Characterization of the Plumbing System Beneath Askja Caldera, Iceland, Revealed by Microgravity and Deformation Data During Uplift Between 2022 and 2023

Abstract

In July 2021, an episode of uplift started at Askja caldera, Iceland, following over 50 years of subsidence. The initial inflation rate was approximately $700mm/yr$, declining to $350mm/yr$ within 1 month. This deformation has been attributed to a magma intrusion into a pre-existing body of mush. To investigate the physical properties of both the intruded magma and the sub-volcanic mush zone, we jointly inverted GNSS and InSAR ground deformation observations, along with microgravity data from 2022 to 2023 surveys, employing a pressurized, inflating ellipsoid embedded in an elastic half space. Our results indicate a mass increase of $8.14\times 10^{10}kg$ (95$\%$ confidence interval (7.01–9.22) $\times 10^{10}kg$) and a mean pressure change of 2.67 MPa in a volume that extends from ∼0.4–4.6 km below surface, which we interpret as a mush zone. If magma is intruded into a mush, some proportion of the intruded volume is accommodated by compression of the mush, limiting the magnitude of surface deformation. The trade-off between intrusion density and the compressibility of the host mush means that density cannot be directly constrained from joint inversion of deformation and gravity data. However, by assuming typical density values for a basaltic intrusion, we were able to estimate a bulk compressibility range of (0.7–1.2) $\times 10^{-10}$ $P{a}^{-1}$ for the host mush. This is consistent with a crystal-rich basaltic mush and provides a key constraint on the mechanical behavior of Askja's subsurface mush system.