Page last updated at 14:54 GMT, Monday, 8 March 2010

Why Turkey suffers earthquake misery

Turkish rescue teams in Okcular village, March 8 2010
Some of Turkey's vulnerable areas are densely populated

The earthquake that struck near the eastern Turkish village of Basyurt is the latest in a series of deadly tremors to hit the country.

Now, as before, high numbers of casualties have been blamed on traditional or sub-standard buildings.

But Turkey is fundamentally vulnerable to earthquakes because of its geographical location.

The country sits between two huge tectonic plates, Eurasia and Africa/Arabia, which are inexorably grinding into one another, north to south.

The Anatolian plate, on which most of the Turkish landmass lies, is being squeezed westwards towards the Aegean Sea.

Periodic movements happen along two main faults, the North Anatolian fault and the East Anatolian fault.

The earthquake in Basyurt looks likely to have been triggered by movement on the eastern fault, said Brian Baptie, a seismologist at the British Geological Survey.

'Domino' effect

But it is along the 1,000km (621-mile) North Anatolian fault that there has been a "gradual migration" of big earthquakes since 1939, he said.

The fault is not one long single break in the rock, but is made up of lots of sections.

Map of Turkey
1. Basyurt, March 2010, magnitude 6.0, at least 57 dead
2. Bingol, May 2003, magnitude 6.4, more than 160 dead
3. Duzce, November 1999, magnitude 7.2, about 400 dead
4. Izmit, August 1999, magnitude 7.4, more than 17,000 dead

When an earthquake occurs, the stress is passed along to the next section, and tremors have been migrating west.

The migration is "a result of different segments of the fault rupturing at different times - it's a bit like incrementally tearing a piece of paper, or you hit one domino and the other one falls down", said Mr Baptie.

On the basis of this pattern, Istanbul is thought to be especially at risk - even though seismologists believe there are no warning signs that allow the timing and size of an earthquake to be predicted with any accuracy.

This means safe building practices are essential in earthquake-prone areas.

Many of the losses in earthquake regions arise from non-compliance with earthquake building codes.

Severe damage can be caused by relatively small earthquakes.

"Many densely populated parts of Turkey are at risk from earthquakes," said Mr Baptie. "The risk is often increased by poor construction.

"Istanbul is a city that's at risk from a significant earthquake at some point in the future," he said.

"But we've got no way of saying where, when and exactly how big an earthquake might be."

Reconstruction plan

Earthquake safety was forced firmly on to Turkey's political agenda following the Izmit tremors in 1999, which killed more than 17,000 people.

But experts and politicians have warned that more needs to be done.

Turkish Prime Minister Recep Tayyip Erdogan blamed casualties in the quake that struck near Basyurt on traditional mud-brick buildings.

He said he had instructed Turkey's housing agency to start a reconstruction project building earthquake-proof homes in the area.

Both the North Anatolian and East Anatolian faults are "strike/slip" faults, in which tectonic plates slide past each other horizontally.

The Enriquillo-Plaintain Garden fault that caused the recent earthquake in Haiti falls into this category, as does the San Andreas fault in California.

The westward push of Turkey is being accommodated in the Aegean, which is shrinking as one part of the sea-floor is being thrust under another.

These plate movements are ultimately caused by the slow-moving hot rock in the Earth's mantle.

When the rock's breaking point is eventually reached, it fractures on a fault and the two sides grind past each other, causing an earthquake.

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