Change in microgravity during an inflation episode at Askja Volcano, Iceland, 2023 – 2025

Askja is one of the most monitored volcanoes in Iceland. Since 1966, annual ground deformation measurements have been carried out in Askja along a leveling line. In 1993 the first Global Navigation Satellite System (GNSS) measurements were made in Askja and in 1992 the first Interferometric Synthetic Aperture Radar (InSAR) images of Askja were gathered. Since 2021 there has been uplift at Askja volcano, after decades of subsidence. The uplift is monitored with GNSS and InSAR measurements. The net uplift from June 2021 to December 2025 is approximately 90 cm with a decreasing rate. Previous geodetic models of the observed ground deformation inferred an inflation source at a median depth of 2.7 – 2.8 km. Gravity surveys have been carried out regularly since 1988, and annually since 2018. Gravity measurements show mass or density changes in the sub-surface. From 1988 – 2017 there was a net gravity decrease, while measurements from 2017 – 2023 show a net gravity increase during that period.

We carried out GNSS campaigns and gravity surveys in August of 2024 and 2025. We measured 18 gravity stations and 20 GNSS stations scattered around Askja. The gravity was measured with two relative spring gravimeters (Scrintex CG5 and CG6). Gravimeters are very sensitive and prone to sudden data tares, to mitigate this we used two meters. We can evaluate the uplift between years with GNSS and InSAR data and apply the theoretical Free Air gradient to correct for the gravity change due to elevation change.  The yearly uplift rate 2023 - 2025 is up to about 10 cm/year. After correcting for the height changes, preliminary evaluation suggests that the net gravity change from 2023-2025 does vary between stations, with increase at some stations and decrease at others. By analyzing the gravity change we are adding another parameter to our dataset, which helps us to identify the process responsible for the current uplift episode.