The quake struck at 9:47 am local time from a depth of about 112 kilometres (69 miles), according to the USGS, and was centred in a patch of ocean about 198 kilometres (123 miles) north of Tonga's capital Nuku'alofa.
There were no immediate reports of damage in Tonga.
It struck near the sparsely populated islands of Kao and Tofua, and the Pacific Tsunami Warning Centre said "there is no tsunami threat from this earthquake".
Local resident Mary Lyn Fonua who was already at work in Nuku'alofa, told AFP the two-storey building she was in swayed strongly for a few seconds.
"Then we evacuated to higher ground, locals are feeling nervous," she added.
Earthquakes are common in Tonga, a low-lying archipelago home to around 100,000 people that straddles the seismic Ring of Fire.
The Ring of Fire is an arc of intense tectonic activity that stretches through Southeast Asia and across the Pacific basin.
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