Shared magma supply at Santorini and Kolumbo constrained by amphibious seismological and geodetic analyses of the 2025 dike intrusion

In January 2025, Santorini and its neighbouring islands experienced an intense earthquake swarm, prompting the Greek authorities to declare a state of emergency followed by the island’s evacuation of the majority of the population. Following a gradual inflation and rise in seismic activity beneath the Santorini caldera, the main seismic swarm began on January 27, close to the submarine volcano Kolumbo, 10 km offshore NE of Santorini at 18 km depth. The Santorini and Kolumbo volcanoes have both produced highly explosive (VEI 5) eruptions in historical times, including the 1650 eruption of Kolumbo, which formed a 2.5 km-wide and 500 m-deep submarine crater and triggered a tsunami that devastated the surrounding islands. Although petrological, seismological, and geodetic analyses identified distinct shallow- and mid-crustal magma reservoirs, there has been debate over whether the two volcanic centres are connected and share a common deep magma source, or whether they result from distinct plumbing systems. The 2025 seismic crisis provided an unprecedented opportunity to observe the volcanic system and investigate the potential deep coupling. Integrating seismic and geodetic data from onshore and offshore instruments, we observe and model the dynamic emplacement of a 13-km long intrusion with a volume of 0.31 km³ into the upper crust offshore Santorini, reactivating principal regional faults and arresting 3–5 km below the seafloor. We determine a gradual inflation of Santorini's shallow reservoir 6 months before the crisis, during the intrusion a mid-crustal reservoir beneath Kolumbo at ~7.6 km depth rapidly deflated. This suggests that both volcanoes share, and potentially compete for, a common deep magma supply. In December 2025, we recovered additional ocean-bottom seismometers and pressure sensors, enabling us to refine our seismological catalogues and deformation modelling during and after the seismic crisis. Our analyses highlight the importance of shoreline-crossing monitoring and the need for real-time access to submarine sensor data for a more robust crisis response.