The USGS reported that the quake hit at a depth of 10 kilometres (6.2 miles) about 149 kilometres from Vanuatu's capital Port Vila.
The Pacific Tsunami Warning Center said there was no threat of a tsunami.
A second, smaller 5.1 magnitude quake hit the same area about 20 minutes later, the USGS said.
Vanuatu is part of the "Ring of Fire", a zone of tectonic activity around the Pacific that experiences frequent earthquakes and volcanic eruptions.
The South Pacific island was jolted by a powerful 6.8-magnitude tremor in late January, but there were no reports of damage.
In 2013 the neighbouring Solomon Islands were struck by a tsunami after an 8.0-magnitude earthquake rattled the region. The tsunami killed least 10 people, destroyed hundreds of homes and left thousands of people homeless.