Discovery of the origin of the largest Mediterranean tsunami in two centuries
An international team coordinated by French researchers has just 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 up new perspectives for the assessment of seismic and tsunami risks in the Mediterranean.
The Ariane HROV on board the N/O Europe © F. Leclerc
Publication date: 27/11/2024
Press, Research
Related teams :
Marine Geosciences
Related themes : Natural Hazards
In the Mediterranean, the threat of tsunamis is very real: scientists estimate that there is a non-negligible probability of such an event striking its coasts in the next 30 years. To better anticipate these risks, it is crucial to understand the origins of historical tsunamis. An international team made up of members of the IPGP, the Géoazur Laboratory (Université Côte d’Azur), the National and Kapodistrian University of Athens (Greece), the Laboratoire de Géologie de l’ENS, and the Vicorob Laboratory at the University of Girona (Spain), has studied the largest tsunami of modern times: the Amorgos tsunami of 9 July 1956. Triggered by a major earthquake in the Cyclades archipelago, with an estimated magnitude ranging between 7.2 and 7.8, it generated waves of up to 20 metres on some Greek coasts, causing considerable damage. However, its precise source has remained a mystery until now, as the seismological networks of the time were unable to determine the exact location of the event.
To unravel this mystery, researchers conducted two large-scale campaigns at sea in 2022 and 2023: AMORGOS-22 and AMORGOS-23. Aboard the Oceanographic Vessel Europe of the French Oceanographic Fleet, they deployed cutting-edge underwater technologies to explore the fault system of the Santorini-Amorgos graben1, where the earthquake had been approximately located. The ship’s echo sounders first mapped the entire fault system, revealing spectacular escarpments that form veritable underwater cliffs, with vertical drops of up to several hundred metres. The exploration was then refined by two sophisticated underwater vehicles: the AUV IdefX2, which mapped the escarpments in ultra-precise metric terms, and the HROV Ariane3, which systematically explored them visually using its onboard cameras.
This meticulous methodology has paid off: researchers have discovered irrefutable evidence of recent deformation at the foot of the Amorgos fault, an imposing underwater feature rising to a height of more than 700 metres. The many photos and videos collected by the Ariane HROV were used to produce three-dimensional digital models using photogrammetry4, revealing the extent of the rupture caused by the 1956 earthquake. The measurements are spectacular: the earthquake caused a displacement of the seabed of between 9 and 16 metres, a value comparable to that of the largest earthquakes recorded on Earth. This discovery suggests that this tectonic movement alone was sufficient to generate the gigantic waves observed in 1956, challenging the previous hypothesis of an underwater landslide.
This study, published in Communications Earth & Environment, shows that it is now possible to identify and study the traces of undersea earthquakes several decades after they have occurred.
Notes
- Graben = Tectonic collapse trench bordered by normal faults
- AUV IdefX = Autonomous underwater vehicle capable of high-resolution bathymetric surveys
- Ariane HROV = Hybrid remotely operated underwater robot for visual exploration of the seabed
Photogrammetry = A technique for reconstructing objects in 3D from photographs taken from different angles.
Ref: F. Leclerc, S. Palagonia, N. Feuillet, et al. Large seafloor rupture caused by the 1956 Amorgos tsunamigenic earthquake, Greece. Commun Earth Environ 5, 663 (2024). DOI : 10.1038/s43247-024-01839-0
