2023

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 20^th 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.

Die schweren Beben waren von Israel bis Kroatien spürbar und haben in einer Region von etwa 400 auf 300 km zu schweren Schäden geführt. Bis Dienstagmittag waren bereits über 5’000 Todesopfer und 25‘000 Verletzte bekannt, davon etwa zwei Drittel in der Türkei und ein Drittel in Syrien. Diese Zahlen werden voraussichtlich noch weiter ansteigen. 

Das Magnitude 7.8 Erdbeben hat in einer seismisch aktiven Region stattgefunden, in der sich die Anatolische, Arabische und Afrikanische Platte treffen. Nach bisherigen Erkenntnissen lag das Beben auf der vom Golf von Iskenderun in Richtung Nordosten verlaufenden Ostanatolischen Störung. Ein Beben dieser Grösse versetzt die Erdplatten entlang einer Verwerfung von 180 bis 200 km Länge um mehrere Meter. Der gesamte Bruchvorgang dauerte 30 bis 40 Sekunden, was eine Bruch-Fortpflanzungsgeschwindigkeit weit über Schallgeschwindigkeit ergibt. Die Bruchfläche reichte dabei von einer Tiefe von rund 20 km bis an die Erdoberfläche. Das Beben von letzter Nacht ist eines der stärksten bekannten Beben in der Region. Die bisher grössten historischen Beben mit Magnituden von ungefähr 7 haben in den Jahren 1138 und 1822 die Stadt Aleppo in Syrien komplett verwüstet. Das Beben von 1822 forderte einige zehntausend Todesopfer. Seit 1970 blieb es in der Region verhältnismässig ruhig mit nur drei Beben mit Magnituden von 6 oder mehr. Das grösste davon erschütterte die Region am 24. Januar 2020 mit einer Magnitude von 6.7.

Die ersten Wellen (P-Wellen) der Beben in der Südosttürkei trafen mit einer Verzögerung von gut 5 Minuten in der Schweiz ein, die stärkeren, aber langsameren Oberflächenwellen legten die 2’700 km in etwa einer Viertelstunde zurück. Beim Hauptbeben von Montagnacht wurde die Erdoberfläche in der Schweiz um bis zu einem Zentimeter vertikal ausgelenkt – vergleichbar mit den Bodenbewegungen, die hierzulande nach dem Magnitude 9.2 Beben von Tohoku (Japan) im Jahr 2011 registriert wurden. Allerdings treffen in so grossem Abstand vom Erdbebenherd nur noch langperiodische Wellen ein: Das heisst, die Bodenoberfläche bewegt sich innerhalb von etwa zwei Minuten erst  einen Zentimeter nach oben und dann wieder nach unten. Das ist mit geeigneten Seismometern deutlich messbar und kann zur Lokalisierung des Erdbebens wie auch zur Magnitudenbestimmung verwendet werden. Für uns Menschen sind diese Wellen aber weder spürbar noch haben sie irgendwelche Schadensfolgen.