Using remote sensing and earth observation data to determine the sources behind the 2024-2025 unrest at Santorini and Kolumbo volcanoes.

Intense seismicity started on 27 January 2025 in the Aegean sea about 10 km NE of Santorini and lasted for over a month. This event was preceded by a period of elevated seismicity within the Santorini caldera since September 2024. Continuous GNSS stations on Santorini and neighboring islands as well as Sentinel-1 InSAR acquisitions over Santorini recorded significant deformation during this time period. Ocean-bottom pressure sensors also recorded subsidence after 27 January.

Here, we focus on the inversion of the geodetic data to infer the potential sources behind the deformation. We find a source of inflation at 3.8 km depth within the Santorini caldera between July 2024 and January 2025. Its location matches the location of the source behind the previous unrest in 2011-2012. Between 27 January and end of February, the deformation pattern is found to be consistent with a deflating source at 7.6 km depth below Kolumbo volcano and 13-km long opening dislocation between Kolumbo and Anhydros. Using these results, we were also able to divide this latter episode into smaller time intervals to study the propagation of the opening dislocation upward and to the NE. These results, in combination with the seismicity, lead to the conclusion that there is a coupling between the Santorini and Kolumbo volcanoes and that a dike was intruded in the crust on 27 January, coming from a low velocity anomaly body at 18 km depth.

This study shows that even in difficult settings (deformation occurring underwater with sparse islands around), remote sensing and earth observations can provide essential information to explain an on-going unrest crisis. It is therefore critical to ensure that such data collection is secured onward to help with the understanding of future volcanic and tectonic crisis.