Canterbury's tsunami hazard

A tsunami is a series of waves that move through the ocean (or a lake) caused by a sudden movement of the sea floor or by something falling into the ocean - just like the ripples on a pond when you throw a stone into it.

They are most often caused by undersea earthquakes, but can also be caused by landslides and volcanic eruptions.

The entire Canterbury coast is at risk from tsunamis, created both close to our shore and further away.  The sort of warning you get and how much time you have to move to higher ground depends on how far away the tsunami is coming from. The biggest tsunami threat for most of the Canterbury coast is from a tsunami coming from across the Pacific Ocean.  In contrast, along the Kaikōura and northern Hurunui coast the biggest tsunami threat comes from earthquake faults close to shore, created by the collision of two tectonic plates.

How are tsunami waves different to normal ocean waves?

Tsunami waves are not like normal ocean waves, which are created by the wind. Normal ocean waves only involve the top few metres of water in the ocean and come and go off the coast quickly without flooding higher areas.

Tsunami waves involve the whole depth of the ocean, not just the top few metres. They are long waves and travel across the deep, open ocean as fast as a jet plane. When tsunamis get to shallower water they slow down, bunch up and get higher.

Because a tsunami is a series of waves, it causes many waves or surges that arrive over several hours or even days. The first wave is often not the largest – the largest wave can arrive hours after the first wave.

When they reach shore, tsunami waves are often no higher than normal ocean waves, but they are more dangerous because they are they have much more water behind them and move very fast - a tsunami wave less than half a metre high will knock you off your feet. Often tsunamis look like the tide is coming in or going out very quickly, hence them also being known as 'tidal waves’.

Check out this video of one of the 2010 Chile tsunami waves coming into the Avon Heathcote Estuary (Ihutai) – it doesn’t look that high, but it packs a punch.

Most tsunamis are not big enough flood land, but they can cause strong and unpredictable currents and surges around the coast, which can be dangerous for people in the water or on the beach or rocks.  Occasionally, perhaps a few times in your lifetime if you live in Canterbury, a tsunami will be big enough to flood land.

One of the key messages is that although the tsunami waves didn't look that high, they were incredibly fast. That is what causes so much of the damage and so many injuries in a tsunami. 

GNS Science also have some great videos showing how tsunamis work.

Tsunamis created close to our shore

The Marlborough, Kaikōura and northern Hurunui coast lies in the zone where the Pacific and Australian tectonic plates collide. The enormous forces involved in this collision have broken the Earth’s crust to bits here, creating earthquake faults on land and under the sea in this area.  These faults can create tsunamis which can reach the shore in less than one hour (‘local source tsunamis’).

There are also earthquake faults in Pegasus Bay, off the Christchurch coast, but these are smaller and move less frequently than the faults further north.  These faults are unlikely to generate a tsunami that would be damaging along the open Pegasus Bay coast, including Christchurch, but that could be damaging at the heads of the northern Banks Peninsula bays, because of the way these bays tend to ‘funnel’ tsunami waves.

There are no known earthquake faults in the Canterbury Bight off the South Canterbury coast. 

The tsunami generated by the magnitude 7.8 November 2016 earthquake is the only damaging tsunami created close to our shore in Canterbury since European settlement, but there are pūrākau (Māori stories) of taniwha coming out of the sea and grabbing people off the beach. 

A damaging tsunami created close to our shore is less likely than one from across the Pacific Ocean, but there is less time to act if you need to. Because these tsunamis are created so close to our shore there is no time for Civil Defence to issue an official warning, or to sound any sirens (where they are installed). 

If you feel a long (more than a minute) or strong (hard to stand up) earthquake evacuate the red and orange tsunami evacuation zones as soon as the shaking stops. Don't wait for sirens, an emergency mobile alert, or an official warning. You can see if you are in a red or orange tsunami evacuation zone on our interactive map.

(Note that in other parts of the country, with a different tsunami risk, you may need to evacuate red, orange and yellow zones after a long or strong earthquake – check out the local evacuation zones and what they mean if you are going somewhere else.) 

While there are no known earthquake faults large enough to create a damaging local source tsunami off the South Canterbury coast, it is still wise to move inland or to higher ground if you feel a long or strong earthquake. 

Kaikōura Canyon

The Kaikōura Canyon lies just off Goose Bay, south of Kaikōura Peninsula. If the sands and gravels at the top of the canyon slide into the canyon, most likely during a strong earthquake, a tsunami could be created. 

Whether a landslide in the Kaikōura Canyon creates a tsunami, and how big it could be, depends on how much sediment moves and how fast, which is hard to determine beforehand. Research in 2004 showed that the area between South Bay and Oaro could be flooded with several metres of water within 10 minutes.

We, along with NIWA researchers on the RV Ikatere, are re-investigating the canyon tsunami hazard using data and techniques that weren’t available in 2004. Initial analysis of sea floor sediment suggests there are several small potential landslides at the head of the canyon rather than one large landslide. This could still create a tsunami, but a smaller one than previously thought. You can read the initial report here.  

Investigations prior to November 2016 were concentrating on how big these landslides are and how likely they are to move, and then mapping potential flooding from a tsunami. NIWA are currently resurveying the head of the canyon to reassess the underwater landslide hazard and potential tsunami flooding.

If a tsunami happened in the Kaikōura Canyon, it would affect the coastline between Kaikōura Peninsula and Oaro, but it would be too small by the time it got to Pegasus Bay and Christchurch to do any damage there or further south. This is because this tsunami would come from one point (a landslide) rather than a line (an earthquake fault) so the waves decrease in size very quickly from the source. 

Tsunamis from Fiordland or the North Island

The Canterbury coast could be affected by a tsunami created in the Puysegur Trench off the Fiordland coast, or in the Hikurangi and Kermadec trenches off the North Island. These tsunamis are called regional source tsunamis and would take between 1 and 3 hours to reach the closest piece of Canterbury coast.

Canterbury hasn’t experienced a regional source tsunami in its written history, so we don’t know much about them, apart from what geologists and tsunami modellers can tell us. The Kaikoura coastline is the most vulnerable part of Canterbury to these tsunamis as it is closest to the Hikurangi Trench.

We may get an official warning of a regional source tsunami, if there is time. It generally takes at least half an hour to confirm that a tsunami has been created from an earthquake, which doesn’t leave much time to issue a warning and get it out to people. The best warning is the earthquake itself, which in this case would be felt as a long, rolling earthquake throughout most of Canterbury.

If you feel a long (more than a minute) or strong (hard to stand up) earthquake evacuate the red and orange tsunami evacuation zones as soon as the shaking stops. Don't wait for sirens, an emergency mobile alert, or an official warning. You can see if you are in a red or orange tsunami evacuation zone on our interactive map.

(Note that in other parts of the country, with a different tsunami risk, you may need to evacuate red, orange and yellow zones after a long or strong earthquake – check out the local evacuation zones and what they mean if you are going somewhere else.)

NIWA have mapped what a ‘worst case’ regional source tsunami from the Hikurangi Trench might look like for Canterbury to help us with evacuation planning.  The modelled tsunami is an extreme event, only expected once every 2500 years. Most tsunamis we experience will be much smaller than this.  You can download the NIWA Hikurangi tsunami report here. 

Tsunamis from the Pacific Islands or across the Pacific Ocean

All of Canterbury's coastline can be affected by tsunamis from the Pacific Islands or from across the Pacific Ocean. These tsunamis are called distant source tsunamis and they take at least three, but usually more than 12 hours to reach our coast. 

For the Canterbury coast between Pegasus Bay and the Waitaki River mouth these tsunamis are the biggest and most likely tsunami threat. Four of the five damaging tsunamis that Canterbury has experienced - 1868, 1877, 1960 and 2010 – came from the South American coast. 

Pegasus Bay and Banks Peninsula are particularly vulnerable to tsunamis from across the Pacific Ocean because the Chatham Rise, the undersea ridge between the Chatham Islands and Banks Peninsula, tends to focus tsunami waves towards this area. Once the tsunami waves arrive the shape of the bays and harbours tend to channel and amplify the waves. 

You won’t feel the earthquake that creates a tsunami from the Pacific Islands or across the Pacific Ocean because it is too far away, but there will be time for Civil Defence to issue an official warning, and to say which tsunami evacuation zones people need to leave.    

In most official warnings you will only need to stay out of the red zone, which covers beaches, river mouths, estuaries and harbours, because of the strong and unpredictable currents and surges that even small tsunamis can create. 

NIWA have mapped what a ‘worst case’ distant source tsunami, created by a magnitude 9.5 earthquake off the coast of Peru, might look like for Canterbury to help with evacuation planning.

This is an extreme event, only expected once every 2500 years. Most tsunamis we experience will be much smaller than this.  You can download the NIWA South American tsunami report here. 

Why didn’t the 2010/11 Canterbury earthquakes cause tsunamis?

The September 2010, February 2011 and June 2011 earthquakes did not cause tsunamis because they happened on land and did not disrupt the sea floor.

The December 2011 and February 2016 earthquakes were off the coast, in Pegasus Bay, but the faults that moved were too small to get all the way to the surface and break the sea floor to create a tsunami.

The aftershocks from these earthquakes were also too small to cause a tsunami. An earthquake under the sea floor usually needs to be greater than magnitude 7 for the fault to cut all the way up through the rocks to the sea floor to cause even a small tsunami.