In July 1956, the largest earthquake in Greece in the 20th century occurred in Amorgos, while it was followed by the largest tsunami of the last two centuries in the Mediterranean. Their source remained enigmatic and the question of which fault was responsible for this earthquake, plagued scientists for years. An international research team provided the answer using state-of-the-art equipment.

The Amorgos earthquake occurred on July 9, 1956, in the underwater area between Amorgos and Santorini. Its magnitude ranged between 7.2 and 7.8 on the Richter scale.

Its exact location and depth were the subject of debate, as only a few seismometers had been installed at the time. A few minutes later, a second earthquake followed, closer to Santorini.

Both vibrations had as resulting in 53 deaths and much damage.

Also, a few minutes after the first vibration, one tsunami it inundated most of the nearby shores with waves a few meters high. Two shepherds perched on the rocks along the southern coast of the island of Amorgos, witnessed a particularly high wave of at least 20 meters. The island of Astypalaia was also flooded with ten meter high waves.

Sixty-eight years after this devastating earthquake, an international team of researchers from the Geoazur-Universite Cote d’Azur, the Institut de Physique du Globe de Paris, the National and Kapodistrian University of Athens, the Geology Laboratory of the Ecole Normale Superieur de Paris and the Vicorob laboratory of the University of Girona, after many years of research, identified the fault that caused the earthquake, as well as the movement that took place on the seabed.

In 2015, the international oceanographic expedition mapped the sea floor between Santorini and Amorgos revealing three main faults located about 700 meters below the sea surface. In 2022, the next expedition with the French oceanographic vessel Europe departed from Heraklion, Crete and headed for these rifts. Specific parts of the faults were mapped at a very high resolution of less than one meter, by an autonomous underwater vehicle (AUV IdefX), which was able to dive and travel close to the seabed.

A year later, on a new mission with the same research vessel, the scientific team was accompanied by an underwater robotic vehicle, Ariane, which was equipped with cameras that allowed scientists to observe the seabed in 4K. As the professor of the Department of Geology and Geoenvironment of EKPA, Paraskevi Nomiko, who participated in the mission and the publication, characteristically tells APE-MPE, the robotic vehicle “was essentially the eyes of the scientist”. The aim was to further investigate the selected fault segments and identify the source of these large events.

This is not the first time that marine geologists have detected seismic faults on the sea floor with robotic vehicles. This has been achieved in the Caribbean and in Japan for the 2004 and 2011 earthquakes, respectively, that caused tsunamis. Here, however, the fault was sought for an earlier and less well-documented earthquake and tsunami, the source of which remained enigmatic.

As found during the investigation, the movement of the seabed during this great earthquake was at least nine meters and in some places it even reached 16 meters. “It is a very long movement, but it is justified for an earthquake that “touched” the 7.8 Richter scale,” observes Mrs. Legal.

According to the data analysis, the fault that was activated during the earthquake was the Amorgos fault. Ms. Nomikou describes that “we studied all three faults, in order to find signs of activation, which would show us who was responsible for the earthquake. The only thing that had a sign of activation was the Amorgos fault.”

Also, the researchers determined that the tsunami was caused by the large movement of the fault, and not by submarine landslides, as previously claimed. The results of the research were published in the Nature group’s journal Communications Earth & Environment.

The research is part of the “AMORGOS” program, funded by the National Research Foundation of France and French research institutions and will continue until 2029. The next goal of the research, according to Ms. Legal, is to study the continuity of the fault in the land area and its movement.