A 10-km-long crack belt running south to north was found on Thursday in Zhongba county of the Tibet Autonomous Region, which elevated the scale of the earthquake that struck the area on Monday to magnitude 8, a local official said.
Zhu Quan, head of the Tibet earthquake bureau, said the crack was 30 km to the west of the town of Longgeer and 5,200 m above sea level, and there were also sand blast and water gushing along it.
"All these take the magnitude up to 8 from 6.8 reported on Monday," Zhu told the China News Service.
The tremor will have no effect on the Qinghai-Tibet railway, which is several hundred kilometers from the quake zone, he said.
Although the earthquake had, as of Wednesday, affected 7,600 people and damaged 622 houses and 54 schools, conditions in the area were stable, he said.
No casualties have been reported.
Zhongba Party Secretary Zhu Jiang said life and work had gone back to normal by Friday.
The local government has allocated an emergency fund of 3 million yuan ($440,000) for food and clothing, he said.
The county has also received 400 tents and 3,500 quilts from local governments, he said.
Health and quarantine departments in the country have also prepared medicines in case of emergencies, Zhu said.
"More relief materials are on the way," Zhu Jiang was quoted by the Xinhua News Agency as saying.
Primary schools in the three worst-hit towns were told to postpone the new term, which was due to start on Monday, he said.
Monday's quake occurred at 9:22 pm and its epicenter was about 10 km underground.
More than 2,000 people were evacuated from their homes on Tuesday.
A hydropower station was slightly damaged in the quake, Zhu Quan said.
The counties of Gerze and Coqen in neighboring Ali Prefecture also felt the quake, but sustained no damage, he said.
The lack of casualties is due to the "comfortable-living project" implemented by the Tibet regional government, he said.
Under the project, farmers have been moved to new homes built to withstand earthquakes.
Zhongba, which has a population of 18,000 and an area of 43,594 sq km, experiences frequent quakes.
It is 4,700 m above sea level.
A 6.7-magnitude quake occurred in the same area in 2004, followed by a 6.5-magnitude quake on April 8, 2005, but neither caused casualties, the regional earthquake bureau said.