Mix of factors exacerbated New Zealand Christchurch earthquake

10:15, March 18, 2011      

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A splitted road is seen in east suburb of Christchurch after the earthquake, March 1, 2011. (Xinhua File Photo/Li Qiuchan)

A rare combination of factors are being blamed for the New Zealand Christchurch magnitude-6.3 earth quake, the latest scientific investigations into Feb. 22 earthquake show.

Violent, unprecedented ground-shaking, unusually high levels of energy release, a fault pointing at the city like a loaded gun and trampoline-like bouncing of ground layers under the city combined over about 20 seconds on February 22 to cause the disaster, according to New Zealand's Institute of Geological and Nuclear Sciences (GNS).

GNS Science seismologist Bill Fry said on Friday the quake was so well documented by the network of instruments around Christchurch that it would change international scientific thinking about earthquakes.

"We don't have many such good recordings from so near the source. Seismologists globally will be looking at this for a long time to come," Fry was quoted as saying by The Press.

He said that ground acceleration more than twice that of gravity in some parts of Christchurch was the strongest recorded in a New Zealand quake and up to four times greater than the maximum acceleration measured in last week's magnitude-9.0 quake 130 km off northeast Japan.

Fry said the faults that broke, causing Canterbury's big quakes last month and in September last year, were unusually strong.

The ground breaking along the fault only lasted about three seconds but the city was feeling the first waves before the rupture was over because of its speed.

Ground shaking was worse because the waves from the rupture, and the rupture, moved in the same direction. The fault was sloping down towards the southeast and so it directed much of its energy northwest across the city.

When the earthquake hit, the weaker top few meters of ground under the city "trampolined" further upwards than stronger layers lower down and separated from them.

"Think of jumping on a trampoline. If you change the rate at which you jump, and get out of synch with the trampoline, when you come down and land as the trampoline is still heading upwards, you get quite a jolt to your knees," Fry said.

The magnitude 6.3 quake devastated Christchurch central city and killed an estimated 182 people.

Source: Xinhua

 
 
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