Huge energy of shake puzzles scientists

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

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."

Canterbury University structural geologist and active tectonics specialist Jocelyn Campbell said a "spaghetti junction" of faults beneath the Canterbury Plains and through the foothills had been known about for many years.

Others were gradually forming under the Plains.

"They're basically all pointing at one place, Christchurch. It was always a matter of if, not when, these would start to arrive in our neck of the woods.

"We've known for quite a long time that there's stirrings under a placid surface," she said.

GeoNet manager Ken Gledhill said the earthquake was the country's most important in almost 80 years. GeoNet received 7300 reports from people who felt the quake and 60,000 reports of people feelingaftershocks.

When the network was set up it was with the idea of collecting information on an Alpine Fault or Hope Fault quake, he said. The site had 564 million hits in the six days after the main shake.

The Press