At least 359 people were injured yesterday in the county - already one of the worst-hit areas in the May 12 earthquake.
One death was reported in Guangyuan city.
The aftershock also destroyed more than 70,000 homes and left 200,000 others in danger of collapse, said Mr Wang Fei, an official with the disaster relief office in Guangyuan city.
In Ningqiang county in Shaanxi province, at least 20 people were injured and many houses collapsed.
The tremor was also felt in Xian, Shaanxi's capital, where many fled their homes and offices for open spaces.
Longnan city in Gansu province saw one death and 109 injured.
The aftershock even caused office towers to sway in Beijing, 1,300km away.
Nearly 8,000 aftershocks have been reported in Sichuan since the massive earthquake 15 days ago.
As yesterday's aftershock hit, some 1,800 soldiers and police were hiking to a blocked 'quake lake' - just 3.2km upstream from Beichuan county, which was hardest-hit by the quake.
They were each carrying 10kg of explosives to blast through the barrier before it bursts and causes a flash flood. Fog had prevented the airlift of personnel and equipment to the lake. Heavy rain and high winds were expected in the area yesterday and today.
Concerned by a steep rise in the water level of a giant lake at Tangjiashan, the authorities have evacuated thousands of residents from below the lake as a precaution.
Premier Wen Jiabao has said the main concerns now are secondary disasters such as flooding and landslides and epidemics, as well as providing shelter for the five million displaced.
Earlier yesterday, the Cabinet said the confirmed death toll from the quake had risen to 62,664, with a further 23,775 people missing.
The Tangjiashan quake lake rose 1.93m last Saturday to 723m, Xinhua said.
The person in charge of overseeing the blast mission, General Zhou, said the Tangjiashan lake was formed by a mudslide.
'The relief work for Tangjiashan quake lake is now at a critical stage,' Vice-Minister of Water Resource E Jingpin said in Beijing. He added that 20,000 people had been evacuated from areas threatened by 19 quake lakes.
Mr E said China had a window of about one month before the rainy season begins in Sichuan and must use the 'precious time' to shore up as many dams as possible.
The mountainous region along the faultline is densely packed with dams, and the ministry said yesterday that 69 dams in Sichuan were in danger of bursting due to quake-related damage.
REUTERS, ASSOCIATED PRESS
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