Some people know them as 'tidal waves' but they are not caused by tides. A tsunami is a series of sea waves generated by an earthquake, a large underwater landslide or volcanic activity.
An international warning system, based in Hawaii, attempts to predict any activity that could lead to a tsunami in the Pacific Ocean.
The Ministry of Civil Defence and Emergency Management issues warnings about tsunami through the Police and Civil Defence organisations, and via radio and television broadcasts.
Wanganui has records of tsunami occurring after the 1868 earthquake. A 'bore' travelled some distance up the Whanganui River. There is no record of the damage this may have caused or how far up the river it went.
Another tsunami was recorded on Christmas Day, 1922, at Castlecliff Beach. Again, there is no record of damage.
The most tsunami-prone areas in New Zealand are between East Cape and Napier, the Cook Strait area, the area around Banks Peninsula and the East Coast of the South Island.
Civil Defence sirens have been installed at Castlecliff and Mowhanau to help warn people of tsunamis.
The continuous sounding of sirens sited along the coast and the lower Whanganui River estuary up to Putiki will give warning that an emergency is taking place.
Click here to listen to “The Sting”, the electronic warning sound which will be broadcast for official Civil Defence warnings. The local Wanganui urban sirens use the exact same sound. To then find out what is happening, please listen to your radio.
Near source tsunami
A "near source" tsunami is generated close to the affected coastline. The water level may fall very quickly past the normal low tide mark, then return just as quickly. If this happens, there won't be enough time to issue warnings.
What you can do to help yourself
When a tsunami threatens:
- Turn on your radio and follow all instructions.
- If you are told to evacuate, do so immediately.
- Leave the area immediately if you are on the beach or near a river mouth when a strong earthquake occurs.
- Go at least one kilometre inland or 35 metres above sea level.
- Tsunamis usually 'draw up' water before they come inland, so if you see the water level at a beach or river start to go down quickly (after an earthquake) it is probably a tsunami. Leave the area immediately.
- NIWA has more information on tsunamis.
The following article was sourced from the NIWA/GNS Natural Hazard Centre.
Tsunami: how will you know one's coming?
First, GeoNet monitors earthquakes, volcanoes, and underwater landslides in the New Zealand region round the clock.
Second, a system of sea-level gauges is required to confirm whether there is a tsunami or not. For example, on Christmas Eve 2004, there was a magnitude 8.1 quake to the west of Auckland Islands, but it generated only small waves up to 20cm high which arrived at NIWA's Dog Island gauge in Foveaux Strait 3 hours later.
Tsunami from the other side of the Pacific are detected by the Pacific Tsunami Warning Center.
Closer to home, NIWA coordinates and disseminates data on tsunami, storm-tides, and other coastal hazards from a network of 21 gauges around New Zealand, including Chatham Islands and Antarctica (Scott Base). Over half of the gauges are operated by outside organisations who have partnered with us. Data are uploaded nightly and some sites are displayed at www.niwa.co.nz/services/sealevels/. Currently, any emergency response in detecting and confirming a tsunami is undertaken on a best-endeavours basis. Plans for the near future include upgrading the service so it can deliver a round-the-clock service to emergency managers.
Once a tsunami is confirmed, emergency managers can activate public warnings and evacuation plans. However, if a tsunami is generated very close to shore, it may take only a few minutes to reach the coast with little time for warnings. In vulnerable coastal areas, people need to be forewarned of the risk and move to safer ground when they feel a strong earthquake or notice unusual sea behaviour.
For more information, contact:
07 859 1894
Or call free on 0800 RING NIWA (0800 746 464)