A major earthquake occurred in Christchurch on Tuesday 22 February 2011 at 12:51 p.m. local time (23:51 UTC, 21 February). The Mw 6.2 (ML 6.3) earthquake struck the Canterbury Region in the South Island, centred 6.7 kilometres (4.2 mi) south-east of the central business district. It caused widespread damage across Christchurch and killed 185 people, making it New Zealand's fifth-deadliest disaster. Scientists classified it as an intraplate earthquake and a potential aftershock of the September 2010 Canterbury earthquake.
Christchurch's central city and eastern suburbs were badly affected, with damage to buildings and infrastructure already weakened by the 2010 Canterbury earthquake and its aftershocks. Significant liquefaction affected the eastern suburbs, producing around 400,000 tonnes of silt. The earthquake was felt across the South Island and parts of the lower and central North Island. While the initial quake only lasted for approximately 10 seconds, the damage was severe because of the location and shallowness of the earthquake's focus in relation to Christchurch as well as previous quake damage. Subsequent population loss saw the Christchurch main urban area fall behind the Wellington equivalent, to decrease from second- to third-most populous area in New Zealand. Adjusted for inflation, the 2010–2011 Canterbury earthquakes caused over $54.8 billion in damage, making it New Zealand's costliest natural disaster and one of the most expensive disasters in history.
The 2011 Christchurch Earthquake was a 6.3-magnitude intraplate earthquake. It may have been an aftershock of the 7.1-magnitude 4 September 2010 Canterbury earthquake. New Zealand's GNS Science has stated that the earthquake was part of the aftershock sequence that had been occurring since the September magnitude-7.1 quake, however a seismologist from Geoscience Australia considers it a separate event given its location on a separate fault system.
Although smaller in magnitude than the 2010 earthquake, the February earthquake was more damaging and deadly for a number of reasons. The hypocentre was closer to Christchurch and was shallower, 5 kilometres (3 mi) underground, whereas the September quake was measured as having a depth of 10 kilometres (6 mi). The February earthquake occurred during lunchtime on a weekday when the CBD was busy, and many buildings were already weakened from the previous quakes. The peak ground acceleration (PGA) was extremely high, and simultaneous vertical and horizontal ground movement was "almost impossible" for buildings to survive intact. Liquefaction was significantly greater than that of the 2010 quake, causing the upwelling of more than 200,000 tonnes of silt which needed to be cleared. The increased liquefaction caused significant ground movement, undermining many foundations and destroying infrastructure, damage which "may be the greatest ever recorded anywhere in a modern city". 80% of the water and sewerage system was severely damaged. The earthquake also caused an increased spring activity in the Avon River / Ōtākaro and Ōpāwaho / Heathcote River.
GNS Science stated that the earthquake "arose from the rupture of an 8 km x 8 km fault running east-northeast at a depth of 1–2 km beneath the southern edge of the Avon-Heathcote Estuary and dipping southwards at an angle of about 65 degrees from the horizontal beneath the Port Hills."
While both the 2010 and 2011 earthquakes occurred on "blind" or unknown faults, New Zealand's Earthquake Commission had, in a 1991 report, predicted moderate earthquakes in Canterbury with the likelihood of associated liquefaction.
Initial measurement of peak ground acceleration (PGA) in central Christchurch exceeded 1.8 g (i.e. 1.8 times the acceleration of gravity), with the highest recording 2.2 g, at Heathcote Valley Primary School, a shaking intensity equivalent to MMI X+. Subsequent analysis revised the Heathcote Valley Primary School acceleration down to 1.37 g, with the 1.89 g reading at Pages Road Pumping Station in Christchurch revised down to 1.51 g. Nevertheless, these were the highest PGAs ever recorded in New Zealand; the highest reading during the September 2010 event was 1.26 g, recorded near Darfield. The PGA is also one of the greatest-ever ground accelerations recorded in the world, and was unusually high for a 6.2 quake, and the highest in a vertical direction. The central business district (CBD) experienced PGAs in the range of 0.574 and 0.802 g. As a comparison, the 7.0 Mw 2010 Haiti earthquake had an estimated PGA of 0.5 g. The acceleration occurred mainly in a vertical direction, with eyewitness accounts of people being tossed into the air.
Based on the local intensity reports, the maximum intensity assigned was XI (Extreme).
The upwards (positive acceleration) was greater than the downwards, which had a maximum recording of 0.9 g; the maximum recorded horizontal acceleration was 1.7 g. The force of the earthquake was "statistically unlikely" to occur more than once in 1000 years, according to one seismic engineer, with a PGA greater than many modern buildings were designed to withstand. Although the rupture was subsurface, satellite images indicated that the net displacement of the land south of the fault was 50 cm westwards and upwards; the land movement would have been greater during the earthquake. Land movement was varied around the area horizontally—in both east and west directions—and vertically; the Port Hills were raised by 40 cm.
The earthquake was a "strike-slip event with oblique motion" which caused mostly horizontal movement with some vertical movement, with reverse thrust causing upwards vertical movement. The vertical acceleration was far greater than the horizontal acceleration.
At the time of the earthquake, the New Zealand building code required a building with a 50-year design life to withstand predicted loads of a 500-year event. Initial reports by GNS Science suggested that ground motion "considerably exceeded even 2500-year design motions", beyond maximum considered events (MCE). By comparison, the 2010 quake—in which damage was predominantly to pre-1970s buildings—exerted 65% of the design loading on buildings. The acceleration experienced in February 2011 would "totally flatten" most world cities, causing massive loss of life; in Christchurch, New Zealand's stringent building codes limited the disaster. It is also possible that "seismic lensing" contributed to the ground effect, with the seismic waves rebounding off the hard basalt of the Port Hills back into the city. Geologists reported liquefaction was worse than the 2010 earthquake.
The earthquake generated a significant series of its own aftershocks. More than 361 aftershocks occurred in the first week following the magnitude 6.3 earthquake.
The largest was a magnitude 5.9 tremor which occurred just under two hours after the main earthquake.
A 5.3-magnitude aftershock on 16 April, the largest for several weeks, caused further damage, including power cuts and further rock falls on the Port Hills.
An aftershock centred near Rolleston measuring Mw 4.9 (ML 5.3) hit the region on 10 May. It caused minor damage to buildings in the city centre and was felt as far away as Dunedin and Greymouth, but no casualties were reported.
On 6 June, a large aftershock measuring 5.5 on the Richter scale occurred and was felt as far away as Kaikōura and Oamaru. It caused liquefaction in the eastern suburbs and 20,000 homes to lose power.
A series of aftershocks occurred on 13 June. A tremor of 5.7 on the Richter scale was felt at 1 pm NZT, with a depth of 9 km, and its epicentre being at Taylors Mistake. A 6.0 tremor occurred just over an hour later, with a depth of 7 km, located 5 km south-east of the city. Power was cut to around 54,000 homes, with further damage and liquefaction in already weakened areas. The Lyttelton Timeball Station collapsed and Christ Church Cathedral sustained more damage. At least 46 people were reported injured. These were followed by a magnitude 5.4 quake at a depth of 8 km and centred 10 km south-west of Christchurch at 10:34 pm on 21 June 2011.