Another Strong Aftershock Rattles China

A strong aftershock rattled cities in a region of China already devastated by last month's earthquake.

The U.S. Geological Survey measured the temblor Monday afternoon at magnitude 5.

China's official Xinhua News Agency says the aftershock shook the Sichuan provincial capital of Chengdu and the hard-hit city of Mianyang for about 10 seconds.

It was not known whether the aftershock affected the brimming Tangjiashan lake, which was threatening to burst its dam and flood more than 1 million people downstream.

An additional 120 troops were sent Monday to help soldiers with explosives and anti-tank weapons blast rocks and mud slowing the drainage of the still-rising lake.

By Monday morning, the water level in the earthquake-formed Tangjiashan lake had reached more than six feet above a spillway carved into the dam last week to divert water and release pressure on the unstable dam wall, the official Xinhua News Agency said.

An earlier magnitude 5.0 aftershock Sunday sent landslides down surrounding mountains and underscored the threat of flooding. Authorities had been on alert both for increased rainfall and new aftershocks that could weaken the dam or send more debris plunging into the lake.

Television and official Web sites showed People's Liberation Army troops firing 82-millimeter recoilless guns at debris on Sunday. The troops dislodged enough debris to speed the drainage, but the level continued to rise with the inflow from the blocked river behind the dam, Xinhua reported.

David Petley, a geography professor at Britain's University of Durham, said the situation at the lake appeared to be reaching a crisis level. "The teams on the dam are fighting a desperate battle now," he said. "The outcome is very uncertain."

The rising water level indicates the outflow was not fast enough, Petley said. From news photos, he said, there are indications that the top of the dam was holding, instead of eroding slowly as it should, while the channel farther down was eroding too quickly.

That could potentially place increased pressure on the dam by suddenly sucking down large volumes of water, overwhelming the barrier, Petley said.

More than 250,000 people downstream have been evacuated in recent weeks, adding to the turmoil created by last month's massive earthquake in China's Sichuan province. Many were living in improvised camps on surrounding hillsides, surviving on instant noodles and suffering from heat, mosquitoes and a lack of water for bathing.

The death toll from the quake climbed Sunday to 69,136, with 17,686 people still missing.

Tangjiashan lake was formed when rubble from a massive landslide set off by the May 12 quake blocked the flow of the Tongkou River, also known as the Jianjiang.

Wooden houses, boulders and other debris have been blasted to speed the flow of water into the spillway. Other troops have been deepening the channel and digging on a second spillway.

Managing the Tangjiashan lake has become a priority for a government working to head off another catastrophe even as it cares for millions left homeless by the 7.9 magnitude quake. More than 1.3 million people live downriver from Tangjiashan.

The Tangjiashan lake is the largest of more than 30 created by last month's quake. Government experts quoted by state media have played down the threat of imminent flooding, though a variety of factors could set off a dam collapse: rain, aftershocks, landslides, increased leakage from the barrier.