Geologists warn in Friday's issues of Science that recent quakes have left Istanbul, Turkey's largest city, increasingly vulnerable to a powerful earthquake.

Quakes in August and November of last year of magnitude 7.4 and 7.1, respectively, increased stress in the ground faults below Istanbul, according to Tom Parsons, the team's leader. He estimated a 70 percent chance of a major quake within 30 years — a prediction similar to that of a major quake hitting California.

"We don't want to cause panic with this," Parsons told The Associated Press. "In all likelihood we have time to re-engineer many buildings to better protect the people who live and work there. We hope that our research will prove useful to Turkish leaders and scientists as they evaluate various options."

The scientists did not predict the intensity or the timing of such a quake, but they did offer recommendations for possible preparatory measures. Last year's quakes killed 18,000 people and caused up to $25 billion in damage.