A powerful 7.3-magnitude earthquake struck off Vanuatu in the Pacific Ocean on Wednesday, but no tsunami threat was detected and no damage reported, seismologists said.

The quake hit at a depth of 131 kilometres (81 miles) about 335 kilometres from the capital Port Vila, the United States Geological Survey said.

"Based on all available data a destructive Pacific-wide tsunami is not expected," the Pacific Tsunami Warning Centre said.

Government agency Geoscience Australia said the general threshold for an undersea earthquake that could potentially generate a tsunami was a magnitude above 6.5 and a depth below 100 kilometres.

"It was probably felt but unlikely to cause any damage," seismologist Dan Jackson told AFP, adding that there had been 27 tremors above magnitude 7.0 around Vanuatu over the past 20 years.

"Quakes like this are a common occurrence over there. The region around Vanuatu and the Solomon islands is the most seismic in the world."

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 South Pacific island was jolted by a 6.8-magnitude tremor in late January and another of 6.5 in February, but there were no reports of damage.