DEEP-BLUE

Based on current knowledge, we can identify four main phenomena that trigger tsunamis: the occurrence of an earthquake along a submarine fault (the most common cause), an underwater volcanic eruption, an underwater landslide, or, very rarely, the fall of a meteorite. While European historical records mention the occurrence of large tsunamis and detail their impacts on Mediterranean civilizations, the causes of many of them are still unknown. Today, the development of underwater vehicles allows us to research the sources of these tsunamis by observing the seafloor.

A study, conducted by the team from 2013 to 2017 on the fault responsible for the 2004 Saintes tsunami in the Caribbean, highlighted long-lasting traces of past underwater deformation generated by fault slip. While traces of seismic events occurring on land disappear rapidly due to erosion, deformation of the seabed persists longer. It is therefore possible to observe old deformations on the seabed, and thus determine the source of historical tsunamis. The DEEP_BLUE project stems from this observation.

This project aims to understand one of the largest European earthquakes in the last two centuries. On July 9, 1956, off Greece's coast between Santorini and Amorgos, an earthquake greater than magnitude 7 caused the twentieth century's largest tsunami in the Mediterranean Sea. Seismometers were scarce, leading to uncertainty in localizing the quake and fault. Identifying this fault is crucial for assessing seismic and tsunami risks in this tourist region, which hosts more than two million visitors each year.

Previous studies identified three potentially ruptured faults near the earthquake's epicenter. But the characteristics of the tsunami waves, which spread to the Turkish and mainland Greek coasts, suggested there were multiple triggers: seafloor movements generated by fault slip during the earthquake and large underwater landslides.

Two missions conducted on the Research Vessel Europe (French Oceanographic Fleet) shed light on these events. During the AMORGOS-22 cruise, a submarine was used to acquire very fine bathymetric data of the three faults, which are essential for identifying ruptures and escarpments on the seafloor. In 2023, the AMORGOS-23 mission, supported by DEEP_BLUE, conducted seven dives with an HROV (Hybrid Remotely Operated Vehicle) equipped with cameras, aiming to observe the seabed and find traces of these events.

Observations confirmed that one of these faults did indeed recently slip. Since the HROV has a limited field of vision, the video data was subsequently processed by photogrammetry to reconstruct in 3D the outcrops that show evidence of this recent seabed rupture. The slip could be. Sediment cores were also sampled by the HROV to confirm the age of this rupture and to associate these deformations with the 1956 earthquake. This discovery thus challenges the hypothesis that two sources are needed to generate the tsunami, in favor of a single massive underwater rupture of the fault, which could have generated the observed waves.