A magnitude 7.1 earthquake occurred near Christchurch at 4.35 a.m. on Saturday 4 September 2010. It was widely felt over the South Island and the southern North Island, and caused considerable damage in central Canterbury, especially in Christchurch. It was the largest earthquake to affect a major urban area since the 1931 Hawke’s Bay earthquake.
The epicentre was around 37 kilometres west of Christchurch, near the town of Darfield. Because of this, its scientific name is the Darfield earthquake, though it is more widely known as the Canterbury earthquake. It was a relatively shallow earthquake – about 10 kilometres below the surface of the Canterbury Plains – and produced the strongest shaking ever recorded in New Zealand. Ground near the epicentre moved up to 1.25 times the acceleration due to gravity. The earthquake was accompanied by a large surface rupture.
The Darfield earthquake was the first in New Zealand where social media were used for the sharing of information. GeoNet earthquake reports were sent out using both Twitter and Facebook after each major aftershock. Twitter was regularly swamped with messages reporting what people have felt within seconds, as well as demands that GeoNet hurry up and post the location and magnitude.
There was no loss of life, and few serious injuries. The quake occurred at a time when most people were in bed and the streets were largely deserted. The lack of casualties was also due to strict building regulations and partial strengthening of many older buildings.
The worst damage was suffered by older (mainly pre-1940s) buildings constructed of brick and masonry, and lacking adequate reinforcement. Some walls crumbled, with bricks cascading on to the streets. Brick chimneys toppled through tile roofs. One of the few cases of serious injury was caused by a falling chimney. A number of historic stone churches were badly damaged, although both the Anglican and Catholic cathedrals survived with minor cracking. An early Treasury estimate of the cost of the earthquake was $4 billion.
The Greendale Fault
A fault rupture occurred along a previously unknown faultline, which has been named the Greendale Fault. Movement along the fault has broken the surface, creating a fault trace that extends for 30 kilometres west from Rolleston. Roads, fences, shelter belts and irrigation channels have been offset sideways, in places up to 5 metres, with up to 1.3 metres vertical offset. The area to the north of the fault rupture has moved eastwards and the area to the south has moved westwards.
Aftershocks continued for several months after the earthquake, some strong enough to cause damage to already-weakened structures. The aftershocks were mainly clustered along the Greendale Fault, reflecting gradual readjustment of the earth’s crust to a major rupture. As some of the aftershocks at the eastern end of the Greendale Fault were close to Christchurch, they were felt particularly strongly.
Liquefaction and lateral spreading
The earthquake’s shaking turned water-saturated layers of soft sand and silt into liquid mush, a process known as liquefaction. The most badly affected areas were close to the coast, especially the town of Kaiapoi and the Christchurch suburb of Bexley, where a number of modern houses were damaged. The ground above the liquidised layers subsided unevenly, cracking footpaths, roads and houses. Water and sewer pipes broke, leading to local flooding. Silty sludge squirted upwards through cracks to produce small sand volcanoes.
Cracks opened up close to rivers and streams where the ground moved downslope towards the water, causing damage to man-made structures – a process known as lateral spreading.