The quake struck at a depth of 4.8 kilometres (3 miles), some 85 kilometres (52.8 miles) south of the port city Luganville — known locally as Santo — USGS said.
Vanuatu is part of the “Ring of Fire”, a zone of tectonic activity around the Pacific that is subject to frequent earthquakes and volcanic eruptions.
This month has seen a series of tremors reported, including a powerful 7.0-magnitude quake on Friday which hit 209 kilometres from the capital Port Vila.
That powerful earthquake briefly prompted a tsunami warning that was cancelled after locals reported no significant damage.
A 6.9-magnitude tremor also hit the country at the beginning of April but no tsunami alerts were issued and there were also no reports of damage.