Istanbul quake likely by 2030

Istanbul suffered in 1999

By BBC Science's Ania Lichtarowicz

The Turkish city of Istanbul has a 60% chance of suffering a large earthquake within the next 30 years, scientists believe.

An international team of researchers has calculated the probability of a seismic disaster hitting the region by using new methods of earthquake forecasting.

There's no need to panic but we hope our work can be used effectively to improve safety

Tom Parsons

They believe that major faults near the city have been stressed by previous seismic shifts and are probably near the end of their dormant periods.

On 17 August, 1999, an earthquake measuring 7.4 on the Richter Scale shook the city of Izmit in Turkey. Just three months later, a similar quake hit Duzce.

Over 18,000 people were killed and more than 15,000 buildings collapsed, causing up to $25bn of damage.

Seismic dynasty

However, these events were only the latest in a long history of large earthquakes. Istanbul, which lies northwest of the North Anatolian fault, has been heavily hit at least a dozen times in the last 1500 years.

Tom Parsons from the United States Geological Survey led the team behind the work and he says last year's events in Turkey will have an effect on seismic activity in the area.

Many buildings collapsed in Sakarya

"It looks like that earthquake has increased stress on some of the faults near Istanbul, which would then increase the likelihood of an earthquake in the near term," he told the BBC.

Traditional methods of predicting the possibility of earthquakes have underestimated events in the region, as they do not take into account the build-up of stress.

Whenever an earthquake occurs at one point on a fault, it may create higher stress levels further down the fault, and possibly trigger an earthquake.

Dr Parsons explained: "We've looked at the occurrence of large earthquakes along the North Anatolian Fault system and since 1939 there's been a mostly westwards progressing sequence of large earthquakes - it looks like these earthquakes have triggered the next.

"Each time there's been a large earthquake along the fault zone, in many cases there's been another earthquake to the west."

Stress build-up

Using historical earthquake records over the last 2,000 years, the scientists worked out the approximate epicentre, magnitude and rupture lengths of each quake. This data allowed them to take into account stress build up in the area.

According to their calculations, over the next 30 years, Istanbul faces a 62% probability of strong shaking, similar in magnitude to that experienced last year in Turkey.

The risk of a major earthquake in the next 25 years is 50%, and over the next 10 years, 32%, give or take an uncertainty of 12%. They hope their data can be used to better prepare for any future earthquakes.

Dr Parsons said: "In California, the San Andreas fault zone runs through San Francisco and Los Angeles, major urban centres, and we have comparable probabilities here. For example, in the Bay area we have a 70% probability of a large earthquake occurring.

"And the response here is to try a retrofit programme where roads and buildings are being upgraded. In addition, individuals can do a lot to improve the safety of their environment, for example bolting large pieces of furniture to walls and having a response plan for what to do when an earthquake strikes."

Despite these high percentages, the scientists stress that people should not worry about an immediate quake. Their work only shows the probability of an earthquake happening and does not predict when one will definitely occur.

"There's no need to panic because of this," said Dr Parsons. "It's just an estimate of high probability and it shouldn't come as a complete surprise. We hope our work can be used effectively to improve safety."