A magnitude 7.1 earthquake occurred near Christchurch at 4.35 a.m. on Saturday 4 September 2010. Widely felt over the South Island and the southern North Island, it 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 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. During the strong shaking, the ground near the epicentre moved at 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 in which social media were used to share 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 had felt, as well as demands that GeoNet hurry up and post the location and magnitude.
There were few serious injuries, although one woman, Lillian Daniels-Witika, suffered a fatal heart attack due to the earthquake. Most people were in bed and the streets were almost 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 onto the streets. Brick chimneys toppled through tile roofs. A falling chimney caused one of the few serious injuries. A number of historic stone churches were badly damaged, although both the Anglican and Catholic cathedrals suffered only 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 fault line, which was named the Greendale Fault. Movement along the fault broke the surface, creating a fault trace extending for 30 kilometres west from Rolleston. Roads, fences, shelter belts and irrigation channels were offset sideways by up to 5 metres, with up to 1.3 metres vertical offset. The area north of the fault rupture moved eastwards and the area south of it 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. Aftershocks at the eastern end of the Greendale Fault, close to Christchurch, 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 up through cracks to produce small sand volcanoes.
Cracks opened up close to rivers and streams where the ground moved down towards the water, causing damage to man-made structures – a process known as lateral spreading.