At 19:00 UTC on 27 January 2025, the first shock rolled through the Aegean, a low-magnitude quake centered ten kilometers northeast of Santorini and just two kilometers southeast of the submarine Kolumbo volcano. Residents on the island later described a faint thump beneath the caldera, a vibration that set glass rattling in kitchens and drew worried calls to local authorities. Within an hour, the swarm multiplied. Instruments at the Global Navigation Satellite System (GNSS) station SANT recorded abrupt deformation, while ocean-bottom seismometers deployed weeks earlier near Kolumbo began logging a steady hum of coherent tremor. By midnight, more than 150 small quakes had struck, the strongest approaching magnitude 3, and the crisis that had been building for months was now undeniable.
This latest Nature study reconstructs the entire sequence with precision. It shows that the unrest did not erupt suddenly, but followed a gradual ramp-up visible to instruments as early as July 2024. That summer, satellites detected 50 millimeters of uplift inside the Santorini caldera, subtle enough to escape tourist notice but clear in interferometric synthetic aperture radar (InSAR) records. GNSS logs confirmed 45 millimeters of displacement at the SANT station, building pressure in a shallow reservoir at 3.8 kilometers depth. By September, earthquake counts had tripled from background rates. The national seismic network recorded dozens of low-magnitude events each week, their hypocenters clustering beneath the northern caldera basin. Dispatch notes from that period mention “felt tremors at Kameni, duration short, no damage,” reflecting an official uncertainty about whether tectonic faulting or magmatic movement was responsible.
Gas measurements provided another signal. On 8 July 2024, a survey team used a Dräger X-am 8000 portable multigas analyzer inside the Nea Kameni craters. They measured baseline CO₂ flux across eleven stations. When they repeated the measurements on 3 February 2025, days after the swarm ignited, several sites showed order-of-magnitude increases in hydrogen and carbon dioxide emissions. One operator remarked in field notes, “Flux jump consistent with recharge. Chamber pressurizing.” That data, combined with uplift trends, suggested magma was not only intruding but also degassing toward the surface.
Then came the main swarm. At 19:00 on 27 January seismicity initiated between 12 and 18 kilometers depth, five kilometers west of Santorini. Early quakes measured between magnitude 1 and 3. They were accompanied by coherent tremor in the 1–10 Hz band, the kind associated with microquakes inside a propagating dike. For the first five days, labeled Phase I in the new catalogue, hypocenters migrated toward Santorini before shifting northeast. GNSS stations logged rapid subsidence to the northeast, while ocean-bottom pressure sensors at Kolumbo measured crater floor subsidence up to 32 centimeters. At this point, scientists concluded magma was moving away from Santorini into the faulted Anhydros block.
On 1 February, Phase II began. Earthquake foci shallowed, migrating northeastward at a velocity of about one kilometer per hour. Hypocenters clustered around eight kilometers depth. Several exceeded magnitude 5. Seismologist Dimitris Anastasiou, stationed at the National Technical University of Athens, told colleagues over radio, “We are watching a dike cut northeast. The speed matches intrusion.” His assessment proved correct. Joint inversion of GNSS, InSAR, and pressure sensor data revealed a 13-kilometer-long dike intruding between Kolumbo and Anhydros, opening from five to eleven kilometers depth and inflating by 0.313 cubic kilometers. Meanwhile, a mid-crustal reservoir beneath Kolumbo deflated by 0.076 cubic kilometers, proving the intrusion was drawing directly from Kolumbo’s magma storage.
The following days traced a shifting battle underground. By 6 February, Phase III directed activity southwest, focusing on the uppermost five kilometers of crust. From 10 to 11 February, Phase IV saw seismicity jump northeast again, with deep quakes at 12 kilometers racing laterally 25 kilometers across the Anhydros block at peak propagation velocities of one kilometer per hour. Records show twelve distinct seismic surges over the month, each marked by rapid quake bursts and coherent tremor. One OBSP station captured high-frequency energy bursts above 5 Hz, characteristic of dense microquake swarms triggered by dike propagation. “Every surge felt like the ground inhaling, holding, then releasing again,” a Santorini resident told a visiting Athens reporter at the time.
The crisis peaked in mid-February. GNSS logs at SANT showed cumulative surface displacement exceeding 10 centimeters. Kolumbo crater subsided by as much as 32 centimeters. On Santorini’s northern flank, stations logged six to thirteen centimeters of subsidence. By 20 February, activity slowed. Scientists identified this as Phase VI, a gradual tapering of the intrusion. On 24 February, magma inflow rates collapsed to near zero. By then, the dike had consumed four times the volume lost from Kolumbo’s reservoir, implying additional deeper magma contributions beyond what was directly observed.
For hazard planners, the findings are stark. Santorini’s last eruption occurred in 1950, a modest dome-forming event. Kolumbo’s last major eruption, in 1650, killed hundreds with pyroclastic flows and a tsunami. Since 2011, Santorini has experienced repeated inflation episodes, each delivering about 0.01 cubic kilometers of magma, but none producing a dike or eruption. Kolumbo saw earthquake swarms in 2003 and 2006, attributed to magma ascent toward its shallow 2.5-kilometer reservoir, but again without eruption. This time was different. Sequential inflation beneath Santorini followed by deflation at Kolumbo, coupled with a 13-kilometer dike injection, confirms hydraulic linkage between the two volcanoes.
The study identifies two low seismic velocity anomalies, labeled L3 and L4, beneath the Anhydros block. These act as mid-crustal storage branches, containing up to 14 percent partial melt. In January, the dike initiated at 18 kilometers depth inside anomaly L4, then wrapped around L3 as it migrated upward. Stress changes reactivated normal faults, triggering thousands of quakes. By the end, more than 30,000 seismic events had been catalogued, with a magnitude of completeness of 1.3, making this one of the most densely recorded swarms in Greek volcanic history.
The hazard implications are double-edged. On one hand, the ratio of dike volume to reservoir drainage, about four, suggests a deep primitive magma source rather than a shallow gas-rich system like Etna. That reduces immediate explosivity risk. On the other hand, Kolumbo and Santorini’s shallow reservoirs remain primed. Historical accounts from 1650 describe simultaneous ash emissions at Kameni and Kolumbo, implying coupled eruptions are possible. The 2025 dike intruded at water depths of about 200 meters, shallower than the Kolumbo chain cones, raising the chance that future intrusions could breach into explosive phreatomagmatic environments. The report stresses that the ascent path through shallow crust will determine eruption style and tsunami potential.
The final analysis is clear. Between July 2024 and March 2025, magma moved from mid-crustal storage into both volcanoes, inflating Santorini’s shallow reservoir, draining Kolumbo’s mid-crustal body, and building a lateral dike through the Anhydros block. More than 0.31 cubic kilometers of magma shifted in a matter of weeks, a volume comparable to past eruptions in the Kolumbo Volcanic Chain. The event forced Greek authorities to declare a state of emergency from 6 February to 3 March. Tourists were evacuated from high-risk zones. Ferry operators logged delays due to quake-related port inspections. In the words of one Santorini civil protection officer: “We prepared for an eruption that never came. But the system is awake now.”
As of late September 2025, both Santorini and Kolumbo remain under heightened monitoring. Continuous GNSS, InSAR passes, and ocean-bottom sensors are still in operation. Gas flux at Nea Kameni remains elevated compared to 2024 baselines. Scientists emphasize that while the immediate dike intrusion has ceased, both volcanoes are replenishing. The present status: magma is still being supplied from depth. The next factual step is extended observation through winter 2025 into spring 2026, when seasonal seismic noise is lowest and satellite interferometry is clearest. That is where attention now turns.
Source:
(2025). Volcanic crisis reveals coupled magma system at Santorini and Kolumbo. Nature, 645, 939–947. https://doi.org/10.1038/s41586-025-09525-7
