The quake, which struck at a depth of 20 kilometres (12.4 miles) and about 250 kilometres northwest of Vanuatu's Santo island, was first reported as a 6.0 magnitude before being downgraded by the USGS.
No tsunami warning was issued.
Vanuatu is part of the "Ring of Fire," a zone of tectonic activity around the Pacific frequently subject to earthquakes and volcanic eruptions.
Last year the island nation was rocked by a series of quakes, including some that prompted tsunami warnings.