A relatively strong aftershock rocked the dam of the Tangjiashan "quake lake" for about 20 seconds on Sunday and caused massive landslides on surrounding mountains.

The aftershock, striking Beichuan County of Sichuan Province at 6:51 PM, measured 4.8 on the Richter Scale, the China Earthquake Administration said on its website.

The aftershock's impact on the dam was still under surveillance.

The Tangjiashan "quake lake" was formed after a massive quake-triggered landslide from Tangjiashan Mountain blocked the Tongkou River, which ran through the Beichuan County, one of the worst-hit areas in the May 12 quake.

The swollen lake is the largest of more than 30 quake lakes in Sichuan following the primary 8.0-magnitude quake, posing a threat to 1.3 million people downstream.

More than 250,000 people in low-lying areas in Mianyang have been relocated under a plan based on the assumption that a third of the lake volume breached its banks.

Two other plans require the relocation of 1.2 million people if half the lake volume is released or 1.3 million if the barrier is fully opened.

Some 600 armed police and soldiers worked for six days and nights to dig a 475-meter channel to divert water from the lake.

Soldiers are still widening and deepening the sluice channel with the help of 30 bulldozers and excavators. They are also digging a second sluice channel on another side of the lake barrier.

Drainage of the dangerous Tangjiashan "quake lake" has gone smoothly since Saturday. However, local government is still on high alert as the water level of the lake continued to rise.

The May 12 quake had killed 69,136 people nationwide as of Sunday noon, the State Council Information Office said.

A total of 374,061 people were injured and 17,686 others remained missing after the 8.0-magnitude quake that jolted southwest China's Sichuan Province and neighboring regions.

(Xinhua News Agency June 9, 2008)