The 2010 Canterbury earthquake (also known as the Darfield earthquake) struck the South Island of New Zealand with a moment magnitude of 7.1 at on 4 September, and had a maximum perceived intensity of X (Extreme) on the Mercalli intensity scale. Some damaging aftershocks followed the main event, the strongest of which was a magnitude 6.3 shock known as the Christchurch earthquake that occurred nearly six months later on 22 February 2011. Because this aftershock was centred very close to Christchurch, it was much more destructive and resulted in the deaths of 185 people.
The earthquake on 4 September caused widespread damage and several power outages, particularly in the city of Christchurch, New Zealand's second largest city at that time. Two residents were seriously injured, one by a collapsing chimney and a second by flying glass. At least two people died and over 1,700 were injured. Mass fatalities were avoided partly due to there being few houses of unreinforced construction, although this was also aided by the quake occurring during the early hours of the morning when most people were off the street.
The earthquake's epicentre was 40 kilometres (25 mi) west of Christchurch, close to the town of Darfield. The hypocentre was at a depth of 10 km. A foreshock of roughly magnitude 5.8 hit five seconds before the main quake, and strong aftershocks were reported, up to magnitude 5.4. The quake was felt as lasting up to 40 seconds, and was felt widely across the South Island, and in the North Island as far north as New Plymouth. As the epicentre was on land away from the coast, no tsunami occurred.
The National Crisis Management Centre in the basement of the Beehive in Wellington was activated, and Civil Defence declared a state of emergency for Christchurch, the Selwyn District, and the Waimakariri District, while Selwyn District, Waimakariri and Timaru activated their emergency operation centres. Initially, a curfew was established for parts of Christchurch Central City from 7 pm to 7 am in response to the earthquake. The New Zealand Army was deployed to the worst affected areas in Canterbury.
Insurance claims totalled between NZ$2.75 and $3.5 billion, although it is unclear how much cost can be attributed to each of the earthquake events in the 2010–2011 Canterbury earthquake sequence. The total estimated damage bill was up to $40 billion, making it the fifth-biggest insurance event in the world since 1953.
Geology
Background
In the first eighty years of European settlement in Christchurch (1850–1930), four earthquakes caused significant damage, the last of these centred at Motunau on the North Canterbury coast in 1922.
Modelling conducted for the New Zealand Earthquake Commission (EQC) in 1991 found that earthquakes with a Mercalli intensity of VIII (significant property damage, loss of life possible) could recur on average in the Christchurch area every 55 years. The study also highlighted the dangers of soil liquefaction of the alluvial sediments underlying the city, and the likelihood of significant damage to water, sewer and power supply services.
Geologists have recognised about 100 faults and fault segments in the region, some as close as 20 km to central Christchurch. The closest faults to Christchurch capable of producing powerful earthquakes occur in the Rangiora-Cust area, near Hororata, and near Darfield. However, the 2010 quake ruptured the ground surface on a previously unknown fault and involved slip on up to seven individual faults.
Event
The main quake occurred as a result of strike-slip faulting within the crust of the Pacific plate, near the eastern foothills of the Southern Alps at the western edge of the Canterbury Plains. The earthquake epicentre was located about 80–90 km (50–56 mi) to the south and east of the current surface expression of the Australia–Pacific plate boundary through the island (the Alpine and Hope Faults). Though removed from the plate boundary itself, the earthquake likely reflects right-lateral motion on one of a number of regional faults related to the overall relative motion of these plates and may be related to the overall southern propagation of the Marlborough fault system in recent geologic time. The peak ground acceleration (PGA) measured was 1.26 g (12.36 m/s2), recorded near Darfield. GNS scientists considered this an "extremely rare seismic recording made near a fault rupture". The February 2011 Christchurch earthquake experienced PGA of 1.51 g.
GNS seismologists and geophysicists, who believed the 7.1 Mw quake consisted of three or four separate quakes, quickly proposed at least two models for the quake.
John Beavan proposed a four-fault model consisting of a 6.5 Mw quake on the Charing Cross fault, followed by a 7.0 Mw quake on the Greendale Fault, and a 6.2 Mw quake near Hororata, with a 6.5 Mw quake on the fourth fault, which could run between West Melton, Sandy Knolls and Burnham. Caroline Holden proposed a three-fault model, consisting of a 6.3 Mw quake of 2–4 seconds on the Charing Cross fault, followed by a 6.9 Mw quake lasting 7–18 seconds on the Greendale Fault, and an approximately 6.5 Mw quake of 15–18 seconds near Hororata. Ultimately, the event appears to have been an extremely complex earthquake that involved up to seven faults.
Aftershocks
By 7 August 2012, the area had experienced 11,000+ recorded aftershocks of magnitude 2 or more, including 26 over 5.0 magnitude, and 2 over 6.0 magnitude. Many caused further damage to buildings in the Christchurch central business district, and some were felt as far away as 300 km to the south in Dunedin.
Notable aftershocks, in chronological order, include:
- On 8 September 2010 a 5.1 magnitude aftershock had an epicentre just 7 km from the Christchurch city centre.
- On 19 October 2010 a magnitude 5.0 aftershock with a depth of just 9 km caused surface shaking – reported at the time to be the worst since the original earthquake.
- On 26 December 2010, Boxing Day, a swarm of more than 32 shallow aftershocks, many centred directly under the city, occurred throughout the day, beginning with a 4.2 jolt at 2:07 am. The largest of these, the "Boxing Day aftershock", with a magnitude of 4.9, at a depth of 12 km below Opawa was felt very strongly. It caused further damage to at least 20 buildings, the closure of central Christchurch, and cut power to more than 40,000 residents for some time.
- On 20 January 2011 a magnitude 5.1 aftershock with a depth of 10 kilometres (6.2 mi) struck at 6:03 am, waking many residents. The shock caused some minor damage and was felt as far away as the West Coast and Oamaru.
- On 22 February 2011 an earthquake of magnitude 6.3 was centred at Heathcote Valley, 5 kilometres (3.1 mi) from Lyttelton and at a depth of 5 kilometres (3.1 mi). Seismologists regard it as an aftershock of the 2010 earthquake. Many buildings collapsed or were significantly damaged in the quake, among them Christchurch's iconic ChristChurch Cathedral. The earthquake resulted in 185 deaths.
- On 6 June 2011 a magnitude 5.5 aftershock with a depth of 15 km centred in Rolleston occurred at 9:09 am, causing power cuts and supermarket closures. The shock was felt widely in the South Island, with reports from as far south as Dunedin and as far north as Nelson. It was the sixth-largest aftershock since the magnitude 7.1 shake in September 2010.
- On 13 June 2011 a magnitude 6.3 earthquake struck Christchurch at a depth of 6 kilometres (3.7 mi). It was preceded by a magnitude 5.6 tremor, and affected many structures previously damaged by other earthquakes. The earthquake injured a total of 46 people, while many others were evacuated from the city centre.
- On 21 June 2011 a magnitude 5.4 aftershock with a depth of 8 km centred south-west of Halswell occurred at 10:34 pm. The shock was felt heavily in Christchurch, causing moderate-heavy content damage in western suburbs such as Upper Riccarton and Hornby. Halswell New World reported that more stock had fallen from shelves in this shake than in the 22 February and 13 June quakes.
- On 22 July 2011 a magnitude 5.1 aftershock with a depth of 12 km occurred at 5:39 am.
- 23 December 2011 featured another series of strong shocks, including a 5.8 at 1:58 pm and a 6.0 at 3:18 pm. The earthquakes interrupted power and water supplies, three unoccupied buildings collapsed and there was again liquefaction in eastern suburbs and rockfalls in hill areas. One person died after tripping on uneven ground caused by the earthquake.
Geotechnical reports
The Earthquake Commission has published two Geotechnical Land Damage Assessment & Reinstatement Report[s]. The first Stage-1 report was published on 21 October 2010. The report briefly describes the mechanics of the earthquake, underlying geology, residential land damage assessment, reconstruction considerations, land and building reinstatement, and remediation options.
The second Stage-2 report was published on 1 December 2010. It divides the quake-affected areas into three zones, and outlines the remediation plans for these areas.
Casualties, damage, and other effects
Most of the damage was in the area surrounding the epicentre, including the city of Christchurch, New Zealand's second-largest urban area with a population of 386,000. Minor damage was reported as far away as Dunedin and Nelson, both around 300–350 kilometres (190–220 mi) from the earthquake's epicentre.
The September 2010 earthquake caused two Christchurch residents to be seriously injured, one by a falling chimney and a second by flying glass, and led to many with less serious injuries. One person died of a heart attack suffered during the quake, with the coroner's report identifying the earthquake as being a contributing factor. Another person died after falling during the quake, although Coronial Services was unable to find the coroner's report for that death.
The subsequent 22 February 2011 aftershock caused a large number of deaths and casualties, along with significant damage to buildings and infrastructure.
Financial exposure
Total Earthquake Commission (EQC), private insurance and individual costs were estimated to reach as high as NZ$4 billion according to the New Zealand Treasury. Another projection of the cost covered by insurance (including EQC) was lower at $2.1 billion to $3.5 billion, but would still rank the quake as the world's fifth most costly to insurers.
The Earthquake Commission covers only domestic residences with private insurance and does not provide cover for businesses. The EQC payout is limited to the first $100,000 plus GST of any individual claim, with any amount above that covered by the insurance company holding the policy. The insurance companies themselves had limited or no exposure, having offloaded most of their risk to reinsurance companies. The EQC had reinsurance of $2.5 billion with a $1.5 billion excess, so its exposure was the first $1.5 billion and any amount after the first $4 billion. It had a total fund prior to this earthquake of approx $6 billion.
For comparison, the 1931 Hawke's Bay earthquake cost NZ£7 million, equal to approximately $650 million in 2010 dollars.
Effects in Christchurch
Reports of the quake's intensity in Christchurch generally ranged from V to VIII (Moderate to Severe) on the Modified Mercalli Intensity Scale. A strong smell of sulphur was widely reported in Christchurch following the earthquake. Sewers and water lines were damaged or broken. The water supply at Rolleston, located to the southwest of Christchurch, was contaminated. Power to up to 75 percent of the city was disrupted. Christchurch Hospital was forced to use emergency generators in the immediate aftermath of the quake. About 90% of the electricity in Christchurch had been restored by 6:00 pm the day of the earthquake. The repair of electricity was estimated to be more difficult in the rural areas. One building caught fire after its electricity was turned back on, igniting leaking LPG in the building. The fire was quickly extinguished by the Fire Service before it could spread. Damage to buried pipes may have allowed sewage to contaminate the residential water supply. Residents were warned to boil tap water before using it for brushing teeth, drinking, and washing or cooking food. Several cases of gastroenteritis were reported. By 7 September 28 cases had been observed at the city's welfare centres. The boil water notice for Christchurch and Banks Peninsula was lifted late on 8 September 2010, after more than 500 tests conducted over three days found no contamination.
Christchurch International Airport was closed following the earthquake and flights in and out of it cancelled. It reopened at 1:30 pm, following inspection of the terminals and main runway. All schools and early childhood centres in Christchurch City, Selwyn and Waimakariri Districts were ordered shut until Monday 13 September for health and safety assessments. The city's two universities, the University of Canterbury and Lincoln University, and the Christchurch campus of the University of Otago were also closed until 13 September awaiting health and safety assessments.
Crime in Christchurch decreased eleven percent compared with the previous year following the earthquake, although there were initial reports of looting in the city centre and "known criminals" trying to pass off as council workers to get into the central city cordon area. Police also observed a fifty-three percent rise in the rates of domestic violence following the earthquake.
Many more heart attacks than usual were reported in the days following the quake. Usually the heart unit at Christchurch Hospital handles two to three heart attacks a day, but the rate had risen to eight to ten a day after the earthquake. A record number of babies for a Saturday were born at Christchurch Women's Hospital in the twenty-four hours after the quake, with the first baby arriving six minutes after the initial shock. This was at least in part due to outlying birthing centres being closed, pending structural checks, forcing more mothers to deliver at hospitals than was generally the case.
Effects outside Christchurch
The quake's epicentre was near Darfield, approximately 40 kilometres (25 mi) from Christchurch. Four metres (13 ft) of sideways movement has been measured between the two sides of the previously unknown fault.