Distant source tsunamis are created more than three hours' travel time from the Canterbury coast.
For the Canterbury coast between the Waitaki River mouth and the Hurunui coast the biggest tsunami threat is a distant-source tsunami generated off the coast of South America, and possibly Central America. The four largest recorded tsunamis to hit Canterbury, in 1868, 1877, 1960 and 2010, were all distant-source tsunamis from South America. Investigations into pre-historic tsunami sediments in Canterbury also suggest South and Central America as the most likely source of tsunamis. We have 12-15 hours' warning of tsunamis from South America.
Tsunami waves travel across the open ocean very quickly - up to 900 km/h. The waves are very long and involve the whole depth of the water, not just the surface of the water like normal waves or swell. When they get to shallower water the waves slow down and bunch up - they get slower, shorter and higher. When they arrive at the shore, they may not be very high, but they still have all that energy behind them, and can move a long way inland, or create strong currents in the water.
When coming from a distant source, like South America, a tsunami will generally arrive in Canterbury as a series of surges. Often it will look like the tide is coming in and going out many times over a short period of time.
Banks Peninsula is particularly vulnerable to distant source tsunamis because the Chatham Rise tends to focus tsunami waves towards it and the elongated bays and harbours channel and amplify tsunami waves.
Our 2010 Chilean tsunami page holds more information on what happened along the Canterbury coast during this distant source event.
Environment Canterbury is currently reassessing potential flooding in Christchurch and Kaiapoi from a worst-case distant tsunami, given the changes in ground level in these areas after the 2010 and 2011 earthquakes. This will likely result in changes to coastal evacuation zones for distant tsunamis in these areas.
New Zealand receives warnings of distant source tsunamis from the Pacific Tsunami Warning Center (PTWC) in Hawaii. The PTWC continuously monitors earthquake activity and sea levels in the Pacific region using data from participating nations, including New Zealand.
PTWC tsunami watches and warnings are sent to the Ministry of Civil Defence and Emergency Management (MCDEM) in New Zealand, and are then passed on to regional Civil Defence Emergency Management (CDEM) groups, local authorities and emergency services. Scientists from the Institute of Geological and Nuclear Sciences model the likely tsunami surge heights and pass these on to MCDEM and CDEM groups. Decisions to evacuate areas are made by the local council. While scientists can model what a tsunami is likely to do, many factors influence the impact a tsunami will have, and these are difficult to predict. Also, the behaviour of a tsunami, and the impact it can have, can vary a lot over a very short length of coast.
Most tsunamis only cause strong currents in the sea and surges on beaches. In this case a tsunami warning from MCDEM or your local council will only advise to stay off beaches and out of the water. If the tsunami is big enough that it is likely to flood land your local council will make a decision to evacuate certain areas.
Check with your local city or district council or community group to see if you are in a tsunami evacuation zone - an area that will be evacuated if a confirmed warning of a tsunami that is likely to flood land is received - and where you should go if you are evacuated. Current evacuation zones for Christchurch City can be found in Christchurch City Council's Evacuation information for coastal Christchurch brochure.
The Pacific Tsunami Warning Centre can only warn us about distant source tsunamis, it cannot provide us with warnings for local source and most regional source tsunamis because they happen too close to shore. Your only warning of a local source tsunami will be an earthquake, or the sea behaving strangely.
Regional source tsunamis are created between one and three hours' travel time from the Canterbury coast.
The threat to Canterbury from regional source tsunamis is smaller than that from distant source tsunamis. However, an earthquake in the Hikurangi Trench, which marks the Hikurangi subduction zone off the east coast of the North Island, could generate a tsunami that could affect North Canterbury. Modelling by GNS Science shows that a tsunami from this area could cause 1-2 metre surges along the Canterbury coast within 30 minutes (Kaikoura) to 2 hours (Banks Peninsula) of the earthquake. This makes this tsunami a local source tsunami for Kaikoura. The surges may be higher than this is localised areas where tsunami waves slosh back and foward, such as in Pegasus Bay. Earthquakes in the Hikurangi Trench are thought to happen once every few thousand years, but we do not know when the last earthquake happened there.
Environment Canterbury is planning to further investigate the potential impacts of a Hikurangi Subduction Zone tsunami on the Canterbury coast within the next few years.
Other regional tsunami sources include the Kermadec and Tonga trenches to the northeast of the North Island, and the Puysegur Trench to the south of Fiordland. These sources are further away and any tsunami generated by earthquakes in these areas would not be directed straight at Canterbury. A tsunami from these sources is unlikely to be damaging along the Canterbury coastline, although it may cause damage along coastlines closer to the source.
You may get official warning of a regional source tsunami, but only if it happens far enough away so there is time to issue a warning. Because regional tsunamis are created one to three hours' travel time away, there is little time between when the tsunami is generated to when it hits the shore. By the time GeoNet has located the earthquake that caused the tsunami, and their tsunami gauges confirm that a tsunami has been generated, there may only be an hour or two to issue a warning and evacuate. Your best warning for a regional source tsunami is a long rolling earthquake. If you are near the coast (within 1-2 km) and feel a long rolling earthquake move to higher ground or inland. Use your common sense and do not wait for an official warning to evacuate.
Local source tsunamis are created less than one hour away from the coast. In Canterbury the only known potential local sources big enough to generate a damaging tsunami are off the Kaikoura coast (shown as bold black lines on the map to the right). These include active faults on the sea floor, and a possible undersea landslide into the Kaikoura Canyon. A tsunami from one of these sources would reach the Kaikoura coastline within minutes and could be very damaging. It would be a very localised event though - the waves would reduce quickly and would be unlikely to cause much damage beyond Kaikoura district.
There is no written record of a local-source tsunami having affected the Kaikoura coast, although there are Maori myths of taniwha coming out of the sea and grabbing people off the beach. Two local-source tsunami scenarios have been modelled for the Kaikoura coast, however these remain credible, but not proven, possibilities.
Scientists have discovered some earthquake faults in Pegasus Bay during surveys undertaken after the February 2011 earthquake. These faults are thought to move very infrequently (once every few thousand or tens of thousands of years) and are not thought to be big enough to generate significant tsunamis. There are no known local tsunami sources offshore of mid or south Canterbury. The biggest known tsunami threat for areas south of Hurunui district remains a distant-source tsunami.
Although the only known potential local tsunami sources in Canterbury are off the North Canterbury coast we still recommend that, like anywhere in New Zealand, if you feel strong ground shaking (where you have difficulty standing) or mild shaking that goes on for a minute or more, that you move to higher ground or as far inland as you can. If a tsunami has been generated locally, it tsunami will reach the coast within an hour.
There are no local tsunami warning systems in New Zealand. This is because the time between the tsunami being generated and it hitting the shore is too short to be able to give a warning. Remember, it takes at least 10 minutes for GeoNet to accurately locate an earthquake, let alone whether it has created a tsunami or not. A tsunami may have arrived at the coast near the earthquake in that time.
There will not be time to give an official warning or to sound any warning sirens for a local source tsunami. Do not wait for an official warning, or for any sirens to sound, to evacuate.
The sirens planned for the Christchurch coastline, and those already in place along other parts of the Canterbury coast are NOT intended to warn you about a local source tsunami. They are for distant source and perhaps regional source (if it is generated far enough away) tsunami warnings only. A strong earthquake will be your only warning.
If you evacuate after strong ground shaking at the coast, walk or cycle if you can, to avoid traffic jams. Only use your car if you have to. You will need to use common sense to work out the best route to use from where you are. Be aware that if the ground shaking is strong, there may be unstable cliffs and rockfalls. Think about weather conditions.
There are many things you can do to lessen the impact of a tsunami on you and your family.
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