A mysterious swarm of earthquakes that occurred near the Greek island of Santorini in early 2025 was caused by rebounding sheets of magma slicing through Earth's crust, finds a new study by an international team involving a UCL (University College London) researcher.
Between late January and early March, the team analysed over 25,000 earthquakes that occurred between Santorini and Amorgos Islands. Hundreds of these were large enough to be felt at the time, with magnitudes exceeding 4.5. The seismic unrest prompted a local state of emergency, school closures, and alarm among residents and tourists on Santorini.
At the beginning, it was not known whether the quakes were linked with volcanic activity either in Santorini's volcanic centre or the nearby underwater volcano Kolumbo, and were possibly the signal of a coming eruption, or were due to tectonic fault slip and possibly the prelude to a larger earthquake (like the destructive, magnitude 7.7 quake that struck the same region in 1956).
In the new study, published in the journal Science, researchers looked at ground vibrations recorded by seismometers over two months. They applied advanced machine learning (artificial intelligence) techniques to firstly detect and then precisely determine the position in Earth's crust of over 25,000 quakes.
They inferred from the earthquakes' distribution the stress changes underground that triggered them, allowing the team to image the movement of pressurised magma with unprecedented detail in space and time.
They found that sheets of magma sliced in waves or pulses at a depth of more than 10km below the surface. These magma intrusions, or dikes, smashed horizontally* through layers of rock across an area of 20km. The volume of magma could have filled 200,000 Olympic-sized swimming pools, the researchers estimated.
The intrusions shot out of an underground reservoir of magma that links the Santorini volcano (caldera) and the nearby Kolumbo underwater volcano. But the intruded magma did not have the pressure or buoyancy needed to reach the surface and cause an eruption.
Co-author Dr Stephen Hicks, based at UCL's Department of Earth Sciences, 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."
The distribution of quakes, the researchers found, matched zones consistent with magma opening cracks, not zones where tectonic slip would be expected.
In addition, they looked at GPS data from satellites showing the uplift and bulging of the Earth's surface caused by the upward-slicing magma. From these changes to the Earth's surface, they were able to estimate the size of the intrusions and their total volume.
Lead author Anthony Lomax, an independent researcher, 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 from Aristotle University of Thessaloniki in Greece said: "This rebounding, wave-like process of magma intrusion may not be unique to Santorini but may be a fundamental mechanism by which magma is transported beneath volcanoes worldwide."
Santorini is part of the Hellenic arc, a chain of mountains and volcanoes on the boundary of the African and Eurasian tectonic plates. The area has a history of devastating eruptions, including the "Minoan eruption" around 1620 BC, one of the largest eruptions in human history.
The 2025 seismic crisis, although it did not culminate in an eruption, highlights the potential hazards facing local populations and underscores the importance of high-resolution monitoring, the researchers said.
*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.
