Fracture dynamics and topographic controls at Mount Thorbjörn during the Svartsengi 2023–2024 volcanic unrest, Iceland

Regions with complex topography experience varying degrees of deformation during volcano-tectonic events. Few studies have investigated the relationship between topography and fracturing in detail, and therefore insights into this interplay are limited. In this work, we analysed deformation patterns during the 2023–2024 volcanic unrest on the Reykjanes Peninsula, Iceland, focusing on the dyke-induced graben episode of 10 November 2023. Our study focuses on Mount Thorbjörn, the ~200 m high glaciovolcanic edifice located on the western shoulder of the graben. We collected four drone photogrammetric datasets between 2022 and 2024, covering different phases of the volcanic unrest. By comparing cm-resolution orthophotos and digital elevation models, we identified transtensional reactivation of pre-existing grabens, with normal offsets reaching 80 cm and 20–30 cm of dextral strike-slip. Larger offsets in the south-eastern sector of the mountain suggest tilting in line with the regional deformation trend, although the displacement in the mountain was high compared to flat adjacent regions. Orthophoto comparison revealed ~9 km of new cumulative surface fractures and over 200 sinkholes, with 88 % of the fractures and 59 % of the sinkholes formed during the November 2023 graben event. Statistical analyses indicate a strong topographic influence on fracture development. Fracture density increased with elevation, clustering around local peaks. Our results provide a conceptual model for fracture dynamics in complex topographies during volcanic unrest, where higher elevations are more sensitive to deformation, leading to greater damage.