At least six aftershocks, the strongest with a preliminary magnitude of 4.6 shook Kutahya, the observatory said.
NTV television cited a resident in Simav as saying some buildings were damaged. Most of the town was without electricity.
Hatice Unlu, a resident of the town of Tavsanli in Kutahya province, told TV8 channel that the temblor caused great panic.
"Furniture swayed and plates fell on the floor. I struggled to save my computer before rushing out," Unlu said. "We fear to return home."
The quake was felt as far as the Aegean city of Izmir, northwestern city of Bursa, Istanbul and the city of Tekirdag in the Thrace.
Earthquakes are frequent in Turkey, which is crossed by faultlines.
In March last year, a 6.0-magnitude quake knocked down stone and mud-brick houses in five villages in Elazig province in eastern Turkey, killing 51 people. In 2003, a 6.4-magnitude earthquake killed 177 people in the southeastern city of Bingol, including 84 children whose school dormitory collapsed.
In 1999, two earthquakes with a magnitude of more than 7 struck northwestern Turkey, killing about 18,000 people.