Tsunami information for Christchurch after the earthquakes

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Tsunami information for Christchurch after the earthquakes

This page provides information on Christchurch and Pegasus Bay's tsunami threat in light of the earthquakes. Essentially, the known risk to Christchurch and Pegasus Bay has not changed, and our most likely threat remains a distant source tsunami from across the Pacific Ocean.

Local source tsunamis

• The local source tsunami risk in Christchurch and Pegasus Bay is low and has not changed as a result of the recent earthquakes.
• From the information we have, a local source tsunami from an earthquake fault in Pegasus Bay is thought to be unlikely. Scientists have discovered some earthquake faults in Pegasus Bay, but they appear to move very infrequently (once every few thousand or tens of thousands of years). They are not thought to be big enough, and would not produce enough vertical movement, to create a significant tsunami that would flood land.
• However, we cannot entirely rule out a local source tsunami, as there is a chance that there are faults we don’t know about, or that the known faults may behave differently than predicted. If you are near the coast (within, say, a couple of kilometres) and feel strong ground shaking that makes it hard to stand up, move inland or to higher ground as quickly as you can, using the safest route that you can.
• 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 in hillside suburbs. Think about weather conditions.
• 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.

Regional source tsunamis

• Regional source tsunamis may come from the east and north of the North Island and off the Fiordland coast.
• The most likely regional tsunami source is the Hikurangi subduction zone fault, off the Wairarapa/Hawkes Bay coast. Earthquakes are thought to happen here roughly every few thousand years but the timing of the last earthquake is not known.
• An earthquake on this fault will be felt in Christchurch as a moderate, rolling earthquake that lasts for more than a minute.
• A tsunami from this fault would take around 1-2 hours to reach Christchurch. It is unlikely to be large here, but it may flood very low lying areas. Sea heights above normal tide height at the coast here would probably be around 1-2 metres, but there may be localised areas where they would be more than that because of the way tsunamis slosh around in Pegasus Bay.
• It is likely that there will be time to sound warning sirens, when installed, for a Hikurangi subduction zone tsunami.
• However, the most important warning will be the long ground shaking from the earthquake. Do not wait for an official warning, or for the sirens to sound, to evacuate.

Distant source tsunamis

• Distant source tsunamis come from across the Pacific Ocean, most likely South America.
• This is our most likely tsunami threat. The four largest recorded tsunamis to hit Christchurch – 1868, 1877, 1960 and 2010 – were all distant source tsunamis from South America.
• Distant source tsunamis take 12-15 hours to reach New Zealand, so there is time to issue warnings and evacuate people if necessary.
• Tsunami sirens will be sounded for a distant source tsunami, once they are installed.
• You may not need to evacuate in a distant source tsunami – you may just need to stay out of the water and off the beach because of strong currents.
• If it is thought that a distant source tsunami could flood coastal land the Police and the Fire Service will evacuate areas indicated on the Coastal Evacuation Plan for Christchurch.
• The distant source tsunami evacuation zones are currently being reassessed given the changes to ground levels in Christchurch and Kaiapoi after the earthquakes. This may result in changes to the distant source tsunami evacuation zones in Christchurch and Kaiapoi.

Where should I evacuate to?

Think about possible evacuation routes for local, regional and distant source tsunamis, practice these with your family or friends, and write them on your Household Emergency Plan. Find out if there are vulnerable people nearby who may need your help.
The best route to take will depend on where you are and the type of tsunami. If it is a distant source tsunami, where there is time for official warnings to be given, evacuation routes are given in the Coastal Evacuation Plan for Christchurch and the Police will help people to evacuate. If you feel strong ground shaking and need to evacuate quickly, you may use the same route as for a distant source tsunami, or you may need to determine a faster route inland or to higher ground.

What about the Kaikoura Canyon?

If a tsunami happens in the Kaikoura Canyon, just south of Kaikoura, it would affect the coastline immediately south of Kaikoura (South Bay, Goose Bay and Oaro), but it would be too small by the time it got to Christchurch to do any damage. This is because this tsunami comes from one point, rather than an offshore earthquake fault line, so the waves created decrease very quickly from the source.

Why didn’t the September 2010 and February 2011 earthquakes cause tsunamis?

The September 2010, February 2011 and June 2011 earthquakes did not cause tsunamis because they happened on land and did not move the sea floor. The aftershocks out to sea from these earthquakes are too small to cause a tsunami. An earthquake at sea usually needs to be greater than magnitude 6.5 to cause even a small tsunami.

Are the warning sirens for Christchurch still coming?

Christchurch City Council is still working through the process of installing tsunami warning sirens. The sirens will not be used for a local tsunami. Sirens will only be used in the event of a distant source tsunami from across the Pacific Ocean, and possibly for a regional source tsunami from the Hikurangi subduction zone. There will not be time to sound the warning sirens if there is a local source tsunami in Pegasus Bay – earthquake shaking will be your only warning.

http://ecan.govt.nz/advice/emergenci...info-chch.aspx

[Sally Furniss]

bump this

we cannot entirely rule out a local source tsunami, as there is a chance that there are faults we don’t know about, or that the known faults may behave differently than predicted. If you are near the coast (within, say, a couple of kilometres) and feel strong ground shaking that makes it hard to stand up, move inland or to higher ground as quickly as you can, using the safest route that you can.

• 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.

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Christchurch City Council is installing sirens along nearly 11km of the coast to warn of incoming tsunamis . ... 47 sirens will be installed from Waimairi Beach in the north to Sumner in the south.

http://www.radionz.co.nz/news/canter...o-be-installed

[Sally Furniss]

Dr Kelvin Berryman, of GNS Science, ... a magnitude 7 quake offshore would create a small tsunami of 1 to 2 metres on the beach.

"There probably was a very small tsunami related to the [magnitude] 6 [on December 23]. That raises the tsunami question that we dealt with after February.

"Some modelling work was done at that time. We ran a hydrodynamic model ... If a magnitude 7 were to occur just offshore here there would be a small tsunami of one to two metres on the beach. The situation with respect to tsunami has not changed ... These magnitude 6s do not produce tsunamis. That is no problem at all.

"If there's a 7, don't wait for the siren to go or be told by Civil Defence to move off the beach. Self-evacuate the beach and go to somewhere above the beach."

Christchurch GNS Science quake press conference - full video - Video