Fault lay quiet for at least 16,000 years

The previously hidden fault, captured here in a GNS Science aerial image after Saturday's Christchurch earthquake. Photo / Supplied

The fault that ruptured the surface of Canterbury paddocks and produced the magnitude 7.1 earthquake has been quiet for at least 16,000 years.

The quake produced a 22km scarp 30km west of Christchurch, scything through rural roads, breaking riverbeds and snapping two houses in half.

Underground, it split alluvial terraces deposited about 16,000 years ago at the end of the last ice age.

Kelvin Berryman, GNS Science's manager of the natural hazards platform, said that before Saturday, nothing in the landscape suggested there was an active fault beneath the Darfield and Rolleston areas.

"Geologists have no information on when the fault last ruptured as it was unknown until last weekend.

"All we can say at this stage is that this newly revealed fault has not ruptured since the gravels were deposited about 16,000 years ago."

The fault had accumulated extreme pressure over that period and collapsed "catastrophically" when the stress reached a certain threshold.

The earth was still shaking out the last of that stress - nearly 100 aftershocks have been recorded so far, the largest of them at magnitude 5.5.

GNS warned that the aftershocks could be large - within one order of the magnitude of the original quake - so there was still the chance of a magnitude 6 shake hitting the unreinforced masonry around Christchurch and in surrounding towns of mid-Canterbury.

The aftershocks would continue for weeks, diminishing in size and frequency.

Dr Berryman said it was highly likely there were other "hidden" faults around New Zealand which might be capable of producing large earthquakes in the future.

Seismologists still believed the major earthquake risk to Christchurch came from known faults in North Canterbury, in the Canterbury foothills, and from the Alpine Fault.

Dr Berryman noted the tremor had not awakened the more active Alpine Fault which ran along the spine of the South Island.

Preliminary tests showed there had been very little effect on the stress regime of the 400km faultline.

Dr Mark Quigley, a geology lecturer at the University of Canterbury, said the tremor had moved the earth up to nearly 5m.

"It was thrust up about a metre in some places. And it has lurched horizontally 4.6 metres to the right."

GNS also found that the jarring jolt which woke Cantabrians early on Saturday was the strongest ground-shaking ever recorded in an earthquake in New Zealand.

Seismologists measured the shaking at 1.25 times the strength of gravity at Greendale, the closest township to the epicentre of the quake.

Geophysics professor Euan Smith, from Victoria University, said there was reason to believe the Canterbury quake could be part of a series of large tremors.

"In 1929 there occurred, in west Canterbury, a magnitude 7 earthquake which turned out to be the first of a series of seven major, magnitude greater than 7, earthquakes over the next 13 years. The series included the second and third largest earthquakes in European times.

"It is improbable that this occurrence of such large earthquakes in rapid succession was coincidental. There is no reason to think that such a series could not happen again."

- additional reporting: NZPA