The analysis of the earthquake swarm around Santorini reveals the origin and development of the seismic crisis.
At the start of 2025, the Greek island of Santorini and its surroundings were rocked by tens of thousands of earthquakes. A new study in Nature from the GFZ Helmholtz Centre for Geosciences and GEOMAR Helmholtz Centre for Ocean Research Kiel, conducted with international partners, now offers the first detailed geological explanation of this seismic crisis.
By combining measurements from land-based earthquake stations with data collected by instruments on the seafloor near the Kolumbo underwater volcano (about 7 km from Santorini), researchers applied an advanced AI-supported technique to pinpoint earthquake activity. Their analysis shows that nearly 300 million cubic meters of magma rose from deep within the crust and stalled roughly four kilometers beneath the ocean floor. As it traveled upward, the magma fractured surrounding rock, producing thousands of quakes and tremors.
A seismically unstable region – geological background
Santorini itself lies in the eastern Mediterranean within the Hellenic volcanic arc, one of the region’s most geologically active areas. The island group forms the edge of a caldera created by a colossal eruption about 3,600 years ago, a landmark event in volcanic history.
The active underwater volcano Kolumbo lies in the immediate vicinity. In addition, the region is crossed by several active geological fault zones, which is the result of the African Plate pushing north-east against the Hellenic Plate. The Earth’s crust beneath the Mediterranean region has broken up into several microplates that shift against each other, and in some cases subduct and melt, thus, sourcing volcanic activity.
Santorini has produced multiple eruptions is historic times, most recently in 1950. In 1956, two severe earthquakes occurred in the southern Aegean Sea, only 13 minutes apart, between Santorini and the neighboring island of Amorgos. These had magnitudes of 7.4 and 7.2 respectively, triggering a tsunami.
The earthquake swarm that initiated in late January 2025 took place in exactly this region. During the crisis, more than 28,000 earthquakes were recorded. The strongest of these reached magnitudes of over 5.0. The severe shaking caused great public concern during the seismic crisis, partly because the cause was initially unclear, being potentially either tectonic or volcanic.
What happened underground? – Findings from the current study
The new study now shows that the earthquake swarm was triggered by the deep transport of magma. The chain of events had already begun in July 2024, when magma rose into a shallow reservoir beneath Santorini. This initially led to a barely noticeable uplift of Santorini by a few centimeters. At the beginning of January 2025, seismic activity intensified, and from the end of January, magma began to rise from the depths, accompanied by intense seismic activity.
However, the seismic activity shifted away from Santorini over a distance of more than 10 kilometers to the northeast. During this phase, the foci of the quakes moved in several pulses from a depth of 18 kilometers upwards to a depth of only 3 kilometers below the seafloor. The high-resolution temporal and spatial analysis of the earthquake distribution, combined with satellite radio interferometry (InSAR), GPS ground stations and seafloor stations, made it possible to model the events.
Dr Marius Isken, geophysicist at the GFZ and one of the two lead authors of the study, says: “The seismic activity was typical of magma ascending through the Earth’s crust. The migrating magma breaks the rock and forms pathways, which causes intense earthquake activity. Our analysis enabled us to trace the path and dynamics of the magma ascent with a high degree of accuracy.”
As a result of the magma movement, the island of Santorini subsided again, which the authors interpret as evidence of a previously unknown hydraulic connection between the two volcanoes. Dr Jens Karstens, marine geophysicist at GEOMAR and also lead author of the study, explains: “Through close international cooperation and the combination of various geophysical methods, we were able to follow the development of the seismic crisis in near real time and even learn something about the interaction between the two volcanoes. This will help us to improve the monitoring of both volcanoes in the future.”
View from many perspectives – methods
Two factors in particular enabled the exceptionally detailed mapping of the subsurface. For one, an AI-driven method developed at the GFZ for the automatic evaluation of large seismic data sets. Secondly, GEOMAR had already deployed underwater sensors at the crater of the underwater volcano Kolumbo at the beginning of January as part of the MULTI-MAREX project. These sensors not only measured seismic signals directly above the reservoir, but also pressure changes resulting from the subsidence of the seabed by up to 30 centimeters during the intrusion of magma beneath Kolumbo.
Scientific research activity on Santorini is continuing despite the decline in seismic activity. The GFZ is conducting repeated gas and temperature measurements on Santorini, while GEOMAR currently has eight seabed sensor platforms in operation.
Prof. Dr Heidrun Kopp, Professor of Marine Geodesy at GEOMAR and project manager of MULTI-MAREX, says: “The joint findings were always shared with the Greek authorities in order to enable the fastest and most accurate assessment of the situation possible in the event of new earthquakes.” Co-author Prof. Dr Paraskevi Nomikou is Professor of Geological Oceanography at the University of Athens and works closely with the German partner institutes on the MULTI-MAREX project. She adds: “This long-standing cooperation made it possible to jointly manage the events at the beginning of the year and to analyze them so precisely from a scientific point of view. Understanding the dynamics in this geologically highly active region as accurately as possible is crucial for the safety and protection of the population.”
Reference: “Volcanic crisis reveals coupled magma system at Santorini and Kolumbo” 24 September 2025, Nature.
DOI: 10.1038/s41586-025-09525-7
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