Interactions between an inflating magma chamber and caldera ring faults at Askja volcano, Iceland, during the 2021-2023 unrest episode

Abstract

Unrest episodes at volcanic systems are often associated with ground displacements produced by the pressure changes inside magma chambers at depth. The interactions with preexisting faults can complicate the deformation pattern observed at the surface. Calderas are volcanic systems characterized by a surface depression formed when the roof block collapses along ring faults, triggered by the rapid depressurization of an underlying magma chamber. As caldera ring faults are zones of weakness in the subsurface, we can expect stress interactions between the magma source and the ring faults during unrest episodes or eruptions. An example of such interaction likely occurred at Askja volcano during the 2021-2023 unrest episode. The Askja volcanic system is located in the North Volcanic Zone of Iceland. It consists of a central volcano with three nested calderas in the middle of a fissure swarm. Geodetic data have shown that the Askja caldera floor continuously subsided from 1983 until August 2021, when the volcano entered a period of unrest with rapid uplift and increased seismic activity. The seismicity decreased after three months, while the uplift continued for two years until it substantially slowed down in September 2023.We use Sentinel-1 SAR images to study the ground deformation at Askja volcano between 2016 and 2023. Only Summer acquisitions can be used since the area is covered by snow during the rest of the year, preventing retrieval of the deformation signal due to lack of coherence. The InSAR time series shows steady subsidence of the Askja caldera floor between July 2016 and July 2021, and then, in early August 2021, the displacement changed to uplift. In only one month, the uplift matched the subsidence of the previous five years. By September 2023, the maximum uplift reached ~70 cm in the center of Askja caldera. Deformation maps show an asymmetric pattern that follows the ring faults in the northwestern part of Askja caldera. The pattern is similar for both the subsidence and uplift periods, suggesting the same magma body deflated before the unrest and then inflated when the pressure increased in August 2021. Using boundary element models, we assessed the ground deformation resulting from the interaction between an inflating sill and the caldera ring faults. Then, we estimated the source parameters using Bayesian inference. While a magmatic sill source can account for the broad uplift, triggered ring fault movement localizes the deformation close to the caldera rim, yielding an asymmetric deformation pattern that better fits the observed data. Even if this unrest didn’t culminate in an eruption, it highlights the importance of closely monitoring this volcanic system with InSAR technique. Indeed, this provides spatial data, enabling us to observe the peculiar deformation pattern. In synergy with the mapped faults and fractures, this allowed us to better understand the volcano's behavior and interactions with the rift and obtain an accurate image of its magmatic system.