No tsunami alert was issued, said the Pacific Tsunami Warning Center, based in Hawaii.
The quake hit the Moro Gulf off Mindanao Island at 7:15 a.m. (2315 GMT), the USGS said.
It was centered some six miles under the Moro Gulf, the agency said, and was some 75 miles (121 kilometers) southwest of the city of Cotabato on Mindanao, and was about 566 miles (910 kilometers) southeast of Manila.