The U.S. Geological Survey put the quake's magnitude at 5.0, while the Istanbul-based Kandilli Observatory said it measured 5.2. The earthquake was centered in the Gemlik Bay near the city of Bursa, Kandilli said. It struck at 5 p.m. local time (1300 GMT).

A magnitude 5 quake can cause considerable damage.

The U.S. Geological Survey said the quake's epicenter was about 65 kilometers (40 miles) south of Istanbul and 30 kilometers (20 miles) north of Bursa.

Two people were lightly injured in Gemlik when they jumped from their houses in panic during the quake, the Dogan news agency said. At least three other people fainted, it said.

On Friday, an earthquake of the same magnitude shook the Turkish city of Balikesir and was also felt in Istanbul, about 400 kilometers (250 miles) to the north.

Earthquakes are frequent in Turkey, which lies on active fault lines. Two devastating earthquakes hit northwestern Turkey in 1999, killing some 18,000 people.

Geologists have been warning that Istanbul, a city with an estimated population of more than 11 million people, would be woefully unprepared for a large earthquake, an event which many feel is probable in the near future.

"This is a dangerous region. We have to take these little quakes as a warning for the big one," said Naci Gorur, a professor in the geology department at Istanbul Technical University. "When the big quake comes, ten thousand, 100,000 lives will be in danger."

Other geologists warned of aftershocks following Tuesday's quake. The Associated Press

/The International Herald Tribune/