Seasonal ground deformation at subglacial Katla Volcano, Iceland: observations and models

Changes at the surface of a volcanic edifice, such as snow or hydrological loading, ice cap melting, and flank destabilization, can cause significant surface deformation. Understanding the contribution of surface processes to ground deformation is therefore important for monitoring the state of the underlying volcanic system. The Katla Volcano in Iceland lies under Mýrdalsjökull, the fourth largest glacier in Iceland, and undergoes the largest seasonal deformation of all Icelandic volcanoes: up to 4 cm horizontally and 3 cm vertically at the Austmannsbunga (AUST) Global Navigation Satellite System (GNSS) station. The last confirmed eruption of Katla occurred in 1918. Since then, episodes of elevated seismicity and jökulhlaups (sudden glacial outburst floods) have been recorded at the volcano, the most noticeable in 1955, 1999, 2011, and 2024. During the 2024 jökulhlaup, horizontal displacement of up to 7 cm was recorded at AUST. In this work, elastic three-dimensional finite element method models were implemented to quantify surface deformation from seasonal load changes. The models include realistic bedrock topography and ice unloading based on recent data collected at Mýrdalsjökull. A deformation model considering only seasonal snow unloading can reproduce within uncertainty the observed GNSS vertical surface displacements. It can also explain the horizontal signals at GNSS stations located outside the glacier, although not at GNSS stations located on nunataks inside Mýrdalsjökull. An additional deformation source must be considered to explain the residual cm-scale horizontal displacement at these stations. We model the residual signal using a thermo-poro-elastic cylindrical source. The best-fit source is a cylinder located at the surface that produces cm-scale horizontal deformation with limited vertical deformation. The shallow source depth derived from the inversion and the surface deformation recorded at AUST during the jökulhlaup series in 2024 and 2025 suggests that seasonal deformation is not strictly related to magmatic activity. We infer that changes within the hydrologic system of the glacier are responsible for most of the horizontal deformation at Katla.