The tremor shook buildings sending panicked residents scurrying for cover, but no major damage was immediately reported.
The quake struck 39 kilometres (24 miles) northwest of Kangding in the mountainous west of the province at 4:55 pm local time (0855 GMT) at a depth of 14 kilometres, the USGS said.
The quake was initially reported at 5.8 magnitude at a depth of nine kilometres.
The official Xinhua news agency reported that a woman in her 70s died after being hit in the head by a glass window that fell during the tremor. It added that 60 others were injured and taken to the Ganzi People’s Hospital, where three were said to be in a critical condition.
Forty-two students at a primary school in Tagong Town at the epicentre were injured in a stampede following the quake, Xinhua said.
China’s national CCTV television showed footage taken with a mobile phone of panicked residents running in the streets in what appeared to be a commercial area.
Kangding county, with a population of about 100,000 people, is located in an area of Sichuan traditionally populated by ethnic Tibetans.
– 55,000 people affected –
About 55,000 people have been affected by the quake, Xinhua reported, citing the provincial civil affairs department.
A woman reached by phone in the area told CCTV that buildings around her withstood the shaking. Xinhua reported minor cracks in some airport buildings, adding that flights had been unaffected.
But the quake sent household items falling onto the floor, Xinhua quoted a resident of Kangding as saying. He added, however, that he did not see any houses collapse.
“The house window was shaking fiercely,” said a woman in Chengdu, Sichuan’s capital, located around 300 kilometres from the epicentre.
“Some people rushed out of the building,” she told Xinhua.
More than 100 vehicles remain trapped on the G318 Highway linking Sichuan and Tibet after part of the road caved in, the news agency said, adding that traffic was expected to resume by mid-day Sunday.
Passenger trains in affected areas were halted by Chengdu railway authorities but service has now resumed, Xinhua said.
China Earthquake Networks Center measured the quake at 6.3, according to Xinhua. China uses a different magnitude scale to the US.
There have been 95 aftershocks so far, Xinhua reported, adding that medical response teams from nearby cities had been mobilised, while the civil affairs department had sent thousands of tents, quilts and coats to those affected.
China’s southwestern provinces of Sichuan and Yunnan are acutely vulnerable to earthquakes.
The region sees frequent seismic activity from the collision of the Indian and Eurasian tectonic plates, which form the vast Himalayan mountain range.
In May 2008, a 7.9 magnitude quake rocked Sichuan, killing more than 80,000 people and flattening swathes of the province. It was the worst quake disaster to hit China in more than three decades.
Last month, hundreds of people were injured and more than 100,000 displaced after a shallow 6.0 magnitude tremor hit Yunnan province, close to China’s borders with Myanmar and Laos.
And in August, a 6.1-magnitude quake struck Yunnan killing more than 600 people.
More than 3,000 people were injured, while more than 80,000 homes were completely or partially destroyed.
In 1976, the industrial city of Tangshan, 200 kilometres east of Beijing, was levelled by an earthquake measuring 7.5 according to the US Geological Survey.
Beijing puts the official death toll from that disaster at 242,000, with 164,000 seriously injured, although Western sources say the number of victims could have been much higher. -AFP