Dynamics of Santorini Volcano During Earthquake Swarm 2025 Based on Gravity Monitoring Supported by GNSS

Abstract This study investigates gravity changes in the Santorini volcanic island (South Aegean Sea, Greece) before, during and after the ‘February 2025’ seismic crisis, with the aim of detecting subsurface mass changes. Four gravity campaigns were conducted between September 2023 and June 2025 using a Scintrex CG-6 gravimeter. 31 stations were spread over Thera and Nea Kameni islands. GNSS data from four permanent stations were used to correct for surface deformation effects. It was found that even shortly before the beginning of the unrest and volcanic inflation the gravity field had changed (June 2024): in general, the data showed an increase of gravity in a few areas, including part of the central Nea Kameni island, as well as in the area of Fira, Imerovigli and Oia. The positive gravity changes are located on an annular area with the centre corresponding to the Mogi source inverted from the GNSS data. This indication was interpreted as a deeper magma intrusion that consequently caused a significant inflation, lasting from August 2024 to January 2025. In this period, increased seismicity (above background level) occurred within the Santorini volcanic structure. The next gravity campaign, conducted in February 2025 during the seismic crisis, showed the end of this extensive mass increase; however, there was an indication of possible dyke or hydrothermal fluids intrusion within the Kolumbo fault zone. Such intrusion was modelled, and it was confirmed that fluids saturating high-porosity volcanic formations can produce a gravity signal relevant to the observed ones. At this time, the previously observed inflation was followed by fast and steep deflation of the surface, while the principal earthquake swarm migrated towards NE, reaching the Anydros islet. The last campaign in June 2025 was characterised by only minor changes, except a mass increase in Fira area, where another dyke/fluids intrusion could be inferred. At that period, another small earthquake swarm occurred again inside the Santorini caldera, and also a minor inflation was observed at this part of Santorini. Gravity monitoring proved to be a useful contribution to the control of volcanic activity in this short-term regime, with the gravity campaigns separated by months, rather than years.