The U.S. Geological Survey reported the aftershock struck at 12:41 p.m. (12:41 a.m. EDT) just south of the town of Qingchuan at the relatively shallow depth of 6 miles.
There were no immediate reports of new damage or injuries. Authorities had earlier begun evacuating people downstream from Tangjiashan lake, formed above the devastated town of Beichuan, after water rose to within 6 feet of flooding its bank.
Authorities plan to drain lake water through a diversion channel as early as Thursday. By noon, people in the downstream city of Mianyang had begun moving to higher ground. Several tent camps set up to house quake victims were empty and police and army troops were blocking roads into low-lying areas.
The flood threat has been exacerbated by waves formed by aftershocks and continued heavy rain in the mountains upstream, the report said, citing unnamed officials monitoring the situation.
Steep hillsides that plunged into river valleys during the May 12 quake blocked rivers and formed more than 30 new lakes. Explosives and heavy machinery have been used in several cases to clear the rubble.
"We must prepare for dealing with the worst situation but strive for the best results," Water Resource Minister Chen Lei was quoted as saying by the official Xinhua News Agency.
Speaking at a Wednesday meeting in Sichuan's provincial capital of Chengdu, Chen stressed the need for caution in draining Tangjiashan to ensure the safety of people downstream.
Skies in the nearby city of Mianyang were clear around noon on Thursday, although rain was expected by the weekend.
Aftershocks have continued to cause landslides throughout the quake zone, prompting the evacuation of more than 14,000 around the town of Hanwang, Xinhua said.
Elsewhere in the area, parents whose children were crushed to death in their classrooms vowed to continue grass-roots campaigns for investigations into alleged corruption and shoddy construction.
On Wednesday, security forces blocked grieving parents and reporters from some collapsed schools in a sign that Beijing was becoming increasingly nervous over their protests.
The quake has so far killed more than 69,000 people. Lu Guangjin, spokesman for the State Council, or China's Cabinet, said there was no tally on how many of the victims were schoolchildren.
Another 17,991 remain missing and about 5 million people have been left homeless, forced to seek shelter in tent camps and hastily built prefabricated housing units.
The government has said about 7,000 classrooms were destroyed, while angry parents and even rescuers have pointed to steel reinforcing rods in broken concrete slabs that were thinner than a ballpoint pen.
Authorities promised to investigate the school collapses, but there had not yet been any word on the findings. Lu said that officials were analyzing samples of the rubble but that the work would take time.
Still, Lu stressed that the government took the issue seriously, with the rescue and relief headquarters convening a special meeting several days ago to discuss how it would handle the collapsed schools.
Impatient parents had tried to file a lawsuit a day earlier in Dujiangyan, saying they wanted compensation along with an explanation and apology from the government. But officials refused to accept their papers and turned them away from the courthouse.
Searchers have yet to find wreckage or survivors from a military helicopter that went missing on Saturday afternoon with 19 people aboard, Xinhua said.