Undersea earthquakes and faults: Only 30% of the Greek seabeds have been systematically studied

“The Greek seas hide great wealth and great threats at the same time. We have every reason to exploit the marine wealth and we have an obligation to reduce the risk from marine threats. Both can be done in only one way: by strengthening marine research,” geologist Dr. Dimitris Sakellariou, Research Director at the Institute of Oceanography of the Hellenic Center for Marine Research (COME), member of the Seismic Risk & Reduction of Seismic Risk Committee of the OASP, coordinator of the research program “Athens vs. England” (ELIDEK) and coordinator of the “New Research Vessel” project.

Mr. Sakellariou mentions that in recent years the researchers of the Institute of Oceanography of ELKETHE have collected and processed all the available data from past and recent marine seismic and bathymetric surveys.

The purpose of this effort was to map the undersea faults in the Aegean, the Ionian Sea and the Hellenic Trench, the study of their kinematic characteristics, the assessment of their seismic potential and the understanding of the tectonic deformation of the wider area.

From the mapping a database was created of several hundred major and minor, definite and probable, submarine faults. About 100-150 of the most important faults are shown on a related map (see attachment). According to Mr. Sakellariou, “the map of submarine faults looks complete, but unfortunately it is not. In contrast to mainland Greece, only 30% of the Greek seabeds have been systematically studied. For the remaining 70%, our knowledge is very limited and we can only make estimates.”

New facts about the faults

However, from the available scientific data, new evidence and conclusions emerged about the main kinematic characteristics of the faults. This resulted in a better understanding of the long-term deformation processes of the so-called “Aegean microplate”, but also the derivation of realistic estimates for the seismic potential of submarine faults.

The Aegean region is a puzzle of numerous smaller lithospheric blocks, the boundaries of which are defined by larger or smaller fault zones. Each fault zone has its own kinematic characteristics, which determine the type of movement between adjacent blocks.

For example, the tectonic widening (stretching) of the Gulf of Corinth in a North-South direction causes the Peloponnese to move away from Central Greece at a rate of 10-12 meters per 1,000 years.

Lemnos, which is located in the southern part of the dextral zone of the North Aegean (North Anatolia Rift), is moving southwest with respect to Mount Athos at a rate of 12-13 meters per 1,000 years. Astypalaia is moving to the southwest relative to Amorgos at a rate of 4 meters per 1,000 years.

“If you consider that an earthquake with a magnitude of 6.5-7.0 is needed to move one meter on a fault, it is easy to see how many earthquakes are needed to create the movements mentioned above”, Mr. Sakellariou points out.

Overall, the wider area of ​​the “Aegean microplate” is expanding towards the South. The movement of Crete to the south-southwest at a rate of 30-35 meters per 1,000 years relative to Northern Greece represents the total deformation of the microplate. This deformation occurs as an overall result of the movements that take place along the numerous faults and fault zones that run through the area.

“It is easy to see that submarine faults have a very important contribution to the deformation of the Aegean, they are characterized by a high seismic potential and can cause strong earthquakes with devastating results,” he emphasizes.

The contribution of ELKETHE

The database of underwater active faults of ELKETHE will be used and integrated into the digital fault map of Greece. This project is implemented by the Hellenic Geological and Mineral Research Authority (EAGME) in collaboration with all the geological and seismological bodies of the country, led by the Organization for Earthquake Planning & Protection (OASP).

By next summer, a new marine seismic survey system is expected to be installed on the Oceanographic Vessel AEGEO, which will upgrade the existing capabilities and contribute to the collection of new data and a better understanding of submarine faults. This system will be acquired within the framework of the “Athens vs. England” program of the Institute of Oceanography of ELKETHE and is financed by the Hellenic Research & Innovation Foundation (ELIDEK). The supply and operation of underwater seismographs is another goal of ELKETHE that will significantly help in the monitoring and understanding of underwater earthquakes.

“The construction of the new, modern research vessel, which is financed 75% by the European Investment Bank and 25% by the Greek State, will also give a great boost to marine research in our country. Despite the delay of at least three years in securing national participation, we hope that the various relevant government and state actors will quickly be convinced of the usefulness and necessity of the new research vessel and its construction will soon begin,” says Mr. Sakellariou.

The undersea earthquakes of the past

Referring to the past, he recalls that “four of the largest and most destructive earthquakes that have occurred in the wider area of ​​the Aegean, the Ionian and the Hellenic Trench have a common feature: they were underwater and caused by submarine faults”.

– On July 21, 365 AD an undersea earthquake southwest of Crete with a magnitude of about 8 – 8.5 lifted the entire West Crete and caused the largest known tsunami in the Mediterranean.

– On August 8, 1303 AD an undersea earthquake south of Karpathos with a magnitude of about 8 created major devastation on all the nearby islands and caused an equally devastating tsunami in the Eastern Mediterranean.

– On August 9, 11 and 12, 1953, four earthquakes with magnitudes of 6.4, 6.8, 7.2 and 6.3 in the Ionian Sea almost completely leveled Kefalonia, Ithaca and Zakynthos.

– On July 9, 1956, an undersea earthquake with a magnitude of about 7.5 in the area between Amorgos, Santorini, Anafi and Astypalaia created great destruction in all the nearby islands and caused the largest tsunami of the 20th century in the Mediterranean.

– A few decades ago, on February 24 and 25 and March 4, 1981, there were three earthquakes with epicenters in the sea area of ​​Alkyonides and magnitudes 6.7, 6.4 and 6.3 respectively, which caused human losses and significant destruction not only in the neighboring areas but also in Athens.

– In the recent past, three undersea earthquakes with magnitudes greater than 6 in Lesbos (June 17, 2017, magnitude 6.3), Kos (July 21, 2017, magnitude 6.2) and Samos (October 30, 2020, magnitude 6.7) ) caused human losses, major disasters and, respectively, low-intensity tsunamis.

“All of the above examples of earthquakes, past and recent, point to one important conclusion: submarine earthquakes and submarine faults pose as great a threat as onshore earthquakes and onshore faults. Bearing in mind that underwater earthquakes have the potential to cause tsunamis, in some cases they prove to be more destructive than terrestrial earthquakes”, says Mr. Sakellariou.

RES-EMP

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