Frequently asked questions about tsunami.
- What causes a tsunami?
- Tsunami are most commonly cause by a seafloor earthquake. Other triggers are landslides, undersea volcanic eruptions, and meteorite impact. Sudden changes to the seafloor cause the ocean to flow away from the disturbance causing waves.
- How fast do tsunami waves travel?
- In the open ocean, tsunami waves travel at 600km/h to 700km/h. In the deep ocean, waves from a large tsunami may be as little at 60cm high. As they encounter shallow water they slow down to about 30km/h and increase in height.
- Why does seawater recede a long way out before a tsunami?
- Tsunami are not just moving waves on the ocean surface; they also include troughs, and sometimes the trough reaches the coast first. When this happens, the ocean first draws down and sucks water away from the coastlines. Then it rushes back in with enormous speed and force when the waves arrive. People who notice the receding water have as little as 5 minutes to flee inland to higher ground.
- Has New Zealand been hit by tsunami?
- New Zealand has experienced about 10 tsunami higher than 5m since 1840. Some were caused by distant earthquakes, but most by seafloor earthquakes not far from the coast.
- How vulnerable is New Zealand to tsunami?
- New Zealand is quite vulnerable to tsunami. Tsunami hazard for the Pacific basin is higher than for other oceans because of the 'ring-of-fire' - the zone of earthquakes and volcanoes associated with the tectonic plate boundaries around the Pacific. Because tsunami research is relatively new, the knowledge about the severity and frequency of local and distant tsunami is incomplete.
- If there is a tsunami warning what should I do?
- Authorities will give warnings about tsunami if they are coming from far away. Turn on your radio and follow instructions, take essential items if you are told to evacuate, and do not go to the beach to watch the waves. If you are near a beach and feel a strong earthquake or notice the water receding unusually, move to higher ground immediately. If you cannot go to higher ground, go at least 1.5 km inland. Do not return for at least one hour or until advised by authorities.