By Theodore Lainas

 

On July 9, 1956, a very strong and destructive earthquake of magnitude 7.5 occurred, centered on the sea area between Amorgos, Anidros and Anafis. The limited means of the time resulted in our knowledge of this earthquake being limited and the exact location of the epicenter impossible. Sixty-seven years later, an international oceanographic mission is trying to locate, with the help of a sophisticated robot, information about this strongest Mediterranean earthquake in the last 200 years.

 

During previous oceanographic expeditions, geophysical studies were carried out, where the morphology of the seabed between Santorini, Amorgos, Anafi and Astypalaia was mapped and three submarine faults parallel to each other, at a distance of 70 kilometers, were identified at a maximum depth of 750 meters.

 

An international team has been conducting underwater surveys in the same area for the past two years to shed light on the mechanism that triggered the devastating earthquake and subsequent tsunami. The team includes the associate professor of the Department of Geology and Geoenvironment of the National and Kapodistrian University of Athens Evi Nomikou, as well as scientists from research institutions in France, Frédéric Leclerc from the University of the Cote d'Azur, from the Institut du Physique du Globe of Paris and from the Ecole Normale Superieure de Paris. The submarine fault of Amorgos, explains Ms. Nomikou, “is one of the most active faults in the Greek maritime space and it is important to have data on its geometry, but also for the first time visual data from the ‘mirror’ of the fault. Τhe smooth surface created when the ground moves after an earthquake. Our goal is to understand the genesis mechanism of the 1956 earthquake and the resulting tsunami”.

 

The robot diver 

 

A valuable assistant in their research is the Ariane robotic vehicle, which has the ability to dive to a depth of up to 2,500 meters and stay there for up to six hours on each dive. Its 4K cameras record the morphology of the seabed and allow researchers to see and study the relief at a maximum speed of 0.5 meters per second. Equipped with two mechanical arms, it is also able to collect sediment in 30 cm long tubes called corers. The eight sediment cores collected will help date the underwater relief.

 

For the research, Ariane dived nine times for a total of 26 hours and traveled a total of 13 kilometers exploring the Amorgos fault zone and other faults. It took thousands of photos, which will be used to produce 3D models that will help scientists measure seafloor deformation.

 

In addition, the French oceanographic ship “L' Europe” which is equipped with a multiband mapping system to record the underwater morphology in coastal areas, also participated in the research. During the night but also during the days when there were stormy winds that prevented the robotic vessel from diving, “L' Europe” collected bathymetric data in the coastal zone of Anafi and Amorgos.

 

The high-resolution bathymetric data will be used in modeling the propagation of the 1956 tsunami, in order to better understand its generation mechanism.

 

In addition, the researchers identified several undersea landslides possibly linked to the 1956 earthquake, which could have amplified the tidal waves. Currently, the processing and interpretation of the data is in progress, in order to form a better picture of these past events and to have a better understanding of the future in one of the most geodynamically active areas of the Greek underwater space.

 

The first result from the research, as stated by Ms. Nomikou, is that the sea area between Amorgos and Santorini “is geodynamically active with many geohazards. However, we now have at our disposal high-resolution geophysical data and three-dimensional relief models, which will help us better study the deformation of the bottom after the 1956 earthquake”.