Interpreting Long-term Ground Deformation Signals at Subglacial Katla Volcano, Iceland: Combined Effects of Surface Loading and Magmatic Processes.

Surface ground deformation at active volcanoes is commonly attributed to fluid processes at depth, such as magma storage, propagation, and geothermal activity. Geodetic modeling of ground deformation of these processes is important for understanding and assessing the potential hazard posed by volcanic unrest.  However, changes in surface loading, such as ice retreat, can also cause ground deformation. Katla volcano in Iceland underlies Mýrdalsjökull, the fourth largest glacier in Iceland, which has been retreating since about 1890. Historically, Katla has had an average repose time of ~50 years, but the last confirmed eruption occurred in 1918. Katla has been continuously uplifting since the installation of GNSS in 1993. Over the past 10 years, Katla has had an average uplift rate of ~17 mm/yr, as recorded at a GNSS station located on a nunatak on the caldera rim. GNSS stations outside of the glacier record vertical deformation rates of ~10 mm/yr.

In this work, we investigate the effects of the long-term ice retreat and magmatic processes on the recorded ground deformation. It is necessary to understand the contribution of surface load changes on recorded ground deformation to be able to isolate and monitor volcanic signals. We model a long-term deformation source at Katla, using an analytical inversion of GNSS data, after the removal of seasonal signals, plate spreading, and estimated rates of deformation due to glacial retreat (glacial isostatic adjustment, GIA) at Mýrdalsjökull and other glaciers in Iceland. We find the best-fit deformation source parameters are highly dependent on the GIA correction, so evaluation of the GIA contributions to a surface deformation signal is needed. We generate a 3D Finite Element (FE) model, using COMSOL Multiphysics, including realistic topography and ice unloading, based on 10 years of GNSS data from Mýrdalsjökull, to investigate the contribution of ice retreat on the deformation signal. We furthermore evaluate the applicability of Iceland country-wide GIA models considering ice retreat in all of Iceland. A previous study of the seasonal snow loading signal at Katla was able to reproduce observed horizontal deformation, several mm/yr, at the edge of the glacier. This implies that long-term glacial retreat may contribute to the observed inflation at Katla, rather than deformation of only volcanogenic origin.