What are fault lines, and how do they cause earthquakes?

A 7.8 magnitude earthquake struck Turkey and neighbouring countries early Monday, February 6. This caused the death of thousands of people, injured tens of thousands and the collapse of a large number of buildings in Turkey and Syria.

Situated between two major fault lines and compressed by three major tectonic plates, Turkey is one of the most seismologically complex and active areas in the world.

Much of the country rests on the relatively small Anatolian plate, which is bounded to the north by the Eurasian plate and to the south and east by the African and Arabian plates, which compress Turkey's territory, producing devastating earthquakes.

In 2022 alone, the Turkish Disaster and Emergency Management Authority recorded more than 22,000 earthquakes in the country.

What are fault lines?

A fault is a fracture or fracture zone between two blocks of rock. Faults allow blocks to move relative to each other. This movement can happen quickly, in the form of an earthquake, or it can happen slowly, in the form of landslides.

Faults can vary in length from a few millimetres to thousands of kilometres. Most faults produce repeated displacements through geologic time.

During an earthquake, the rock on one side of the fault suddenly slides relative to the other. The failure surface can be horizontal or vertical or some arbitrary angle in between.

The hit region was not prepared

Turkey is one of the most active seismic areas in the world, it is located on the Anatolian plate, between two major fault lines.

You have to understand that the surface of the Earth is divided into several parts that fit together like a cosmic puzzle. Those pieces are always rubbing against each other, moving the ground very slowly, so we don't notice it.

But sometimes the tension increases and one or both plates slide, releasing an incredible amount of energy, causing what we call an earthquake.

Most modern cities, particularly those on major fault lines, have strict regulations on buildings, this ensures that buildings do not easily collapse during such an event. In the Turkish city of Istanbul, most high-rise buildings are designed to withstand earthquakes.

And that is not the case in Gaziantep, which is located near the epicentre. Although it is a major city and a provisional capital, Gaziantep is not as modern as Istanbul and many of its high-rise buildings were not built to the same standard, which ultimately caused several of them to collapse.

In a video posted on social networks, a building can be seen collapsing and the upper floors crushing the floors below.

US Geological Survey structural engineer Kishor Jaiswal told the Associated Press that such "pancake" collapses are a sign that the building could not withstand the strong movements produced by earthquakes.

But what made this earthquake one of the deadliest in history?

The power

An earthquake of magnitude 7.8 is very powerful, especially for one with an epicentre on land. The most powerful tectonic movements occur under the ocean, far from human civilization.

Such ocean events can be deadly by causing a tsunami, as evidenced by the 2011 tsunami that hit Japan following a magnitude 9.0 earthquake that shook the ocean floor nearly 45 miles offshore, but on the earth can be just as devastating.

By way of comparison, the most devastating earthquake in US history, by the amount of damage caused, was Northridge in 1994.

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• Series of earthquakes in Turkey, What are the reasons?
• Earthquakes cannot be predicted, says U.S Geological Survey
• Top 10 most deadliest earthquake ever

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