The tremor struck 40 kilometres west of Panguna on Papua New Guinea's Bougainville Island at a depth of 153 kilometres at 3:30pm local time (0430 GMT), the USGS said.
It its preliminary assessment it added that light to moderate damage was possible on Bougainville Island.
The Pacific Tsunami Warning Center warned that waves of between 0.3 and one metre (1-3 feet) above tide level were possible for some coastal areas of PNG and the neighbouring Solomon Islands.
"Even though it is quite deep at 150 kilometres, because it is such a large earthquake, it will produce shaking on the surface," Geoscience Australia seismologist Spiro Spiliopoulos told AFP.
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He added that some damage was possible.
A 7.9-magnitude quake struck off Papua New Guinea in mid-December, but no casualties were reported and an initial tsunami threat was later deemed to have passed.
Earthquakes are common near Papua New Guinea, which lies on the 4,000-kilometre-long Pacific Australia plate.
It forms part of the "Ring of Fire", a hotspot for seismic activity due to friction between tectonic plates.
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