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  Services » Regional Services » Regional hazards and emergency management » Earthquakes

Earthquakes

On this page: Photograph of damage caused during the 1987 Edgecumbe earthquakeWaikato earthquakes; How earthquakes happen; Earthquake hazards;  Measuring earthquakes ; What we are doing; What you can do; Useful links.

 

Earthquakes are natural hazards that occur when the earth’s tectonic plates move against each other. They disturb the earth's surface, damaging people and property.

There are some specific areas in the Waikato region that contain active earthquake fault lines which are more likely to experience earthquake activity.

Major earthquakes in New Zealand include:

  • The Christchurch earthquake of 2011 (Magnitude 6.3 on the Richter Scale).
  • The Edgecumbe earthquake in the Bay of Plenty, March 1987 (Magnitude 6.5 on the Richter Scale).
  • The Napier (Hawkes Bay) earthquake of 1931 (Magnitude 7.8 on the Richter Scale).
  • The Wairarapa earthquake of 1855 (Magnitude 8.2 on the Richter Scale). This is New Zealand’s largest recorded earthquake.

Waikato earthquakes

New Zealand straddles the boundary between two tectonic plates - the Pacific Plate and the Australian Plate. These plates are moving towards each other by about 50 mm per year. This movement can be felt as tremors in the Waikato region.

There are three large active faults considered 'risky' in the Waikato region:

  • The Kerepehi Fault - extending along the Thames Estuary and the Hauraki Plains.
  • The Rangipo Fault - on the eastern side of Mount Ruapehu.
  • The Wairoa North Fault - running along the Bombay Hills north-east of Mercer.

There have been long gaps between earthquakes in the Waikato region. The area between Taupo and Rotorua is the most vulnerable part of our region for earthquakes. It contains many active fault lines running in a north-east direction.

Check out our map of earthquake zones in the Waikato region.

How earthquakes happen

Our map of New Zealand’s tectonic plates shows the relative movement between the Pacific and Australian Plates in millimetres per year.

small image of how earthquakes happen

Earthquakes occur when tectonic plates grind and scrape against each other, or when there's a sudden slip on a fault. A fault slip is caused when stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the earth's crust. These waves cause the shaking that we feel during an earthquake.

The size (magnitude) of an earthquake depends on the geology of the area, the basement rock and the soil that covers the rock. As earthquake waves pass through the ground, the degree to which they are felt depends on:

  • how close a person is to the epicentre of the earthquake
  • how well compacted the soils are covering the basement rock, and
  • the type of building a person is in during the earthquake.

During an earthquake waves of energy spread out from the epicentre, causing:

  • ground movement - greatest near the epicentre
  • soft, uncompacted soils (such as sands) to shake more violently than rock - moving building foundations
  • liquefaction - where there is water present in the ground. Sand spits and sites containing artificial fill are particularly at risk.

Earthquake hazards

Smaller earthquakes or tremors can shake items off shelves or walls, and cause minor cracking of building foundations and walls. In stronger earthquakes, people may be injured by falling objects, collapsing buildings or through falling into cracks or holes opening up in the ground.

Earthquakes can also threaten ‘lifeline’ services such as water, power, telecommunication and transportation networks.

Earthquakes can create other related hazards including:

'Aftershocks' may occur following an earthquake. They are smaller ground shakes that occur as rocks rearrange around the edges of ruptured faults. Aftershocks usually stop soon after an earthquake. However, small aftershocks can continue for several years after a large earthquake.

Measuring earthquakes

The size of an earthquake is measured by:

  • Magnitude - size of the earthquake from the vibration measured on a seismograph.
  • Intensity - the size of the earthquake as measured by the earthquake effects felt and seen by people.

The Richter Scale measures the magnitude of an earthquake, ranging from 0 to 10. An earthquake with a magnitude greater than 4.5 on this scale can damage buildings. Severe earthquakes have magnitudes greater than 7.


Map showing the application of the Modified Mercalli Intensity Scale to an hypothetical earthquake centred near Whakatane

The 'Modified Mercalli Intensity Scale' is used in New Zealand to:

  • explain the effects of earthquakes on buildings, roads and other structures
  • describe people's perceptions of the strength of an earthquake.

The Scale ranges from MM 'I' (not generally felt by people) to MM 'XII' (virtually total damage to buildings and land features). This information is used to create isoseismal maps.

Waikato Regional Council receives isoseismal maps from the Institute of Geological and Nuclear Sciences following an earthquake. The maps allow us to identify communities likely to be affected.

The isoseismal map (above right) shows the Modified Mercalli Scale for an earthquake near Whakatane.

What we are doing

Under the Resource Management Act 1991(external link)  Waikato Regional Council has responsibilities for:

  • identifying hazards through the resource consent and regional planning processes
  • ensuring risk assessments are undertaken
  • preparing and implementing risk mitigation plans (such as our Earthquake Risk Mitigation Plan) to minimise the effect of natural hazards on the Waikato economy and community
  • providing information on natural hazards to the public
  • providing early warnings of natural hazard events.

Waikato Regional Council has produced a ‘regional Ground Shaking Risk Zones and Active Fault Lines’ map showing active faults in the region and the extent of the Taupo Volcanic Zone and is used in our Waikato region Lifelines Study.

For policy information on natural hazards, see our Regional Policy Statement.(external link)

What you can do

The Ministry of Civil Defence and Emergency Management(external link) has a guide to preparing yourself for an earthquake, and what to do when an earthquake happens.

For more information on how to prepare (external link)for an emergency and who to contact when an emergency happens, see our Civil Defence and Emergency Management pages.

Ministry for the Environment(external link) also has proposed guidelines for building near active fault lines.

Useful links

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