An international team coordinated by French researchers has made a major discovery in the Aegean Sea: the precise identification of the underwater rupture that caused the largest modern tsunami in the Mediterranean. This discovery, made more than 60 years after the event, opens new perspectives for the evaluation of seismic and tsunami risks in the Mediterranean.
In the Mediterranean, the threat of tsunamis is very real: scientists estimate there is a significant probability that such an event could strike its coasts within the next 30 years. To better anticipate these risks, it is crucial to understand the origins of historic tsunamis.
A research team from CNRS Terre & Univers (see boxed text) focused on the most significant modern event: the Amorgos tsunami of July 9, 1956. Triggered by a major earthquake in the Cyclades archipelago, with an estimated magnitude between 7.2 and 7.8, it generated waves reaching up to 66 feet (20 meters) on certain Greek coasts, causing significant damage. However, its precise source remained a mystery until now, as the seismological networks of the time did not allow for the exact localization of the event.
To solve this mystery, researchers conducted two large-scale oceanographic campaigns in 2022 and 2023: AMORGOS-22 and AMORGOS-23. Aboard the French Oceanographic Fleet's research vessel Europe, they deployed cutting-edge underwater technologies to explore the fault systems of the Santorini-Amorgos graben, where the earthquake was approximately located.
The ship's sonar equipment first mapped the entire fault system, revealing spectacular escarpments forming true underwater cliffs with vertical drops reaching several hundred meters. The exploration was then refined with two advanced underwater vehicles: the AUV IdefX, which created ultra-precise metric-scale maps of the escarpments, and the HROV Ariane, which conducted systematic visual explorations using its onboard cameras.
3D model of the Amorgos fault plane, derived from videos captured by the HROV Ariane (modified from Leclerc et al., 2024). The base of the plane, approximately 51.5 feet (15.7 meters) here, was recently exposed, most likely during the 1956 earthquake.
This meticulous methodology bore fruit: researchers discovered irrefutable evidence of recent deformation at the base of the Amorgos fault, a massive underwater relief rising over 2,296 feet (700 meters). Numerous photos and videos collected by the HROV Ariane allowed the team to create three-dimensional digital models using photogrammetry, revealing the scale of the rupture caused by the 1956 earthquake.
The measurements are spectacular: the earthquake caused a seabed displacement of 30 to 52 feet (9 to 16 meters), an amplitude comparable to the most significant earthquakes recorded on Earth. This discovery suggests that this tectonic movement alone was sufficient to generate the gigantic waves observed in 1956, challenging the previously favored hypothesis of an underwater landslide.
This study, published in
Communications Earth & Environment, demonstrates that it is now possible to identify and study traces of underwater earthquakes several decades after their occurrence. These efforts continue, particularly as part of Sylvain Palagonia's doctoral research (EDSFA, Géoazur, Université Côte d'Azur) and the ANR AMORGOS project (2025–2030), which will further refine our understanding of seismic and tsunami risks in the eastern Mediterranean.
Learn more:
Large seafloor rupture caused by the 1956 Amorgos tsunamigenic earthquake, Greece.
Commun Earth Environ 5, 663 (2024).
https://doi.org/10.1038/s43247-024-01839-0