Natural Hazards in New Zealand

Background on natural hazards in New Zealand (download pdf 350 kb)



Introduction.

You may be looking for information or you have purchased a Property Hazard Report from PropertyInsight but would like to have more background information on natural and man-made hazards that may occur. This report contains information on natural hazards in New Zealand and explains the meaning of the expressions used in the Property Hazard Report. It also includes additional references to organizations capable of providing even more information.

What is a natural hazard?

A natural hazard is a process taking place in the natural environment that potentially impacts on human life, property, or both. More specifically the term "natural hazard" refers to atmospheric (e.g cyclones), hydrologic (e.g. flooding), geologic (e.g. earthquake or volcanic eruption) and bush fires. The qualifier "natural" eliminates such exclusively man-made phenomena as pollution and chemical contamination.

What are common hazards in New Zealand?

Hazards having a human and financial impact on New Zealand are:

• Landslides


• Flooding


• Earthquakes


• Coastal hazards


• Climate and weather related hazards


• Volcanic and geothermal hazards

When is a hazard of concern?

A hazard assessment is based around three major elements:

• The susceptibility of an area to specific hazard


• The likely strength or intensity of an event when it occurs (ie what is the potential for damage)


• The likelihood or chance of such an event occuring

The Property Hazard Report primarily assesses the "susceptibility" of an area to a hazard. When sufficient information is available on the intensity and likelihood of such an event occurring, an indication of the level of "risk" is provided.
A hazard is of concern when the area is susceptible to the hazard, the hazard can result in considerable damage and the chance of the event occurring is high ("high risk"). Sometimes the risk can not be fully assessed due to insufficient information. In that case the assessment is limited to "susceptibility" instead of "risk". It should also be noted that "risk" is a technical term. The "perceived" risk by communities may differ from the risk as calculated by experts. Tolerance of risk also differs from person to person. Hence the information provided in the Property Hazard Report is as inclusive as possible enabling individuals to validate their own assessment.

Landslide hazard

What is a landslide hazard?


A landslide is the movement of rocks and/or soil down a slope under the influence of gravity. Some landslides are extremely fast moving and others are slow. Some involve large amounts of material that can cause extensive damage to properties; others are only a minor inconvenience. Landslides can vary in size from a single boulder in a rock fall to huge volumes of rocks and soil that can cover many kilometres. Landslides can occur naturally after rain, floods, earthquakes, volcanoes or even vibrations from thunder. They can be caused or made worse by human activity such as: vegetation removal; creating unsupported slopes such as roadside cuttings; other excavation; or, increased soil moisture (from leaking water pipes etc).

A large landslide looks a bit like a snow avalanche. It makes a loud noise, and can have enough force to wipe out anything in its path. Many landslides happen in remote areas and don't affect people, but others can be a danger to life and property. Fast moving landslides can cause damage, injury, or loss of life, but even the slow creep of rock and soil down a slope can be destructive over time.Whether an area is susceptible to landslides depends on the slope, geology, soil and the proximity to cliffs or steep slopes (above or below the area of interest).

How is a landslide hazard determined?

The landslide hazard can be determined in different ways, depending on the availability of information. The hazard can be assessed based on field investigations, correlation with similar areas with historical recorded landslides or the use of a susceptibility model. The Property Hazard Report based on a susceptibility model combine a slope-geology and a runout-collapse model. The model results are complemented with historical information when available.
Valley floors may have a low susceptibility according to the slope-geology model but proximity to steep valley walls can substantially increase the susceptibility in some instances.

How much damage can a landside cause ?

It is a scientific fact that all hills eventually wear down and landslides are one of the processes involved. New Zealand is a hilly and mountainous country and as a result, landslides are widespread - they are one of the natural hazards that people are most likely to encounter. Landslides have partially or totally blocked State Highway 1 several times in recent history, disrupting travel plans and causing delays to thousands of motorists.

Landslides can be very dangerous (several New Zealanders have been killed by landslides) and can cause significant damage. In August of 1979, a huge landslide gave way under the Dunedin suburb of Abbotsford, destroying 69 homes (see picture).
New Zealand houses have been seriously damaged or destroyed by landslides in Northland, Auckland, Coromandel, Raurimu, Central Otago, the West Coast, and the Kapiti Coast.
The impact of landslides can also be more far-reaching than on just the people and buildings in their path. In 1998 a landslide in Kelson, Lower Hutt destroyed a sewer line, allowing raw sewage to enter the Hutt River. This raised the level of pollution in Wellington Harbour, so shellfish beds had to be closed.

Flooding hazards

What is a flooding hazard?

Flooding is the most common natural hazard in New Zealand and it is therefore the hazard most likely to affect you as a property owner. They are also the most common cause of a Civil Defence Emergency.
Flooding can be caused by:

• River flooding


• Flooding in flat areas due to heavy rainfall


• Flooding due to blocked drains or waterways block by debris

Areas prone to flooding are:

• Low-lying flood plains with active river systems


• Valley floors of steep river catchments that are susceptible to intense rainfall
  (e.g. tropical storms)


• Areas near sea-level and close to the coast.

Flood prone land may be protected by flood control schemes (e.g. stopbanks)

How is a flood hazard determined?

A flood hazard is determined in several ways, often depended on the information available:

• The extent of historical floods


• A qualitative assessment based on the best available knowledge


• Flood modeling techniques

With a flood model, local authorities use records of river flows and rainfall in their area to calculate how frequently floods of different heights occur and to determine the areas likely to be flooded. For example, the "1 in 100 year flood" zone is the area that has a one-percent or more chance of being flooded in any given year. Flood levels would be expected to reach this high, or higher, on average only once in a hundred years. Over the course of 1,000 years, these events would be expected to occur 10 times. However, it is still possible to have several "100-year floods" in a relatively short period. Changes to the rivers and catchments may also change the frequency of floods of various magnitudes.

How much damage can floods cause?

The amount of damage that a flood may cause can vary, depending on how deep the water is and how fast it is moving. The ponds of water that result from a blocked stormwater drain are less likely to cause as much damage as an overflowing stream or river. Floodwater contaminated by sewage, however, can cause substantial damage because items soaked by the water may no longer be usable.

Earthquake hazards

What is an earthquake hazard?

Elements taken into account for assessing earthquake hazard are:

• Likelihood of a certain intensity occurring


• Susceptibility to amplification (or increased shaking) due to local soil conditions


• Susceptibility to liquefaction due to local soil conditions


• Susceptibility to fault rupture

How many damaging earthquakes do occur?

New Zealand is a seismically active country, and earthquakes occur regularly in most regions The Institute of Geological and Nuclear Sciences records about 14,000 earthquakes in and around New Zealand each year. Most are small, but between 100 and 150 are big enough to be felt.

Only a small few of these will cause damage. In the last 50 years there have been more than 300 large earthquakes, but many occurred in unpopulated areas or were deep and so did not cause damage. The chance of a damaging earthquake affecting a property varies considerably across the country, as does the likely strength of the earthquake shaking that will be experienced. Most of the devastation caused by earthquakes is a direct result of strong ground shaking. Earthquake shaking can create other hazards, causing liquefaction of the ground (see "liquefaction" section below). The rupture and movement of the ground along a fault line can also be devastating. Their effects, however, are usually concentrated within relatively small areas it is the shaking that causes the most widespread damage because it affects all buildings and man-made structures over a large area.

What is a damaging earthquake: Richter Scale and Mercalli Intensity



Earthquakes are measured a number of different ways. The strength or intensity of an earthquake is expressed as the Modified Mercalli Intensity (MMI), which describes the effects of ground shaking. The MMI describes "how it actually feels", i.e. it indicates how an earthquake is actually experienced by humans, buildings, furniture etc. This should not be confused with magnitude, which represents a unique value for a particular earthquake.
The Richter Scale measures the magnitude of an earthquake where it occurs (i.e. the epicentre), but it does not take into account factors such as the distance of the earthquake from the surface of the earth, ground conditions/geology and other factors.



What is the likelihood or chance of a damaging earthquake?

A comparison of the average expected frequency of earthquake shaking for New Zealands main centres is given below. Under New Zealand standards, the earthquake frequency is regarded "high" in Wellington, "moderate" in Christchurch and "low" in Auckland and Dunedin.



What is the likelihood of increased shaking?

Different rock or soil types will behave different in any given earthquake. Like the ripples that radiate from a stone dropped in a pool, ground shaking decreases with increasing distance from the earthquake. However, unlike the water in the pool, the materials of the earth vary in strength and density.

In some places, shaking can be affected by the type and thickness of soil resulting in an increased intensity of shaking (ground shaking amplification). Higher levels of shaking typically lead to more damage to buildings and property.



Past large distant earthquakes have shown that very soft soils can amplify the
shaking intensity compared to neighbouring sites on other materials. The resultant shaking can be stronger and last longer and can cause damage to property that would not otherwise have occurred.



When an earthquake is large and located close by, shaking intensities can be very strong (MM8 or greater) over most rock or soil types, but there is an exception: past earthquakes have shown that very soft soils may reduce the high shaking intensities compared to neighbouring sites on other materials. This is because at these high intensities of shaking, the very soft materials can absorb some of the shock rather than transmitting it. In general this tends to protect houses from much of the earthquake shaking and resulting damage but may not protect high-rise buildings.
Classification of a property in terms of its behaviour during ground shaking is based on the best available geological maps as these are used to obtain a description of the rock/soil in the vicinity of a property and an assessment of the age of the materials. This information is used to estimate the thickness, strength,and stiffness of the soil or rock beneath a property. Comparison with similar materials elsewhere in New Zealand where earthquake shaking has been amplified allows an assessment to be made.

When is a property susceptible to liquefaction?

Liquefaction can occur at sites with very wet, loose, sandy or silty soils when they are strongly shaken during an earthquake. The shaking causes the soil particle structure to collapse, and any water present lubricates the soil, so the whole mass behaves more like a liquid than a solid. Cracks can form to the surface, and then slurry of sandy water can spurt upward as fountains at the surface. These features are called sand boils. They can be spectacular but rarely cause damage, though cleaning up the sand may be a sizeable task. With stronger shaking, liquefied soil can flow; causing buildings and other structures on the soil to tilt or sink, and structures buried beneath the ground can float towards the surface. Buildings and large objects either on or in a soil layer that liquefies tend to behave like ships at sea; they can tilt, sink or even float.
Liquefaction can have very damaging effects when it occurs near the bank of rivers, the edges of lakes, and along shorelines. When the underlying soil liquefies, the ground above tends to shift towards the low or unsupported side; a phenomena called lateral spreading. Few buildings or structures can survive the resulting ground movement or distortions.
The kinds of soil and surface materials most susceptible to liquefaction are those that have formed in watery environments, such as silted-up lakebeds and coastal lagoons behind some sand dunes. Reclaimed land that has not been well compacted is also susceptible. River terraces (gravel) are usually well drained, however, and thus are not likely to liquefy, nor are beachfront sands (which usually are well compacted) and, of course, dry soils. Seismic engineers are well aware of the potential dangers of liquefaction, so modern high-rise buildings are often supported by piles that penetrate through the surface soils to solid ground beneath. As a result, the buildings themselves are protected from the liquefaction, but their service connections can still be cut if the ground around the building settles.

What is fault rupture?

A fault is a fracture or zone of fracturing in rock along which there has been
displacement (possibly cm or km) of one side relative to the other. An active fault is one that has broken through to the surface and along which the areas on either side have moved in recent geological time. Studies have shown most active faults have moved several times in the past, in the same place and as a result active faults are considered likely to move again in the future. Buildings sited across a fault that ruptures are likely to suffer considerable damage. Not all faults can be located accurately and the information on how often movement occurs and how large future movements might be is often uncertain.
The fault rupture hazard is based on the accuracy with which the locations of active faults are known. In many places the exact position of the fault is not accurately known, and so the zone in which the rupture or associated ground deformation may possibly occur is broad. It is also likely that a number of active faults in New Zealand have yet to be discovered.
The Ministry for the Environment (MfE) has released interim guidelines on planning for development of land on or near to active faults. For more details on the proposal see http://www.mfe.govt.nz/

Tsunami hazard

What is a tsunami?

Tsunami is a Japanese word meaning " harbour wave ". A tsunami is a natural phenomenon consisting of a series of waves generated when a large volume of water in the sea, or in a lake, is rapidly displaced. Tsunami are known for their capacity to violently inundate coastlines, causing devastating property damage, injuries, and loss of life, but tsunami can be small as well. Even small tsunami can be life-threatening and damaging.

The principal sources of tsunami are:

• Large submarine or coastal earthquakes (in which significant uplift or subsidence of the seafloor or coast occurs).


• underwater landslides (which may be triggered by an earthquake, or volcanic activity).


• Large landslides from coastal or lakeside cliffs.


• Volcanic eruptions (e.g., under-water explosions or caldera collapse, pyroclastic flows and atmospheric pressure waves).


• A meteor (bolide) splashdown, or an atmospheric air-burst over the ocean.
  

In a tsunami, the whole water column from the ocean floor to its surface is affected, the initial disturbance creating a series of waves radiating outwards, until the waves either dissipate or collide with a shoreline. Tsunami waves can arrive at nearby shores within minutes, or travel across the deep ocean basins at speeds in excess of 500 kilometres per hour (km/hr). A very large disturbance (eg an earthquake above magnitude 8.5) is required to cause a tsunami that is damaging at great distances from its source. On the other hand, tsunami that are generated locally do not need such a large source to be large and damaging along nearby shores.

Tsunami hazards in New Zealand

New Zealand is located well out into the Pacific Ocean in such a place as to be vulnerable to tsunami generated by large earthquakes along the Pacific Rim, as far away as Alaska or as close as the east coast of the North Island, for example . Tsunami from South American sources pose the greatest distant source tsunami threat to New Zealand. A graphic reminder of this was the 22–23 May 1960 tsunami from Chile that produced a maximum height of nearly 3 m in Lyttelton Harbour (see picture below - courtesy of Lyttleton Port Co), and also affected other harbours all along the east coast from Northland to Stewart Island. Some west Coast harbours also experienced surges. The tsunami in August 1868 caused by a very large earthquake in southern Peru was larger. The tsunami took 15 hours or so to travel to New Zealand and had a wave height that reached up to 10 m in the Chatham Islands and up to 4 m along parts of the east coast, particularly affecting Banks Peninsula. One person was killed on the west coast of Chatham Island.

New Zealand is more at risk from local source tsunami, that is, tsunami generated "locally" by earthquakes, underwater or coastal landslides (which might be triggered by earthquakes) or volcanoes close to our coast.
The local source tsunami hazard and risk is high because of the potentially large tsunami wave heights and short warning time. The most recent example of a local source tsunami is the one that hit 120 km of coastline from Mahia Peninsula northwards on 25 March 1947 with heights up to 10 m. It was caused by an earthquake 50-60 km off the coast. The 1855 Wairarapa magnitude 8.2 earthquake also caused a tsunami that reached 10 m - at Te Kopi, in Palliser Bay. Water also swept over the beach at Lyall Bay and into shops along Lambton Quay. The tsunami affected the whole Cook Strait region and beyond, at least as far north as Otaki and at least as far south as the Clarence River mouth in northeastern Marlborough.
Tsunami with run-up heights of a metre or more have occurred about once every 10 years on average somewhere around New Zealand, a similar frequency to Hawaii and Indonesia, but about one third that in Japan. Smaller tsunami occur more frequently, the smallest of which are only detectable on sea-level recorders. Tsunamis of 10 m or more some where in New Zealand have occurred three times since European settlement in 1840. Maori oral histories have several stories that have been interpreted as tsunami.
New Zealand can expect tsunami in the future. Some coasts are more at risk than others because of their proximity to areas of high local seismic activity, or exposure to tsunami from more distant sources. No part of the New Zealand coastline is completely free from tsunami hazard
Unlike distant source tsunami that take many hours to reach New Zealand, local source tsunami can arrive in minutes, allowing almost no time to issue official warnings. The strong shaking that accompanies a large earthquake is nature’s tsunami warning sign. A tsunami may follow a strong earthquake and people should leave the coast immediately (see other section) and go to a safe place. Another natural warning sign is the strange behaviour of the sea – either a sudden and unusual recession or rise, or a strange roaring, likened by some to the sound of a jet or motorbike.

Potential damage from tsunamis

Tsunami damage and casualties are usually from four main factors:

• Impact of swiftly-flowing torrent (up to 40 km/hr), or travelling bores, on vessels in navigable waterways, canal estates and marinas, and on buildings, infrastructure and people where coastal margins are inundated. Torrents (inundating and receding) and bores can also cause substantial erosion both of the coast and the sea-floor. They can scour roads and railways, land and associated vegetation. The receding flows, or "out-rush", when a large tsunami wave recedes are often the main cause of drowning.


• Debris impacts—many casualties and much building damage arise from the high-velocity impulsive impacts of floating debris picked up and carried by the in-rush (inundating) and out-rush (receding) flows.


• Fire and contamination—fire may occur when fuel installations are floated or breached by debris, or when home heaters are overturned. Breached fuel tanks, and broken or flooded sewerage pipes or works can cause contamination. Homes and many businesses contain many harmful chemicals that can be spilled.


• Inundation and saltwater-contamination by the ponding of potentially large volumes of seawater can cause medium- to long-term damage to buildings, electronics, fittings, and to farmland.

The GNS report "Review of the Tsunami Hazard and Risk in New Zealand" made a comparison with direct losses from earthquakes, which GNS modelled previously. In summary, the damage to property from tsunami is about twice what GNS expect from earthquakes with similar return period, and the deaths and injuries are many times more. A caveat here is that the projected deaths and injuries numbers in the report were based on there being no self-evacuation in response to natural warnings or no directed evacuation in response to official warnings,. For distant source tsunami, the relatively long time that is available to implement an appropriate response and evacuation means that the risk can be reduced or eliminated with appropriate public response to official warnings. In New Zealand, the Ministry of Civil Defence and Emergency Management is responsible for the distant source tsunami warning system. It receives tsunami warnings from the Pacific Tsunami Warning Center, evaluates the potential danger for New Zealand by consulting an expert panel and informs regional and local Civil Defence Groups. These Groups are responsible for determining the appropriate response in their area.
Early warning systems for local source tsunami pose a much greater scientific and operational challenge than those developed for distant source tsunami, and no system is presently available in New Zealand to give warnings for tsunami that may arrive minutes after the initiating event. Recognition of natural tsunami warning signs and preparedness for effective and immediate response to them by people will significantly reduce the risk to life-safety from locally generated tsunami.

Why are tsunami waves different from other waves?

The amplitude of tsunami waves in deep water is generally less than one metre, producing only a gentle rise and fall of the sea surface that is not noticed by ships, nor able to be seen by aircraft, although new satellites with sea-surface elevation technology can detect large tsunami in the deep ocean. When tsunami waves reach shallower waters, their speed decreases rapidly from their deep-ocean values, and at the same time their height increases (as the front of each wave slows down and the back of the wave, which is moving faster, catches up on the front, piling the water higher). A tsunami wave that is only half a metre high in the open ocean can increase to a devastating 10 m high wave travelling at 10-40 km/hr at impact with the shore.
Tsunami waves differ from the usual waves we see breaking on the beach or in the deep ocean, particularly in the distance between successive waves, and because tsunami waves occupy the whole ocean depth and not just the top few tens of metres as in storm waves. Both of these factors contribute to the huge momentum of water in a tsunami at the coast. In a tsunami, the distance between successive waves (called wavelength) can vary from several kilometres to over 400 km, rather than around 100 metres for normal waves at the beach. The time between successive tsunami wave crests (called period) can vary from several minutes to
a few hours, rather than the few seconds usual for beach waves. Hence, when tsunami waves reach the shore, they continue to flood inland over many minutes, and then the waves may retreat over as many minutes, before the arrival of the next wave. The waves may come in at irregular intervals, often without complete withdrawal of the inundating water from previous waves due to retardation of the outflow and impoundments. The first wave to arrive may not be the largest wave.


Information sources

Property Hazard Report

• PropertyInsight (www.propertyinsight.co.nz/)


• Institute of Geological and Nuclear Sciences Limited (http://www.gns.cri.nz/)


• Quotable Value Limited (http://www.qv.co.nz/)

Other sources of information

• Earthquake Commission - EQC (http://www.eqc.govt.nz/)


• Building Research Association New Zealand - BRANZ (http://www.branz.co.nz/)


• Land Information New Zealand - LINZ (http://www.linz.govt.nz/)


• Ministry of Civil Defence and Emergency Management (http://www.civildefence.govt.nz/)


• GeoNet ( www.geonet.org.nz/ )


• Encyclopedia of New Zealand (www.teara.govt.nz)

Photos and images are provided for education purposes are courtesy of GNS and GeoNet.

HazardInfo.pdf
Background_Report_v3%201%20.pdf
Background_Report_v3_1.pdf
Background_Report_v3_1.pdf