There were no immediate reports of damage or casualties, and no tsunami warning was issued.
The quake hit at about 7:30 a.m. local time. It was deep, some 368.5 miles below the surface, the USGS said.
The temblor hit some 264 miles west of Nuku'alofa, Tonga, and 281 miles southeast of Suva, Fiji.
The Associated Press contributed to this report.