The offshore quake hit just before 2:30 p.m. at a depth of 27 kilometers (17 miles), about 70 km southwest of Okinawa's Yonakuni Island, not far from Taiwan, according to the USGS report.
Taiwan's Central Weather Bureau said the shake, the fifth this year above a magnitude of 6.0, was felt across the island, though there were no immediate reports of damage.
"Hope everyone is safe ... there could be some aftershocks and please be cautious," New Taipei City Mayor Hou You-yi posted on Facebook.
Japan's Meteorological Agency estimated the tremor at a magnitude of 6.6 and said it may prompt minor sea-level fluctuations, but has not issued a tsunami warning.
Taiwan and the surrounding area are regularly hit by earthquakes due to the nearby junction of two tectonic plates.
In March, a 6.7-magnitude earthquake that struck off Taiwan's eastern coast rattled buildings and roused people from sleep, leaving one injured and collapsing a half-constructed bridge in the island's east.
Earthquakes of 6.0 or more can prove deadly, although much depends on where they strike and at what depth.
Taiwan experienced a quake of a similar magnitude in January when a 6.2 tremor hit the east coast. There was no widespread damage or injuries.
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