Earthquakes happen every day in New Zealand. Instruments record the ground shaking from over 20,000 earthquakes in and around the country each year. Most are too small to be noticed, but between 150 and 200 are big enough to be felt. Between 1840 and 2016, earthquakes in New Zealand caused 501 deaths directly or indirectly.
Earth’s seemingly solid outer surface – the continents with their submarine shelves, and the rocky floor of the deep ocean – is in fact divided like a jigsaw puzzle into huge sections called plates. Driven by convection currents deep within the earth, these plates move a few centimetres per year across the surface of the planet – the rate at which your fingernails grow.
Earthquakes are most frequent in regions where two moving plates meet and press against each other. New Zealand is in such a region – it straddles the boundary between the Pacific Plate, which covers almost a quarter of the earth’s surface, and the Australian Plate.
Where plates collide, the brittle top layer of the plate – the crust – slowly distorts, and stress builds up over many years until the crust ruptures.
Earthquakes usually occur along faults, which are existing fractures in the crust. Sometimes the blocks of rock on either side of a fault abruptly shift to a new position in just a few seconds. This sudden release of energy sends out waves, which are felt on the surface as an earthquake. The strength of the quake depends on the area of fault that has shifted and the amount of movement.
In 1935, the American seismologist Charles Richter invented a scale to indicate the strength of an earthquake. The Richter magnitude was based on the largest amplitude ‘wiggle’ recorded on a seismograph. Richter’s method worked well for small to moderate earthquakes, but the magnitudes of very large earthquakes were underestimated. Today, with better instruments, scientists can measure the energy of different types of earthquake waves. The size of earthquakes is referred to as their magnitude, but this is no longer measured on the Richter scale.
The most powerful New Zealand earthquake on record occurred in Wairarapa in 1855. During this quake, land moved along at least 140 kilometres of the Wairarapa Fault. Where the movement was greatest, the land shifted horizontally more than 18 metres, and part of the adjacent Remutaka Range rose more than 6 metres.
Not all of the movement may occur during the initial earthquake. Subsequent quakes, called aftershocks, occur nearby as the earth adjusts to the dislocation of land along the fault. Aftershocks may occur for weeks, months or even years after a large earthquake. For example, the Canterbury area experienced over 10,000 aftershocks in the five years after the 2010-2011 earthquakes.
One measure of the energy released by an earthquake at its source is earthquake magnitude. Magnitude is commonly determined from the shaking recorded on a seismograph. Each unit of magnitude on the scale represents a substantial increase in energy. A magnitude 5 earthquake, for example, releases more than 30 times more energy than a magnitude 4, and a magnitude 6 more than 900 times more energy than a magnitude 4. The powerful 1855 Wairarapa earthquake – the largest recorded in New Zealand in the last 200 years – had a magnitude of about 8.2.
The damage caused by earthquakes depends in part on the type of ground beneath buildings. Bedrock generally shakes the least, while soft sediment or artificial fill can amplify shaking. Some loose, water-saturated soils may even liquefy, producing geysers of sand or mud. Buildings on liquefied soils may settle, fracture or list like ships. Liquefaction was a major cause of damage to thousands of residential houses and the water and sewerage network in eastern Christchurch in the 2010–11 earthquake sequence. Maps showing zones with different ground-shaking amplification, based on ground conditions, are available for several New Zealand cities.
As they travel outward from the earthquake source, earthquake waves lose energy and the shaking they cause gradually diminishes in intensity. Shallow earthquakes, originating near the surface, cause more severe shaking and damage than deep earthquakes of the same magnitude.
To study the effects of a particular earthquake in different parts of the country, the intensity of shaking at a number of localities is determined using the Mercalli scale. This scale ranks the severity of shaking by its effects on people, buildings and the environment – from barely perceptible tremors to ground movements that can topple large buildings. The amount of damage to buildings caused by a given amount of shaking, however, varies from country to country, depending on the way the buildings are constructed. New Zealand uses a modified version of the Mercalli scale that is based on the local building types.
New Zealand’s earthquakes originate from the collision between the Australian and Pacific plates.
New Zealand is the visible part of a largely submerged small continent. The islands New Zealanders live on are the continent’s highlands, thrust above sea level by the collision of the Australian and Pacific plates. The boundary between these two moving plates runs diagonally through the country.
The North Island and its continental shelves, which lie under the sea, are on the Australian Plate, as is the land west of the Alpine Fault in the South Island. The rest of the South Island is part of the Pacific Plate. The pattern of earthquakes in New Zealand reflects the activity of the plates along their boundaries.
The huge plates that make up the surface of the planet have two main types of rocky outer layer, or crust. Beneath the floor of the deep oceans is oceanic crust: this is about 8 kilometres thick, and made of dense rock. But most land areas and their offshore continental shelves are made of continental crust. This averages 35 kilometres in thickness and is made of lighter, relatively buoyant rock. Both types of crust may occur on a single plate.
Volcanic activity can produce several types of seismic activity and earthquakes. Magma and volcanic gases moving through underground conduits can generate volcanic tremor, a fairly continuous ground vibration. As molten rock rises beneath a volcano, however, it may break the surrounding rock, producing small to moderate volcanic earthquakes.
Where a plate with thin oceanic crust collides with a plate with continental crust, the plate with the oceanic crust is forced down under the continental plate and into the subsurface – a process called subduction. Friction, however, prevents the down-going plate from sliding under smoothly. As it descends, it drags against the overlying plate, and eventually the crust of the overlying plate fractures, shifts or crumples, causing frequent shallow earthquakes.
Much more rarely, a large area of the down-going oceanic plate overcomes friction and abruptly shoves its way further under the overlying plate. This can produce very powerful earthquakes, which scientists call subduction earthquakes. Some of the largest earthquakes in the world, with magnitudes greater than 9, have been subduction earthquakes
Subduction is responsible for earthquakes in many New Zealand regions. In the North Island, no subduction earthquakes have occurred during the period of European settlement. Sediment deposits from subduction earthquakes, including tsunami deposits, have been found in coastal areas in the North and northern South Island.
Ocean crust of the Pacific Plate is descending under the eastern North Island and Marlborough. Here the land has splintered into long blocks separated by major faults. Along these faults, the blocks have intermittently shifted both horizontally and vertically. Some have been tilted upward, forming mountains such as the Tararuas in the North Island and the Kaikōuras in the South Island. Movement on these long faults has produced several earthquakes of magnitude 7 or greater, such as the 1848 earthquakes along the Awatere Fault in Marlborough, the 1855 Wairarapa earthquake and the 1888 earthquake along the Hope Fault in North Canterbury. The 2016 Kaikōura magnitude 7.8 earthquake resulted from the progressive rupture of a series of shorter faults along the north-eastern coast of the South Island.
In the central North Island, the brittle crust of the overlying plate is being pulled apart, and parts of the region are subsiding. The 1987 Edgecumbe earthquake occurred in this area. Small earthquakes, related to volcanic activity rather than crustal stress, also occur in the central North Island volcanic zone.
Near the south-western end of the South Island, the roles of the plates are reversed. Here the Australian Plate has a thin crust of oceanic rock. Just offshore from Fiordland, it descends beneath the thicker continental crust of the South Island. The magnitude 7.8 Fiordland earthquake on 15 July 2009 was a subduction earthquake – the result of the oceanic rock under the Tasman Sea moving inward under the South Island.
Subduction also causes very deep earthquakes. These earthquakes occur within the sinking oceanic crust as the stiff slabs are bent downward. There are distinctive zones of deep earthquakes beneath the North Island and Marlborough, and under Fiordland.
Sometimes slow slippage occurs along the subduction boundary over a period of days, weeks or months. These ‘silent earthquakes’ produce no seismic tremors and can be detected only by GPS measurements of movement of land above the subduction zone. They occur most frequently in the Gisborne, Hawke’s Bay, Manawatū and Kapiti regions. The 2016 Kaikōura earthquake triggered slow slip movement in all of these areas.
In the central South Island, the colliding Australian and Pacific plates are both thick continental crust, so one plate cannot sink under the other. Instead, the crust of the Pacific Plate is being buckled, broken and forced upward, creating the Southern Alps. The boundary between the plates is the huge Alpine Fault. Earthquakes along the Alpine Fault have often been very large – the last occurred in about 1717 AD, with movement along 375 kilometres of the fault. East of the Alpine Fault, earthquakes also occur on numerous smaller faults that criss-cross the region. There have been earthquakes in the magnitude 7 range near Arthur’s Pass and Murchison in 1929 and Īnangahua in 1968.
New Zealand has a national network of instruments and data centres, GeoNet, which detects and monitors earthquakes, volcanic activity, large landslides, tsunamis and the slow deformation that precedes large earthquakes. Data from instrument stations throughout the country is transmitted via satellite, radio and computer links to centres at Wairākei and Wellington, which are operated by GNS Science. GeoNet’s computers automatically analyse any earthquakes above a given magnitude and post the information on their website. This website has become New Zealand’s primary source of immediate information when people feel an earthquake. GeoNet staff can provide information to help emergency services respond rapidly, and high-quality data is freely available to researchers. GeoNet centres routinely locate more than 20,000 earthquakes per year. The year 2016 was exceptional, however, with 32,828 quakes recorded due to numerous aftershocks from several major earthquakes.
New Zealand scientists have found that some large earthquakes may be preceded by distinctive changes in the pattern of smaller ones over several years. However, it is not yet possible to predict where and when earthquakes will strike, or their likely magnitude.
Some areas of New Zealand have a higher probability than others of damage from earthquakes. The frequency of large earthquakes can be estimated from historical records of earthquakes since European settlement, from geological evidence of past large earthquakes, and from instrumental records of smaller quakes.
Geological studies provide evidence of the timing and size of past earthquakes. By examining faults where they cut the surface and digging trenches through them, scientists can determine when these last ruptured and how frequently they moved in prehistoric times. As earthquakes commonly recur along faults, their history may indicate their likely future behaviour.
Additional information about possible future earthquakes comes from measuring land movement in areas that are being deformed, using techniques such as Global Positioning System (GPS) measurements and radar. This information can be used to identify areas where the likelihood of earthquakes is increased or reduced.
Accurately predicting ‘the big one’ remains a conundrum for scientists. Over the centuries, many signs of an impending earthquake have been proposed. Animals are thought to behave strangely – sheep and cattle were restless 15 minutes before the 1968 Īnangahua earthquake. Unusual lights in the sky have preceded some earthquakes. These may be related to changes in the ground’s electrical conductivity. But the claim that earthquakes are preceded by ‘earthquake weather’ – sultry, ominous conditions – is unproven, as no direct correlation has been found between weather and seismic activity.
Earthquake swarms or foreshocks have heralded some earthquakes, including the 1888 North Canterbury, 1929 Murchison and 1987 Edgecumbe quakes. In the weeks before the North Canterbury and Murchison earthquakes, people reported booming noises – small earthquakes that were heard rather than felt. More often, however, powerful earthquakes have struck with little warning, or even following a quiet period.
The ability to accurately predict an impending earthquake could result in life-saving measures, including evacuation of a region. But the social and economic cost could be high if the prediction proved to be wrong. The ultimate goal of prediction is to save lives and minimise damage, disruption and the cost of recovery. As yet there is no sound scientific basis for earthquake prediction, so communities in regions with a high probability of earthquakes must be prepared, by designing earthquake-resistant buildings and structures, planning for civil defence and educating the public on emergency measures.
New Zealand is one of the few countries in the world with national government earthquake insurance for homeowners. In 1944, two years after earthquakes seriously affected Wairarapa and Wellington, a national commission was set up to cover damage from earthquakes. Cover for landslides, tsunamis, volcanic eruptions and hydrothermal activity was added later. In 1993, war damage was excluded and the commission became the Earthquake Commission (EQC). EQC insurance covers homes and their contents up to a specified monetary limit, but not commercial properties or motor vehicles. Homeowners automatically get EQC cover when they buy fire insurance.
EQC maintains a natural disaster fund, invested both in New Zealand and overseas, and buys catastrophe reinsurance. EQC provides core funding for the GeoNet monitoring network, and funds hazard research and public education programmes.
Traditionally the response to disasters such as earthquakes was the responsibility of local communities, with assistance from the government on a case-by-case basis. The scale of the 1931 Hawke’s Bay earthquake showed the need for a national response to disasters as well as for public education.
A national emergency management system has gradually evolved since the Second World War. The current system is based on a network of emergency response teams in regional centres, with a core of trained staff and volunteers, co-ordinated by the Ministry of Civil Defence and Emergency Management.
Engineers have a saying: ‘Earthquakes don’t kill people, buildings do.’ Destructive earthquakes have taught New Zealanders hard lessons in designing safe buildings. In early Wellington, buildings of brick and masonry collapsed in the 1848 earthquake. As a result, the town was largely rebuilt in wood, and suffered less damage during the magnitude 8.2 earthquake of 1855.
Widespread damage from the 1929 Murchison and 1931 Hawke’s Bay earthquakes had a profound effect on public perceptions of the hazard posed by earthquakes. Attention was focused on weaknesses in building construction, especially poor building standards and the lack of any provision for earthquake-resistant design. This led to a draft by-law in 1931, which was incorporated into a building code in 1935. The code recommended standards of design and construction so that buildings could resist the horizontal motions created by ground shaking. Masonry buildings had to be firmly bonded, with parts tied together so the structure would move as one unit.
Building codes in 1965, 1976, 1984 and 1992 have added requirements to accommodate changes in building materials and design. Rather than prescribing specific materials, designs or construction methods, the 1992 New Zealand code outlines how a building must perform to withstand the forces expected during an earthquake. This allows builders to use innovative design and construction methods to create earthquake-resistant buildings. For a moderate earthquake, the main aim is to protect a building from structural damage. For a major earthquake, however, the goal is to protect life by ensuring a building will not collapse and people can escape from it, even if the building itself is badly damaged.
Earlier building codes applied only to new construction, but current codes require many older buildings to be brought up to specified safety standards. A number of historic buildings have been strengthened, including Parliament Buildings in Wellington.
Major earthquakes in 2010–11 and 2016 have tested the building codes. The magnitude 7.1 Canterbury earthquake in September 2010 damaged a number of older brick and masonry buildings in Christchurch, causing several major injuries. More violent shaking in February 2011 caused the collapse of two relatively new office buildings that was responsible for 133 of the 186 deaths in the Christchurch earthquake.
Most older buildings in central Wellington were built between 1880 and 1930, and were not designed to resist earthquakes. In the 1970s, the city council required such buildings to be demolished or strengthened. Such measures do not bring a building up to modern construction standards, but are aimed at avoiding collapse and minimising loss of life. No buildings collapsed from shaking during the November 2016 Kaikōura earthquake, but a number of buildings suffered structural damage and needed to be demolished.
The department of civil engineering at the University of Canterbury in Christchurch has gained international recognition for its research into the behaviour of reinforced and pre-stressed concrete in buildings and bridges during earthquakes. Their analysis and design methods have been used in structural design codes in New Zealand and overseas. Books by New Zealand scientists and engineers have become standard texts.
Many buildings and bridges in New Zealand and overseas are protected with lead dampers and lead and rubber bearings invented by a New Zealander, Bill Robinson. These devices in building foundations can reduce the motion caused by ground shaking. Te Papa Tongarewa, the national museum of New Zealand, and Parliament Buildings have been fitted with these bearings.
Earthquakes also threaten city lifelines – water, sewerage and drain pipes, gas, electricity, telecommunications and transport networks. Lifeline engineering aims at reducing both the damage and the time needed to restore services. In New Zealand, several measures have been undertaken to protect utilities. Flexible joints or ductile pipes have been used for water pipelines across unstable ground to prevent rupture. Similarly, gas pipelines have been welded to prevent breakage, or replaced by polythene pipes. Some Wellington bridges and overpasses have bearings or dampers to reduce movement, preventing concrete decks from collapsing.
The Clyde Dam in Central Otago is built to withstand intense shaking, even though it is in a region where the probability of a major earthquake is low. The dam is built across the River Channel Fault, and has been constructed with a specially designed slip joint. If the land on either side of the fault moves during an earthquake, the joint will allow sections of the dam to shift up to 2 metres horizontally and 1 metre vertically without the dam failing.
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