A 6.1-magnitude aftershock hit Sichuan province at 4:32 pm on Friday, leaving 231 people injured, four critically, China Earthquake Networks Center and local government said.

The epicenter was located between Pingwu and Beichuan counties. The tremor caused severe landslides in the northern part of the province but no fatalities were reported.

The two counties were among the hardest hit by the May 12 earthquake.

In Pingwu, 130,000 people were affected, 540 houses destroyed and 2,450 others damaged, Meng Xiancai, deputy head of the county's publicity department, said. Most of the communication facilities were damaged in 25 towns of Pingwu.

Zhao Guangmin, a primary school headmaster in Pingwu, said he saw tiles falling from the roofs of buildings. Aftershocks had occurred repeatedly in the area, he said.

"We have to get used to life with aftershocks," Zhao, whose county has lost more than 5,000 lives as a result of the May 12 quake, said, "A day without aftershocks is abnormal for us."

Yin Xianjin, a lawyer in Deyang, a city close to the epicenter, was on the first floor of a building when he started to feel it move.

"Luckily, the building stopped moving very soon," he said, adding it was one of the strongest aftershocks since May 12.

People rushed outside when they felt the aftershock, but most of them went back indoors once the tremor stopped.

"We've learned from experiences that aftershocks rarely strike twice in the same day," Huang Shizhen, a 55-year-old housewife, said.

Seismologists said the aftershocks of a major earthquake could go on for years.

"A big earthquake could release most, but not all of the underground energy, and its aftershocks may last for a long time," Chen Xuezhong, a senior researcher with the geophysics institute of the State Seismological Bureau, said.

For example, the biggest aftershock of the 7.8-magnitude Tangshan earthquake, which took place on July 28, 1976, occurred on Nov 15, he said.

Aftershocks could be devastating as many buildings have already been damaged, he said.

(China Daily August 2, 2008)