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Abstract

A gravity and vertical displacement dataset compiled at the Krafla Volcanic System provides unprecedented insights into the evolution of the magmatic system during the Krafla Fires (19751984). The strong correlation between gravity and vertical displacement indicate that the main subsurface processes involved mass input and withdrawal from the shallow magmatic system. Although the eruptive period has been extensively studied, there are still some uncertainties related to the active magmatic system, partly due to the lack of detailed gravity investigations and numerical models that jointly reproduce gravity and surface deformation. We recompiled and reprocessed legacy gravity and vertical displacement measurements to investigate the magmatic system during the Krafla Fires. By grouping the observations into three-year intervals (1975-1978, 1978-1981, 1981-1984), we reduce the influence of high-frequency temporal variations and reveal larger-scale gravity and deformation signals associated with the dynamics and architecture of the magmatic system. For 1978-1984 period, this analysis reveals an elongated low-gravity (-150 𝜇Gal) and subsidence (-1.2 m) anomaly centered on the Leirhnjúkur geothermal area – extending 2 km north-south and 1 km east-west – bounded by positive gravity anomalies (150 𝜇Gal) and inflation (1 m) lobes on either side. We investigate the potential contributions of structural and magmatic processes to the observed anomaly for this interval, using a series of finite-element models (FEMs). We first explore different magmatic source geometries and pressure-density changes, and then assess how mechanical heterogeneity associated with the caldera and fissure swarm affects the surface anomalies. Accounting for this mechanical heterogeneity substantially improves the fit to the observations and constrains the pressure and density changes required to reproduce the observed signals. Within this heterogeneous framework, the best fit is achieved by a combination of two magmatic sources within the upper 3 km: a shallow north-south striking dike at 0.2 km depth and 1.5 km height, with a pressure increase and density reduction, overlying a sill at 2.5 km depth with a pressure decrease and density increment. More broadly, the results demonstrate how integrating gravity modelling, surface deformation, and numerical approaches can improve resolution of subsurface processes in rifted calderas, with implications for volcanic hazard assessment and geothermal resource evaluation in Iceland and other active volcanic regions.