Christchurch earthquake: Hidden faultline to blame

A group of damaged shops in the suburb of St Albans. Photo / Simon Baker.

The quake that wreaked devastation across Canterbury yesterday may have occurred on a previously unknown faultline, seismologists say.

The 7.1 magnitude tremor that shook the South Island before dawn was the eighth-strongest recorded in this country and caused the most damage since the 1931 earthquake that flattened Napier.

John Ristau of GNS Science said: "At this stage it appears the earthquake has not occurred on a known fault."

Ristau said the destruction would provide a frightening reminder to Wellingtonians as they await "the big one".

He added: "It would be a lot worse in Wellington because the fault goes right through, and it would be a lot bigger earthquake than this one."

Scientists warned the aftershocks, some of which measured more than 5 on the Richter scale, could continue for weeks or even months.

Big earthquakes are rare in Christchurch. Ristau said such a quake would normally be expected along the Alpine faultline which runs for 600km up the spine of the South Island.

GNS researchers are investigating whether yesterday's earthquake had increased the stress on the Alpine faultline and whether the quake has unsettled the Wellington fault, potentially sparking further earthquakes.

Experts said the earthquake caused such widespread damage because of its shallow depth of 10km.

Dozens of aftershocks rattled Christchurch throughout the day, the strongest with a magnitude of 5.1.

Mark Quigley, a geologist lecturer at Canterbury University, said: "With a major earthquake you get the biggest shock then a series of aftershocks that are sticky parts of the fault which didn't move during the main earthquake that slip and rupture themselves."

Scientists from GNS Science, Victoria University and Stanford University in the United States are heading to Canterbury to join colleagues from Canterbury University to find out more about the geological and environmental impacts of the quake and its effects.

They will deploy battery-powered earthquake instruments to record aftershocks for the next three weeks.

The quake was a combination of reverse fault and strike slip quake, meaning the earth shifted horizontally and was thrust up vertically.

Quigley estimated the quake would have displaced the earth by 3m or 4m. His team of researchers spent yesterday scouring the Darfield area for a zone where the quake had ruptured the surface.

"It will look like a step in the land," said Quigley. "We expect to see broken rivers and large fissures, large cracks in the earth's surface, broken roads - generally things that have been offset."

GNS Science has received reports of the earthquake being felt as far north as Northland as well as Wanganui, Taranaki, Wellington and Dunedin.

How earthquakes are felt depends on the geology of the earth. Quigley's Christchurch suburb of Avonside was severely affected by flooding, broken roads and soil erupting out of the ground because it is built on a soft foundation of the Avon River flood plain.

"There is a lot of water in the soil that gets shaken up and liberated during the earthquake and that causes sand volcanoes," said Quigley.

Coastal areas, such as Brighton, suffered damage for the same reason. Other areas of Christchurch built on more solid bedrock experienced a lesser degree of damage.