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, said Hu Peng, a media officer at the disaster relief headquarters of the lake. 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.