The epicentre was detected at an estimated depth of 12 kilometres (seven miles) at 8:46 am local time (2146 GMT Monday), the USGS said.
The quake hit about 20 kilometres (12 miles) off the coast, a short distance from the town of Wewak, capital of the Pacific island state's East Sepik Province.
There is "no tsunami threat", the Pacific Tsunami Warning Centre said in a separate bulletin.
"I think it lasted for almost a minute," said Danny Seolo, a worker staying at the Wewak Village Inn Hotel and Apartments.
"Not very violent but kind of like you are on a boat or something -- it was swaying," he told AFP by telephone, adding that he had not seen any damage.
Earthquakes are common in Papua New Guinea, which sits on top of the seismic "Ring of Fire" -- an arc of intense tectonic activity that stretches through Southeast Asia and across the Pacific basin.
Although they seldom cause widespread damage in the sparsely populated jungle highlands, they can trigger destructive landslides.
At least seven people were killed in April this year when a 7.0-magnitude quake hit a jungle-clad area in the country's interior.
About 180 homes were destroyed in the heavily rainforested Karawari area, near the quake's epicentre.
In September last year, 10 people were killed when a 7.6-magnitude earthquake levelled hundreds of homes, split roads and caused power outages across the rugged north of the country.
It was the largest quake to rock Papua New Guinea since 2018, when almost 150 people were killed following a shallow 7.5-magnitude tremor in Hela province.
Many of Papua New Guinea's nine million citizens live outside major towns and cities, where the difficult terrain and lack of sealed roads can seriously hamstring search-and-rescue efforts.
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