More than 250,000 people downstream have been evacuated in recent weeks.
The effect of the 20-second temblor on Tangjiashan lake, as it is known, was not clear and its dam was under surveillance, said the state-run Xinhua News Agency, which had a reporter at the site. A diversion channel draining the lake appeared to be operating smoothly following the aftershock and a rainstorm about the same time, the agency said.
The temblor was centered about seven miles northwest of the lake, according to data reported by the U.S. Geological Survey. It set off landslides on nearby mountains, Xinhua said, without indicating whether any rubble landed in the lake and pushed water levels even higher.
One evacuee in a mountainside tent camp about 25 miles downstream said the situation remained stable.
"We felt the aftershock but it wasn't anything bad. We're numb to this by now," said Yu Taichun, a doctor living with about 2,500 people on Taohua Mountain, overlooking the riverside town of Qinglian.
The Tangjiashan lake was formed when rubble from a landslide set off by the deadly May 12 earthquake blocked the flow of the Tongkou River. Despite a hastily dug diversion channel that began draining the lake early Saturday, water levels have continued rising dangerously.
Soldiers blew up wooden houses, boulders and other debris Sunday to speed the flow of water into the spillway. Other troops were deepening the channel and digging on a second spillway.
Managing Tangjiashan lake has become a priority for the government as it works to head off another catastrophe even as it cares for millions left homeless from the quake. More than 1.3 million people live downriver from the lake.
The death toll from the quake climbed Sunday to 69,136, with 17,686 people still missing.
The Tangjiashan lake is the largest of more than 30 created by last month's quake, which was centered in Sichuan province. Government experts quoted by state media have played down the threat of imminent flooding, though a variety of factors could set off a dam collapse: rain, aftershocks, landslides, increased leakage from the barrier.
"Controlling the lake outflow is critical for the dam's safety," Zhang Ting, head of the Sichuan provincial hydro-meteorological bureau, was quoted as saying by Xinhua. "If the water flows out too slowly, the accumulating inflow will increase pressure on the barrier, but too strong an outflow will also erode the diversion channel and lead to the dam collapse."
Evacuees, some of whom were forced to relocate from more permanent camps more than two weeks ago, were impatient for the lake to drain. Officials estimated it would be a week before people could return home, even if all goes well.
"I wish they'd hurry, look at us here," said rice farmer Cai Yuhua, gesturing at a cluster of mostly homemade tents built on a hillside in Jiuling village, where she and hundreds of others waited.
"The last time we could go back to our homes was May 22. I want to go home and look at my things," said Cai, who was living under a striped plastic tarp tied over bamboo poles.
Meanwhile, a cargo train derailed in northeastern Sichuan province early Sunday after being struck by rocks falling from a mountain, Xinhua said. One railway worker was killed and another was seriously injured.
It was not known if the falling rocks were related to the May 12 quake or its aftershocks. The rocks may have been loosened by recent heavy rains, Xinhua reported, citing a Chengdu railway administration official.
The damaged line was repaired by Sunday evening.