At the beginning of the year, tens of thousands of earthquakes shaked the Greek island of Santorini and its surroundings within 30 days. Geologists have now analyzed how this seismic crisis came about. Accordingly, a month -long and complex relocation of magma triggered the swarm quake underground. A total of 300 million cubic meters of fluid magma rose from the deep earth crust, gathered in reservoirs and generated the seismic vibrations. Incidentally, they raised Santorini and let it drop again when the magma was aberred via a previously unknown path across the neighboring volcano. In the future, the findings could help to better monitor the volcanic area.
The Greek archipelago around Santorini in the eastern Mediterranean was created in its current form around 3,600 years ago by a huge volcanic eruption. Since then, the islands have formed the edge of the Vulcan Caldera. It also includes the still active underwater volcano Kolumbo, which is about seven kilometers next to Santorini, and a historically less active volcano directly under Santorini. The surrounding region is referred to as a Hellenic volcanic arch and is also highly active because the African plate presses against the Hellenic plate and forms several geological break zones. Regular earthquakes in the area testify to these plate movements.
Most recently, in January 2025 there was a real swarm of earthquake in exactly this region, in which thousands of people were evacuated by the islands. During this seismic crisis, more than 28,000 earthquakes were registered. The strongest of magnitudes they achieved of more than 5.0. However, it was initially unclear whether the causes were predominantly tectonic or volcanic nature.
Magma promotion triggered the swarm of earthquake
Researchers led by Marius Isken from the GFZ Helmholtz Center for Geo research in Potsdam have now examined more closely on how the swarm life came about. To do this, they evaluated data from satellites, earthquake and GPS stations on land as well as sensors that were placed on the crater of the underwater volcano Columbo on the sea floor. These underwater sensors measure seismic signals and pressure change on the sea floor. With the help of artificial intelligence, the geologists determined exactly where the earthquakes appeared and what happened in detail in the subsoil. So they reconstructed the temporal and spatial process of events.
These modeling showed that the seismic crisis was a volcanic nature. A total of around 300 million cubic meters of liquid magma rose from the deep earth crust and came to a standstill around four kilometers below the ocean floor. During this transport by the earth’s crust, which began in July 2024, the magma first filled a flat reservoir under Santorini. From September, the relocating magma generated the first seismic vibrations and earthquakes. This activity increased at the beginning of January 2025. From January 27, further magma suddenly rose from the depths and triggered the intensive swarm of earthquake, which lasted over 30 days, as the geophysicists report.
“The seismic activity was typical of the rise of magma through the earth’s crust. The rising magma breaks its way through the rock and leads to intensive earthquake activity,” explains Isken. Meanwhile, the epicenters of the individual quakes hiked along a distance of over ten kilometers, which led past Santorini in the northeast. At the same time, the herd of earth was shifted upwards: the first quakes were created 18 kilometers below the sea floor, the last only three kilometers below, as the team stated.
New connection between two volcanoes discovered
However, the magmabe movements had another effect: The island of Santorini, initially hardly noticeable, rose by less than one centimeter, then up to ten centimeters. Later, however, she lowered herself because the magma from the reservoir under Santorini was off. In parallel, a chamber of the neighboring Columbo was strikingly emptied and filled, as the measurement data unveiled. The seabed there temporarily dropped by up to 30 centimeters.
This suggests that there is a previously unknown hydraulic connection between the Columbo and the Santorini. The two volcanoes and their chambers in around four and three kilometers deeply combine a vertical pipeline through which the magma flow back and forth and also trigger earthquakes, the geologists close. This also indicates that both volcanoes are probably fed from the same magma source starting about eight kilometers depth. Similar volcanic compounds like the now discovered in the Aegean geologists suspect under Hawaii, Iceland and Kamchatka for a long time.
“By combining different geophysical methods, we were able to follow the development of the seismic crisis almost in real time and even learn something about the interaction of both volcanoes. This will help us to improve the surveillance of both volcanoes in the future,” says co-author Jens Karstens from Geomar. The researchers continue to monitor the region around Santorini via various measuring devices. This not only helps the scientists, but also to the Greek authorities to better assess the situation and deal with future tremors. “Knowing the dynamics in this geologically highly active region as precisely as possible is important for the security and protection of the population,” says co-author Paraskevi Nomikou from the University of Athens.
Source: Marius Isken (GFZ Helmholtz Center for Geof research) et al.; Nature, DOI: 10.1038/S41586-025-09525-7