Likely ring-fault activation at Askja caldera (Iceland) during the 2021 unrest

The Askja volcanic system, located in the North Volcanic Zone of Iceland, consists of a central volcano with three nested calderas (Kollur, Askja, and Öskjuvatn) and a 20 km wide and ~190 km long fissure swarm with a NNE-SSW trend. Kollur caldera is ~5 km wide and formed in the Pleistocene while the younger 8-km wide Askja caldera, the largest among the three, formed in the Holocene. The smaller (~4 km) and lake-filled Öskjuvatn caldera is located within the Askja caldera and formed following the 1875 Plinian eruption. This event was followed by several localized eruptions along the Öskjuvatn ring fault system (1921, 1922, and 1929) and the last eruption occurred in 1961 in correspondence with the Askja northern caldera border. After this eruption, the Askja caldera first underwent inflation for several years followed by slow (< 1 cm/yr) subsidence over decades. In early August 2021, the volcano entered a period of unrest with new earthquake activity located below the central volcano, and the GNSS station OLAC, located near the center of Askja caldera, started to uplift at a high rate (~3 cm/week). The uplift continued until the end of November 2021. Here we use SAR images acquired from four different orbits (two ascending and two descending) by the Sentinel-1 satellites to study the ground deformation during this unrest period. Only data from the first half of the unrest period could be used (until the end of September). Later, heavy snow resulted in the loss of interferometric coherence within the caldera, preventing retrieval of the deformation signal. The maximum ground displacement of ~10 cm (from the end of July to the end of September) was found at the center of the Askja caldera, near the western shore of Öskjuvatn Lake. Interestingly, the interferograms show an asymmetric deformation pattern that follows the ring faults in the northwestern part of Askja caldera. Analytical models suggest that a roughly 7 x 3 km² NW-SE elongated sill inflated at a shallow depth of ~2 km below the Askja caldera. However, simple sill models cannot explain the asymmetrical deformation pattern observed in the InSAR data. Therefore, using boundary element modeling, we find that while the magmatic intrusion accounts for the broad uplift, possible ring-fault activity would localize the deformation close to the caldera rim. Furthermore, an elongated sill, like the one obtained from the first source estimation, would probably activate only a part of the ring-fault system, leading to an asymmetric deformation pattern.