The undersea quake hit at a depth of 23 kilometres (14 miles) about 118 kilometres from the capital Nuku`Alofa at 8:42 am (1842 GMT Sunday), the USGS said.
There were no tsunami warnings in effect, according to the Hawaii-based Pacific Tsunami warning Center.
Tonga, almost 2,000 kilometres (1,200 miles) northeast of New Zealand, lies on the so-called Pacific "Ring of Fire", where continental plates collide causing frequent volcanic and seismic activity.
Nine people died in Tonga in an earthquake-triggered tsunami in October 2009 that also killed 143 people in Samoa. (*)
Editor: Kunto Wibisono
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