Sichuan quake was once-in-4,000-year event: scientists

September 27, 2009
Residents search among the rubble of a collapsed building in Dujiangyan southwest China Sichuan province in May 2008. People who were killed, injured or bereaved in the 2008 Sichuan earthquake had the cruel misfortune to be victims of an event that probably occurs just once in four millennia, seismologists said on Sunday.

People who were killed, injured or bereaved in the 2008 Sichuan earthquake had the cruel misfortune to be victims of an event that probably occurs just once in four millennia, seismologists said on Sunday.

In a paper published in the journal , Shen Zhengkang of the China Earthquake Administration and colleagues said the May 12, 2008 quake comprised a strong , unusual geology and the failure of three subterranean "barriers" to resist the shock.

Using (GPS) markers and data from satellite-borne interferometric radar, the scientists built up a picture of the Longmen Shan fault, on the northwest rim of the Sichuan basin, as it was gouged open by the 7.9-magnitude temblor.

Nearly 88,000 people were killed in what was the largest seismic event in China in more than 50 years.

The investigators said the sub-surface geometry is complex, varying significantly along the length of the .

In the southwest of the zone, the fault's plane dips slightly to the northwest. It then rides up, becoming nearly vertical, in the zone's northeast.

Added to this is a change in motion -- principal movement -- along the fault.

The motion initially starts out as a thrust, a vertical movement in which lower layers of rock are pushed up and on top of higher layers. Farther along the fault, this changes to so-called strike-slip movements, which are lateral.

Three junction segments which have held up for years received extraordinary blows that day when the main shock was unleashed about 30 kilometers (18 miles) southwest of the city of Yingxiu, the paper says.

A wave of energy rocketed along the fault, ripping open the rock beneath Yingxiu as well as Beichuan and Nanba, which is where the biggest earth slippages -- and fatalities -- occurred.

These three locations were "barriers" that were smashed down in a single, exceptional event and caused the rest of the fault to rip open, the study said.

"We estimate that the failure of barriers and rupture along multiple segments takes place approximately once in 4,000 years," it said.

Offering a morsel of comfort, Shen told AFP: "There are still aftershocks, but I don't think there is big chance in that region of another big one" for the foreseeable future.

(c) 2009 AFP

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Sep 27, 2009
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not rated yet Sep 28, 2009
2000 bc is right about the time that the Sichuan indigenous people started with metalwork. Possibly corrolary but there are no indictaions in their history or myth that a similar event occured.
not rated yet Sep 28, 2009
Agree the past evidence may not be applicable.

Also, shortly after the event, I was talking to the owner of a local Chinese grocery. When I mentioned the possibility that weight of the water behind the new Three Gorges Dam was responsible -- apparently that thought had already occurred to the Chinese community.

We in California are regularly exploring and discovering new factors about our situation. Some reassuring, many not. It's just a little hard to believe the result of a Chinese study that effectively says, "We couldn't have anticipated that this would happen, so the government isn't at fault."
not rated yet Sep 28, 2009
That's an interesting thought that I didn't look at previously. Question is what is the additional weight of the water behind the 3GD. If it's of note perhaps one can "detect" earthquake impact of urban and resource pool programs due to location and geological activity.

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