Tsunami in New Zealand

Local, "near-source" tsunami are generated by large offshore earthquakes, landslides, and volcanic eruptions close to New Zealand. If the tsunami is caused by a local earthquake, the shaking associated with the event may be the only warning people get before this kind of tsunami arrives. Other natural signs could also include loud or unusual noises from the sea or a sudden unexpected change in sea level at the coast. Such a tsunami can arrive within minutes, before there is time to issue an official warning.

Pacific-wide events can be caused by a distant earthquake on the other side of the Pacific, like South America or Alaska. For these, there will be time to issue warnings before the tsunami arrives. Regional sources can arrive in New Zealand between one to three hours after the event. For these sources the there is still time for a warning, but it can be quite small.

There has been only one recorded fatality from a tsunami since European settlement. As a result, the size and nature of New Zealand’s tsunami risk is poorly understood. GNS Science modelling has highlighted that New Zealand’s major population centres risk exposure to tsunami of varying sizes from a range of sources. The worst-case scenario of a 1/500-year tsunami is projected to result in up to about 33,000 casualties, 27,000 injuries and $45bn in property loss.

These losses are based on a night-time event when people are expected to be at home. Casualties and injuries may be worse for a daytime event. However, the losses are also based on a scenario of very limited evacuation. Our science contributes to accurate predictions and warnings of the arrival of tsunami on our coasts and underpins the design of localized evacuation plans.

Tsunamis are usually caused by an earthquake under the seafloor. Other triggers include landslides, undersea volcanic eruptions, and meteorite impacts. These sudden disturbances cause the ocean to flow away from the disturbance, creating large tsunami waves.

In the open ocean, tsunami waves are small and can pass a ship unnoticed. As they encounter shallow water, they increase in height. Tsunamis are not just moving lumps on the ocean’s surface, they also include hollows. Sometimes the hollow reaches the coast first. When this happens, the ocean first draws back, sucking water away from coastline. It then rushes in with enormous speed and force as the lump arrives. In low-lying coastal areas, tsunamis can travel a long way inland.

A tsunami wave is not just seawater. It picks up debris as it moves, which gives it added destructive power. A retreating tsunami wave can cause as much damage as the initial forward surge.