Why was the destruction to buildings in Turkey so catastrophic?

The images of collapsed, and collapsing buildings are truly distressing.

After all, structural failure is the main reason for the catastrophic loss of life in this earthquake.

So why has it been so bad in an area of known risk?

Above all else, this was a very severe earthquake. Possibly even more severe than even Turkey’s knowledgeable and experienced seismic risk scientists had calculated in worst-case scenarios.

Evidence of this comes from the seismic sensor network in Turkey which measures the amount of ground shaking during quakes – in this case around the East Anatolian Fault which caused the disaster.

Data is preliminary, and could be revised, but some of the highest measurements from some of those sensors exceed the limits for shaking that are assumed in Turkey’s earthquake building design codes.

These typically require buildings to cope with a severity of ground shaking expected to occur once every 475 years. And resist collapsing in a once-every-2,475-year event.

But some sensors recorded peak ground accelerations – a key measure of the earthquake’s force – of well in excess of 7m per second squared.

Turkey-Syria earthquake – latest updates

These values, say experts, appear to exceed the shaking predicted to occur in these areas, even in that rarest, one-in-2,475-year earthquake event.

“Even the very well-designed, very well-executed buildings would have suffered and would have been challenged,” says Prof Yasemin Didem Aktas, a structural engineer at University College London.

“But this doesn’t rule out the building stock we are seeing collapsing was free of defects and problems,” she adds.

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Evidence of that can be seen in cities like Gaziantep – 70km from the quake’s epicentre – that suffered much less severe shaking.

Many large buildings there failed catastrophically, either collapsing into rubble or “pancaking” with floors remaining intact but falling one on top of the other.

Risks to cities like Gaziantep were known too. In a publication last year, a team of Turkish hazard experts used existing risk maps to show many neighbourhoods with older, or poorly constructed buildings were a severe risk from an earthquake.

However their modelling exercise assumed a quake 10 times less severe than the one that struck in the early hours of Monday morning.

Another important factor is that there wasn’t just one major earthquake, but two. The first event was followed nine hours later by a magnitude 7.5 aftershock. Some buildings that survived the initial shock collapsed in the second.

Thousands more buildings survived, but are seriously damaged. And this raises a major challenge for authorities dealing with the grim aftermath.

Tens of thousands of survivors are already homeless, but many thousands more may not be able to safely return to homes that may still be standing, but broken beyond repair.

There will be a special programme called Disaster Zone: The Turkey-Syria Earthquake on Sky News on Friday evening at 9.30pm