A tsunami warning was issued for Fiji and New Zealand but later lifted.
The earthquake was at 1526 GMT, said the website monitored in London. It was not immediately clear what damage it caused, but a report on Sky Television said it was felt in several islands. A resident of Tonga told Sky he felt tremors lasting over 30 seconds.
The earthquake, classified by the USGS as a "great" quake, struck 155 km (95 miles) south of Tonga's Neiafu Island and 2145 km (1340 miles) northeast of Auckland, New Zealand, the USGS said.
The Tonga islands are an archipelago east of Australia and northeast of New Zealand.
An earthquake in December 2004 off Indonesia caused a tsunami across the Indian Ocean that killed more than 200,000 people in several countries
A police officer in the Tongan capital, Nuku'alofa, reached by telephone, said there were no immediate reports of damage.
More than 200,000 people were
killed in the tsunami in 2004
"There was a big earthquake here, but we have no (reports of) damage in Nuku'alofa," said Constable Fonny Mahe. "We have no injuries reported, nothing at all."
The quake also was felt strongly to the north of the capital.
"It was strong but not long" and "lasted about one minute and 30 seconds," duty constable Salesi Baongo told The Associated Press by telephone from Neiafu, 300km (180 miles) to the north.
"It shook the building, it was strong but not a powerful shake," he said. "There is no building damage and nobody injured."