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 , 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 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 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.Christchurch Quake Map 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 a 5.1 magnitude aftershock had an epicentre just 7 km from the Christchurch city centre. On 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 2:07am. The largest of these, the "Boxing Day aftershock", with a magnitude of 4.9, at a depth of 12 km below OpawaGeoNet – New Zealand Earthquake Report – 26 December 2010 at 10:30 am (NZDT) was felt very strongly. It caused further damage to at least 20 buildings, the closure of central Christchurch,"Inner City Buildings Feel The Brunt of Shakes" , The Press and cut power to more than 40,000 residents for some time. On a magnitude 5.1 aftershock with a depth of struck at 6:03am, waking many residents. The shock caused some minor damage and was felt as far away as the West Coast and Oamaru. On an earthquake of magnitude 6.3 was centred at Heathcote Valley, from Lyttelton and at a depth of . 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 . 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 10:34pm. 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 5:39am. 23 December 2011 featured another series of strong shocks, including a 5.8 at 1:58pm and a 6.0 at 3:18pm. 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 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 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.. Based on £7,000,000 in 1931 quarter 1 compared to 2010 quarter 2.
Effects in Christchurch