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.

Today's shallow quake centred near Darfield has caused millions of dollars worth of damage to homes, commercial buildings, and infrastucture such as water and sewer networks in mid-Canterbury.

Despite a lot of work in recent years identifying active faults under the Canterbury Plains and in the Canterbury foothills, "at this stage it appears the earthquake has not occurred on a known fault", said GNS Science duty seismologist John Ristau.

"We had this unknown fault generate a significant quake 30km or so from Christchurch: the fact that there is so little structural damage to modern buildings is quite remarkable," Victoria University's Professor Euan Smith told NZPA .

"We got off extremely lightly," said Prof Smith.

The energy recorded from the quake indicated the fault line would be about 40km long, and that there had been a couple of metres of horizontal displacement of rock on each side of the fault, said Prof Smith.

"The suggestion is that it is a fault that is fairly inactive," he said.

This raised the question of why that fault had triggered, and not the Porters Pass fault just to the north. Recent research indicated the Porters Pass fault generates earthquakes over 7.1 magnitude every 1500 years, with ground displacement of up to 8m.

"We just don't know enough about how the earth works to be able to say why one should go, and not the other," said Prof Smith.