The quake, which was 6.3 kilometers in depth, hit near Kurayoshi city to the west of Osaka, the USGS said.
Almost 80,000 households suffered from initial blackouts, but most power has been restored. A handful of light injuries were reported.
There have been reports of housing damage, with state media showing properties flattened by the temblor, but no tsunami warning has been issued.
No irregularities were reported from nuclear power plants in the region, authorities said.
The operator of the Chugoku and Okayama Highways, NEXCO West, said the roads had been closed for safety inspection, but no damage has been recorded.
Aid groups were standing by to offer material support if needed.
"We're hearing reports of homes suffering damage, several major highways have been closed off while the authorities assess their safety and tens of thousands of people have been left without power," said Kunio Senga, CEO of Save the Children Japan.
"We know children are always among the most vulnerable in disasters like this, so we're monitoring how we can support them in the recovery process."
The Japan Meteorological Agency has warned of possible strong aftershocks in the next couple of days.
Japan, which sits along the so-called Ring of Fire, is no stranger to earthquakes.
The largest recorded quake to hit Japan
came on March 11, 2011, when a magnitude-9.0 centered 231 miles (372 kilometers) northeast of Tokyo devastated the country.
That quake triggered a massive tsunami that swallowed entire communities in eastern Japan. It killed about 22,000 people -- almost 20,000 from the initial quake and tsunami, and the rest from health conditions related to the disaster.