The quake hit at a depth of 218 kilometres (135 miles) about 84 kilometres from Vanuatu’s capital Port Vila, at 5.31pm local time (0331 GMT), the USGS said.
The Pacific Tsunami Warning Center said there was no threat of a tsunami. The USGS originally put the magnitude at 6.9.
Government agency Geoscience Australia duty seismologist Marco Maldoni said the threshold for an undersea earthquake that could potentially generate a tsunami was a magnitude of 6.5 and a depth of 100 kilometres.
“This one, being 200 kilometres deep, does not meet that requirement. The deeper the earthquake is, the less likely it is to generate a tsunami,” Maldoni told AFP.
“They (Vanuatu) would have felt some tremor, but no damage. It’s quite normal for that part of the world to have these sorts of earthquakes.”
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.
The last powerful tremor that struck off the South Pacific island was in July. It had a magnitude of 6.3 and was 114 kilometres deep.
In 2013 the neighbouring Solomon Islands were hit by a tsunami after an 8.0-magnitude earthquake rattled the region. That tsunami left at least 10 people dead, destroyed hundreds of homes and left thousands of people homeless. (AFP)