There was no immediate tsunami warning or any reports of damage or casualties from the quake, which hit at a depth of 73 km (47 miles) about 474 km (295 miles) north of Vanuatu's island of Santo, the USGS said.
The archipelago was hit by a tremor of similar magnitude on July 13.
On that occasion, people were seen running from buildings and items falling from shelves as a result of the quake.
Vanuatu is a volcanic island with 24 active and inactive volcanoes on its territory.
