It was initially unclear whether the seismic activity signaled an impending volcanic eruption or tectonic fault movement. Researchers used advanced machine learning analysis of seismic data to precisely locate each quake and reveal stress changes underground associated with the migration of pressurized magma. This enabled detailed visualization of magma movement.
Sheets of magma, or magmatic dike intrusions, moved in waves more than 10 kilometers below the surface, cutting horizontally through rock over a 20-kilometer span. The total volume was estimated at enough to fill 200,000 Olympic swimming pools. The magma originated in a reservoir connecting the Santorini volcano and the nearby Kolumbo underwater volcano, but lacked the conditions to reach the surface.
Dr Stephen Hicks of the Department of Earth Sciences at UCL said, "We used a new method to work out the cause of a swarm of earthquakes, treating each of the 25,000 precisely located quakes as 'virtual stress meters' - clues as to how stress was changing underground. This gave us a robust and higher-resolution picture of what was happening, allowing us to rule out fault slippage as the earthquakes' main cause.
"Our technique could be applied to future earthquake swarms almost in real time and could allow us to better forecast the likelihood of volcanic eruptions or larger earthquakes.
"Our evidence suggests the magma causing the Santorini earthquakes wasn't getting close to the surface. If we apply our technique to similar swarms of earthquakes in future, we could pinpoint where the magma would likely come out and potentially the amount.
"Our approach only uses data from seismometers recording ground vibrations, and so it is especially useful for underwater events where satellite images, or on-land GPS, used to spot changes in the position of the land, may not be available."
Earthquake patterns corresponded to regions where magma opened cracks in the crust, not typical fault slip areas. Satellite GPS confirmed the surface bulged upward, showing further evidence of upward magma movement and allowing estimates of intrusion scale and volume.
Lead author Anthony Lomax said, "Our results show that magma intrusions, which help build Earth's crust, generate earthquakes, and can lead to hazardous volcanic eruptions, do not involve a simple one-way process of magma moving laterally or vertically. Most striking was that the intrusion did not move smoothly. Instead, it rebounded in waves - opening new fractures, closing others, and pumping magma forward in pulses. These pulses of magma pressure created a vast, dynamic and cascading pattern of stress and triggered earthquakes in the surrounding crust."
Co-author Eleftheria Papadimitriou of Aristotle University of Thessaloniki said the process may be common beneath volcanoes worldwide.
Santorini sits on the Hellenic arc between the African and Eurasian plates and has a history of powerful eruptions, including the ancient Minoan eruption. The 2025 seismic events did not result in an eruption but highlighted the risks to local populations and the importance of high-resolution monitoring.
*The sheets of magma were vertically oriented but travelled horizontally. If the sheets were a knife pushing through bread, the blade would be pointing downwards, but the knife would be moving sideways.
Research Report:The 2025 Santorini unrest unveiled: Rebounding magmatic dike intrusion with triggered seismicity
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