In July 1956, an incident occurred on Amorgos. the most powerful earthquake in Greece of the 20th century, which was followed by the largest tsunami in the last two centuries in the Mediterranean Sea.
The source of this earthquake remained a mystery, and the question of which fault was responsible for this event has plagued scientists for many years. An international research team has provided the answer using the latest generation equipment.
The Amorgos earthquake occurred on July 9, 1956 in the underwater zone between Amorgos and Santorini. Its magnitude fluctuated between 7.2 and 7.8 points. The exact location and depth of the earthquake became a matter of debate, as only a few seismometers were installed at the time. A few minutes after the first earthquake, a second one occurred, closer to Santorini.
Both aftershocks resulted in 53 deaths and significant damage. In addition, within minutes of the first shock, the tsunami inundated most of the nearby shores, creating waves several meters high. Two shepherds climbing the cliffs along the southern coast of Amorgos island witnessed a particularly high wave, at least 20 meters high. The island of Astypalaia was also flooded by waves up to ten meters high.
Sixty-eight years after this devastating earthquake, an international team of researchers from Geoazur-Universite Cote d'Azur, the Institute of Physics of the Earth in Paris, the National and Kapodistrian University of Athens, the Laboratory of Geology Ecole Normale Superior in Paris and the Vicorob Laboratory of the University of Girona, after many years of research, discovered a faultwhich caused an earthquake, as well as movement that occurred on the underwater floor.
In 2015, an international oceanographic expedition mapped the seabed between Santorini and Amorgos, revealing three main faults, located approximately 700 meters below the surface of the sea. In 2022, the next expedition on the French oceanographic vessel Europe set off from Heraklion to these faults. Specific areas of the faults were mapped at very high resolution, less than one meter, using an autonomous underwater vehicle (AUV IdefX) that was able to dive and navigate close to the bottom.
A year later, on a new expedition with the same research vessel, the research team carried an Ariane underwater robotic device equipped with cameras that allowed scientists to observe the seafloor in 4K. As noted by Professor of the Department of Geology and Geoenvironment of EKPA Paraskevi Nomikou, who participated in the expedition and publication, the robotic device “was actually through the eyes of a scientist.” The goal was to examine selected fault areas in more detail and determine the source of these major events.
This is not the first time that marine geologists have discovered seismic faults on the seafloor using robotic devices. This was achieved in the Caribbean and Japan for the 2004 and 2011 earthquakes respectively, which caused tsunamis. However, they were looking for the fault of an older and less documented earthquake and tsunami, the source of which remained a mystery.
As was established during the study, the movement of the underwater bottom during this strong earthquake was at least nine meters, and in some places even reached 16 meters. “This is a very large movement, but it is justified for an earthquake, which “reached” 7.8 points”notes Ms. Nomiku.
According to data analysis, the fault that activated during the earthquake was the Amorgos fault. Ms. Nomiku describes: “We studied all three faults to look for signs of activation that would indicate which one was responsible for the earthquake. The only one that showed a sign of activation was the Amorgos fault.”.
In addition, the researchers found that the tsunami was caused by large movement of the fault, and not by underwater landslides, as previously stated. The results of the study were published in the journal “Communications Earth & Environment” Nature group.
The study is part of the program “AMORGOS”funded by the French National Research Foundation and French research institutions, and will continue until 2029. The next goal of the study, according to Ms. Nomik, is to study the continuation of the fault on land and its movement.
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