Jiangyou: A "relatively strong" aftershock on Sunday shook the massive earthquake-formed lake in China that has been threatening to flood more than 1 million people and triggered landslides in surrounding mountains, a state news agency reported.

The effect of the 20-second temblor on the Tangjiashan lake was not immediately known, Xinhua said. The dam of unstable mud and rocks was under surveillance following the aftershock.

The magnitude of the aftershock was not immediately known, Xinhua said.

Though water has been draining from a hastily dug diversion channel for nearly two days, the lake has continued to swell.