Story: Historic earthquakes
Page 13 – The 2011 Christchurch earthquake and other recent earthquakes
On Tuesday 22 February 2011 at 12.51 p.m. Christchurch was badly damaged by a magnitude 6.3 earthquake, which killed 185 people and injured several thousand. The earthquake epicentre was near Lyttelton, just 10 kilometres south-east of Christchurch’s central business district. The earthquake occurred more than five months after the 4 September 2010 earthquake, but is considered to be an aftershock of the earlier quake.
Casualties and damage
The earthquake occurred during lunch time, when many people were on the city streets. More than 110 fatalities were from the collapse of two multi-storey office buildings – the Canterbury Television and Pyne Gould Corporation buildings. Falling bricks and masonry on Manchester Street and Cashel Mall killed 11 people, and six died in two city buses crushed by crumbling walls. Rock cliffs behind houses collapsed in the Sumner and Redcliffs area, and boulders tumbled from the Port Hills summits, with five people killed by falling rocks.
A violent earthquake
Although not as powerful as the magnitude 7.1 earthquake on 4 September 2010, this earthquake occurred on a faultline that was shallow and close to the city, so the shaking was particularly destructive. In the February 2011 quake, the fault movement and structure of the bedrock produced exceptionally strong ground motion – up to 1.8 times the acceleration due to gravity in the eastern suburbs. In the city centre ground accelerations were three to four times greater than the ground motion produced by the September 2010 earthquake.
The earthquake brought down many buildings previously damaged in the September 2010 earthquake, especially older brick and mortar buildings. Many heritage buildings were heavily damaged, including the Provincial Council Chambers, Lyttelton’s Timeball Station, and both the Anglican Christchurch Cathedral and the Catholic Cathedral of the Blessed Sacrament. Among the modern buildings damaged, and eventually demolished, was Christchurch’s tallest building, the Hotel Grand Chancellor. Over a quarter of the buildings in the central business district were demolished.
Liquefaction was much more extensive than in the September 2010 earthquake. Eastern sections of the city were built on a former swamp. Shaking turned water-saturated layers of sand and silt beneath the surface into sludge that squirted upwards through cracks. Properties and streets were buried in thick layers of silt, and water and sewage from broken pipes flooded streets. House foundations cracked and buckled, wrecking many homes. Despite the damage to homes, there were few serious injuries in residential houses in liquefaction areas. However, several thousand homes will have to be demolished, and some sections of suburbs will probably never be re-occupied.
Aftermath and exodus
The government immediately activated its National Crisis Management Centre, and declared a national state of emergency the day after the quake. Christchurch’s central business district remained cordoned off for more than two years after the earthquake. Electricity was restored to 75% of the city within three days, but water supplies and sewerage systems took several years to restore in some areas affected by liquefaction.
In the weeks following the earthquake about 70,000 people were believed to have left the city due to uninhabitable homes, lack of basic services and continuing aftershocks. Timaru’s population swelled by 20% and thousands of pupils registered at schools in other cities and towns. However, many were expected to return to Christchurch as conditions improved.
Fault beneath the Port Hills
The earthquake was caused by the rupture of a 15-kilometre-long fault along the southern edge of the city, from Cashmere to the Avon-Heathcote estuary. The fault slopes southward beneath the Port Hills and didn’t break the surface, so scientists have used instrumental measurements to determine its location and movement.
2013 Cook Strait and Lake Grassmere earthquakes
At 5.09 p.m. on Sunday 21 July 2013 a magnitude 6.5 earthquake rocked central New Zealand, causing minor damage. The earthquake was centred in Cook Strait, about 20 kilometres east of Seddon in Marlborough. In Wellington city goods fell from supermarket shelves and plate glass and masonry fell from downtown buildings. Four people were hospitalised with minor injuries but there were no fatalities. The following day much of the central city was closed for business as buildings were assessed for structural damage.
The period of seismic activity continued with a 6.6 magnitude quake at 2.31 p.m. on Friday 16 August. Centred 10 kilometres south-east of Seddon, close to Lake Grassmere, the quake caused significant damage to buildings in Seddon and nearby towns. It was felt strongly in Wellington and through much of central New Zealand.
Acknowledgements to GeoNet and GNS Science