The quake struck at a depth of 314 kilometres (195 miles) at 12:37 pm (2337 GMT Thursday), more than 400 kilometres from Samoa's southwest coast, the USGS said.
The US tsunami warning agency said there was "no tsunami threat".
Earthquakes are common in the South Pacific, which skirts the seismic "Ring of Fire" -- an arc of intense tectonic activity that stretches through Southeast Asia and across the Pacific basin.
Although they seldom cause widespread damage in sparsely populated areas, they can trigger destructive landslides.
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