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Turkey: Continental Collision Zone To Blame For Deadly Earthquakes

The earthquake that struck western Turkey overnight, killing hundreds of people, has proven yet again how singularly prone the region is to seismic activity. As a seismologist tells our correspondent Charles Recknagel, the reasons for the earthquakes' deadly force lie both in the geology of the region and its poor housing.

Prague, 17 August 1999 (RFE/RL) -- The region from Turkey west to Iran and including the Caucasus is so prone to earthquakes that it is regularly home to some of the world's most deadly natural disasters.

In the summer of 1990, an earthquake measuring 7.7 on the Richter scale devastated the Caspian regions of Gilan and Zanjan in Iran, killing 35,000 people. Two years earlier, in 1988, an earthquake of 6.9 magnitude killed more than 25,000 people in northwest Armenia. And in 1939, another 33,000 people died in eastern Turkey in an earthquake with a strength of 7.9.

Scientists who study the causes of earthquakes say that there are so many deadly tremblers in the region because it is located at a point where several of the Earth's great landmasses i.e. tectonic plates meet.

Roger Musson is a seismologist at the British Geological Survey. He explained the cause of the earthquakes to RFE/RL by telephone from Edinburgh, Scotland:

"This is what we call a continental collision zone. The earth's crust may seem to be a stable place, but in fact the continents are moving around at a rate which is roughly equivalent to the rate at which your fingernails grow. And the main block that accommodates Europe and Asia is colliding with Africa, and where these two plates are meeting then everything is broken up into a series of small pieces, which are moving about with respect to one another."

Musson said the earthquakes are the result of pressure that builds up along the faultlines where the constantly shifting landmasses squeeze against one another. When the pressure becomes too great, one of the pieces must give way in a sudden, sharp movement.

It is that tremor which collapses manmade structures in the area, often killing those inside or nearby. Afterward, the realigned pieces of the earth's crust subside for a time, until new patterns of stress force them to readjust their positions elsewhere along their borders.

The seismologist says the reason Turkey is particularly prone to earthquakes is that the bulk of the country is being squeezed in a westerly direction between Africa and Europe. He says that most of the motion takes place along two major faultlines. One, along which last night's earthquake occurred, runs east-to-west along the north of Turkey. The second major fault line runs northeast-to-southwest through the eastern part of Turkey.

But beyond knowing where the major faultlines in Turkey run, seismologists have made little progress in being able to predict where along them earthquakes will occur, or when.

Musson says seismologists try to study how conditions change locally in an area prior to an earthquake in an effort to gather enough data to be able to recognize the same signs elsewhere later. But in Turkey, such data gathering has not progressed well. An international effort to monitor Turkish earthquakes was carried out in the 1970s and 1980s in northwest Turkey, but those decades saw too few earthquakes to produce enough data to help the prediction efforts.

Musson says the other major earthquake-prone areas of the region -- the Caucasus and Iran -- are also continental collision zones. But the pressures upon them are due less to the collision of the Eurasian and African plates than to interplays between the Eurasian, Arabian and Indian landmasses.

"Instead of [Turkey's situation] of the Anatolian fault being sandwiched between Europe and Africa, the Eurasian [plate] and Arabia are also colliding and the Caucasus is a complex tectonic zone made up of faults that are largely active because of this collision process. As the Arabian plate is being pushed northwards, then on the one hand it impacts directly upon the Caucasus, but also there is pressure to the northeast, which is pushing Iran from that direction. And at the same time, India is also moving north and that is producing earthquakes along the line of the Himalayan mountains and that is also contributing to the stress in Iran."

The seismologist says that despite the tremendous movements of the earth's crust in the area of Turkey, Iran and the Caucasus, the region is far from being the planet's most active site for earthquakes. That distinction belongs to the south Pacific Ocean, where powerful earthquakes regularly occur but do not make headlines because they are mostly along the ocean floor. Other highly active zones are South America, China and Japan.

But Musson says that Turkey, Iran and the Caucasus routinely suffer some of the world's highest casualties from earthquakes because they lie in an area where the most common building material is stone rather than wood. He says that whereas wooden structures bend but may not collapse, the poorly built high-rises which are common throughout the region only contribute to the earthquakes' death toll.

"The problem with the region from Turkey to Iran is that it is a densely populated area and although wooden houses are good at withstanding earthquakes, there isn't very much timber available for building. So people have to build houses out of the local stone. And stone houses, particularly ones that aren't very well built, are the worst sort of houses to be in an earthquake because they don't have a lot of strength and if they collapse they are heavy and they kill anybody that is inside."

As rescuers now clear the rubble left by Turkey's latest major earthquake, officials are only beginning to discover how many lives it has taken. The actual toll from the tremors, which measured 6.7 on the Richter scale, is feared to be much higher than the some 300 confirmed dead so far.