Earthquakes

New Zealand experiences thousands of earthquakes every year. Most are either very deep or centred well offshore, and cause little damage or injury. But a large earthquake could happen any time and can be followed by aftershocks that continue for a long time.

 

 

Aerial earthquake photograph.

New Zealand lies on the boundary of the Pacific and Australian tectonic plates.

Tectonic plates are always on the move. Tension builds up as they scrape over, under or past each other. In some places movement between the plates is happening all the time, and this causes frequent small or moderate earthquakes. Other areas, where the movement is not constant, are likely to get stronger quakes separated by longer periods of time because the tension builds up and then is released in a bigger movement.

Most (though not all) earthquakes occur at faults, which are breaks that go deep within the Earth, caused by the movement of these plates. The point under the ground where the earthquake actually begins is called the hypocentre or focus. The place directly above it on the surface is called the epicentre.

Earthquakes cause vibration waves to travel through the ground. The first sign of a quake is often the rumbling sound caused by the ‘P’ (primary or push) waves travelling at about 20,000 kilometres an hour. That is twenty times faster than a jet aircraft. The ‘S’ (secondary or shear) waves follow along at about 10,000 kilometres an hour. They cause the main rolling and shaking effects of an earthquake.

There are two ways of measuring earthquakes:

  • The Richter scale measures the energy released by an earthquake called the earthquake's magnitude. The Magnitude (M) scale ranges from one to ten (the largest so far was the 9.5 Chilean earthquake in 1960). It is a logarithmic scale, which means that a magnitude seven earthquake is 32 times as powerful as a magnitude six quake. The 1855 Wellington earthquake had an estimated magnitude of 8.2, Napier was struck by an M 7.8 quake in 1931, and Kaikoura by an M 7.8 quake in 2016.

  • The Modified Mercalli (MM) scale is a measure of intensity based on the effects of the earthquake on people and buildings. This scale ranges from MM1 (smallest) to MM12 (largest). The 1855 Wellington and 1931 Napier earthquakes were both MM10 at their epicentres. The shaking intensity lessens as you get further away from the epicentre. An earthquake can only have one Magnitude value (how much energy), but it can have a range of intensities from very strong shaking if you are close, to very mild shaking if you are further away.

Strong earthquakes change the land, lifting it up or dropping it down. They cause landslides and rockfalls on hills and sometimes make solid land behave like a wobbling liquid (liquefaction). All of these effects can damage buildings, roads, pipes in the ground, electricity and telephone networks. Earthquakes create very strong shaking, which can damage and sometimes collapse buildings. Very big earthquakes in the sea floor sometimes create tsunami waves.

 

 

Drop, cover, hold

 

What do we do before an earthquake?

  • Practise your earthquake drill: DROP, COVER AND HOLD.

  • Identify safe places close to you at home or school.

What do we do during an earthquake?

  • DROP, COVER AND HOLD.

  • Stay where you are until the
    shaking stops.

  • If inside remain inside, if outside
    stay outside.

  • Do not attempt to run outside.

What do we do after an earthquake?

  • Listen to and follow all instructions from adults or the radio.

  • Stay Calm.
    If you can, help others who may need it.

  • Watch out for possible hazards.

  • Remember there may be some aftershocks.

Home learning

AmberDraw a plan of your house showing places to shelter in an earthquake. Show where to turn off water, electricity and gas. Share your plan with a friend and ask if they have thought of doing their own.

Make a plan with your family to get through an emergency. Think about the things you need every day and work out what  you would do if you didn't have them. Fill in this online form  then print it out, stick it on the fridge and make sure everyone knows the plan. 

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