The July 2011 tsunami alert caused by an earthquake near the Kermadec Islands reminds us New Zealand’s entire coast may be at risk of tsunami. A large tsunami could flood coastlines and cause devastating property damage, injuries and possibly loss of life.
A tsunami is a sea wave caused by large submarine or coastal earthquakes, under-sea landslides, under-sea volcanic eruptions or even big objects, such as meteorites, falling into the sea.
Tsunami waves can travel at about 600km per hour across deep oceans – as fast as some jet planes – and can be 10-30 metres high when they reach land.
The Pacific Tsunami Warning Center in Hawaii detects and warns countries about tsunami that may affect them. However, a tsunami generated close to NZ’s coast could arrive so quickly there is little or no time to warn people.
Three types of tsunami
There are three distinct types of tsunami. The type you encounter depends on the distance you are from the place where it is generated.
• Distant tsunami are generated from a long way away, such as Japan. In this case, we will have more than three hours warning time for New Zealand.
• Regional tsunami are generated between one and three hours’ travel time away from their destination. An eruption from an underwater volcano in the Kermadec Trench to the north of New Zealand could generate a regional tsunami.
• Local tsunami are generated very close to New Zealand. This type of tsunami is very dangerous because we may only have a few minutes warning.
Know the natural warning signs
If you are at the coast and experience any of the following, move immediately to the nearest high ground, or as far inland as you can:
• Feel a strong earthquake that makes it hard to stand up, or a weak rolling earthquake that lasts a minute or more.
• See an unusual or sudden rise or fall in sea level.
• Hear loud and unusual noises from the sea.
Prepare before it happens
• Develop a household emergency plan and have a getaway kit ready.
• Know where the nearest high ground is and how you will reach it. Plan to get as high up or as far inland as you can. Plan your escape route for when you are at home, as well as for when you may be working or holidaying near the coast.
What to do during a tsunami
• Take your getaway kit with you if possible. Do not travel into the areas at risk to get your kit or belongings.
• Move immediately to the nearest higher ground, or as far inland as you can. If evacuation maps are present, follow the routes shown.
• Walk or bike if possible and drive only if essential. If driving, keep going once you are well outside the evacuation zone to allow room for others behind you.
• If you cannot escape the tsunami, go to an upper storey of a sturdy building or climb onto a roof or up a tree.
• Never go to the shore to watch for a tsunami. Stay away from at-risk areas until the official all-clear is given.
• Listen to your local radio stations as emergency management officials will be broadcasting the most appropriate advice for your community and situation.
Our experience with recent tsunamis has shown us strong surges are experienced flowing in and out of the Tauranga Harbour entrance.
These surges have been up to four knots in the case of the Japan tsunami, changing direction roughly every 10-20 minutes. The surges peaked several hours after the initial predicted wave and lasted – gradually decreasing in intensity – for several days.
This effect would be likely anywhere there are strong currents, for example at river mouths, around prominent headlands and between islands.
After a tsunami
• Continue to listen to the radio for Civil Defence advice and do not return to the evacuation zones until authorities have given the all-clear.
• Be aware there will probably be more than one wave and it may not be safe for up to 24 hours, or longer. The waves that follow the first one are likely to be bigger.
• Stay away from boat ramps and beaches.
Gisborne’s 1947 tsunami
In March 1947 an earthquake off Poverty Bay, probably accompanied by an underwater landslide, produced a 10m high tsunami north of Gisborne.
The force of the wave was enough to break off large fence posts at ground level. Seaweed was found in overhead wires. At Tatapouri Hotel outbuildings were damaged and destroyed, and fish were collected from inside the hotel itself.
Kermadec Trench tsunami
An earthquake deep under the sea in the Kermadec Trench off the north east of New Zealand could generate a tsunami that strikes the coast less than an hour after the event. Initially, depending on the severity of the under-sea quake, waves up to five metres could hit the shore, rising to eight metres on shallower stretches of coast. Large waves are likely to hit the coast for several hours after the first strikes.
Boats are usually safer in water deeper than 20 metres than if they are near the shore. Move your boat out to sea only if there is time and it is safe to do so. If your boat is kept in a marina and there’s not sufficient time to motor into deep water, double up on all your mooring lines, disconnect any shore power connections and get yourself to a safe place on high ground in good time.
After the tsunami, expect unusual currents in harbour and river entrances and assume there will be unpredictable tide changes possibly lasting for up to several days.
Fishing is reputedly poor around the time of a tsunami, so there’s no reason to head out in the boat until it all settles down.