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Thursday, August 19, 1999 Published at 11:39 GMT 12:39 UK


The reasons for Turkey's seismic suffering

By BBC News Online's Dr Damian Carrington

The misery for the earthquake-struck people of Izmit, Turkey is unlikely to end for months, as heavy aftershocks will compound the damage and hamper rescue work.

The BBC's Margaret Gilmore: "There were ten powerful aftershocks"
The warning comes from Dr Roger Musson, Head of Seismic Hazards at the British Geological Survey in Edinburgh, UK who is analysing the magnitude 6.8 to 7.0 tremor.

"The largest aftershock so far was 5.5 but they are coming all the time," he told BBC News Online. "This fault is very much prone to large aftershocks, because it doesn't make a clean break when it moves."

This means further movements, leading to further tremors, are needed to release the stress built up in the fault.

Shallow and deadly

Turkey Earthquake
The damage from the initial tremor was particularly intense because of the unusually shallow depth of the earthquake's focus, just 10km below the surface. This means the shock waves are felt much more strongly at the surface than for a deeper earthquake.

Initial estimates of the magnitude of the earthquake varied from 6.4 to 7.8 on the Richter scale but the latest calculations, which include damage reports, have narrowed the range to 6.8 to 7.0.

[ image: Falling buildings kill people, not earthquakes themselves]
Falling buildings kill people, not earthquakes themselves
The devastating quake is only the latest in a long line of severe shocks to strike the region.

The geological fault involved is called the North Anatolian Fault. Disaster last struck in July 1967 just 50km west of Izmit, with a magnitude 7.1 tremor. And since 1939 there have now been 11 quakes over magnitude 6.7 along the 1000km length of the North Anatolian Fault.

Prediction impossible

Russ Evans of the British Geological Survey, who spent eight years working in the Izmit area
Seismologists believe there were no warning signs before the earthquake, but the risk of a disastrous rupture in the Izmit are has been known for many years. A 1992 study estimated a 12% chance of such a quake before 2020.

However, seismologists can only assess probabilities, not make specific predictions. This means safe (and expensive) building practices are essential in earthquake-prone areas if many lives are not to be lost.

Turkey's susceptibility to seismic catastrophes is due to the country's geological setting. It sits between two huge tectonic plates, Eurasia and Africa/Arabia, which are inexorably grinding into one another, north to south.

Squeezed like a pip

The Turkish landmass is a small tectonic plate which is being squeezed like a pip between the two giants. It is moving westwards across the surface of the Earth, towards the Aegean.

The periodic movements happen on two main faults, the North Anatolian Fault and the East Anatolian Fault. These are both "strike/slip" faults in which tectonic plates slide past each other horizontally. The San Andreas fault in California is a well-known example.

The last movement on the East Anatolian Fault was in June 1998 when 6.3 magnitude quake killed 144 people and injured over 1,500 in and around the city of Adana.

Breaking point

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, ultimately caused by the swirling, hot rock in the Earth's mantle, cause stress to build up in the rocks affected.

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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