The undersea quake struck at a shallow depth of 15.2 kilometres around 185 kilometres south-west of the South Pacific island of Vanua Levu, Fiji, at 4:01am local time, the USGS said in a revised statement.
Initially the USGS said the quake measured 5.9 but seismologists later revised the strength.
There was no warning in effect on the Pacific Tsunami Warning Centre website.