There were no immediate tsunami alerts or reports of damage.
The USGS said the 7.4-magnitude quake at a depth of 106.5 miles struck just after 5 a.m. about 175 miles southwest of Vaini, Tonga; and 178 miles southwest of the capital Nukualofa; and 442 miles southeast of Suva, Fiji.
It was followed about 4 hours later by a magnitude-6.3 quake at a depth of 95.7 miles, 52 miles southwest of Nukualofa and 531miles south-southwest of Apia, Samoa.
Matangi Tonga Online reported the first quake shook homes and rattled window in Nukualofa but no damage had been reported.
The Tonga archipelago, northeast of New Zealand, has a population of fewer than 105,000.
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