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Factors combined to intensify Chch quake - GNS

Published: 11:06AM Friday March 18, 2011 Source: ONE News

An unusual combination of at least four seismic effects caused last month's deadly magnitude 6.3 earthquake in Christchurch, scientists have found.

Police say the final death toll from the February 22 quake is expected to be 182.

GNS Science said the intense shaking in Christchurch was caused by the high amount of energy released in the rupture of the fault, the direction that energy was released -towards the city, a recently discovered trampoline-like interaction between geological layers under the city, and the close proximity of the earthquake to the city.

The ground accelerations in Christchurch, the largest ever recorded for a New Zealand earthquake, were as much as four times higher than the highest accelerations measured in last Friday's magnitude 9.0 earthquake off the east coast of Japan, GNS Science says.

The largest ground accelerations in Christchurch during the February 22 quake were more than twice the acceleration due to gravity.

Seismologists have calculated that the energy released in Christchurch was equivalent to a magnitude 6.7 earthquake.

GNS Science Seismologist, Bill Fry, said it was the acceleration that caused the 'jolting' feeling people experienced during an earthquake.

"A Formula 1 racing car accelerates on average at less than twice the force due to gravity, and during the magnitude 9.0 'mega-quake' off Japan's east coast last Friday, peak ground acceleration was recorded at only about 0.53 the strength of gravity, or less than one quarter that recorded in Christchurch," Fry said.

Faults failed

The faults that failed in both the September magnitude 7.1 and February magnitude 6.3 Christchurch quakes were very strong, with high amounts of friction holding the two sides together, Fry said.

"When a 'strong' fault breaks, it releases more energy than an equal-sized 'weak' fault would. 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 analogous to the plywood, but most earthquakes are more like breaking the styrofoam."

In addition, seismologists have discovered that the earthquake rupture was faster than most earthquakes of this magnitude.

"In fact, the rupture travelled at about the same speed as some of the waves that were generated by it," Fry said.

This implies that as the waves from the first second or so of the rupture were travelling toward Christchurch, new energy was being released during the next couple of seconds, and these were added together to make the shaking particularly 'punchy'.

This resulted in something compared to the supersonic boom of a jet aircraft, he said.