The first quake, with a magnitude of 6.4, was centred 265 kilometres (165 miles) southeast of the capital Suva and 165 kilometres northwest of the southern island of Ndoi, the USGS said.
It struck at 9:11 am local time (2111 GMT Sunday) at a depth of 687.6 kilometres.
Another 6.2 magnitude quake struck nearby less than an hour later.
The quakes occurred near fault lines in the Pacific "Ring of Fire", where continental plates in the earth's crust meet and earthquakes and volcanic activity are common.
