Monday, November 15, 1999 Published at 13:29 GMT
Istanbul quake more likely but unpredictable
The stress in the rocks is being "pumped" westwards
The latest devastating earthquake to rock Turkey has significantly increased the risk of a large seismic strike near Istanbul, says an expert on the region's geology.
But Dr Russ Evans of the British Geological Survey warned that it is not possible to say when any quake might occur. It could hit any time in the next few months or years, or even not at all.
In the short term, Dr Evans said, the daily chances of a major earth movement have increased from about one in 30,000 to as low as one in 2,000. The reason is that the fragmented North Anatolian Fault passes on the rock stress from one segment to the next, and the Sea of Marmara section appears next in line.
Dr Evans has spent long periods in Turkey working with Turkish and international seismologists, including several weeks after the August quake.
His comments follow those by Professor Ahmet Mete Isikara, director of the Kandilli Observatory in Istanbul. His predictions of future earthquakes in the "seismic void" between the towns of Sapanca and Akyazi led to people in the area setting up tents in expectation.
But Dr Evans said: "Excepting those houses built on very poor land and those already damaged in the earthquakes, there is not a strong reason for people to change their lifestyles immediately.
"The actual seismic risk has not changed that much but people's perception of it has. They will need to think about strengthening their buildings and I know from the contact I have had that the Turkish government is pursuing this with all due haste. But it can't be done overnight."
The seismic threat to Istanbul comes from possible movement along the section of the North Anatolian Fault (NAF), under the Sea of Marmara and to the west of Izmit. This area has been identified as a danger zone by both Dr Evans and Professor Isikara.
The NAF is not one long single break in the rock but is made up of lots of sections. When an earthquake occurs, the stress which led to it is passed along to the next section, a process called "stress pumping".
"This seems to happen all to often on the NAF," said Dr Evans. "It is the classic example of this kind stress transfer. It moves the stress into a new area. There is then a fight between the rock healing and weakening processes. Eventually the rock weakening processes win and the next section goes."
Examination after the August earthquake showed that there had been no displacement on the NAF section under the Sea of Marmara. The fear is that this section is now bearing the additional stress passed on by the other earthquakes to the west and is therefore more likely to rupture.
The one piece of better news is that Istanbul is expected to withstand any earthquake far better than the areas hit recently.
"Undoubtedly some buildings would be damaged but much of Istanbul is built on better, firmer ground," said Dr Evans. "I wouldn't expect to see the kinds of problems we saw before where whole towns were flattened.
"But not all of Istanbul is safe as some parts of the city suffered ground collapses during the recent earthquakes even though they were relatively far away - there are areas of poor ground which have been built upon."
Slow motion jellies
Seismic waves are amplified when they travel through softer ground, made of sands and clays. Whole areas can shake like slow-motion jellies and this produces a much greater damaging effect than quakes which hit houses built on hard rock.
In the previous Turkish earthquakes, Dr Evans said: "The amplification of ground shaking accounted for the large number of collapses and significant loss of life."
For 30 years up until August, north-western Turkey had suffered far fewer earthquakes than expected, compared to its seismological track record. This lull reduced the awareness of the seismic hazards just at time when huge numbers of buildings were being erected.
The two recent tragic reminders of the power of the restless Earth have now jolted Turkey out of that lull and made identifying and strengthening dangerous buildings more urgent than ever.