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Sichuan quake once-in-4000-year event

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 report.

In a paper published in the journal Nature Geoscience, Dr Shen Zhengkang of the China Earthquake Administration and colleagues say the 12 May 2008 quake comprised a strong seismic wave, unusual geology and the failure of three subterranean 'barriers' to resist the shock.

Using global positioning system (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 say the sub-surface geometry is complex, varying significantly along the length of the fault zone.

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, or 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.

Wave of energy

Three junction segments, which have held up for years, received extraordinary blows that day when the main shock was unleashed about 30 kilometres southwest of the city of Yingxiu, the authors say.

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.

The paper's authors say that these three locations were 'barriers' that were smashed down in a single, exceptional event and caused the rest of the fault to rip open.

"We estimate that the failure of barriers and rupture along multiple segments takes place approximately once in 4000 years," they say.

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