Tens of thousands of people have been killed and scores more injured by a huge earthquake which struck south-eastern Turkey, near the Syrian border, in the early hours of Monday morning.

The earthquake, which hit near the town of Gaziantep, was closely followed by numerous aftershocks - including one quake which was almost as large as the first.

Why was it so deadly?

The first earthquake was big - it registered as 7.8, classified as "major" on the official magnitude scale. It broke along about 100km (62 miles) of fault line, causing serious damage to buildings near the fault.

Prof Joanna Faure Walker, head of the Institute for Risk and Disaster Reduction at University College London, said: "Of the deadliest earthquakes in any given year, only two in the last 10 years have been of equivalent magnitude, and four in the previous 10 years."

But it is not only the power of the tremor that causes devastation.

This incident occurred in the early hours of the morning, when people were inside and sleeping.

The sturdiness of the buildings is also a factor.

Dr Carmen Solana, reader in volcanology and risk communication at the University of Portsmouth, says: "The resistant infrastructure is unfortunately patchy in South Turkey and especially Syria, so saving lives now mostly relies on response. The next 24 hours are crucial to find survivors. After 48 hours the number of survivors decreases enormously."

This was a region where there had not been a major earthquake for more than 200 years or any warning signs, so the level of preparedness would be less than for a region which was more used to dealing with tremors.

Earthquake destruction in before and after picturesThe survivors' choice - danger inside or freezing outside

What caused the earthquake?

The Earth's crust is made up of separate bits, called plates, that nestle alongside each other.

These plates often try to move but are prevented by the friction of rubbing up against an adjoining one. But sometimes the pressure builds until one plate suddenly jerks across, causing the surface to move.

In this case it was the Arabian plate moving northwards and grinding against the Anatolian plate.

Friction from the plates has been responsible for very damaging earthquakes in the past.

On 13 August 1822 it caused an earthquake registering 7.4 in magnitude, significantly less than the 7.8 magnitude recorded on Monday.

Even so, the 19th Century earthquake resulted in immense damage to towns in the area, with 7,000 deaths recorded in the city of Aleppo alone. Damaging aftershocks continued for nearly a year.

There have already been several aftershocks following the current earthquake and scientists are expecting it to follow the same trend as the previous big one in the region.