The quake, which hit at 8:19 am (0019 GMT), was centred 61 kilometres (38 miles) under the floor of the Sulawesi Sea, some 288 kilometres north of Sangihe island near the maritime border with the Philippines, said Yudo of the meteorology agency here.
"It was too deep to be felt on the surface," he said, adding that several aftershocks also registered but all were weaker than the initial jolt.
The Indonesian archipelago sits on the so-called Pacific Ring of Fire where continental plates meet, causing frequent seismic and volcanic activity.
Indonesia was the nation worst hit by the earthquake-triggered Asian tsunami in December 2004, which killed some 168,000 people in Aceh province. A 7.7-magnitude earthquake in July on the south coast of the main island of Java also killed more than 600 people.

Copyright AFP (Agence France-Presse), 2006