A tsunami is a series of waves created by the sudden movement of the ocean floor typically caused by earthquakes, but can also be from underwater landslides or volcanic eruptions. In deep ocean waters tsunami waves are generally widely spaced with small wave heights. When they enter shallower water as they approach land, they increase in height (tsunami can reach 10 metres and more), travel more slowly and the waves move closer together (though they remain much more widely spaced than wind waves or swell - typically tsunami waves can be 15 to 60 minutes or longer apart).
Large tsunami are very destructive to the coastline and adjacent land and present a significant hazard to human lives. Large tsunami coming ashore can be as a breaking wall of water or a rapid surge that may continue for up to 30 minutes or more before receding. Often, the next wave will begin coming ashore before the first one has had time to recede. The first wave may not be the largest. The surging water collects and carries much debris. The impacts from the debris carried by the inflowing and outflowing water can be very destructive.
Smaller tsunami, whilst less destructive to coastal communities can still present a considerable hazard with water surging onto beaches and low lying coastal land, with strong and unusual currents up into rivers, estuaries and harbours.
If you feel a long (more than a minute) or strong (hard to stand up) earthquake, and you are in a tsunami hazard zone, immediately get out of all tsunami evacuation zones. The earthquake will be the major natural warning of a tsunami. In the event of tsunami people should immediately head inland until they are well outside of the evacuation zone.
If there is an official warning from Civil Defence (in Nelson Tasman this includes media and social media alerts), then evacuate from the zones (red, orange or yellow) stated in the warning. Further information about warnings is contained below.
Natural warnings of a tsunami include:
- Strong earthquake shaking (i.e. it is hard to stand up or walk steadily, there is significant household contents damage and building damage)
- Weak, rolling earthquake shaking of unusually long duration (i.e. a minute or more)
- Out of ordinary sea behaviour, such as unusual and sudden sea level fall or rise
- The sea making loud and unusual noises, especially roaring like a jet engine
If there is time, an official warning will be issued:
- The Ministry of Civil Defence and Emergency Management issues official tsunami alerts for New Zealand
- The Nelson Tasman CDEM Group will relay these warnings and add local information
- To be informed, listen to radio/TV, and follow social media
- If you become aware of an urgent tsunami alert, please tell your neighbours. Think about vulnerable people you may know such as elderly or disabled people.
- New Zealand does not have a siren network for tsunami alerts. Sirens are used only in some places in NZ (mainly eastern coastline locations).
All of New Zealand’s coast is subject to tsunami hazard. The greatest hazard is on the east coast of both the North and South islands as these are directly exposed to greater number of tsunami sources.
Tasman Bay and Golden Bay are located on the northern coast of the South Island and open into the South Taranaki Bight. A tsunami approaching New Zealand from the east will still make its way through Cook Strait to Tasman and Golden Bay, however its size will be diminished.
Tasman Bay and Golden Bay are subject to tsunami hazard from various local, regional and distant sources. Local sources are where a tsunami will arrive onshore within one hour while regional sources are a tsunami with a one to three hour travel time. A distant tsunami has a travel time of greater than three hours.
Local sources that could impact Tasman Bay and Golden Bay include the offshore extension of the Waimea Fault and other undersea faults in the Taranaki Bight and Cook Strait.
Regional sources include the Hikurangi and Puysegur trenches and offshore faults of Wellington and Marlborough.
Distant sources include tectonic subduction zones around the Pacific rim from south America through Alaska and Japan to the New Hebrides and Solomon Islands. The Kermedec/Tonga trench, although closer than the Pacific rim, is also a distance source of tsunami for Tasman and Golden bays.