The magnitude-7.8 and magnitude-7.5 earthquakes in Türkiye on 6 February 2023 are devastating events with a massive impact on the lives of millions of people in Türkiye and Syria. The hypocentre of the Gaziantep earthquake (magnitude 7.8) was 18 km below the earth's surface and around 9 km east of Sakçagözü. The hypocentre of the second quake (Kahramanmaraş earthquake, magnitude 7.5) was at a depth of 10 km and about 100 km north of the first major quake, on a second fault line. The metropolis of Gaziantep in Türkiye is very close to the epicentre of the earthquake bearing its name, while Aleppo in Syria is only 100 km away. Another ten major cities are located directly on the fault zone.
Read more...The earthquakes occurred due to the movement of tectonic plates. The region where the earthquakes took place is known as a triple junction, where three tectonic plates meet: the Anatolian, the Arabian and the African. According to current information, the Gaziantep earthquake is believed to have been caused by the movement of the southern part of the East Anatolian Fault Zone (see Figure 1). This fault zone runs through southeastern Türkiye and is responsible for many of the earthquakes that happen in the region. Over time, tremendous underground stresses built up in the fault zone. When these were suddenly released, it triggered the ground motions that caused devastating damage in the region. However, the effects were not only felt locally: as far away as Cyprus, 400 km from the epicentre of the Gaziantep earthquake, a dynamic displacement of approximately 20 cm was measured by GPS (Global Positioning System).
According to preliminary calculations by the United States Geological Survey (USGS), the Gaziantep earthquake caused a large section of the fault to shift by about 3.4 m over a vertical area of around 220 x 30 km (roughly the size of the canton of Grisons). The second earthquake, with a magnitude of 7.5, had a fracture surface of 40 x 20 km (a similar size to the canton of Jura) and a slip of approximately 10 m. As more data becomes available in the coming days, this information on the size and movement of the fracture surfaces will be further refined and is therefore liable to change significantly.
The earthquakes brought down many buildings. In the city of Kahramanmaraş, for example, an estimated 16,000 buildings have been damaged (Report 6.2.23, Kandilli Observatory and Earthquake Research Institute). This was due not only to the strength of the shaking but also to inadequate construction. Although Türkiye revised its building standards after the 1999 İzmit earthquake, there are still many older buildings that were constructed prior to this, meaning that their seismic safety is unknown and in many cases inadequate. A similar situation exists in many countries, including Switzerland, since renewal of the building stock is usually a slow process and upgrading existing buildings can be very expensive. Moreover, in Turkish cities, the ground floor is often designed as an open shop area with few load-bearing walls. Such walls are vital to a building's stability. Without them, a strong ground motion can cause the ground floor to collapse, bringing down the entire building.
The earthquakes also caused widespread power outages and communications disruptions, making it difficult for rescue and clearance teams to reach affected areas and coordinate relief efforts. According to the USGS, large areas are also expected to be affected by landslides and soil liquefaction caused by the shaking. Soil liquefaction weakens the subsurface, resulting – like landslides – in additional damage to buildings, roads and infrastructure.
The region has been hit by several strong earthquakes in the past. The last earthquake of comparable strength was in 1939 at Erzincan on the North Anatolian Fault and had a magnitude of 7.8. It was one of the most powerful quakes to hit Turkey in the 20th century, causing major damage in a number of towns and villages. Immediately prior to the two recent earthquakes in southern Turkey, the annual probability of an earthquake with a magnitude greater than 7.7 in this region was 0.13%, compared with 0.25% for an earthquake with a magnitude greater than 7.4 (Danciu et al., 2021). This is rather high by European standards, as can be seen in Figure 2, where the affected region is located in the purple-coloured area. The occurrence of strong earthquakes like those on 6 February is therefore not unexpected. Regions with a lower hazard level are coloured yellow in Figure 2.
A large earthquake is typically followed by further quakes. It is therefore possible that more earthquakes of a similar or even greater magnitude will occur. Most aftershocks happen within minutes or hours of the mainshock. Further aftershocks can occur for weeks, months or even years afterwards, but their number tends to decrease over time. This pattern is also clearly visible in the current sequence, as shown by the grey bars in Figure 3. The number of earthquakes expected per day is already around 95-97% lower after one week than on the first day. Nevertheless, in rare cases, further major earthquakes can take place.