There were no immediate reports of casualties or damage.
The epicenter was in the Banda Sea about 120 miles southwest of Ambon, the capital of Maluku province, the observatory said. The quake was recorded at 4 a.m. local time.
A magnitude 9.1 earthquake on Dec. 26, 2004, off the coast of Indonesia's Sumatra Island triggered a tsunami that killed more than 131,000 people in nearby Aceh province and 100,000 others in countries around Asia.
A magnitude 6 earthquake can cause severe damage.