cavities or void spaces were then verified with field observations, LiDAR surface deformation, and targeted shallow excavations. Comparison with historical aerial photographs indicates several anomalies correspond to reactivated, and further opened, pre-existing fractures along the main graben that cuts through the town. This integrated drone and ground-based approach enabled rapid mapping of the major fracture networks in inaccessible terrain, maintaining operator safety. In turn this guided near-real time hazard assessments, and supported stakeholder decision-making, by revealing fracture continuity, areas of sinkhole development within the fracture lineaments, as well as aperture variability and branching patterns along the fractures. Forward magnetic modelling shows that the anomaly shapes and amplitudes are compatible with fracture and void sources within the upper ~10-20 m of bedrock. This study demonstrates the first combined application of drone and ground magnetometry for rapid real-time fracture mapping in an urban post-volcano-tectonic crisis event setting which has affected the >3,700 local residents of the town of Grindavík.