The quake, which struck at 2:33 am (1833 GMT Saturday), hit 26 kilometres (16 miles) south of the city of Dezhou in Shandong province, at a depth of 10 kilometres, the USGS said.
It was the strongest to hit the province in more than a decade, state-run tabloid the Global Times said.
The quake was felt as far away as Beijing and Tianjin, as well as in Shanghai, about 800 kilometres (500 miles) from the epicentre.
Videos on social media showed shaking light fixtures, trembling ground and people evacuating their buildings, with one clip showing people walking past bricks scattered on the ground.
"The tremor was so strong... during the earthquake my head was shaking on the pillow, I thought I was having a nightmare," one person posted on social media platform Weibo from Shandong's neighbouring Hebei province.
USGS's PAGER system, which provides preliminary assessments on the impact of earthquakes, issued a red alert, estimating extensive damage and some casualties were probable based on previous quake data.
Citing Shandong authorities, state broadcaster CCTV said there were at least 21 injured and that 126 houses or other buildings had "collapsed" from the earthquake, which was followed by 52 aftershocks.
An AFP team saw cracked walls and bricks strewn on the ground near the epicentre of the quake in Shandong's rural and sparsely populated Pingyuan county, but the damage appeared relatively minor.
Locals helped with the cleanup operation in one village, with a group of four elderly women putting a low brick wall back together outside an overgrown yard.
Deng Hongqiang, 55, who lives outside the village but came back to prop up the wall on his uninhabited property, told AFP he had been jolted awake by the quake.
"At the time, all I knew was the ground was shaking... so I went outside," he said.
There was "no way" to repair his old house in the area, he said. "We'll have to demolish and rebuild."
- 'It's scary' -
China's Ministry of Emergency Management has launched a level-four emergency response and sent a team to Shandong province to lead the rescue work, according to state news agency Xinhua.
Footage from CCTV showed rescue personnel in red uniforms marching past first aid tents that had been set up on a school athletics field surrounded by seemingly undamaged buildings.
"Only specific old dirt buildings that were uninhabited have collapsed," CCTV said, showing footage of piles of crumbled bricks between undamaged buildings and pieces of exterior wall stripped from a still-standing house.
"Some yard perimeter walls have collapsed and been damaged," the broadcaster said.
Water and communications infrastructure were functioning normally in the area but hundreds of train services were suspended on Sunday morning, according to CCTV.
"I can't say anything except that it's scary," another Weibo user said.
Earthquakes are not uncommon in China but it is rare for them to hit the eastern part of the country, where most of the population and big cities are located.
An official from the Shandong Seismological Bureau said the possibility of a larger earthquake was "very small", according to local media.
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