Chinese earthquake expert Zhang Peizhen said an aftershock zone of about 300 km was formed after the Wenchuan quake when he briefed the country's senior lawmakers on Thursday on the cause of the May 12 quake.

"More than 13,000 aftershocks have been monitored in the quake zone, with the strongest reaching 6.4 magnitude on the Richter scale. Aftershocks will still remain for quite a long time," Zhang told the third session of the Standing Committee of the 11th National People's Congress, the top legislature.

The president of the Institute of Geology of the China Earthquake Administration (CEA) said the aftershock zone was located at the Longmen fracture, which links the counties at the quake epicenter -- Wenchuan and Beichuan.

He said after the quake, the CEA immediately sent a team of more than 500 specialists to the zone to evaluate the losses and conduct geological surveys. Studies showed that the quake was the faulting result of the Longmen fracture, lasting from Yingxiu township of Wenchuan to Beichuan.

"During the more than one minute of shaking, a rupture of more than 60 km occurred in Guanxian and Jiangyou counties, a bordering area between Longmen Mountain and the Chengdu Plain," said Zhang, head of an expert committee investigating the cause of the quake.

He said ridge-lines, rivers and construction were destroyed by the surface rupture, which gave rise to a large number of geological disasters such as landslides and mud-rock flows.

Studies showed that as Yingxiu and Beichuan witnessed a serious rupture, a focus of energy release, the two places were hardly hit by the quake.

Zhang said a global positioning satellite (GPS) system showed the quake caused a massive surface deformation. "The Sichuan Basin has relatively descended, with Anxian and Dujiangyan sinking about 30 to 60 centimeters."

"Instead, areas around Chongqing Municipality goes up about several millimeters," he said, adding the deformation was only temporary, and with time passing by, the earth would be back to a stable movement state.

The expert said according to the GPS monitoring data, the gliding speed of the fault line suddenly slowed at the Longmen Mountain, which enabled energy to be accumulated here so that a 8.0-magnitude quake was formed.

"It's estimated that massive quakes would recur at least every 3,000 to 5,000 years. That's why thousands of years of historical records did not register any massive quake on the fault line of the Longmen Mountain," he said.

"The unpredictable and devastating May 12 quake is a new type that deserved to be thoroughly studied," Zhang said.

(Xinhua News Agency June 27, 2008)