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.
One of them, located on Ambae island, is currently threatening 10,000 people living on its foot, as the thick ash it has been spewing since it started its activity in September is killing crops, polluting water supplies and fouling the air.
According to the Daily Mail, Brad Scott, a New Zealand volcanologist with GNS Science who has been seconded to help Vanuatu authorities, said more than one quarter of Ambae has been severely impacted by the volcano's activity, with many traditional thatch roofs collapsing under the weight of the thick ash.
The archipelago sits on the so-called Pacific Ring of Fire, the most geologically active area in the world.
More than 90 per cent of earthquakes occur in this area, which has also experienced 22 of the 25 biggest volcanic eruptions in history.