Consequences of mountain shaking during the 2019-2024 oblique rifting episode at Mount Þorbjörn, Reykjanes Peninsula, Iceland

The Reykjanes Peninsula (Iceland) has been experiencing an oblique rifting episode since 2019. Intense and repeated earthquake swarms (magnitude up to M_L~5) and ground shaking accompany the deformation of the region. Magma accumulation in crustal reservoirs and discharge to dikes and eruptive fissures actively participate in the tectonic crisis. The crisis is accompanied by fault offsets and slope instabilities, generating hazards that have not yet been systematically investigated. Using photogrammetry, digital image correlation and seismicity data, we analyse the rockfall activity (442 events mapped) and fault kinematics (up to 0.8 m offset) from Mount Þorbjörn (Thorbjoern) between 2019 and 2024. We find a positive correlation between the timing of rockfall events and earthquakes with energy density (estimated seismic energy that could generate a rock block displacement at a given location) larger than ~1 J/km³ at Mount Þorbjörn's summit. We propose that this energy density is sufficient to trigger rockfall events at steep slopes on the Reykjanes Peninsula. The seismic energy density for steep terrains could allow a quick assessment of the potential for rockfall activity after earthquakes.

The novel strategy to measure ground displacement using high-resolution and high-precision unmanned aerial vehicles presented here, adapted from several existing state-of-the-art methods (Post-Processing Kinematics, 3D point cloud correlation, 2D terrain correlation, mapping on orthophotos), enables the detection and kinematic characterisation of previously unmapped structural features and rockfall events. Multiple NS and NNE-SSW-oriented faults were reactivated in a right-lateral sense of motion, with up to 40 cm offsets. The graben faults dissecting the mountain were reactivated multiple times, with up to 80 cm vertical offsets in the centre of the graben. The methodology described here can serve various applications in research, industry, and local emergency management services after future seismic crises.