QINGLIAN, China — A magnitude 5.0 aftershock shook a brimming, earthquake-formed lake and sent landslides tumbling down surrounding mountains Sunday, 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, though the lake’s level continued to rise, 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.

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. By yesterday morning, the water level had reached more than six feet above the diversion channel carved into the dam, Xinhua reported.