November 25, 2010
By Paul Gorman
Stuff, New Zealand

Scientists are puzzled why the Canterbury earthquake released more than 10 times the energy a quake of its size normally would.

Photo: Aftermath of September 4th Earthquake in Christchurch, NZ. Footbridge over Avon river. September 6, 2010

The United States Geological Survey has calculated the waves travelling from the focus of the September 4 quake had an energy magnitude of between 7.4 and 7.8.

While the quake's actual size remains 7.1, GNS Science seismologist Dr Martin Reyners said the energy magnitude was actually what would be expected from a much larger earthquake.

He told a day-long symposium on the quake at the GeoNZ conference that those "extremely high" figures left the problem of why the recently revealed Greendale Fault and others moved the way they did that morning.

It also meant it was an extremely complicated event.

The shear stress released was 10 to 20 times more than expected. "It took a lot of stress to make that fault move. For its magnitude, this earthquake was unusually efficient at generating seismic waves.

"Compare that to the Dusky Sound earthquake last year. That was magnitude 7.8, but had an energy magnitude of 7.6. So it was very inefficient for its magnitude in generating seismic waves."

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