What causes a tsunami?
Tsunami are usually caused by earthquakes that move the sea floor. Other causes include;
- undersea landslides
- undersea volcanic eruptions
- meteorite impact
Sudden changes to the seafloor raises the water level above and causes the sea to flow away from the movement, creating waves.
In the open ocean, tsunami waves travel at 600 to 700 kilometres per hour (kph). In the deep ocean, waves from a large tsunami may be as little as 60cm high. They pass ships unnoticed. As they reach shallow water, they slow down to about 30kph and gain height.
In low-lying coastal areas waves can travel a long way inland.
Before a tsunami
The seawater may recede a long way out, sometimes hundreds of metres, before returning as a tsunami. Tsunamis are not just moving lumps on top of the ocean surface, they also include hollows. Sometimes the hollow reaches the coast first. When this happens, the ocean first draws down and sucks water away from the beach. It then rushes back in with great speed and force as the lumps arrive. People who notice the receding water have as little as five minutes to get inland to higher ground.
Other signs of a tsunami are a sudden rise or fall in sea level and hearing loud and unusual noises from the sea.
Tsunami in New Zealand
In New Zealand there have been about 10 tsunami higher than 5 metres since 1840. Some were caused by earthquakes that happened a long way away, but most were from seafloor earthquakes closer to New Zealand.
Risk of tsunami
The risk of tsunami in the Pacific Ocean is higher than for other oceans because of the Pacific Plate boundary or ‘Ring of Fire’. This zone has lots of earthquakes because of the movement of the tectonic plates. New Zealand is therefore at risk of tsunami.
Sources of tsunami
Tsunami can come from close or far off sources. If a tsunami is formed close to land then there may be very little warning. Tsunami formed by local earthquakes can reach the nearest coast before scientists can work out the location of the earthquake and issue a warning or start tsunami sirens.
- A strong earthquake may be the only warning of a tsunami.
- If an earthquake lasts for more than a minute or is strong enough that it is difficult to stand then you need to move quickly to higher ground.
- If you are at the beach and notice the sea suddenly recedes you also need to move to higher ground.
For tsunamis caused by far off earthquakes such as in South America, there will be warnings. Tsunamis from South America, Alaska and Japan take more than 12 hours to reach New Zealand.