Canterbury earthquake really three quakes?

Scientists are investigating whether the 7.1 magnitude earthquake which struck Christchurch and its surrounding communities today was actually two or three shocks in quick succession.

The US Geological Survey (USGS ) in Golden, Colorado, has said that scientists are still trying to reconstruct the way today's quake played out.

"We think that this is a very complex event," said geophysicist Paul Caruso. "We think that the main shock may have consisted actually of three earthquakes."

GNS Science in Wellington - which changed its initial calculations of a 7.4 magnitude shake 30km west of Christchurch to a 7.1 shake 40 km west - said it could not confirm the mechanisms of the earthquake.

"There are several parts to this earthquake occurring within seconds of each other and it will take some time to decipher what the waveforms recorded by our seismographs tell us about the sequence of events," said a spokesman.

But GNS scientist Dr Hamish Campbell said there was potential for a quake to propagate from one end of a fault to the other: the Napier quake moved from the southwest to the northeast.

And Pennsylvania State University geoscientist, Professor Kevin Furlong, who is spending a sabbatical year at Canterbury University, told NZPA that the direction of rupture could significantly affect the way ground motion was felt, but detection of "directivity" would have to wait on detailed analysis.

He said there was an initial foreshock of about a 5.8 magnitude, about five seconds before the main impact, and possibly from a slightly different location. "I think a lot of people were woken up by that, without knowing why, then ... whammo!" Prof Furlong said.

"The main shock itself had two main pulses of energy in it."

He spent the day recording the track of the fault, and measured 4m of lateral movement between the two sides of the fault.

"There were several very unlucky people who had the fault go right through their home.

"It was not a pretty sight."

Though the 7.1 magnitude earthquake caused few serious injuries, GNS Science said the effects of the quake look set to be the most significant since the Napier earthquake killed 256 people in 1931 - that 7.8 magnitude shake caused the largest loss of life and most extensive damage of any recorded New Zealand quake.

http://www.stuff.co.nz/national/4096802/Canterbury-earthquake-really-three-quakes