Thursday, April 25, 2013

Experts debate tremor’s cause

Experts debate tremor�s cause
By Hu Qingyun
Global Times
April 22, 2013

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.

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