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A week has passed since the strongest earthquake in southeastern Turkey and northern Syria over the last 85 years. The cause of the earthquake was a tectonic shift on the East Anatolian Fault, at the contact of the Arabian and Anatolian lithospheric plates. Extremely destructive tremors hit these countries on Monday, February 6, 2023, at 01:17:36 UTC. It was 04:17 local time in Turkey and 03:17 in Syria. Scientists from the Earth Science Institute of the SAS summarized knowledge about the earthquake from a geological-geophysical point of view.

The East Anatolian Fault is a 700 km long fault line of a transform (lateral displacement) nature with a left-lateral movement. It separates the Anatolian and Arabian plates (Pic. 1). The permanent speed of movement on the fault is around 10 mm per year in the eastern part and around 1.4 mm per year in the western part. It is several orders of magnitude more in the case of a jump motion during an earthquake.

Between 2007 and 2020, more than 26,000 earthquakes were registered in the fault zone, of which 160 have a determined sense of movement on the fault (the so-called focal mechanism). Seismological and tectonic characteristics show that the fault is tilted to the north and demonstrates a very well-defined boundary between the two microplates along which left-lateral movement occurs. The deformation is not tied to only one specific line, but the entire system of subparallel faults. We should not think about the overall movement on the fault as only sideways displacement. There are segments within the fault where shrinkage (compression) and stretching (extension) occur.   

In terms of timing, the East Anatolian Fault formed geologically from the middle Miocene (about 12 million years) to the early Pliocene (three million years), which is documented by geological-tectonic measurements of the deformation history preserved in sedimentary and volcanic rocks. Left-lateral movement on the fault has been demonstrated since three million years ago.

So far, relevant historical data confirmed devastating earthquakes with M6.0-7.2 in: 1789 – M7.2; 1795 – M7.0; 1866 – M7.2; 1872 – M7.2; 1874 – M 7.1; 1875 - M6.7; 1893 – M7.1; 1971 – M6.7; 2010 – M 6.0; 2020 - M6.8 (Pic. 2). These data document that the current earthquake - M7.8 mainshock and M7.5 aftershock - is also the strongest in history, although similar events in the undocumented past are not excluded. Both mainshocks were also recorded on the territory of Slovakia by the National Network of Seismic Stations (Pic. 3 and 4). One of the causes of the current extremely destructive earthquake may be the cumulative build-up of stress and energy in the fault zone, which has not been seen in a strong earthquake with M above 7.0 for 130 years. If we look at other parts of Turkey - for example, on the North Anatolian Fault, the 1999 Izmit earthquakes in the west of the country were M7.4 and M7.1, and the 2011 Lake Van earthquake in eastern Turkey was M7.1. However, the Van earthquake was more than 400 km away from the East Anatolian Fault and is not expected to have an impact on this fault.

As we can see, strong earthquakes are not uncommon in this region. Turkish and other scientists from all over the world were unpleasantly surprised by these strong earthquakes from the point of view that, based on real data, a strong earthquake was expected south of Istanbul. Seismic activity on the North Anatolian Fault migrated towards Istanbul after the devastating 1939 earthquake and stopped near Izmit (roughly 50 km east of Istanbul) in 1999. This 1999 earthquake caused significant damage in Istanbul. Based on statistical calculations, scientists around the world expect a strong earthquake in the Sea of ​​Marmara region within the next 30 years with a probability of 62 ±15 percent and within the next decade with a probability of 32 ±12 percent.

Adverse factors

The first unfavourable factor from the point of view of the consequences of the earthquake for the inhabitants was the night or early morning when almost everyone was still asleep.  Eleven minutes after the M7.8 mainshock came the first M6.7 aftershock. It was as strong as the last earthquake in the nearby area three years ago, on 1. 2020, with M6.8. It is likely that only after these two shocks those who survived and were not crushed by debris, could get to the open space. However, strong aftershocks continued, on the day of the earthquake on February 6. The ground shook perceptibly almost constantly (statistics according to EMSC data - European Mediterranean Seismological Centre - are shown in the Table). What was not destroyed by the mainshock and the first strong aftershock was destroyed by the M7.5 aftershock nine hours later, on February 6, 2023 at10:24:49 UTC (13:24 local Turkish time), when the first rescue efforts were already underway, and there are also videos from this aftershock of falling buildings damaged in the main event.   

The second unfavourable factor is the high population density. According to UN data, more than 23 million people live in the epicentral area within a radius of 500 km. In the most affected area of ​​southeastern Turkey and northern Syria are the Turkish cities of Gaziantep (1.1 million people), Şanliurfa (450 thousand), Malatya (440 thousand ), Antakya (Hatay) (400 thousand), Kahramanmaraş (380 thousand), Adiyaman (220 thousand), Osmaniye (200 thousand), Iskerderun (160 thousand), Elbistan (80 thousand), Kilis (80 -thousand), the Syrian city of Aleppo (1.6 million), Idlib (165 thousand), Manbij (100 thousand), Afrin (36 thousand)... In addition, millions more refugees from the conflicts in Syria live in the border area. According to the level of destruction of entire residential districts, there is a real threat that the total number of victims may exceed 50 to 70 thousand.

The third factor is the traditional benevolence of the authorities in the construction of buildings in this area of ​​the world, even though in terms of seismic hazard (probability of an earthquake) and seismic risk (probability of damage), the areas around the North Anatolian and East Anatolian Fault are among the most seismically active in the world! The low quality and seismic resistance of the building infrastructure are likely behind the high number of victims.   

The fourth unfavourable factor is the winter season, when the temperature drops below minus 3 to minus 4 degrees of Celsius in the affected area at night and even below minus 6 degrees of Celsius in inhabited mountainous areas, which significantly reduces the chance of survival under the rubble.

Table: Statistics of earthquakes in SE Turkey and N Syria according to EMSC data. On February 13, 2023, 1 M4.6 aftershock occurred until the text deadline.

For comparison, an earthquake with a magnitude of more than 4 has not occurred since 1964 in Slovakia, the last earthquake of M5.0 or more was in 1930 near Dobrá Voda in the area of Malé Karpaty, and the only earthquake probably with M around 6.0 was in Komárno in 1763. To reassure our inhabitants, we can state that there is no danger of a similarly strong earthquake as now, but also in the past in Turkey. In our territory, there is no such extensive seismoactive fault as the East Anatolian or North Anatolian Fault, and the geological-tectonic development of the Western Carpathians had similar dynamics to the current one in Turkey approximately 10 or more million years ago. 

We stated in the report that the East Anatolian Fault was formed approximately 12 million years ago. Seismic activity in the area of ​​the Anatolian plate lasts at least that long. It won't change even in another million years. From a global perspective, in the Mediterranean region (Pic. 1), a huge block of Africa will continue to slide under Eurasia. Between the main plates, tectonic activity is concentrated in several microplates: Arabian, Anatolian, Aegean. The Arabian plate is moving north at a rate of 15-20 mm per year.  In the straight direction, it affects the deformation of the Zagros Mountains. In the lateral direction, it pushes the Anatolian plate to the west. In further development, in millions of years, the Anatolian plate, along with the entire Greco-Aegean region, will disappear in the Hellenic Trench south of Crete. At the same time, along the Dead Sea Fault, Arabia will rotate away from Africa. In its place is already the Red Sea, which may become a new ocean in the distant future. Although natural events in the form of earthquakes and volcanic eruptions are often tragic, we cannot stop the geological development of the Earth and must adapt to it.

Text: Ján Madarás, Kristián Csicsay and Andrej Cipciar, Earth Science Institute of the SAS

Data from seismic stations in Kolačno (KOLL) and Smolenice (SMOL) are published with the consent and based on the cooperation with the Centre for Science and Research, s.r.o.

Sources

Güvercin, S. E., Karabulut, H., Konca, A. Ö., Dogan, U. and Ergintav, S. 2022: Active Seismotectonics of the East Anatolian Fault. Geophysical Journal International. (2022) 230, 50–69. doi.org/10.1093/gji/ggac045

Kalafat, D., Zulfikar, A. C., Akcan S. O. 2021: Academic Platform Journal of Natural Hazards and Disaster Management, 2(2), 96-111, 2021 doi: 10.52114/apjhad.1039670

https://www.usgs.gov/international-programs/seismicity-study-turkey

https://en.wikipedia.org/wiki/2023_Turkey%E2%80%93Syria_earthquake#/media/File:Anatolian_Plate_Vectoral.svg