QINGLIAN, China — A magnitude 5.0 aftershock shook a brimming, earthquake-formed lake and sent landslides tumbling down surrounding mountains yesterday, underscoring the persistent threat of flooding to more than 1 million weary refugees downstream.

No new evacuations were ordered and the lake’s dam of unstable mud and rocks did not collapse, a press officer at the disaster relief headquarters of the lake, Hu Peng, said . He had no additional details.

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 7 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.