Experts debate tremor’s cause

The 7.0 magnitude quake that rattled Lushan county in Sichuan Province on Saturday has stirred heated discussions among experts about whether the quake has any links with a previous 8.0 magnitude quake that hit the province in 2008.

Some pointed out that the Lushan earthquake, similar to the Wenchuan earthquake, occurred on the Longmenshan fault line and was triggered by movement in the earth's crust.

Fan Xiao, chief engineer at the Sichuan Bureau of Geology and Mineral Resources, told the Global Times on Sunday that the earthquake was an aftershock of the Wenchuan earthquake in May 2008.

"After the Wenchuan quake, the fault line became active as its stress wasn't completely released. So it was possible for a 7.0 magnitude aftershock even five years later," Fan said, adding that it was the Longmenshan fault line adjusting its stress.

Zhou Bengang, a researcher with the China Earthquake Networks Center, disagreed, saying the Lushan quake was isolated. "The quake occurred on the southern part of the fault line, while the Wenchuan earthquake occurred in the middle part."

Zhou said the quake happened in different parts of the fracture and earthquakes in the southern part of the fault line are comparatively weak.

Feng Zhiming, a researcher at the Institute of Geographic Sciences and Natural Resources Research of Chinese Academy of Social Sciences, told the Global Times that the death toll of the Lushan earthquake would be lower and the property damage would be less, because of its relatively low strength.

Apart from the natural movement of the earth's crust, recently constructed dams and their reservoirs may have also triggered the two earthquakes, Fan noted.  

The Pubugou reservoir, which holds 5.39 billion cubic meters and was created by a 186 meter tall dam, is 80 kilometers from Lushan. Earthquake monitors have detected 1,834 temblors in the area between October of 2006 and December of 2011.

"The large reservoirs built on the fault line can induce earthquakes as the huge amount of water adds huge pressure to the fracture," Fan said.

A reservoir with a capacity of over 1 billion cubic meters and a dam more than 100 meters tall would have a 30 percent to 40 percent chance of inducing an earthquake, said Fan.

Fan is one of the experts who questioned whether the 2008 earthquake was triggered by the Zipingpu reservoir, which is about 4.5 kilometers from the epicenter and has a capacity of 1.1 billion cubic meters. He said authorities should realize reservoirs built on seismically active fault lines can cause movement in the earth's crust.

Special report: 7.0-magnitude earthquake jolts Ya'an, Sichuan