A 5.3 magnitude aftershock struck China's quake-battered Sichuan province Thursday while authorities watched nervously as rising water behind an earthquake-created dam neared a spillway designed to relieve pressure on it, reported CNN.
The U.S. Geological Survey reported the aftershock struck at 12:41 p.m. (12:41 a.m. EDT) about 65 kilometers (40 miles) west of Guangyuan, Sichuan province.
The quake was recorded at a depth of 10 kilometers (6.2 miles) , according to the USGS.
The threat of aftershocks from the May 12 quake and more rain have put the area in "extreme danger," said Liu Ning, chief engineer of the Ministry of Water Resources.
Heavy rainfall and severe aftershocks could compromise the integrity of the dam and send water cascading out.
In the event of particularly heavy rain and a major aftershock, the chances of a catastrophic failure would stand at 93 percent, Liu estimated.
Authorities have evacuated about 200,000 people who are considered to be at risk if water overflows the dam.
Engineers built the spillway to drain water from the lake in a controlled way once it tops the lowest levels of debris blocking the river.
On Tuesday, water in the so-called Tangjiashan quake lake was within 8 feet (2.4 meters) of the spillway, in Beichuan county, Cheng Dianlong, deputy chief of the State Flood Control and Drought Relief Headquarters, told the Xinhua news agency.
Engineers used heavy equipment and dynamite to chisel a 1,560 foot-long (475-meter-long) spillway through tons of rock and soil that slid off the hillside during the earthquake. They finished work on the channel Sunday.
Thirty-five so-called quake lakes were created by the 7.9-magnitude quake.
The Tangjiashan lake worries Chinese officials most because it could threaten up to 1.3 million people downstream.
The official death toll from the earthquake has risen to 69,122, an increase of 15, the country's Civil Affairs Ministry reported Wednesday, according to state-run media.
Another 373,606 people were injured and 17,991 remain missing, according to the ministry.