Rare mix of factors exacerbated quake
PAUL GORMAN
Last updated 05:00 18/03/2011
Christchurch might be one of the world's unluckiest cities, scientific investigations into last month's earthquake show.
A rare combination of factors are being blamed for the magnitude-6.3 quake, which killed possibly as many as 180 people.
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.
GNS Science seismologist Bill Fry said 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 said 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 kilometres off northeast Japan.
There were four factors that caused the intense shaking from the Port Hills fault earthquake:
The high amount of energy released in the fault's rupture.
The direction in which it was released.
A recently discovered "trampoline effect" between layers of sediment under the city.
The closeness of the earthquake.
Fry said the faults that broke, causing Canterbury's big quakes last month and in September last year, were unusually strong.
"When a strong fault breaks it releases more energy than an equal-sized weak fault. Think about the difference between breaking a 1cm-thick sheet of styrofoam and a 1cm-thick sheet of plywood, and the amount of jarring you would feel in your hands.
"The Canterbury crust is like the plywood, but most earthquakes are more like breaking the styrofoam."
The strength of the fault was also a factor behind the speed of the rupture, which was faster than for most quakes of the same magnitude.
The ground breaking along the fault lasted for only about three seconds, but because of its speed, people in the city were feeling the first waves from the shake before the rupture was over, he said.
Ground shaking was particularly "punchy" because the waves from the rupture, and the rupture, had been moving in the same direction.
"It's like bulldozing snow," he said. "You are pushing snow along that you've already collected, but all the time you are adding more."
The hidden fault under the northern edge of the Port Hills was sloping down towards the southeast and therefore "pointing straight at Christchurch". When it broke, it directed much of its energy northwest across the city.
When the earthquake hit, the weaker top few metres of ground under the city "trampolined" further upwards than stronger layers lower down and separated from them.
"When these upper layers fell back under gravity, they `slapped' against the lower layers coming up again, producing very high impacts," Fry said.
"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."
There had been isolated recordings of the trampoline effect in the past, but Christchurch had provided scientists with a full set of data to study it more closely.
- The Press
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