The Christchurch earthquake
New Zealand’s location on the Pacific ‘Ring of Fire’ makes it particularly susceptible
to tectonic hazards. The country’s wealth, sophisticated monitoring systems and
small population of 4.4 million mean that it is much less vulnerable to the impacts of
natural hazards than countries such as Haiti. The earthquake which struck New
Zealand on 4 September 2010 was similar in magnitude to the one that hit Haiti in
January 2010, measuring 7.1 on the Richter scale, compared with the Haiti
earthquake’s 7.0. In New Zealand there were no deaths and only two serious
injuries, but Haiti suffered more than 300,000 casualties and 1.3 million displaced
people. Moreover, the disruption caused by the September earthquake in New
Zealand was quickly overcome, but there are still displaced people, badly damaged
buildings and serious problems of sanitation in Haiti. However, New Zealand was not
destined to escape unscathed from the tectonic forces at work in its South Island. On
22 February 2011 a magnitude 6.3 aftershock of the September earthquake struck
the country’s second largest city of Christchurch, killing 181 people and seriously
damaging the city. The deaths from this earthquake make it the second most deadly
natural disaster in New Zealand’s history, while the estimated cost of rebuilding of
around $NZ 15–16 billion will mean it is by far the country’s most expensive.
New Zealand’s plate tectonics
New Zealand is located along a section of the Pacific Ring of Fire where the Pacific
plate is converging with Indo-Australian plate. Subduction along this boundary is
responsible for volcanic activity in the North Island, which is also the location of New
Zealand’s major earthquakes. The largest of these, with a magnitude of 8.2, occurred
near the Wairarapa Plains on 23 January 1855. New Zealand experiences between
10,000 and 15,000 earthquakes each year, of which around 100–150 are large
enough to be felt, with several exceeding magnitude 6. On average magnitude 7
earthquakes occur once every 10 years, while magnitude 8 earthquakes occur once a
century. The South Island is less seismically active than the North Island and
experiences fewer earthquakes of a high magnitude. The September 2010
earthquake was the largest recorded in the Canterbury region. The interaction of the
Pacific and Indo-Australian plates in the South Island has resulted in the major Alpine
and Hope fault lines. It is movement along these faults, strike-slip faulting, and
numerous smaller faults that causes the South Island’s earthquakes, including the
deadly February 2011 Christchurch earthquake.
Geology of the 2011 earthquake
The February 2011 earthquake resulted from movement along a previously unknown
fault line running approximately east–west and located to the southeast of
Christchurch. A strike-slip fault resulted in movement both east and west, although
there was also upward movement (reverse thrust) along the fault line. The relatively
shallow (5 km) depth of the earthquake meant that, even though its magnitude was
lower, it was much more destructive than the 2010 earthquake. The February
earthquake’s epicentre, in the Port Hills, 10 km southeast of the centre of
Christchurch, was also much closer to the city (Figure 1). It seems likely that the
February earthquake was an aftershock of the 2010 earthquake, although some
geologists consider that it was a separate event because it occurred along a separate
fault. The February earthquake itself has produced over 300 aftershocks, the largest
of which, in June 2011, again measured magnitude 6.3. Both the location of the
epicentre and the geology of the Canterbury Plains on which Christchurch is located
compounded the destruction caused by the earthquake. The hard volcanic rocks of
the Port Hills resulted in seismic energy being reflected back up to the surface. In
addition, the fault line which ruptured lies below the alluvial sediments of the
Canterbury Plains. Because groundwater levels were close to the surface, ground
shaking during the earthquake caused liquefaction, which in turn led to significant
damage to buildings and infrastructure.
Hazard planning and mitigation in Christchurch
As would be expected, given New Zealand’s wealth and its experience of destructive
earthquakes, the country is well prepared for seismic hazards. A national network of
instruments and data centres, GeoNet, detects and monitors earthquakes and other
hazardous events, and can provide information to emergency services within
minutes of a major earthquake. Funding for GeoNet is provided by an Earthquake
Commission, established by the government to provide earthquake insurance to
homeowners. The commission is also responsible for public education, including a
campaign designed to encourage New Zealanders to ‘Quake Safe’ their homes. New
Zealand adopted a building code for earthquake-resistant buildings as early as 1935.
Codes are designed to protect buildings from structural damage during moderate
earthquakes and, in the case of major earthquakes, to ensure that buildings do not
collapse and people can escape, even if a building is badly damaged. Older buildings
have had to be reinforced, although only in the more earthquake-prone North
Island. In the South Island, such strengthening was not seen as essential, so
Christchurch’s heritage buildings were much more vulnerable.
The impact of the earthquake
New Zealand’s preparedness was not able to prevent the deadly consequences of
the February earthquake for a number of reasons:
 The shallow focus and the relative proximity of the epicentre to Christchurch
meant that the effects measured 8 on the modified Mercalli intensity scale.
 Many buildings had already been weakened by the September earthquake
 The earthquake occurred on a Tuesday at 12.51 p.m., so the city centre of
Christchurch was crowded. The peak ground acceleration was 1.8 times the
acceleration due to gravity. The shaking intensity this caused in central
Christchurch led to enormous damage to the city’s buildings, around half of
which were destroyed or severely damaged. Most disastrously, the six-storey
Canterbury Television building collapsed, killing 85 people. The six-storey PGC
House also collapsed, killing 18 people, and Christchurch’s tallest building,
the 26-storey Hotel Grand Chancellor, was displaced half a metre by the
earthquake, and had to be demolished. In total, more than 100,000 homes
and other buildings in Christchurch were severely damaged or destroyed by
the earthquake, and more than 100 additional buildings were damaged
beyond repair by an aftershock in June 2011.
Liquefaction
Liquefaction was widespread and severe in Christchurch (Figure 2). It has always
posed a risk to Christchurch as large parts of the city are underlain by soft sediments,
but the problem in 2011 was exacerbated by the unseasonably high water content of
the substrata. As the name implies, liquefaction is a natural phenomenon that occurs
when soils behave like a liquid and not a solid. A number of conditions determine
the likelihood of soils liquefying during an earthquake. These include soil properties
such as grain size, the water content of the soil and the severity of the earthquake.
During an earthquake the individual soil particles become re-arranged. The resultant
shrinkage of the pore spaces causes water to be squeezed out. In built up areas
liquefaction can result in the destruction of infrastructure such as roads, pavements
and underground pipes . Rupturing of pipes in Christchurch resulted in many schools
being closed even though the buildings were relatively intact. Because the soil mass
becomes compacted and decreases in volume, subsidence is common. The ground
can also shift sideways, a process known as lateral spreading.
Preventative measures
The most common way to reduce the impact of liquefaction is to increase the
strength of foundations. This can involve sinking deeper piles. Part of the AMI
Stadium, for example, was reinforced by a vast network of 10 m stone columns
covering an area of over 12,000 m2. Unfortunately, even this was not enough to
prevent subsidence of two stands by around
40 cm. Structural engineers are now considering driving piles down 25 m to reach
solid ground.
The February earthquake also resulted in significant slope failure in the southeastern upland Port Hills area. The volcanic rocks forming this southern region have
steep slopes. Some are remnants of ancient sea cliffs and are almost vertical. These
areas experienced rock and debris falls that caused both fatalities and considerable
damage to properties adjacent to the cliff base.
Reconstruction
Following the September 2010 and February 2011 earthquakes, the government
established the Canterbury Earthquake Recovery Authority to spearhead the
reconstruction of Christchurch. The estimated cost is £13 billion. The city has been
divided into four zones based on the amount of damage, the cost of rebuilding and
the ability of the land to sustain future earthquakes.
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The worst affected residential area, the Red Zone, extends along the banks of the
River Avon from the Bexley wetlands at the Avon estuary to Avonside, close to the
inner city. Land in this zone suffered considerable lateral spreading associated with
liquefaction. The government has agreed to buy over 5,000 of the worst-affected
insured properties or the land on which they are built, based on the 2007 rateable
value. Under the scheme these homes could not be repaired for at least 3 years.
In the Orange Zone there are another 10,000 homes, the fate of which is uncertain
until further geotechnical investigation is completed.
About 100,000 homes are located in the Green Zone and these are likely to be
rebuilt.
The process for assessing the White Zone, which includes Port Hills and the central
business district, has yet to be fully developed.
The demographic impact
It was widely reported after the earthquake in Christchurch that 70,000 people had
fled the city. However, research on the impact of natural disasters in developed
countries would suggest that after 12 months a city of the size of Christchurch
(380,000) is likely to have lost only about 10,000 people through outmigration.
Before the earthquakes of 2010 and 2011, Christchurch had population growth of
about 12,000 a year, mainly due to in-migration. Statistics New Zealand had
projected this trend to continue for the next two decades. The rate of increase is
now likely to be lower. Some sectors of the city’s economy have suffered
enormously but that could be partly offset by new activity in the construction sector.
Indeed, 6 months after the earthquake, although unemployment had risen, it was
still below the national average.
http://www.christchurchquakemap.co.nz
Figure 1
Figure 2